Minor fixes from Bernd.

[s-roff.git] / doc / groff.texinfo

\input texinfo   @c -*-texinfo-*-

@c
@c Please convert this manual with `texi2dvi -e groff.texinfo' due to a bug
@c in texinfo regarding expansion of user-defined macros.
@c
@c You need texinfo 4.2 or newer to format this document!
@c

@c %**start of header (This is for running Texinfo on a region.)
@setfilename groff
@settitle The GNU Troff Manual
@setchapternewpage odd
@footnotestyle separate
@c %**end of header (This is for running Texinfo on a region.)


@smallbook

@finalout


@copying
This manual documents GNU @code{troff} version 1.18.

Copyright @copyright{} 1994-2000, 2001, 2002 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover texts being `A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled `GNU Free Documentation
License.''

(a) The FSF's Back-Cover Text is: `You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''
@end quotation
@end copying


@c We use the following indices:
@c
@c   cindex: concepts
@c   rqindex: requests
@c   esindex: escapes
@c   vindex: registers
@c   kindex: commands in font files
@c   pindex: programs and files
@c   tindex: environment variables
@c   maindex: macros
@c   stindex: strings
@c   opindex: operators
@c
@c tindex and cindex are merged.

@defcodeindex rq
@defcodeindex es
@defcodeindex ma
@defcodeindex st
@defcodeindex op
@syncodeindex tp cp


@c to avoid uppercasing in @deffn while converting to info, we define
@c our special @Var{}
@c
@c due to a (not officially documented) `feature' in makeinfo 4.0,
@c macros are not expanded in @deffn (but the macro definition is
@c properly removed), so we have to define @Var{} directly in TeX also

@macro Var{arg}
\arg\
@end macro
@tex
\gdef\Var#1{\var{#1}}
@end tex


@c To assure correct HTML translation, some ugly hacks are necessary.
@c While processing a @def... request, the HTML translator looks at the
@c next line to decide whether it should start indentation or not.  If
@c it is something starting with @def... (e.g. @deffnx), it doesn't.
@c So we must assure during macro expansion that a @def... is seen.
@c
@c The following macros have to be used:
@c
@c One item:
@c
@c   @Def...
@c
@c Two items:
@c
@c   @Def...List
@c   @Def...ListEnd
@c
@c More than two:
@c
@c   @Def...List
@c   @Def...Item
@c   @Def...Item
@c   ...
@c   @Def...ListEnd
@c
@c The definition block must end with
@c
@c   @endDef...
@c
@c The above is valid for texinfo 4.0f.


@c a dummy macro to assure the `@def...'

@macro defdummy
@end macro


@c definition of requests

@macro Defreq{name, arg}
@deffn Request @t{.\name\} \arg\
@rqindex \name\
@end macro

@macro DefreqList{name, arg}
@deffn Request @t{.\name\} \arg\
@defdummy
@rqindex \name\
@end macro

@macro DefreqItem{name, arg}
@deffnx Request @t{.\name\} \arg\
@defdummy
@rqindex \name\
@end macro

@macro DefreqListEnd{name, arg}
@deffnx Request @t{.\name\} \arg\
@rqindex \name\
@end macro

@macro endDefreq
@end deffn
@end macro


@c definition of escapes

@macro Defesc{name, delimI, arg, delimII}
@deffn Escape @t{\name\\delimI\}@var{\arg\}@t{\delimII\}
@esindex \name\
@end macro

@macro DefescList{name, delimI, arg, delimII}
@deffn Escape @t{\name\\delimI\}@var{\arg\}@t{\delimII\}
@defdummy
@esindex \name\
@end macro

@macro DefescItem{name, delimI, arg, delimII}
@deffnx Escape @t{\name\\delimI\}@var{\arg\}@t{\delimII\}
@defdummy
@esindex \name\
@end macro

@macro DefescListEnd{name, delimI, arg, delimII}
@deffnx Escape @t{\name\\delimI\}@var{\arg\}@t{\delimII\}
@esindex \name\
@end macro

@macro endDefesc
@end deffn
@end macro


@c definition of registers

@macro Defreg{name}
@deffn Register @t{\\n[\name\]}
@vindex \name\
@end macro

@macro DefregList{name}
@deffn Register @t{\\n[\name\]}
@defdummy
@vindex \name\
@end macro

@macro DefregItem{name}
@deffnx Register @t{\\n[\name\]}
@defdummy
@vindex \name\
@end macro

@macro DefregListEnd{name}
@deffnx Register @t{\\n[\name\]}
@vindex \name\
@end macro

@macro endDefreg
@end deffn
@end macro


@c definition of registers specific to macro packages, preprocessors, etc.

@macro Defmpreg{name, package}
@deffn Register @t{\\n[\name\]}
@vindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefmpregList{name, package}
@deffn Register @t{\\n[\name\]}
@defdummy
@vindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefmpregItem{name, package}
@deffnx Register @t{\\n[\name\]}
@defdummy
@vindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefmpregListEnd{name, package}
@deffnx Register @t{\\n[\name\]}
@vindex \name\ @r{[}\package\@r{]}
@end macro

@macro endDefmpreg
@end deffn
@end macro


@c definition of macros

@macro Defmac{name, arg, package}
@defmac @t{.\name\} \arg\
@maindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefmacList{name, arg, package}
@defmac @t{.\name\} \arg\
@defdummy
@maindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefmacItem{name, arg, package}
@defmacx @t{.\name\} \arg\
@defdummy
@maindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefmacListEnd{name, arg, package}
@defmacx @t{.\name\} \arg\
@maindex \name\ @r{[}\package\@r{]}
@end macro

@macro endDefmac
@end defmac
@end macro


@c definition of strings

@macro Defstr{name, package}
@deffn String @t{\\*[\name\]}
@stindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefstrList{name, package}
@deffn String @t{\\*[\name\]}
@defdummy
@stindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefstrItem{name, package}
@deffnx String @t{\\*[\name\]}
@defdummy
@stindex \name\ @r{[}\package\@r{]}
@end macro

@macro DefstrListEnd{name, package}
@deffnx String @t{\\*[\name\]}
@stindex \name\ @r{[}\package\@r{]}
@end macro

@macro endDefstr
@end deffn
@end macro


@c our example macro

@macro Example
@example
@group
@end macro

@macro endExample
@end group
@end example
@end macro


@c <text>

@tex
\gdef\angles#1{\angleleft{}\r{#1}\angleright{}}
@end tex

@macro angles{text}
<\text\>
@end macro


@c a <= sign

@tex
\gdef\LE{\le}
@end tex

@macro LE
<=
@end macro


@c due to a bug in texinfo 4.2, the spacing of `<' is bad in @item

@tex
\gdef\LT{\string<}
@end tex

@macro LT
<
@end macro


@c We need special parentheses and brackets:
@c
@c . Real parentheses in @deffn produce an error while compiling with
@c   TeX
@c . Real brackets use the wrong font in @deffn, overriding @t{}.
@c
@c Since macros aren't expanded in @deffn during -E, the following
@c definitions are for non-TeX only.
@c
@c This is true for texinfo 4.0.

@macro lparen
(
@end macro
@macro rparen
)
@end macro
@macro lbrack
[
@end macro
@macro rbrack
]
@end macro


@tex
\gdef\gobblefirst#1#2{#2}
\gdef\putwordAppendix{\gobblefirst}
@end tex


@c Note: We say `Roman numerals' but `roman font'.


@dircategory Miscellaneous
@direntry
* Groff: (groff).               The GNU troff document formatting system.
@end direntry


@titlepage
@title groff
@subtitle The GNU implementation of @code{troff}
@subtitle Edition 1.18
@subtitle Spring 2002
@author by Trent A.@w{ }Fisher
@author and Werner Lemberg (@email{bug-groff@@gnu.org})

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage


@contents


@ifinfo
@node Top, Introduction, (dir), (dir)
@top GNU troff

@insertcopying
@end ifinfo

@menu
* Introduction::
* Invoking groff::
* Tutorial for Macro Users::
* Macro Packages::
* gtroff Reference::
* Preprocessors::
* Output Devices::
* File formats::
* Installation::
* Copying This Manual::
* Request Index::
* Escape Index::
* Operator Index::
* Register Index::
* Macro Index::
* String Index::
* Glyph Name Index::
* Font File Keyword Index::
* Program and File Index::
* Concept Index::
@end menu



@c =====================================================================
@c =====================================================================

@node Introduction, Invoking groff, Top, Top
@chapter Introduction
@cindex introduction

GNU @code{troff} (or @code{groff}) is a system for typesetting
documents.  @code{troff} is very flexible and has been in existence (and
use) for about 3@w{ }decades.  It is quite widespread and firmly
entrenched in the @acronym{UNIX} community.

@menu
* What Is groff?::
* History::
* groff Capabilities::
* Macro Package Intro::
* Preprocessor Intro::
* Output device intro::
* Credits::
@end menu


@c =====================================================================

@node What Is groff?, History, Introduction, Introduction
@section What Is @code{groff}?
@cindex what is @code{groff}?
@cindex @code{groff} -- what is it?

@code{groff} belongs to an older generation of document preparation
systems, which operate more like compilers than the more recent
interactive @acronym{WYSIWYG}@footnote{What You See Is What You Get}
systems.  @code{groff} and its contemporary counterpart, @TeX{}, both
work using a @dfn{batch} paradigm: The input (or @dfn{source}) files are
normal text files with embedded formatting commands.  These files can
then be processed by @code{groff} to produce a typeset document on a
variety of devices.

Likewise, @code{groff} should not be confused with a @dfn{word
processor}, since that term connotes an integrated system that includes
an editor and a text formatter.  Also, many word processors follow the
@acronym{WYSIWYG} paradigm discussed earlier.

Although @acronym{WYSIWYG} systems may be easier to use, they have a
number of disadvantages compared to @code{troff}:

@itemize @bullet
@item
They must be used on a graphics display to work on a document.

@item
Most of the @acronym{WYSIWYG} systems are either non-free or are not
very portable.

@item
@code{troff} is firmly entrenched in all @acronym{UNIX} systems.

@item
It is difficult to have a wide range of capabilities available within
the confines of a GUI/window system.

@item
It is more difficult to make global changes to a document.
@end itemize

@quotation
``GUIs normally make it simple to accomplish simple actions and
impossible to accomplish complex actions.''  --Doug Gwyn (22/Jun/91 in
@code{comp.unix.wizards})
@end quotation


@c =====================================================================

@node History, groff Capabilities, What Is groff?, Introduction
@section History
@cindex history

@cindex @code{runoff}, the program
@cindex @code{rf}, the program
@code{troff} can trace its origins back to a formatting program called
@code{runoff}, written by J.@w{ }E.@w{ }Saltzer, which ran on MIT's CTSS
operating system in the mid-sixties.  This name came from the common
phrase of the time ``I'll run off a document.''  Bob Morris ported it to
the 635 architecture and called the program @code{roff} (an abbreviation
of @code{runoff}).  It was rewritten as @code{rf} for the @w{PDP-7}
(before having @acronym{UNIX}), and at the same time (1969), Doug
McIllroy rewrote an extended and simplified version of @code{roff} in
the @acronym{BCPL} programming language.

@cindex @code{roff}, the program
The first version of @acronym{UNIX} was developed on a @w{PDP-7} which
was sitting around Bell Labs.  In 1971 the developers wanted to get a
@w{PDP-11} for further work on the operating system.  In order to
justify the cost for this system, they proposed that they would
implement a document formatting system for the @acronym{AT&T} patents
division.  This first formatting program was a reimplementation of
McIllroy's @code{roff}, written by J.@w{ }F.@w{ }Ossanna.

@cindex @code{nroff}, the program
When they needed a more flexible language, a new version of @code{roff}
called @code{nroff} (``Newer @code{roff}'') was written.  It had a much
more complicated syntax, but provided the basis for all future versions.
When they got a Graphic Systems CAT Phototypesetter, Ossanna wrote a
version of @code{nroff} that would drive it.  It was dubbed
@code{troff}, for ``typesetter @code{roff}'', although many people have
speculated that it actually means ``Times @code{roff}'' because of the
use of the Times font family in @code{troff} by default.  As such, the
name @code{troff} is pronounced `@w{t-roff}' rather than `trough'.

With @code{troff} came @code{nroff} (they were actually the same program
except for some @samp{#ifdef}s), which was for producing output for line
printers and character terminals.  It understood everything @code{troff}
did, and ignored the commands which were not applicable (e.g.@: font
changes).

Since there are several things which cannot be done easily in
@code{troff}, work on several preprocessors began.  These programs would
transform certain parts of a document into @code{troff}, which made a
very natural use of pipes in @acronym{UNIX}.

The @code{eqn} preprocessor allowed mathematical formul@ae{} to be
specified in a much simpler and more intuitive manner.  @code{tbl} is a
preprocessor for formatting tables.  The @code{refer} preprocessor (and
the similar program, @code{bib}) processes citations in a document
according to a bibliographic database.

Unfortunately, Ossanna's @code{troff} was written in @w{PDP-11} assembly
language and produced output specifically for the CAT phototypesetter.
He rewrote it in C, although it was now 7000@w{ }lines of uncommented
code and still dependent on the CAT.  As the CAT became less common, and
was no longer supported by the manufacturer, the need to make it support
other devices became a priority.  However, before this could be done,
Ossanna was killed in a car accident.

@pindex ditroff
@cindex @code{ditroff}, the program
So, Brian Kernighan took on the task of rewriting @code{troff}.  The
newly rewritten version produced device independent code which was
very easy for postprocessors to read and translate to the appropriate
printer codes.  Also, this new version of @code{troff} (called
@code{ditroff} for ``device independent @code{troff}'') had several
extensions, which included drawing functions.

Due to the additional abilities of the new version of @code{troff},
several new preprocessors appeared.  The @code{pic} preprocessor
provides a wide range of drawing functions.  Likewise the @code{ideal}
preprocessor did the same, although via a much different paradigm.  The
@code{grap} preprocessor took specifications for graphs, but, unlike
other preprocessors, produced @code{pic} code.

James Clark began work on a GNU implementation of @code{ditroff} in
early@w{ }1989.  The first version, @code{groff}@w{ }0.3.1, was released
June@w{ }1990.  @code{groff} included:

@itemize @bullet
@item
A replacement for @code{ditroff} with many extensions.

@item
The @code{soelim}, @code{pic}, @code{tbl}, and @code{eqn} preprocessors.

@item
Postprocessors for character devices, @sc{PostScript}, @TeX{} DVI, and
X@w{ }Windows.  GNU @code{troff} also eliminated the need for a
separate @code{nroff} program with a postprocessor which would produce
@acronym{ASCII} output.

@item
A version of the @file{me} macros and an implementation of the
@file{man} macros.
@end itemize

Also, a front-end was included which could construct the, sometimes
painfully long, pipelines required for all the post- and preprocessors.

Development of GNU @code{troff} progressed rapidly, and saw the
additions of a replacement for @code{refer}, an implementation of the
@file{ms} and @file{mm} macros, and a program to deduce how to format a
document (@code{grog}).

It was declared a stable (i.e.@: non-beta) package with the release of
version@w{ }1.04 around November@w{ }1991.

Beginning in@w{ }1999, @code{groff} has new maintainers (the package was
an orphan for a few years).  As a result, new features and programs like
@code{grn}, a preprocessor for gremlin images, and an output device to
produce @acronym{HTML} output have been added.


@c =====================================================================

@node groff Capabilities, Macro Package Intro, History, Introduction
@section @code{groff} Capabilities
@cindex @code{groff} capabilities
@cindex capabilities of @code{groff}

So what exactly is @code{groff} capable of doing?  @code{groff} provides
a wide range of low-level text formatting operations.  Using these, it
is possible to perform a wide range of formatting tasks, such as
footnotes, table of contents, multiple columns, etc.  Here's a list of
the most important operations supported by @code{groff}:

@itemize @bullet
@item
text filling, adjusting, and centering

@item
hyphenation

@item
page control

@item
font and glyph size control

@item
vertical spacing (i.e.@: double-spacing)

@item
line length and indenting

@item
macros, strings, diversions, and traps

@item
number registers

@item
tabs, leaders, and fields

@item
input and output conventions and character translation

@item
overstrike, bracket, line drawing, and zero-width functions

@item
local horizontal and vertical motions and the width function

@item
three-part titles

@item
output line numbering

@item
conditional acceptance of input

@item
environment switching

@item
insertions from the standard input

@item
input/output file switching

@item
output and error messages
@end itemize


@c =====================================================================

@node Macro Package Intro, Preprocessor Intro, groff Capabilities, Introduction
@section Macro Packages
@cindex macro packages

Since @code{groff} provides such low-level facilities, it can be quite
difficult to use by itself.  However, @code{groff} provides a
@dfn{macro} facility to specify how certain routine operations (e.g.@w{
}starting paragraphs, printing headers and footers, etc.)@: should be
done.  These macros can be collected together into a @dfn{macro
package}.  There are a number of macro packages available; the most
common (and the ones described in this manual) are @file{man},
@file{mdoc}, @file{me}, @file{ms}, and @file{mm}.


@c =====================================================================

@node Preprocessor Intro, Output device intro, Macro Package Intro, Introduction
@section Preprocessors
@cindex preprocessors

Although @code{groff} provides most functions needed to format a
document, some operations would be unwieldy (e.g.@: to draw pictures).
Therefore, programs called @dfn{preprocessors} were written which
understand their own language and produce the necessary @code{groff}
operations.  These preprocessors are able to differentiate their own
input from the rest of the document via markers.

To use a preprocessor, @acronym{UNIX} pipes are used to feed the output
from the preprocessor into @code{groff}.  Any number of preprocessors
may be used on a given document; in this case, the preprocessors are
linked together into one pipeline.  However, in @code{groff}, the user
does not need to construct the pipe, but only tell @code{groff} what
preprocessors to use.

@code{groff} currently has preprocessors for producing tables
(@code{tbl}), typesetting equations (@code{eqn}), drawing pictures
(@code{pic} and @code{grn}), and for processing bibliographies
(@code{refer}).  An associated program which is useful when dealing with
preprocessors is @code{soelim}.

A free implementation of @code{grap}, a preprocessor for drawing graphs,
can be obtained as an extra package; @code{groff} can use @code{grap}
also.

There are other preprocessors in existence, but, unfortunately, no free
implementations are available.  Among them are preprocessors for drawing
mathematical pictures (@code{ideal}) and chemical structures
(@code{chem}).


@c =====================================================================

@node Output device intro, Credits, Preprocessor Intro, Introduction
@section Output Devices
@cindex postprocessors
@cindex output devices
@cindex devices for output

@code{groff} actually produces device independent code which may be
fed into a postprocessor to produce output for a particular device.
Currently, @code{groff} has postprocessors for @sc{PostScript}
devices, character terminals, X@w{ }Windows (for previewing), @TeX{}
DVI format, HP LaserJet@w{ }4 and Canon LBP printers (which use
@acronym{CAPSL}), and @acronym{HTML}.


@c =====================================================================

@node Credits,  , Output device intro, Introduction
@section Credits
@cindex credits

Large portions of this manual were taken from existing documents, most
notably, the manual pages for the @code{groff} package by James Clark,
and Eric Allman's papers on the @file{me} macro package.

The section on the @file{man} macro package is partly based on Susan@w{
}G.@: Kleinmann's @file{groff_man} manual page written for the Debian
GNU/Linux system.



@c =====================================================================
@c =====================================================================

@node Invoking groff, Tutorial for Macro Users, Introduction, Top
@chapter Invoking @code{groff}
@cindex invoking @code{groff}
@cindex @code{groff} invocation

This section focuses on how to invoke the @code{groff} front end.  This
front end takes care of the details of constructing the pipeline among
the preprocessors, @code{gtroff} and the postprocessor.

It has become a tradition that GNU programs get the prefix @samp{g} to
distinguish it from its original counterparts provided by the host (see
@ref{Environment}, for more details).  Thus, for example, @code{geqn} is
GNU @code{eqn}.  On operating systems like GNU/Linux or the Hurd, which
don't contain proprietary versions of @code{troff}, and on
MS-DOS/MS-Windows, where @code{troff} and associated programs are not
available at all, this prefix is omitted since GNU @code{troff} is the
only used incarnation of @code{troff}.  Exception: @samp{groff} is never
replaced by @samp{roff}.

In this document, we consequently say @samp{gtroff} when talking about
the GNU @code{troff} program.  All other implementations of @code{troff}
are called @acronym{AT&T} @code{troff} which is the common origin of
all @code{troff} derivates (with more or less compatible changes).
Similarly, we say @samp{gpic}, @samp{geqn}, etc.

@menu
* Groff Options::
* Environment::
* Macro Directories::
* Invocation Examples::
@end menu


@c =====================================================================

@node Groff Options, Environment, Invoking groff, Invoking groff
@section Options
@cindex options

@pindex groff
@pindex gtroff
@pindex gpic
@pindex geqn
@pindex ggrn
@pindex grap
@pindex gtbl
@pindex grefer
@pindex gsoelim
@code{groff} normally runs the @code{gtroff} program and a postprocessor
appropriate for the selected device.  The default device is @samp{ps}
(but it can be changed when @code{groff} is configured and built).  It
can optionally preprocess with any of @code{gpic}, @code{geqn},
@code{gtbl}, @code{ggrn}, @code{grap}, @code{grefer}, or @code{gsoelim}.

This section only documents options to the @code{groff} front end.  Many
of the arguments to @code{groff} are passed on to @code{gtroff},
therefore those are also included.  Arguments to pre- or postprocessors
can be found in @ref{Invoking gpic}, @ref{Invoking geqn}, @ref{Invoking
gtbl}, @ref{Invoking ggrn}, @ref{Invoking grefer}, @ref{Invoking
gsoelim}, @ref{Invoking grotty}, @ref{Invoking grops}, @ref{Invoking
grohtml}, @ref{Invoking grodvi}, @ref{Invoking grolj4}, @ref{Invoking
grolbp}, and @ref{Invoking gxditview}.

The command line format for @code{groff} is:

@Example
groff [ -abceghilpstvzCEGNRSUVXZ ] [ -F@var{dir} ] [ -m@var{name} ]
      [ -T@var{def} ] [ -f@var{fam} ] [ -w@var{name} ] [ -W@var{name} ]
      [ -M@var{dir} ] [ -d@var{cs} ] [ -r@var{cn} ] [ -n@var{num} ]
      [ -o@var{list} ] [ -P@var{arg} ] [ -L@var{arg} ] [ -I@var{dir} ]
      [ @var{files}@dots{} ]
@endExample

The command line format for @code{gtroff} is as follows.

@Example
gtroff [ -abcivzCERU ] [ -w@var{name} ] [ -W@var{name} ] [ -d@var{cs} ]
       [ -f@var{fam} ] [ -m@var{name} ] [ -n@var{num} ]
       [ -o@var{list} ] [ -r@var{cn} ] [ -T@var{name} ]
       [ -F@var{dir} ] [ -M@var{dir} ] [ @var{files}@dots{} ]
@endExample

@noindent
Obviously, many of the options to @code{groff} are actually passed on to
@code{gtroff}.

Options without an argument can be grouped behind a single@w{ }@option{-}.
A filename of@w{ }@file{-} denotes the standard input.  It is possible to
have whitespace between an option and its parameter.

The @code{grog} command can be used to guess the correct @code{groff}
command to format a file.

Here's the description of the command-line options:

@cindex command-line options
@table @samp
@item -h
Print a help message.

@item -e
Preprocess with @code{geqn}.

@item -t
Preprocess with @code{gtbl}.

@item -g
Preprocess with @code{ggrn}.

@item -G
Preprocess with @code{grap}.

@item -p
Preprocess with @code{gpic}.

@item -s
Preprocess with @code{gsoelim}.

@item -c
Suppress color output.

@item -R
Preprocess with @code{grefer}.  No mechanism is provided for passing
arguments to @code{grefer} because most @code{grefer} options have
equivalent commands which can be included in the file.  @xref{grefer},
for more details.

@pindex troffrc
@pindex troffrc-end
Note that @code{gtroff} also accepts a @option{-R} option, which is not
accessible via @code{groff}.  This option prevents the loading of the
@file{troffrc} and @file{troffrc-end} files.

@item -v
Make programs run by @code{groff} print out their version number.

@item -V
Print the pipeline on @code{stdout} instead of executing it.

@item -z
Suppress output from @code{gtroff}.  Only error messages are printed.

@item -Z
Do not postprocess the output of @code{gtroff}.  Normally @code{groff}
automatically runs the appropriate postprocessor.

@item -P@var{arg}
Pass @var{arg} to the postprocessor.  Each argument should be passed
with a separate @option{-P} option.  Note that @code{groff} does not
prepend @samp{-} to @var{arg} before passing it to the postprocessor.

@item -l
Send the output to a spooler for printing.  The command used for this is
specified by the @code{print} command in the device description file
(see @ref{Font Files}, for more info).  If not present, @option{-l} is
ignored.

@item -L@var{arg}
Pass @var{arg} to the spooler.  Each argument should be passed with a
separate @option{-L} option.  Note that @code{groff} does not prepend
a @samp{-} to @var{arg} before passing it to the postprocessor.
If the @code{print} keyword in the device description file is missing,
@option{-L} is ignored.

@item -T@var{dev}
Prepare output for device @var{dev}.  The default device is @samp{ps},
unless changed when @code{groff} was configured and built.  The
following are the output devices currently available:

@table @code
@item ps
For @sc{PostScript} printers and previewers.

@item dvi
For @TeX{} DVI format.

@item X75
For a 75@dmn{dpi} X11 previewer.

@item X75-12
For a 75@dmn{dpi} X11 previewer with a 12@dmn{pt} base font in the
document.

@item X100
For a 100@dmn{dpi} X11 previewer.

@item X100-12
For a 100@dmn{dpi} X11 previewer with a 12@dmn{pt} base font in the
document.

@item ascii
@cindex encoding, @acronym{ASCII}
@cindex @acronym{ASCII}, encoding
For typewriter-like devices using the (7-bit) @acronym{ASCII}
character set.

@item latin1
@cindex encoding, latin-1
@cindex latin-1, encoding
For typewriter-like devices that support the @w{Latin-1} (@w{ISO
8859-1}) character set.

@item utf8
@cindex encoding, utf-8
@cindex utf-8, encoding
For typewriter-like devices which use the Unicode (@w{ISO 10646})
character set with @w{UTF-8} encoding.

@item cp1047
@cindex @acronym{EBCDIC} encoding
@cindex encoding, @acronym{EBCDIC}
@cindex encoding, cp1047
@cindex cp1047
@cindex IBM cp1047
For typewriter-like devices which use the @acronym{EBCDIC} encoding IBM
cp1047.

@item lj4
For HP LaserJet4-compatible (or other PCL5-compatible) printers.

@item lbp
For Canon @acronym{CAPSL} printers (@w{LBP-4} and @w{LBP-8} series laser
printers).

@pindex pre-grohtml
@pindex post-grohtml
@cindex @code{grohtml}, the program
@item html
To produce @acronym{HTML} output.  Note that the @acronym{HTML} driver
consists of two parts, a preprocessor (@code{pre-grohtml}) and a
postprocessor (@code{post-grohtml}).
@end table

@cindex output device name string register (@code{.T})
@cindex output device usage number register (@code{.T})
The predefined @code{gtroff} string register @code{.T} contains the
current output device; the read-only number register @code{.T} is set
to@w{ }1 if this option is used (which is always true if @code{groff} is
used to call @code{gtroff}).  @xref{Built-in Registers}.

The postprocessor to be used for a device is specified by the
@code{postpro} command in the device description file.  (@xref{Font
Files}, for more info.)  This can be overridden with the @option{-X}
option.

@item -X
Preview with @code{gxditview} instead of using the usual postprocessor.
This is unlikely to produce good results except with @option{-Tps}.

Note that this is not the same as using @option{-TX75} or
@option{-TX100} to view a document with @code{gxditview}: The former
uses the metrics of the specified device, whereas the latter uses
X-specific fonts and metrics.

@item -N
Don't allow newlines with @code{eqn} delimiters.  This is the same as
the @option{-N} option in @code{geqn}.

@item -S
@cindex @code{open} request, and safer mode
@cindex @code{opena} request, and safer mode
@cindex @code{pso} request, and safer mode
@cindex @code{sy} request, and safer mode
@cindex @code{pi} request, and safer mode
@cindex safer mode
@cindex mode, safer
Safer mode.  Pass the @option{-S} option to @code{gpic} and disable the
@code{open}, @code{opena}, @code{pso}, @code{sy}, and @code{pi}
requests.  For security reasons, this is enabled by default.

@item -U
@cindex mode, unsafe
@cindex unsafe mode
Unsafe mode.  This enables the @code{open}, @code{opena}, @code{pso},
@code{sy}, and @code{pi} requests.

@item -a
@cindex @acronym{ASCII} approximation output register (@code{.A})
Generate an @acronym{ASCII} approximation of the typeset output.  The
read-only register @code{.A} is then set to@w{ }1.  @xref{Built-in
Registers}.  A typical example is

@Example
groff -a -man -Tdvi troff.man | less
@endExample

@noindent
which shows how lines are broken for the DVI device.  Note that this
option is rather useless today since graphic output devices are
available virtually everywhere.

@item -b
Print a backtrace with each warning or error message.  This backtrace
should help track down the cause of the error.  The line numbers given
in the backtrace may not always be correct: @code{gtroff} can get
confused by @code{as} or @code{am} requests while counting line numbers.

@item -i
Read the standard input after all the named input files have been
processed.

@item -w@var{name}
Enable warning @var{name}.  Available warnings are described in
@ref{Debugging}.  Multiple @option{-w} options are allowed.

@item -W@var{name}
Inhibit warning @var{name}.  Multiple @option{-W} options are allowed.

@item -E
Inhibit all error messages.

@item -C
Enable compatibility mode.  @xref{Implementation Differences}, for the
list of incompatibilities between @code{groff} and @acronym{AT&T}
@code{troff}.

@item -d@var{c}@var{s}
@itemx -d@var{name}=@var{s}
Define @var{c} or @var{name} to be a string@w{ }@var{s}.  @var{c}@w{ }must
be a one-letter name; @var{name} can be of arbitrary length.  All string
assignments happen before loading any macro file (including the start-up
file).

@item -f@var{fam}
Use @var{fam} as the default font family.  @xref{Font Families}.

@item -m@var{name}
Read in the file @file{@var{name}.tmac}.  Normally @code{groff} searches
for this in its macro directories.  If it isn't found, it tries
@file{tmac.@var{name}} (searching in the same directories).

@item -n@var{num}
Number the first page @var{num}.

@item -o@var{list}
@cindex print current page register (@code{.P})
Output only pages in @var{list}, which is a comma-separated list of page
ranges; @samp{@var{n}} means print page@w{ }@var{n}, @samp{@var{m}-@var{n}}
means print every page between @var{m} and@w{ }@var{n}, @samp{-@var{n}}
means print every page up to@w{ }@var{n}, @samp{@var{n}-} means print every
page beginning with@w{ }@var{n}.  @code{gtroff} exits after printing the
last page in the list.  All the ranges are inclusive on both ends.

Within @code{gtroff}, this information can be extracted with the
@samp{.P} register.  @xref{Built-in Registers}.

If your document restarts page numbering at the beginning of each
chapter, then @code{gtroff} prints the specified page range for each
chapter.

@item -r@var{c}@var{n}
@itemx -r@var{name}=@var{n}
Set number register@w{ }@var{c} or @var{name} to the value@w{ }@var{n}.
@var{c}@w{ }must be a one-letter name; @var{name} can be of arbitrary
length.  @var{n}@w{ }can be any @code{gtroff} numeric expression.  All
register  assignments happen before loading any macro file (including
the start-up file).

@item -F@var{dir}
Search @file{@var{dir}} for subdirectories @file{dev@var{name}}
(@var{name} is the name of the device), for the @file{DESC} file, and
for font files before looking in the standard directories.

@item -M@var{dir}
Search directory @file{@var{dir}} for macro files before the standard
directories (@pxref{Macro Directories}).

@item -I@var{dir}
This option is as described in @ref{gsoelim}.  It implies the
@option{-s} option.
@end table


@c =====================================================================

@node Environment, Macro Directories, Groff Options, Invoking groff
@section Environment
@cindex environment variables
@cindex variables in environment

There are also several environment variables (of the operating system,
not within @code{gtroff}) which can modify the behavior of @code{groff}.

@table @code
@item GROFF_COMMAND_PREFIX
@tindex GROFF_COMMAND_PREFIX@r{, environment variable}
@cindex command prefix
@cindex prefix, for commands
If this is set to@w{ }@var{X}, then @code{groff} runs @code{@var{X}troff}
instead of @code{gtroff}.  This also applies to @code{tbl}, @code{pic},
@code{eqn}, @code{grn}, @code{refer}, and @code{soelim}.  It does not
apply to @code{grops}, @code{grodvi}, @code{grotty}, @code{pre-grohtml},
@code{post-grohtml}, @code{grolj4}, and @code{gxditview}.

The default command prefix is determined during the installation process.
If a non-GNU troff system is found, prefix @samp{g} is used, none
otherwise.

@item GROFF_TMAC_PATH
@tindex GROFF_TMAC_PATH@r{, environment variable}
A colon-separated list of directories in which to search for macro files
(before the default directories are tried).  @xref{Macro Directories}.

@item GROFF_TYPESETTER
@tindex GROFF_TYPESETTER@r{, environment variable}
The default output device.

@item GROFF_FONT_PATH
@tindex GROFF_FONT_PATH@r{, environment variable}
A colon-separated list of directories in which to search for the
@code{dev}@var{name} directory (before the default directories are
tried).

@item GROFF_BIN_PATH
@tindex GROFF_BIN_PATH@r{, environment variable}
This search path, followed by @code{PATH}, is used for commands executed
by @code{groff}.

@item GROFF_TMPDIR
@tindex GROFF_TMPDIR@r{, environment variable}
@tindex TMPDIR@r{, environment variable}
The directory in which @code{groff} creates temporary files.  If this is
not set and @env{TMPDIR} is set, temporary files are created in that
directory.  Otherwise temporary files are created in a system-dependent
default directory (on Unix and GNU/Linux systems, this is usually
@file{/tmp}).  @code{grops}, @code{grefer}, @code{pre-grohtml}, and
@code{post-grohtml} can create temporary files in this directory.
@end table

Note that MS-DOS and MS-Windows ports of @code{groff} use semi-colons,
rather than colons, to separate the directories in the lists described
above.


@c =====================================================================

@node Macro Directories, Invocation Examples, Environment, Invoking groff
@section Macro Directories
@cindex macro directories
@cindex directories for macros
@cindex searching macros
@cindex macros, searching

All macro file names must be named @code{@var{name}.tmac} or
@code{tmac.@var{name}} to make the @option{-m@var{name}} command line
option work.  The @code{mso} request doesn't have this restriction; any
file name can be used, and @code{gtroff} won't try to append or prepend
the @samp{tmac} string.

@cindex tmac, directory
@cindex directory, for tmac files
@cindex tmac, path
@cindex path, for tmac files
@cindex searching macro files
@cindex macro files, searching
@cindex files, macro, searching
Macro files are kept in the @dfn{tmac directories}, all of which
constitute the @dfn{tmac path}.  The elements of the search path for
macro files are (in that order):

@itemize @bullet
@item
The directories specified with @code{gtroff}'s or @code{groff}'s
@option{-M} command line option.

@item
@tindex GROFF_TMAC_PATH@r{, environment variable}
The directories given in the @env{GROFF_TMAC_PATH} environment
variable.

@item
@cindex safer mode
@cindex mode, safer
@cindex unsafe mode
@cindex mode, unsafe
@cindex current directory
@cindex directory, current
The current directory (only if in unsafe mode using the @option{-U}
command line switch).

@item
@cindex home directory
@cindex directory, home
The home directory.

@item
@cindex site-specific directory
@cindex directory, site-specific
@cindex platform-specific directory
@cindex directory, platform-specific
A platform-dependent directory, a site-specific (platform-independent)
directory, and the main tmac directory; the default locations are

@Example
/usr/local/lib/groff/site-tmac
/usr/local/share/groff/site-tmac
/usr/local/share/groff/1.18/tmac
@endExample

@noindent
assuming that the version of @code{groff} is 1.18, and the installation
prefix was @file{/usr/local}.  It is possible to fine-tune those
directories during the installation process.
@end itemize


@c =====================================================================

@node Invocation Examples,  , Macro Directories, Invoking groff
@section Invocation Examples
@cindex invocation examples
@cindex examples of invocation

This section lists several common uses of @code{groff} and the
corresponding command lines.

@Example
groff file
@endExample

@noindent
This command processes @file{file} without a macro package or a
preprocessor.  The output device is the default, @samp{ps}, and the
output is sent to @code{stdout}.

@Example
groff -t -mandoc -Tascii file | less
@endExample

@noindent
This is basically what a call to the @code{man} program does.
@code{gtroff} processes the manual page @file{file} with the
@file{mandoc} macro file (which in turn either calls the @file{man} or
the @file{mdoc} macro package), using the @code{tbl} preprocessor and
the @acronym{ASCII} output device.  Finally, the @code{less} pager
displays the result.

@Example
groff -X -m me file
@endExample

@noindent
Preview @file{file} with @code{gxditview}, using the @file{me} macro
package.  Since no @option{-T} option is specified, use the default
device (@samp{ps}).  Note that you can either say @w{@samp{-m me}} or
@w{@samp{-me}}; the latter is an anachronism from the early days of
@acronym{UNIX}.@footnote{The same is true for the other main macro
packages that come with @code{groff}: @file{man}, @file{mdoc},
@file{ms}, @file{mm}, and @file{mandoc}.  This won't work in general;
for example, to load @file{trace.tmac}, either @samp{-mtrace} or
@w{@samp{-m trace}} must be used.}

@Example
groff -man -rD1 -z file
@endExample

@noindent
Check @file{file} with the @file{man} macro package, forcing
double-sided printing -- don't produce any output.

@menu
* grog::
@end menu

@c ---------------------------------------------------------------------

@node grog,  , Invocation Examples, Invocation Examples
@subsection @code{grog}

@pindex grog
@code{grog} reads files, guesses which of the @code{groff} preprocessors
and/or macro packages are required for formatting them, and prints the
@code{groff} command including those options on the standard output.  It
generates one or more of the options @option{-e}, @option{-man},
@option{-me}, @option{-mm}, @option{-mom}, @option{-ms}, @option{-mdoc},
@option{-mdoc-old}, @option{-p}, @option{-R}, @option{-g}, @option{-G},
@option{-s}, and @option{-t}.

A special file name@w{ }@file{-} refers to the standard input.  Specifying
no files also means to read the standard input.  Any specified options
are included in the printed command.  No space is allowed between
options and their arguments.  The only options recognized are
@option{-C} (which is also passed on) to enable compatibility mode, and
@option{-v} (if it is the only parameter) to print the version number.

For example,

@Example
grog -Tdvi paper.ms
@endExample

@noindent
guesses the appropriate command to print @file{paper.ms} and then prints
it to the command line after adding the @option{-Tdvi} option.  For
direct execution, enclose the call to @code{grog} in backquotes at the
@acronym{UNIX} shell prompt:

@Example
`grog -Tdvi paper.ms` > paper.dvi
@endExample

@noindent
As seen in the example, it is still necessary to redirect the output to
something meaningful (i.e.@: either a file or a pager program like
@code{less}).



@c =====================================================================
@c =====================================================================

@node Tutorial for Macro Users, Macro Packages, Invoking groff, Top
@chapter Tutorial for Macro Users
@cindex tutorial for macro users
@cindex macros, tutorial for users
@cindex user's tutorial for macros
@cindex user's macro tutorial

Most users tend to use a macro package to format their papers.  This
means that the whole breadth of @code{groff} is not necessary for most
people.  This chapter covers the material needed to efficiently use a
macro package.

@menu
* Basics::
* Common Features::
@end menu


@c =====================================================================

@node Basics, Common Features, Tutorial for Macro Users, Tutorial for Macro Users
@section Basics
@cindex basics of macros
@cindex macro basics

This section covers some of the basic concepts necessary to understand
how to use a macro package.@footnote{This section is derived from
@cite{Writing Papers with nroff using -me} by Eric P.@w{ }Allman.}
References are made throughout to more detailed information, if desired.

@code{gtroff} reads an input file prepared by the user and outputs a
formatted document suitable for publication or framing.  The input
consists of text, or words to be printed, and embedded commands
(@dfn{requests} and @dfn{escapes}), which tell @code{gtroff} how to
format the output.  For more detail on this, see @ref{Embedded
Commands}.

The word @dfn{argument} is used in this chapter to mean a word or number
which appears on the same line as a request, and which modifies the
meaning of that request.  For example, the request

@Example
.sp
@endExample

@noindent
spaces one line, but

@Example
.sp 4
@endExample

@noindent
spaces four lines.  The number@w{ }4 is an argument to the @code{sp}
request which says to space four lines instead of one.  Arguments are
separated from the request and from each other by spaces (@emph{no}
tabs).  More details on this can be found in @ref{Request Arguments}.

The primary function of @code{gtroff} is to collect words from input
lines, fill output lines with those words, justify the right-hand margin
by inserting extra spaces in the line, and output the result.  For
example, the input:

@Example
Now is the time
for all good men
to come to the aid
of their party.
Four score and seven
years ago,...
@endExample

@noindent
is read, packed onto output lines, and justified to produce:

@quotation
Now is the time for all good men to come to the aid of their party.
Four score and seven years ago,@dots{}
@end quotation

@cindex break
@cindex line break
Sometimes a new output line should be started even though the current
line is not yet full; for example, at the end of a paragraph.  To do
this it is possible to cause a @dfn{break}, which starts a new output
line.  Some requests cause a break automatically, as normally do blank
input lines and input lines beginning with a space.

Not all input lines are text to be formatted.  Some input lines are
requests which describe how to format the text.  Requests always have a
period (@samp{.}) or an apostrophe (@samp{'}) as the first character of
the input line.

The text formatter also does more complex things, such as automatically
numbering pages, skipping over page boundaries, putting footnotes in the
correct place, and so forth.

Here are a few hints for preparing text for input to @code{gtroff}.

@itemize @bullet
@item
First, keep the input lines short.  Short input lines are easier to
edit, and @code{gtroff} packs words onto longer lines anyhow.

@item
In keeping with this, it is helpful to begin a new line after every
comma or phrase, since common corrections are to add or delete sentences
or phrases.

@item
End each sentence with two spaces -- or better, start each sentence on a
new line.  @code{gtroff} recognizes characters that usually end a
sentence, and inserts sentence space accordingly.

@item
Do not hyphenate words at the end of lines -- @code{gtroff} is smart
enough to hyphenate words as needed, but is not smart enough to take
hyphens out and join a word back together.  Also, words such as
``mother-in-law'' should not be broken over a line, since then a space
can occur where not wanted, such as ``@w{mother- in}-law''.
@end itemize

@cindex double-spacing (@code{ls})
@cindex spacing
@code{gtroff} double-spaces output text automatically if you use the
request @w{@samp{.ls 2}}.  Reactivate single-spaced mode by typing
@w{@samp{.ls 1}}.@footnote{If you need finer granularity of the
vertical space, use the @code{pvs} request (@pxref{Changing Type
Sizes}).}

A number of requests allow to change the way the output looks,
sometimes called the @dfn{layout} of the output page.  Most of these
requests adjust the placing of @dfn{whitespace} (blank lines or
spaces).

@cindex new page (@code{bp})
The @code{bp} request starts a new page, causing a line break.

@cindex blank line (@code{sp})
@cindex empty line (@code{sp})
@cindex line, empty (@code{sp})
The request @w{@samp{.sp @var{N}}} leaves @var{N}@w{ }lines of blank
space.  @var{N}@w{ }can be omitted (meaning skip a single line) or can
be of the form @var{N}i (for @var{N}@w{ }inches) or @var{N}c (for
@var{N}@w{ }centimeters).  For example, the input:

@Example
.sp 1.5i
My thoughts on the subject
.sp
@endExample

@noindent
leaves one and a half inches of space, followed by the line ``My
thoughts on the subject'', followed by a single blank line (more
measurement units are available, see @ref{Measurements}).

@cindex centering lines (@code{ce})
@cindex lines, centering (@code{ce})
Text lines can be centered by using the @code{ce} request.  The line
after @code{ce} is centered (horizontally) on the page.  To center more
than one line, use @w{@samp{.ce @var{N}}} (where @var{N} is the number
of lines to center), followed by the @var{N}@w{ }lines.  To center many
lines without counting them, type:

@Example
.ce 1000
lines to center
.ce 0
@endExample

@noindent
The @w{@samp{.ce 0}} request tells @code{groff} to center zero more
lines, in other words, stop centering.

@cindex line break (@code{br})
@cindex break (@code{br})
All of these requests cause a break; that is, they always start a new
line.  To start a new line without performing any other action, use
@code{br}.


@c =====================================================================

@node Common Features,  , Basics, Tutorial for Macro Users
@section Common Features
@cindex common features
@cindex features, common

@code{gtroff} provides very low-level operations for formatting a
document.  There are many common routine operations which are done in
all documents.  These common operations are written into @dfn{macros}
and collected into a @dfn{macro package}.

All macro packages provide certain common capabilities which fall into
the following categories.

@menu
* Paragraphs::
* Sections and Chapters::
* Headers and Footers::
* Page Layout Adjustment::
* Displays::
* Footnotes and Annotations::
* Table of Contents::
* Indices::
* Paper Formats::
* Multiple Columns::
* Font and Size Changes::
* Predefined Strings::
* Preprocessor Support::
* Configuration and Customization::
@end menu

@c ---------------------------------------------------------------------

@node Paragraphs, Sections and Chapters, Common Features, Common Features
@subsection Paragraphs
@cindex paragraphs

One of the most common and most used capability is starting a
paragraph.  There are a number of different types of paragraphs, any
of which can be initiated with macros supplied by the macro package.
Normally, paragraphs start with a blank line and the first line
indented, like the text in this manual.  There are also block style
paragraphs, which omit the indentation:

@Example
Some   men  look   at  constitutions   with  sanctimonious
reverence, and deem them like the ark of the covenant, too
sacred to be touched.
@endExample

@noindent
And there are also indented paragraphs which begin with a tag or label
at the margin and the remaining text indented.

@Example
one   This is  the first paragraph.  Notice  how the first
      line of  the resulting  paragraph lines up  with the
      other lines in the paragraph.
@endExample
@Example
longlabel
      This  paragraph   had  a  long   label.   The  first
      character of text on the first line does not line up
      with  the  text  on  second  and  subsequent  lines,
      although they line up with each other.
@endExample

A variation of this is a bulleted list.

@Example
.     Bulleted lists start with a bullet.   It is possible
      to use other glyphs instead of the bullet.  In nroff
      mode using the ASCII character set for output, a dot
      is used instead of a real bullet.
@endExample


@c ---------------------------------------------------------------------

@node Sections and Chapters, Headers and Footers, Paragraphs, Common Features
@subsection Sections and Chapters

Most macro packages supply some form of section headers.  The simplest
kind is simply the heading on a line by itself in bold type.  Others
supply automatically numbered section heading or different heading
styles at different levels.  Some, more sophisticated, macro packages
supply macros for starting chapters and appendices.

@c ---------------------------------------------------------------------

@node Headers and Footers, Page Layout Adjustment, Sections and Chapters, Common Features
@subsection Headers and Footers

Every macro package gives some way to manipulate the @dfn{headers} and
@dfn{footers} (also called @dfn{titles}) on each page.  This is text
put at the top and bottom of each page, respectively, which contain
data like the current page number, the current chapter title, and so
on.  Its appearance is not affected by the running text.  Some packages
allow for different ones on the even and odd pages (for material printed
in a book form).

The titles are called @dfn{three-part titles}, that is, there is a
left-justified part, a centered part, and a right-justified part.  An
automatically generated page number may be put in any of these fields
with the @samp{%} character (see @ref{Page Layout}, for more details).

@c ---------------------------------------------------------------------

@node Page Layout Adjustment, Displays, Headers and Footers, Common Features
@subsection Page Layout

Most macro packages let the user specify top and bottom margins and
other details about the appearance of the printed pages.

@c ---------------------------------------------------------------------

@node Displays, Footnotes and Annotations, Page Layout Adjustment, Common Features
@subsection Displays
@cindex displays

@dfn{Displays} are sections of text to be set off from the body of
the paper.  Major quotes, tables, and figures are types of displays, as
are all the examples used in this document.

@cindex quotes, major
@cindex major quotes
@dfn{Major quotes} are quotes which are several lines long, and hence
are set in from the rest of the text without quote marks around them.

@cindex list
A @dfn{list} is an indented, single-spaced, unfilled display.  Lists
should be used when the material to be printed should not be filled and
justified like normal text, such as columns of figures or the examples
used in this paper.

@cindex keep
A @dfn{keep} is a display of lines which are kept on a single page if
possible.  An example for a keep might be a diagram.  Keeps differ from
lists in that lists may be broken over a page boundary whereas keeps are
not.

@cindex keep, floating
@cindex floating keep
@dfn{Floating keeps} move relative to the text.  Hence, they are good for
things which are referred to by name, such as ``See figure@w{ }3''.  A
floating keep appears at the bottom of the current page if it fits;
otherwise, it appears at the top of the next page.  Meanwhile, the
surrounding text `flows' around the keep, thus leaving no blank areas.

@c ---------------------------------------------------------------------

@node Footnotes and Annotations, Table of Contents, Displays, Common Features
@subsection Footnotes and Annotations
@cindex footnotes
@cindex annotations

There are a number of requests to save text for later printing.

@dfn{Footnotes} are printed at the bottom of the current page.

@cindex delayed text
@dfn{Delayed text} is very similar to a footnote except that it is
printed when called for explicitly.  This allows a list of references to
appear (for example) at the end of each chapter, as is the convention in
some disciplines.

Most macro packages which supply this functionality also supply a means
of automatically numbering either type of annotation.

@c ---------------------------------------------------------------------

@node Table of Contents, Indices, Footnotes and Annotations, Common Features
@subsection Table of Contents
@cindex table of contents
@cindex contents, table of

@dfn{Tables of contents} are a type of delayed text having a tag
(usually the page number) attached to each entry after a row of dots.
The table accumulates throughout the paper until printed, usually after
the paper has ended.  Many macro packages provide the ability to have
several tables of contents (e.g.@: a standard table of contents, a list
of tables, etc).

@c ---------------------------------------------------------------------

@node Indices, Paper Formats, Table of Contents, Common Features
@subsection Indices
@cindex index, in macro package

While some macro packages use the term @dfn{index}, none actually
provide that functionality.  The facilities they call indices are
actually more appropriate for tables of contents.

@pindex makeindex
To produce a real index in a document, external tools like the
@code{makeindex} program are necessary.

@c ---------------------------------------------------------------------

@node Paper Formats, Multiple Columns, Indices, Common Features
@subsection Paper Formats
@cindex paper formats

Some macro packages provide stock formats for various kinds of
documents.  Many of them provide a common format for the title and
opening pages of a technical paper.  The @file{mm} macros in particular
provide formats for letters and memoranda.

@c ---------------------------------------------------------------------

@node Multiple Columns, Font and Size Changes, Paper Formats, Common Features
@subsection Multiple Columns

Some macro packages (but not @file{man}) provide the ability to have two
or more columns on a page.

@c ---------------------------------------------------------------------

@node Font and Size Changes, Predefined Strings, Multiple Columns, Common Features
@subsection Font and Size Changes

The built-in font and size functions are not always intuitive, so all
macro packages provide macros to make these operations simpler.

@c ---------------------------------------------------------------------

@node Predefined Strings, Preprocessor Support, Font and Size Changes, Common Features
@subsection Predefined Strings

Most macro packages provide various predefined strings for a variety of
uses; examples are sub- and superscripts, printable dates, quotes and
various special characters.

@c ---------------------------------------------------------------------

@node Preprocessor Support, Configuration and Customization, Predefined Strings, Common Features
@subsection Preprocessor Support

All macro packages provide support for various preprocessors and may
extend their functionality.

For example, all macro packages mark tables (which are processed with
@code{gtbl}) by placing them between @code{TS} and @code{TE} macros.
The @file{ms} macro package has an option, @samp{.TS@w{ }H}, that prints
a caption at the top of a new page (when the table is too long to fit on
a single page).

@c ---------------------------------------------------------------------

@node Configuration and Customization,  , Preprocessor Support, Common Features
@subsection Configuration and Customization

Some macro packages provide means of customizing many of the details of
how the package behaves.  This ranges from setting the default type size
to changing the appearance of section headers.



@c =====================================================================
@c =====================================================================

@node Macro Packages, gtroff Reference, Tutorial for Macro Users, Top
@chapter Macro Packages
@cindex macro packages
@cindex packages, macros

This chapter documents the main macro packages that come with
@code{groff}.

@menu
* man::
* mdoc::
* ms::
* me::
* mm::
@end menu


@c =====================================================================

@node man, mdoc, Macro Packages, Macro Packages
@section @file{man}
@cindex manual pages
@cindex man pages
@pindex an.tmac
@pindex man.tmac
@pindex man-old.tmac

This is the most popular and probably the most important macro package
of @code{groff}.  It is easy to use, and a vast majority of manual pages
are based on it.

@menu
* Man options::
* Man usage::
* Man font macros::
* Miscellaneous man macros::
* Predefined man strings::
* Preprocessors in man pages::
@end menu

@c ---------------------------------------------------------------------

@node Man options, Man usage, man, man
@subsection Options

The command line format for using the @file{man} macros with
@code{groff} is:

@Example
groff -m man [ -rLL=@var{length} ] [ -rLT=@var{length} ]
      [ -rcR=1 ] [ -rC1 ] [ -rD1 ] [ -rP@var{nnn} ]
      [ -rS@var{xx} ] [ -rX@var{nnn} ] [ @var{files}@dots{} ]
@endExample

@noindent
It is possible to use @samp{-man} instead of @w{@samp{-m man}}.

@table @code
@item -rLL=@var{length}
Set line length to @var{length}.  If not specified, the line length
defaults to 78@w{ }en in nroff mode (this is 78@w{ }characters per
line) and 6.5@w{ }inch otherwise.

@item -rLT=@var{length}
Set title length to @var{length}.  If not specified, the title length
defaults to 78@w{ }en in nroff mode (this is 78@w{ }characters per
line) and 6.5@w{ }inch otherwise.

@item -rcR=1
This option (the default if a TTY output device is used) creates a
single, very long page instead of multiple pages.  Use @code{-rcR=0}
to disable it.

@item -rC1
If more than one manual page is given on the command line, number the
pages continuously, rather than starting each at@w{ }1.

@item -rD1
Double-sided printing.  Footers for even and odd pages are formatted
differently.

@item -rP@var{nnn}
Page numbering starts with @var{nnn} rather than with@w{ }1.

@item -rS@var{xx}
Use @var{xx} (which can be 10, 11, or@w{ }12@dmn{pt}) as the base
document font size instead of the default value of@w{ }10@dmn{pt}.

@item -rX@var{nnn}
After page @var{nnn}, number pages as @var{nnn}a, @var{nnn}b,
@var{nnn}c, etc.  For example, the option @option{-rX2} produces the
following page numbers: 1, 2, 2a, 2b, 2c, etc.
@end table

@c ---------------------------------------------------------------------

@node Man usage, Man font macros, Man options, man
@subsection Usage
@cindex @code{man} macros
@cindex macros for manual pages [@code{man}]

@pindex man.local
This section describes the available macros for manual pages.  For
further customization, put additional macros and requests into the file
@file{man.local} which is loaded immediately after the @file{man}
package.

@Defmac {TH, title section [@Var{extra1} [@Var{extra2} [@Var{extra3}]]], man}
Set the title of the man page to @var{title} and the section to
@var{section}, which must have a value between 1 and@w{ }8.  The value
of @var{section} may also have a string appended, e.g.@: @samp{.pm},
to indicate a specific subsection of the man pages.

Both @var{title} and @var{section} are positioned at the left and right
in the header line (with @var{section} in parentheses immediately
appended to @var{title}.  @var{extra1} is positioned in the middle of
the footer line.  @var{extra2} is positioned at the left in the footer
line (or at the left on even pages and at the right on odd pages if
double-sided printing is active).  @var{extra3} is centered in the
header line.

For @acronym{HTML} output, headers and footers are completely suppressed.

Additionally, this macro starts a new page; the new line number is@w{ }1
again (except if the @option{-rC1} option is given on the command line)
-- this feature is intended only for formatting multiple man pages; a
single man page should contain exactly one @code{TH} macro at the
beginning of the file.
@endDefmac

@Defmac {SH, [@Var{heading}], man}
Set up an unnumbered section heading sticking out to the left.  Prints
out all the text following @code{SH} up to the end of the line (or the
text in the next line if there is no argument to @code{SH}) in bold
face, one size larger than the base document size.  Additionally, the
left margin for the following text is reset to its default value.
@endDefmac

@Defmac {SS, [@Var{heading}], man}
Set up an unnumbered (sub)section heading.  Prints out all the text
following @code{SS} up to the end of the line (or the text in the next
line if there is no argument to @code{SS}) in bold face, at the same
size as the base document size.  Additionally, the left margin for the
following text is reset to its default value.
@endDefmac

@Defmac {TP, [@Var{nnn}], man}
Set up an indented paragraph with label.  The indentation is set to
@var{nnn} if that argument is supplied (the default unit is @samp{n}
if omitted), otherwise it is set to the default indentation value.

The first line of text following this macro is interpreted as a string
to be printed flush-left, as it is appropriate for a label.  It is not
interpreted as part of a paragraph, so there is no attempt to fill the
first line with text from the following input lines.  Nevertheless, if
the label is not as wide as the indentation, then the paragraph starts
at the same line (but indented), continuing on the following lines.
If the label is wider than the indentation, then the descriptive part
of the paragraph begins on the line following the label, entirely
indented.  Note that neither font shape nor font size of the label is
set to a default value; on the other hand, the rest of the text has
default font settings.
@endDefmac

@DefmacList {LP, , man}
@DefmacItem {PP, , man}
@DefmacListEnd {P, , man}
These macros are mutual aliases.  Any of them causes a line break at
the current position, followed by a vertical space downwards by the
amount specified by the @code{PD} macro.  The font size and shape are
reset to the default value (10@dmn{pt} roman if no @option{-rS} option
is given on the command line).  Finally, the current left margin is
restored.
@endDefmac

@Defmac {IP, [@Var{designator} [@Var{nnn}]], man}
Set up an indented paragraph, using @var{designator} as a tag to mark
its beginning.  The indentation is set to @var{nnn} if that argument
is supplied (default unit is @samp{n}), otherwise the default
indentation value is used.  Font size and face of the paragraph (but
not the designator) are reset to their default values.  To start an
indented paragraph with a particular indentation but without a
designator, use @samp{""} (two double quotes) as the first argument of
@code{IP}.

For example, to start a paragraph with bullets as the designator and
4@w{ }en indentation, write

@Example
.IP \(bu 4
@endExample
@endDefmac

@Defmac {HP, [@Var{nnn}], man}
@cindex hanging indentation [@code{man}]
@cindex @code{man} macros, hanging indentation
Set up a paragraph with hanging left indentation.  The indentation is
set to @var{nnn} if that argument is supplied (default unit is
@samp{n}), otherwise the default indentation value is used.  Font size
and face are reset to their default values.
@endDefmac

@Defmac {RS, [@Var{nnn}], man}
@cindex left margin, how to move [@code{man}]
@cindex @code{man} macros, moving left margin
Move the left margin to the right by the value @var{nnn} if specified
(default unit is @samp{n}); otherwise the default indentation value
is used.  Calls to the @code{RS} macro can be nested.
@endDefmac

@Defmac {RE, [@Var{nnn}], man}
Move the left margin back to level @var{nnn}; if no argument is given,
it moves one level back.  The first level (i.e., no call to @code{RS}
yet) has number@w{ }1, and each call to @code{RS} increases the level
by@w{ }1.
@endDefmac

@cindex line breaks, with vertical space [@code{man}]
@cindex @code{man} macros, line breaks with vertical space
To summarize, the following macros cause a line break with the insertion
of vertical space (which amount can be changed with the @code{PD}
macro): @code{SH}, @code{SS}, @code{TP}, @code{LP} (@code{PP},
@code{P}), @code{IP}, and @code{HP}.

@cindex line breaks, without vertical space [@code{man}]
@cindex @code{man} macros, line breaks without vertical space
The macros @code{RS} and @code{RE} also cause a break but do not insert
vertical space.

@c ---------------------------------------------------------------------

@node Man font macros, Miscellaneous man macros, Man usage, man
@subsection Macros to set fonts
@cindex font selection [@code{man}]
@cindex @code{man} macros, how to set fonts

The standard font is roman; the default text size is 10@w{ }point.
If command line option @option{-rS=@var{n}} is given, use
@var{n}@dmn{pt} as the default text size.

@Defmac {SM, [@Var{text}], man}
Set the text on the same line or the text on the next line in a font
that is one point size smaller than the default font.
@endDefmac

@Defmac {SB, [@Var{text}], man}
@cindex bold face [@code{man}]
@cindex @code{man} macros, bold face
Set the text on the same line or the text on the next line in bold face
font, one point size smaller than the default font.
@endDefmac

@Defmac {BI, text, man}
Set its arguments alternately in bold face and italic.  Thus,

@Example
.BI this "word and" that
@endExample

@noindent
would set ``this'' and ``that'' in bold face, and ``word and'' in
italics.
@endDefmac

@Defmac {IB, text, man}
Set its arguments alternately in italic and bold face.
@endDefmac

@Defmac {RI, text, man}
Set its arguments alternately in roman and italic.
@endDefmac

@Defmac {IR, text, man}
Set its arguments alternately in italic and roman.
@endDefmac

@Defmac {BR, text, man}
Set its arguments alternately in bold face and roman.
@endDefmac

@Defmac {RB, text, man}
Set its arguments alternately in roman and bold face.
@endDefmac

@Defmac {B, [@Var{text}], man}
Set @var{text} in bold face.  If no text is present on the line where
the macro is called, then the text of the next line appears in bold
face.
@endDefmac

@Defmac {I, [@Var{text}], man}
@cindex italic fonts [@code{man}]
@cindex @code{man} macros, italic fonts
Set @var{text} in italic.  If no text is present on the line where the
macro is called, then the text of the next line appears in italic.
@endDefmac

@c ---------------------------------------------------------------------

@node Miscellaneous man macros, Predefined man strings, Man font macros, man
@subsection Miscellaneous macros

@pindex grohtml
@cindex @code{man} macros, default indentation
@cindex default indentation [@code{man}]
The default indentation is 7.2@w{ }en for all output devices except for
@code{grohtml} which ignores indentation.

@Defmac {DT, , man}
@cindex tab stops [@code{man}]
@cindex @code{man} macros, tab stops
Set tabs every 0.5@w{ }inches.  Since this macro is always executed
during a call to the @code{TH} macro, it makes sense to call it only if
the tab positions have been changed.
@endDefmac

@Defmac {PD, [@Var{nnn}], man}
@cindex empty space before a paragraph [@code{man}]
@cindex @code{man} macros, empty space before a paragraph
Adjust the empty space before a new paragraph (or section).  The
optional argument gives the amount of space (default unit is
@samp{v}); without parameter, the value is reset to its default value
(1@w{ }line for TTY devices, 0.4@dmn{v}@w{ }otherwise).
@endDefmac

This affects the macros @code{SH}, @code{SS}, @code{TP}, @code{LP} (as
well as @code{PP} and @code{P}), @code{IP}, and @code{HP}.

@c ---------------------------------------------------------------------

@node Predefined man strings, Preprocessors in man pages, Miscellaneous man macros, man
@subsection Predefined strings

The following strings are defined:

@Defstr {S, man}
Switch back to the default font size.
@endDefstr

@Defstr {R, man}
The `registered' sign.
@endDefstr

@Defstr {Tm, man}
The `trademark' sign.
@endDefstr

@DefstrList {lq, man}
@DefstrListEnd {rq, man}
@cindex @code{lq} glyph, and @code{lq} string [@code{man}]
@cindex @code{rq} glyph, and @code{rq} string [@code{man}]
Left and right quote.  This is equal to @code{\(lq} and @code{\(rq},
respectively.
@endDefstr

@c ---------------------------------------------------------------------

@node Preprocessors in man pages,  , Predefined man strings, man
@subsection Preprocessors in @file{man} pages

@cindex preprocessor, calling convention
@cindex calling convention of preprocessors
If a preprocessor like @code{gtbl} or @code{geqn} is needed, it has
become common usage to make the first line of the man page look like
this:

@Example
'\" @var{word}
@endExample

@pindex geqn@r{, invocation in manual pages}
@pindex grefer@r{, invocation in manual pages}
@pindex gtbl@r{, invocation in manual pages}
@pindex man@r{, invocation of preprocessors}
@noindent
Note the single space character after the double quote.  @var{word}
consists of letters for the needed preprocessors: @samp{e} for
@code{geqn}, @samp{r} for @code{grefer}, @samp{t} for @code{gtbl}.
Modern implementations of the @code{man} program read this first line
and automatically call the right preprocessor(s).


@c =====================================================================

@node mdoc, ms, man, Macro Packages
@section @file{mdoc}
@cindex @code{mdoc} macros

@c XXX documentation
@c XXX this is a placeholder until we get stuff knocked into shape
See the @cite{groff_mdoc(7)} man page (type @command{man groff_mdoc}
at the command line).


@c =====================================================================

@node ms, me, mdoc, Macro Packages
@section @file{ms}
@cindex @code{ms} macros

The @file{-ms}
macros are suitable for reports, letters, books,
user manuals, and so forth.
The package provides macros for cover pages, section headings,
paragraphs, lists, footnotes, pagination,
and a table of contents.

@menu
* ms Intro::
* General ms Structure::
* ms Document Control Registers::
* ms Cover Page Macros::
* ms Body Text::
* ms Page Layout::
* Differences from AT&T ms::
@end menu

@c ---------------------------------------------------------------------

@node ms Intro, General ms Structure, ms, ms
@subsection Introduction to @file{ms}

The original @file{-ms} macros were included with
@acronym{AT&T} @code{troff} as well as the
@file{man} macros.
While the @file{man} package is intended for brief documents
that can be read on-line as well as printed, the @file{ms}
macros are suitable for longer documents that are meant to be
printed rather than read on-line.

The @file{ms} macro package included with @code{groff}
is a complete, bottom-up re-implementation.
Several macros (specific to @acronym{AT&T}
or Berkeley) are not included, while several new commands are.
@xref{Differences from AT&T ms}, for more information.

@c ---------------------------------------------------------------------

@node General ms Structure, ms Document Control Registers, ms Intro, ms
@subsection General structure of an @file{ms} document
@cindex @code{ms} macros, general structure

The @file{ms} macro package expects a certain amount of structure,
but not as much as packages such as @file{man} or @file{mdoc}.

The simplest documents can begin with a paragraph macro
(such as @code{LP} or @code{PP}),
and consist of text separated by paragraph macros
or even blank lines.
Longer documents have a structure as follows:

@table @strong
@item Document type
If you invoke the @code{RP}
(report) macro on the first line of the document,
@code{groff} prints the cover page information on its own page;
otherwise it prints the information on the
first page with your document text immediately following.
Other document formats found in @acronym{AT&T} @code{troff}
are specific to @acronym{AT&T} or Berkeley, and are not supported in
@code{groff}.

@item Format and layout
By setting number registers,
you can change your document's type (font and size),
margins, spacing, headers and footers, and footnotes.
@xref{ms Document Control Registers}, for more details.

@item Cover page
A cover page consists of a title, the author's name and institution,
an abstract, and the date.
@footnote{Actually, only the title is required.}
@xref{ms Cover Page Macros}, for more details.

@item Body
Following the cover page is your document.
You can use the @file{ms}
macros to write reports, letters, books, and so forth.
The package is designed for structured documents,
consisting of paragraphs interspersed with headings
and augmented by lists, footnotes, tables, and other
common constructs.
@xref{ms Body Text}, for more details.

@item Table of contents
Longer documents usually include a table of contents,
which you can invoke by placing the
@code{TC}
macro at the end of your document.
The @file{ms}
macros have minimal indexing facilities, consisting of the
@code{IX} macro, which prints an entry on standard error.
Printing the table of contents at the end is necessary since
@code{groff} is a single-pass text formatter,
thus it cannot determine the page number of each section
until that section has actually been set and printed.
Since @file{ms} output is intended for hardcopy,
you can manually relocate the pages containing
the table of contents between the cover page and the
body text after printing.
@end table

@c ---------------------------------------------------------------------

@node ms Document Control Registers, ms Cover Page Macros, General ms Structure, ms
@subsection Document control registers
@cindex @code{ms} macros, document control registers

The following is a list of document control number registers.
For the sake of consistency,
set registers related to margins at the beginning of your document,
or just after the @code{RP} macro.
You can set other registers later in your document,
but you should keep them together at the beginning
to make them easy to find and edit as necessary.

@unnumberedsubsubsec Margin Settings

@Defmpreg {PO, ms}
Defines the page offset (i.e.@: the left margin).
There is no explicit right margin setting; the combination of
the @code{PO} and @code{LL} registers implicitly define the
right margin width.

Effective: next page.

Default value: 1@dmn{i}.
@endDefmpreg

@Defmpreg {LL, ms}
Defines the line length (i.e.@: the width of the body text).

Effective: next paragraph.

Default: 6@dmn{i}.
@endDefmpreg

@Defmpreg {LT, ms}
Defines the title length (i.e.@: the header and footer width).
This is usually the same as @code{LL}, but not necessarily.

Effective: next paragraph.

Default: 6@dmn{i}.
@endDefmpreg

@Defmpreg {HM, ms}
Defines the header margin height at the top of the page.

Effective: next page.

Default: 1@dmn{i}.
@endDefmpreg

@Defmpreg {FM, ms}
Defines the footer margin height at the bottom of the page.

Effective: next page.

Default: 1@dmn{i}.
@endDefmpreg

@unnumberedsubsubsec Text Settings

@Defmpreg {PS, ms}
Defines the point size of the body text.

Effective: next paragraph.

Default: 10@dmn{p}.
@endDefmpreg

@Defmpreg {VS, ms}
Defines the space between lines (line height plus leading).

Effective: next paragraph.

Default: 12@dmn{p}.
@endDefmpreg

@unnumberedsubsubsec Paragraph Settings

@Defmpreg {PI, ms}
Defines the initial indent of a @code{.PP} paragraph.

Effective: next paragraph.

Default: 5@dmn{n}.
@endDefmpreg

@Defmpreg {PD, ms}
Defines the space between paragraphs.

Effective: next paragraph.

Default: 0.3@dmn{v}.
@endDefmpreg

@Defmpreg {QI, ms}
Defines the indent on both sides of a quoted (@code{.QP}) paragraph.

Effective: next paragraph.

Default: 5@dmn{n}.
@endDefmpreg

@unnumberedsubsubsec Footnote Settings

@Defmpreg {FL, ms}
Defines the length of a footnote.

Effective: next footnote.

Default: @math{@code{@\n[LL]} * 5 / 6}.
@endDefmpreg

@Defmpreg {FI, ms}
Defines the footnote indent.

Effective: next footnote.

Default: 2@dmn{n}.
@endDefmpreg

@Defmpreg {FF, ms}
The footnote format:
@table @code
@item 0
Prints the footnote number as a superscript; indents the footnote (default).

@item 1
Prints the number followed by a period (like 1.)
and indents the footnote.

@item 2
Like 1, without an indent.

@item 3
Like 1, but prints the footnote number as a hanging paragraph.
@end table

Effective: next footnote.

Default: 0.
@endDefmpreg

@unnumberedsubsubsec Miscellaneous Number Registers

@Defmpreg {MINGW, ms}
Defines the minimum width between columns in a multi-column document.

Effective: next page.

Default: 2@dmn{n}.
@endDefmpreg

@c ---------------------------------------------------------------------

@node ms Cover Page Macros, ms Body Text, ms Document Control Registers, ms
@subsection Cover page macros
@cindex @code{ms} macros, cover page
@cindex cover page macros, [@code{ms}]

Use the following macros to create a cover page for your document
in the order shown.

@Defmac {RP, [@code{no}], ms}
Specifies the report format for your document.
The report format creates a separate cover page.
The default action (no @code{.RP}
macro) is to print a subset of the
cover page on page 1 of your document.

If you use the word @code{no} as an optional argument,
@code{groff} prints a title page but
does not repeat any of the title page information
(title, author, abstract, etc.)
on page 1 of the document.
@endDefmac

@Defmac {DA, [@dots{}], ms}
(optional) Print the current date, or the arguments to the macro if any,
on the title page (if specified) and in the footers.
This is the default for @code{nroff}.
@endDefmac

@Defmac {ND, [@dots{}], ms}
(optional) Print the current date, or the arguments to the macro if any,
on the title page (if specified) but not in the footers.
This is the default for @code{troff}.
@endDefmac

@Defmac {TL, , ms}
Specifies the document title.
@code{groff} collects text following the @code{.TL}
macro into the title, until reaching the author name or abstract.
@endDefmac

@Defmac {AU, , ms}
Specifies the author's name, which appears on the
line (or lines) immediately following.
You can specify multiple authors as follows:

@Example
.AU
John Doe
.AI
University of West Bumblefuzz
.AU
Martha Buck
.AI
Monolithic Corporation

...
@endExample
@endDefmac

@Defmac {AI, , ms}
Specifies the author's institution.
You can specify multiple institutions in the same way
that you specify multiple authors.
@endDefmac

@Defmac {AB, [@code{no}], ms}
Begins the abstract.
The default is to print the word @acronym{ABSTRACT},
centered and in italics, above the text of the abstract.
The word @code{no} as an optional argument suppresses this heading.
@endDefmac

@Defmac {AE, , ms}
End the abstract.
@endDefmac

The following is example mark-up for a title page.
@cindex title page, example markup
@cindex example markup, title page

@Example
@cartouche
.RP
.TL
The Inevitability of Code Bloat
in Commercial and Free Software
.AU
J. Random Luser
.AI
University of West Bumblefuzz
.AB
This report examines the long-term growth
of the code bases in two large, popular software
packages; the free Emacs and the commercial
Microsoft Word.
While differences appear in the type or order
of features added, due to the different
methodologies used, the results are the same
in the end.
.PP
The free software approach is shown to be
superior in that while free software can
become as bloated as commercial offerings,
free software tends to have fewer serious
bugs and the added features are in line with
user demand.
.AE

... the rest of the paper follows ...
@end cartouche
@endExample

@c ---------------------------------------------------------------------

@node ms Body Text, ms Page Layout, ms Cover Page Macros, ms
@subsection Body text
@cindex @code{ms} macros, body text

This section describes macros used to mark up the body of your document.
Examples include paragraphs, sections, and other groups.

@menu
* Paragraphs in ms::
* Headings in ms::
* Highlighting in ms::
* Lists in ms::
* Indents in ms::
* Tabstops in ms::
* ms Displays and Keeps::
* ms Insertions::
* Example multi-page table::
* ms Footnotes::
@end menu

@c ---------------------------------------------------------------------

@node Paragraphs in ms, Headings in ms, ms Body Text, ms Body Text
@subsubsection Paragraphs
@cindex @code{ms} macros, paragraph handling

The following paragraph types are available.

@Defmac {PP, , ms}
Sets a paragraph with an initial indent.
@endDefmac

@Defmac {LP, , ms}
Sets a paragraph with no initial indent.
@endDefmac

@Defmac {QP, , ms}
Sets a paragraph that is indented at both left and right margins.
The effect is identical to the @acronym{HTML} @code{<BLOCKQUOTE>} element.
The next paragraph or heading returns margins to normal.
@endDefmac

@Defmac {XP, , ms}
Sets a paragraph whose lines are indented,
except for the first line.
This is a Berkeley extension.
@endDefmac

The following markup uses all four paragraph macros.

@Example
@cartouche
.NH 2
Cases used in the study
.LP
The following software and versions were
considered for this report.
.PP
For commercial software, we chose
.B "Microsoft Word for Windows" ,
starting with version 1.0 through the
current version (Word 2000).
.PP
For free software, we chose
.B Emacs ,
from its first appearance as a standalone
editor through the current version (v20).
See [Bloggs 2002] for details.
.QP
Franklin's Law applied to software:
software expands to outgrow both
RAM and disk space over time.
.LP
Bibliography:
.XP
Bloggs, Joseph R.,
.I "Everyone's a Critic" ,
Underground Press, March 2002.
A definitive work that answers all questions
and criticisms about the quality and usability of
free software.

@end cartouche
@endExample

@c ---------------------------------------------------------------------

@node Headings in ms, Highlighting in ms, Paragraphs in ms, ms Body Text
@subsubsection Headings
@cindex @code{ms} macros, headings

Use headings to create a hierarchical structure for your document.
The @file{ms} macros print headings in @strong{bold},
using the same font family and point size as the body text.

The following describes the heading macros:

@DefmacList {NH, @Var{curr-level}, ms}
@DefmacListEnd {NH, @t{S} @Var{level0} @dots{}, ms}
Numbered heading.
The argument is either a numeric argument to indicate the
level of the heading, or the letter@w{ }@code{S} followed by numeric
arguments to set the heading level explicitly.

If you specify heading levels out of sequence, such as invoking
@samp{.NH 3} after @samp{.NH 1}, @code{groff}
prints a warning on standard error.
@endDefmac

@Defmac {SH, , ms}
Unnumbered subheading.
@endDefmac

@c ---------------------------------------------------------------------

@node Highlighting in ms, Lists in ms, Headings in ms, ms Body Text
@subsubsection Highlighting
@cindex @code{ms} macros, highlighting

The @file{ms} macros provide a variety of methods to highlight
or emphasize text:

@Defmac {B, [@Var{txt} [@Var{post} [@Var{pre}]]], ms}
Sets its first argument in @strong{bold type}.
If you specify a second argument, @code{groff} prints it in the
previous font after the bold text, with no intervening space
(this allows you to set punctuation after the highlighted text
without highlighting the punctuation).
Similarly, it prints the third argument (if any) in the previous
font @strong{before} the first argument.
For example,

@Example
.B foo ) (
@endExample

prints (@strong{foo}).

If you give this macro no arguments, @code{groff}
prints all text following in bold until
the next highlighting, paragraph, or heading macro.
@endDefmac

@Defmac {R, [@Var{txt} [@Var{post} [@Var{pre}]]], ms}
Sets its first argument in roman (or regular) type.
It operates similarly to the @code{B}@w{ }macro otherwise.
@endDefmac

@Defmac {I, [@Var{txt} [@Var{post} [@Var{pre}]]], ms}
Sets its first argument in @emph{italic type}.
It operates similarly to the @code{B}@w{ }macro otherwise.
@endDefmac

@Defmac {CW, [@Var{txt} [@Var{post} [@Var{pre}]]], ms}
Sets its first argument in a @code{constant width face}.
It operates similarly to the @code{B}@w{ }macro otherwise.
@endDefmac

@Defmac {BI, [@Var{txt} [@Var{post} [@Var{pre}]]], ms}
Sets its first argument in bold italic type.
It operates similarly to the @code{B}@w{ }macro otherwise.
@endDefmac

@Defmac {BX, [@Var{txt}], ms}
Prints its argument and draws a box around it.
If you want to box a string that contains spaces,
use a digit-width space (@code{\0}).
@endDefmac

@Defmac {UL, [@Var{txt} [@Var{post}]], ms}
Prints its first argument with an underline.
If you specify a second argument, @code{groff}
prints it in the previous font after
the underlined text, with no intervening space.
@endDefmac

@Defmac {LG, , ms}
Prints all text following in larger type
(two points larger than the current point size) until
the next font size, highlighting, paragraph, or heading macro.
You can specify this macro multiple times
to enlarge the point size as needed.
@endDefmac

@Defmac {SM, , ms}
Prints all text following in smaller type
(two points smaller than the current point size) until
the next type size, highlighting, paragraph, or heading macro.
You can specify this macro multiple times
to reduce the point size as needed.
@endDefmac

@Defmac {NL, , ms}
Prints all text following in the normal point size
(that is, the value of the @code{PS} register).
@endDefmac

@c ---------------------------------------------------------------------

@node Lists in ms, Indents in ms, Highlighting in ms, ms Body Text
@subsubsection Lists
@cindex @code{ms} macros, lists

The @code{.IP} macro handles duties for all lists.

@Defmac {IP, [@Var{marker} [@Var{width}]], ms}
The @var{marker} is usually a bullet glyph (@code{\[bu]})
for unordered lists, a number (or auto-incrementing number
register) for numbered lists, or a word or phrase for indented
(glossary-style) lists.

The @var{width} specifies the indent for the body of each list item;
its default unit is @samp{n}.
Once specified, the indent remains the same for all
list items in the document until specified again.
@endDefmac

The following is an example of a bulleted list.
@cindex example markup, bulleted list [@code{ms}]
@cindex bulleted list, example markup [@code{ms}]

@Example
A bulleted list:
.IP \[bu] 2
lawyers
.IP \[bu]
guns
.IP \[bu]
money
@endExample

Produces:

@Example
A bulleted list:

o lawyers

o guns

o money
@endExample

@sp 1

The following is an example of a numbered list.
@cindex example markup, numbered list [@code{ms}]
@cindex numbered list, example markup [@code{ms}]

@Example
.nr step 1 1
A numbered list:
.IP \n[step] 3
lawyers
.IP \n+[step]
guns
.IP \n+[step]
money
@endExample

Produces:

@Example
A numbered list:

1. lawyers

2. guns

3. money
@endExample

Note the use of the auto-incrementing number
register in this example.

@sp 1
The following is an example of a glossary-style list.
@cindex example markup, glossary-style list [@code{ms}]
@cindex glossary-style list, example markup [@code{ms}]

@Example
A glossary-style list:
.IP lawyers 0.4i
Two or more attorneys.
.IP guns
Firearms, preferably
large-caliber.
.IP money
Gotta pay for those
lawyers and guns!
@endExample

Produces:

@Example
A glossary-style list:

lawyers
      Two or more attorneys.

guns  Firearms, preferably large-caliber.

money
      Gotta pay for those lawyers and guns!
@endExample

In the last example, the @code{IP} macro places the definition
on the same line as the term if it has enough space; otherwise,
it breaks to the next line and starts the definition below the
term.
This may or may not be the effect you want, especially if some
of the definitions break and some do not.
The following examples show two possible ways to force a break.

The first workaround uses the @code{br}
request to force a break after printing the term or label.

@Example
@cartouche
A glossary-style list:
.IP lawyers 0.4i
Two or more attorneys.
.IP guns
.br
Firearms, preferably large-caliber.
.IP money
Gotta pay for those lawyers and guns!
@end cartouche
@endExample

@sp 1
The second workaround uses the @code{\p} escape to force the break.
Note the space following the escape; this is important.
If you omit the space, @code{groff} prints the first word on
the same line as the term or label (if it fits) @strong{then}
breaks the line.

@Example
@cartouche
A glossary-style list:
.IP lawyers 0.4i
Two or more attorneys.
.IP guns
\p Firearms, preferably large-caliber.
.IP money
Gotta pay for those lawyers and guns!
@end cartouche
@endExample

@sp 1
To set nested lists, use the @code{RS} and @code{RE} macros.
@xref{Indents in ms}, for more information.
@cindex @code{ms} macros, nested lists
@cindex nested lists [@code{ms}]

For example:

@Example
@cartouche
.IP \[bu] 2
Lawyers:
.RS
.IP \[bu]
Dewey,
.IP \[bu]
Cheatham,
.IP \[bu]
and Howe.
.RE
.IP \[bu]
Guns
@end cartouche
@endExample

Produces:

@Example
o Lawyers:

  o  Dewey,

  o  Cheatham,

  o  and Howe.

o Guns
@endExample

@c ---------------------------------------------------------------------

@node Indents in ms, Tabstops in ms, Lists in ms, ms Body Text
@subsubsection Indents

In many situations,
you may need to indent a section of text
while still wrapping and filling.
@xref{Lists in ms},
for an example of nested lists.

@DefmacList {RS, , ms}
@DefmacListEnd {RE, , ms}
These macros begin and end an indented section.
The @code{PI} register controls the amount of indent,
allowing the indented text to line up under hanging
and indented paragraphs.
@endDefmac

@xref{ms Displays and Keeps},
for macros to indent and turn off filling.

@c ---------------------------------------------------------------------

@node Tabstops in ms, ms Displays and Keeps, Indents in ms, ms Body Text
@subsubsection Tab Stops

Use the @code{ta} request to define tab stops as needed.
@xref{Tabs and Fields}.

@Defmac{TA, , ms}
Use this macro to reset the tab stops to the default for @file{ms}
(every 5n).
You can redefine the @code{TA} macro to create a different set
of default tab stops.
@endDefmac

@c ---------------------------------------------------------------------

@node ms Displays and Keeps, ms Insertions, Tabstops in ms, ms Body Text
@subsubsection Displays and keeps
@cindex @code{ms} macros, displays
@cindex @code{ms} macros, keeps
@cindex keeps [@code{ms}]
@cindex displays [@code{ms}]

Use displays to show text-based examples or figures
(such as code listings).

Displays turn off filling, so lines of code are displayed
as-is without inserting @code{br} requests in between each line.
Displays can be @dfn{kept} on a single page, or allowed
to break across pages.

@DefmacList {DS, @t{L}, ms}
@DefmacItem {LD, , ms}
@DefmacListEnd {DE, , ms}
Left-justified display.
The @samp{.DS L} call generates a page break, if necessary,
to keep the entire display on one page.
The @code{LD} macro allows the display to break across pages.
The @code{DE} macro ends the display.
@endDefmac

@DefmacList {DS, @t{I}, ms}
@DefmacItem {ID, , ms}
@DefmacListEnd {DE, , ms}
Indents the display as defined by the @code{DI} register.
The @samp{.DS I} call generates a page break, if necessary,
to keep the entire display on one page.
The @code{ID} macro allows the display to break across pages.
The @code{DE} macro ends the display.
@endDefmac

@DefmacList {DS, @t{B}, ms}
@DefmacItem {BD, , ms}
@DefmacListEnd {DE, , ms}
Sets a block-centered display: the entire display is left-justified,
but indented so that the longest line in the display is centered
on the page.
The @samp{.DS B} call generates a page break, if necessary,
to keep the entire display on one page.
The @code{BD} macro allows the display to break across pages.
The @code{DE} macro ends the display.
@endDefmac

@DefmacList {DS, @t{C}, ms}
@DefmacItem {CD, , ms}
@DefmacListEnd {DE, , ms}
Sets a centered display: each line in the display is centered.
The @samp{.DS C} call generates a page break, if necessary,
to keep the entire display on one page.
The @code{CD} macro allows the display to break across pages.
The @code{DE} macro ends the display.
@endDefmac

@DefmacList {DS, @t{R}, ms}
@DefmacItem {RD, , ms}
@DefmacListEnd {DE, , ms}
Right-justifies each line in the display.
The @samp{.DS R} call generates a page break, if necessary,
to keep the entire display on one page.
The @code{RD} macro allows the display to break across pages.
The @code{DE} macro ends the display.
@endDefmac

@sp 1
On occasion, you may want to @dfn{keep} other text together on a page.
For example, you may want to keep two paragraphs together, or
a paragraph that refers to a table (or list, or other item)
immediately following.
The @file{ms} macros provide the @code{KS} and @code{KE}
macros for this purpose.

@DefmacList {KS, , ms}
@DefmacListEnd {KE, , ms}
The @code{KS} macro begins a block of text to be kept on a
single page, and the @code{KE} macro ends the block.
@endDefmac

@DefmacList {KF, , ms}
@DefmacListEnd {KE, , ms}
Specifies a @dfn{floating keep};
if the keep cannot fit on the current page, @code{groff}
holds the contents of the keep and allows text following
the keep (in the source file) to fill in the remainder of
the current page.
When the page breaks, whether by an explicit @code{bp}
request or by reaching the end of the page, @code{groff}
prints the floating keep at the top of the new page.
This is useful for printing large graphics or tables
that do not need to appear exactly where specified.
@endDefmac

You can also use the @code{ne} request to force a page break if
there is not enough vertical space remaining on the page.

@sp 1
Use the following macros to draw a box around a section of
text (such as a display).

@DefmacList {B1, , ms}
@DefmacListEnd {B2, , ms}
Marks the beginning and ending of text that is to have a
box drawn around it.
The @code{B1} macro begins the box; the @code{B2} macro ends it.
Text in the box is automatically placed in a diversion (keep).
@endDefmac

@c ---------------------------------------------------------------------

@node ms Insertions, Example multi-page table, ms Displays and Keeps, ms Body Text
@subsubsection Tables, figures, equations, and references
@cindex @code{ms} macros, tables
@cindex @code{ms} macros, figures
@cindex @code{ms} macros, equations
@cindex @code{ms} macros, references
@cindex tables [@code{ms}]
@cindex figures [@code{ms}]
@cindex equations [@code{ms}]
@cindex references [@code{ms}]

The @file{ms} macros support the standard
@code{groff} preprocessors:
@code{tbl}, @code{pic}, @code{eqn}, and @code{refer}.
@pindex tbl
@pindex pic
@pindex eqn
@pindex refer
You mark text meant for preprocessors by enclosing it
in pairs of tags as follows.

@DefmacList {TS, [@code{H}], ms}
@DefmacListEnd {TE, , ms}
Denotes a table, to be processed by the @code{tbl} preprocessor.
The optional argument@w{ }@code{H} to @code{TS} instructs @code{groff}
to create a running header with the information
up to the @code{TH} macro.
@code{groff} prints the header at the beginning of the
table; if the table runs onto another page, @code{groff}
prints the header on the next page as well.
@endDefmac

@DefmacList {PS, , ms}
@DefmacListEnd {PE, , ms}
Denotes a graphic, to be processed by the @code{pic} preprocessor.
You can create a @code{pic} file by hand, using the @acronym{AT&T}
@code{pic} manual available on the Web as a reference, or by using
a graphics program such as @code{xfig}.
@endDefmac

@DefmacList {EQ, [@Var{align}], ms}
@DefmacListEnd {EN, , ms}
Denotes an equation, to be processed by the @code{eqn} preprocessor.
The optional @var{align} argument can be @code{C}, @code{L}, or@w{
}@code{I} to center (the default), left-justify, or indent the equation.
@endDefmac

@DefmacList {[, , ms}
@DefmacListEnd {], , ms}
Denotes a reference, to be processed by the @code{refer} preprocessor.
The @acronym{GNU} @cite{refer(1)} man page provides a comprehensive
reference to the preprocessor and the format of the bibliographic
database.
@endDefmac

@menu
* Example multi-page table::
@end menu

@c ---------------------------------------------------------------------

@node Example multi-page table, ms Footnotes, ms Insertions, ms Body Text
@subsubsection An example multi-page table
@cindex example markup, multi-page table [@code{ms}]
@cindex multi-page table, example markup [@code{ms}]

The following is an example of how to set up a
table that may print across two or more pages.

@Example
@cartouche
.TS H
allbox expand;
cb | cb .
Text      ...of heading...
_
.TH
.T&
l | l .
... the rest of the table follows...
.CW
.TE
@end cartouche
@endExample

@c ---------------------------------------------------------------------

@node ms Footnotes,  , Example multi-page table, ms Body Text
@subsubsection Footnotes
@cindex @code{ms} macros, footnotes
@cindex footnotes [@code{ms}]

The @file{ms} macro package has a flexible footnote system.
You can specify either numbered footnotes or symbolic footnotes
(that is, using a marker such as a dagger symbol).

@Defstr {*, ms}
Specifies the location of a numbered footnote marker in the text.
@endDefesc

@DefmacList {FS, , ms}
@DefmacListEnd {FE, , ms}
Specifies the text of the footnote.
The default action is to create a numbered footnote;
you can create a symbolic footnote by specifying
a @dfn{mark} glyph
(such as @code{\[dg]} for the dagger glyph)
in the body text and as an argument to the @code{FS} macro,
followed by the text of the footnote
and the @code{FE} macro.
@endDefmac

You can control how @code{groff}
prints footnote numbers by changing the value of the
@code{FF} register.  @xref{ms Document Control Registers}.

@c ---------------------------------------------------------------------

@node ms Page Layout, Differences from AT&T ms, ms Body Text, ms
@subsection Page layout
@cindex @code{ms} macros, page layout
@cindex page layout [@code{ms}]

The default output from the @file{ms}
macros provides a minimalist page layout:
it prints a single column, with the page number centered at the top
of each page.
It prints no footers.

You can change the layout by setting
the proper number registers and strings.

@menu
* ms Headers and Footers::
* ms Margins::
* ms Multiple Columns::
* ms TOC::
* ms Strings and Special Characters::
@end menu

@c ---------------------------------------------------------------------

@node ms Headers and Footers, ms Margins, ms Page Layout, ms Page Layout
@subsubsection Headers and footers
@cindex @code{ms} macros, headers
@cindex @code{ms} macros, footers
@cindex headers [@code{ms}]
@cindex footers [@code{ms}]

For documents that do not distinguish between odd and even pages,
set the following strings:

@DefstrList {LH, ms}
@DefstrItem {CH, ms}
@DefstrListEnd {RH, ms}
Sets the left, center, and right headers.
@endDefstr

@DefstrList {LF, ms}
@DefstrItem {CF, ms}
@DefstrListEnd {RF, ms}
Sets the left, center, and right footers.
@endDefstr

@sp 1
For documents that need different information printed in the
even and odd pages, use the following macros:

@DefmacList {OH, @t{'}@Var{left}@t{'}@Var{center}@t{'}@Var{right}@t{'}, ms}
@DefmacItem {EH, @t{'}@Var{left}@t{'}@Var{center}@t{'}@Var{right}@t{'}, ms}
@DefmacItem {OF, @t{'}@Var{left}@t{'}@Var{center}@t{'}@Var{right}@t{'}, ms}
@DefmacListEnd {EF, @t{'}@Var{left}@t{'}@Var{center}@t{'}@Var{right}@t{'}, ms}
The @code{OH} and @code{EH} macros define headers for the odd and even pages;
the @code{OF} and @code{EF} macros define footers for the odd and even pages.
This is more flexible than defining the individual strings.

You can replace the quote (@code{'}) marks with any character not
appearing in the header or footer text.
@endDefmac

@c ---------------------------------------------------------------------

@node ms Margins, ms Multiple Columns, ms Headers and Footers, ms Page Layout
@subsubsection Margins
@cindex @code{ms} macros, margins

You control margins using a set of number registers.
@xref{ms Document Control Registers}, for details.

@c ---------------------------------------------------------------------

@node ms Multiple Columns, ms TOC, ms Margins, ms Page Layout
@subsubsection Multiple columns
@cindex @code{ms} macros, multiple columns
@cindex multiple columns [@code{ms}]

The @file{ms} macros can set text in as many columns as will
reasonably fit on the page.
The following macros are available;
all of them force a page break if a multi-column mode is already set.
However, if the current mode is single-column, starting a multi-column
mode does @strong{not} force a page break.

@Defmac {1C, , ms}
Single-column mode.
@endDefmac

@Defmac {2C, , ms}
Two-column mode.
@endDefmac

@Defmac {MC, [@Var{width} [@Var{gutter}]], ms}
Multi-column mode.
If you specify no arguments, it is equivalent to the
@code{2C} macro.
Otherwise, @var{width} is the width of each column and
@var{gutter} is the space between columns.
The @code{MINGW} number register controls the default gutter width.
@endDefmac

@c ---------------------------------------------------------------------

@node ms TOC, ms Strings and Special Characters, ms Multiple Columns, ms Page Layout
@subsubsection Creating a table of contents
@cindex @code{ms} macros, creating table of contents
@cindex table of contents, creating [@code{ms}]

The facilities in the @file{ms} macro package for creating
a table of contents are semi-automated at best.
Assuming that you want the table of contents to consist of
the document's headings, you need to repeat those headings
wrapped in @code{XS} and @code{XE} macros.

@DefmacList {XS, [@Var{page}], ms}
@DefmacItem {XA, [@Var{page}], ms}
@DefmacListEnd {XE, , ms}
These macros define a table of contents
or an individual entry in the table of contents,
depending on their use.
The macros are very simple; they cannot indent a heading based on its level.
The easiest way to work around this is to add tabs
to the table of contents string.
The following is an example:

@Example
@cartouche
.NH 1
Introduction
.XS
Introduction
.XE
.LP
...
.CW
.NH 2
Methodology
.XS
        Methodology
.XE
.LP
...
@end cartouche
@endExample

You can manually create a table of contents
by beginning with the @code{XS} macro for the first entry,
specifying the page number for that entry as the argument to @code{XS}.
Add subsequent entries using the @code{XA} macro,
specifying the page number for that entry as the argument to @code{XA}.
The following is an example:

@Example
@cartouche
.XS 1
Introduction
.XA 2
A Brief History of the Universe
.XA 729
Details of Galactic Formation
...
.XE
@end cartouche
@endExample
@endDefmac

@Defmac {TC, [@code{no}], ms}
Prints the table of contents on a new page,
setting the page number to@w{ }@strong{i} (Roman numeral one).
You should usually place this macro at the end of the
file, since @code{groff} is a single-pass formatter and
can only print what has been collected up to the point
that the @code{TC} macro appears.

The optional argument @code{no} suppresses printing
the title specified by the string register @code{TOC}.
@endDefmac

@Defmac{PX, [@code{no}], ms}
Prints the table of contents on a new page,
using the current page numbering sequence.
Use this macro to print a manually-generated table of contents
at the beginning of your document.

The optional argument @code{no} suppresses printing
the title specified by the string register @code{TOC}.
@endDefmac

The @cite{Groff and Friends HOWTO}
includes a @code{sed} script that automatically inserts
@code{XS} and @code{XE} macro entries after each heading in a document.

Altering the @code{NH} macro to automatically build the table
of contents is perhaps initially more difficult, but would save
a great deal of time in the long run if you use @file{ms} regularly.

@c ---------------------------------------------------------------------

@node ms Strings and Special Characters,  , ms TOC, ms Page Layout
@subsubsection Strings and Special Characters
@cindex @code{ms} macros, strings
@cindex @code{ms} macros, special characters
@cindex @code{ms} macros, accent marks
@cindex accent marks [@code{ms}]
@cindex special characters [@code{ms}]
@cindex strings [@code{ms}]

The @file{ms} macros provide the following predefined strings.
You can change the string definitions to help in creating
documents in languages other than English.

@Defstr {REFERENCES, ms}
Contains the string printed at the beginning of the
references (bibliography) page.
The default is @samp{References}.
@endDefstr

@Defstr {ABSTRACT, ms}
Contains the string printed at the beginning of the abstract.
The default is @samp{ABSTRACT}.
@endDefstr

@Defstr {TOC, ms}
Contains the string printed at the beginning of the table of contents.
@endDefstr

@DefstrList {MONTH1, ms}
@DefstrItem {MONTH2, ms}
@DefstrItem {MONTH3, ms}
@DefstrItem {MONTH4, ms}
@DefstrItem {MONTH5, ms}
@DefstrItem {MONTH6, ms}
@DefstrItem {MONTH7, ms}
@DefstrItem {MONTH8, ms}
@DefstrItem {MONTH9, ms}
@DefstrItem {MONTH10, ms}
@DefstrItem {MONTH11, ms}
@DefstrListEnd {MONTH12, ms}
Prints the full name of the month in dates.
The default is @samp{January}, @samp{February}, etc.
@endDefstr

The following special characters are available@footnote{For an
explanation what special characters are see @ref{Special Characters}.}:

@Defstr {-, ms}
Prints an em dash.
@endDefstr

@DefstrList {*Q, ms}
@DefstrListEnd {*U, ms}
Prints typographer's quotes in troff,
plain quotes in nroff.
@code{*Q} is the left quote and @code{*U} is the right quote.
@endDefstr

Improved accent marks are available in the @file{ms} macros.

@Defmac {AM, , ms}
Specify this macro at the beginning of your document
to enable extended accent marks and special characters.
This is a Berkeley extension.

To use the accent marks, place them @strong{after}
the character being accented.
@endDefmac

The following accent marks are available
after invoking the @code{AM} macro:

@Defstr {\', ms}
Acute accent.
@endDefstr

@Defstr {\`, ms}
Grave accent.
@endDefstr

@Defstr {^, ms}
Circumflex.
@endDefstr

@Defstr {\,, ms}
Cedilla.
@endDefstr

@Defstr {~, ms}
Tilde.
@endDefstr

@deffn String @t{\*[:]}
@ifnotinfo
@stindex : @r{[}ms@r{]}
@end ifnotinfo
@ifinfo
@stindex @r{<colon>} @r{[}ms@r{]}
@end ifinfo
Umlaut.
@end deffn

@Defstr {v, ms}
Hacek.
@endDefstr

@Defstr {_, ms}
Macron (overbar).
@endDefstr

@Defstr {., ms}
Underdot.
@endDefstr

@Defstr {o, ms}
Ring above.
@endDefstr

The following are standalone characters
available after invoking the @code{AM} macro:

@Defstr {?, ms}
Upside-down question mark.
@endDefstr

@Defstr {!, ms}
Upside-down exclamation point.
@endDefstr

@Defstr {8, ms}
German @ss{} ligature.
@endDefstr

@Defstr {3, ms}
Yogh.
@endDefstr

@Defstr {Th, ms}
Uppercase thorn.
@endDefstr

@Defstr {th, ms}
Lowercase thorn.
@endDefstr

@Defstr {D-, ms}
Uppercase eth.
@endDefstr

@Defstr {d-, ms}
Lowercase eth.
@endDefstr

@Defstr {q, ms}
Hooked o.
@endDefstr

@Defstr {ae, ms}
Lowercase @ae{} ligature.
@endDefstr

@Defstr {Ae, ms}
Uppercase @AE{} ligature.
@endDefstr

@c ---------------------------------------------------------------------

@node Differences from AT&T ms,  , ms Page Layout, ms
@subsection Differences from @acronym{AT&T} @file{ms}
@cindex @code{ms} macros, differences from @acronym{AT&T}
@cindex @acronym{AT&T} @code{troff}, @code{ms} macro package differences

This section lists the (minor) differences between the
@code{groff -ms} macros and @acronym{AT&T}
@code{troff -ms} macros.

@menu
* Missing ms Macros::
* Additional ms Macros::
@end menu

@c ---------------------------------------------------------------------

@node Missing ms Macros, Additional ms Macros, Differences from AT&T ms, Differences from AT&T ms
@subsubsection @code{troff} macros not appearing in @code{groff}

Macros missing from @code{groff -ms}
are cover page macros specific to Bell Labs.
The macros known to be missing are:

@table @code
@item .TM
Technical memorandum; a cover sheet style

@item .IM
Internal memorandum; a cover sheet style

@item .MR
Memo for record; a cover sheet style

@item .MF
Memo for file; a cover sheet style

@item .EG
Engineer's notes; a cover sheet style

@item .TR
Computing Science Tech Report; a cover sheet style

@item .OK
Other keywords

@item .CS
Cover sheet information

@item .MH
A cover sheet macro
@end table

@c ---------------------------------------------------------------------

@node Additional ms Macros,  , Missing ms Macros, Differences from AT&T ms
@subsubsection @code{groff} macros not appearing in @acronym{AT&T} @code{troff}

The @code{groff -ms} macros have a few minor extensions
compared to the @acronym{AT&T} @code{troff -ms} macros.

@Defmac {AM, , ms}
Improved accent marks.
@xref{ms Strings and Special Characters}, for details.
@endDefmac

@Defmac {DS, @t{I}, ms}
Indented display.
The default behavior of @acronym{AT&T} @code{troff -ms}
was to indent; the @code{groff} default prints displays
flush left with the body text.
@endDefmac

@Defmac {CW, , ms}
Print text in @code{constant width} (Courier) font.
@endDefmac

@Defmac {IX, , ms}
Indexing term (printed on standard error).
You can write a script to capture and process an index
generated in this manner.
@endDefmac

@sp 1
The following additional number registers
appear in @code{groff -ms}:

@Defmpreg {MINGW, ms}
Specifies a minimum space
between columns (for multi-column output); this takes the
place of the @code{GW} register that was documented but apparently
not implemented in @acronym{AT&T} @code{troff}.
@endDefmpreg

@sp 1
Several new string registers are available as well.
You can change these to handle (for example) the local language.
@xref{ms Strings and Special Characters}, for details.


@c =====================================================================

@node me, mm, ms, Macro Packages
@section @file{me}
@cindex @code{me} macro package

@c XXX documentation
@c XXX this is a placeholder until we get stuff knocked into shape
See the @file{meintro.me} and @file{meref.me} documents in
groff's @file{doc} directory.


@c =====================================================================

@node mm,  , me, Macro Packages
@section @file{mm}
@cindex @code{mm} macro package

@c XXX documentation
@c XXX this is a placeholder until we get stuff knocked into shape
See the @cite{groff_mm(7)} man page (type @command{man groff_mm} at
the command line).


@c =====================================================================
@c =====================================================================

@node gtroff Reference, Preprocessors, Macro Packages, Top
@chapter @code{gtroff} Reference
@cindex reference, @code{gtroff}
@cindex @code{gtroff}, reference

This chapter covers @strong{all} of the facilities of @code{gtroff}.
Users of macro packages may skip it if not interested in details.


@menu
* Text::
* Input Conventions::
* Measurements::
* Expressions::
* Identifiers::
* Embedded Commands::
* Registers::
* Manipulating Filling and Adjusting::
* Manipulating Hyphenation::
* Manipulating Spacing::
* Tabs and Fields::
* Character Translations::
* Troff and Nroff Mode::
* Line Layout::
* Line Control::
* Page Layout::
* Page Control::
* Fonts::
* Sizes::
* Strings::
* Conditionals and Loops::
* Writing Macros::
* Page Motions::
* Drawing Requests::
* Traps::
* Diversions::
* Environments::
* Suppressing output::
* Colors::
* I/O::
* Postprocessor Access::
* Miscellaneous::
* Gtroff Internals::
* Debugging::
* Implementation Differences::
@end menu


@c =====================================================================

@node Text, Input Conventions, gtroff Reference, gtroff Reference
@section Text
@cindex text, @code{gtroff} processing

@code{gtroff} input files contain text with control commands
interspersed throughout.  But, even without control codes, @code{gtroff}
still does several things with the input text:

@itemize @bullet
@item
filling and adjusting

@item
adding additional space after sentences

@item
hyphenating

@item
inserting implicit line breaks
@end itemize

@menu
* Filling and Adjusting::
* Hyphenation::
* Sentences::
* Tab Stops::
* Implicit Line Breaks::
@end menu

@c ---------------------------------------------------------------------

@node Filling and Adjusting, Hyphenation, Text, Text
@subsection Filling and Adjusting
@cindex filling
@cindex adjusting

When @code{gtroff} reads text, it collects words from the input and fits
as many of them together on one output line as it can.  This is known as
@dfn{filling}.

@cindex leading spaces
@cindex spaces, leading and trailing
@cindex extra spaces
@cindex trailing spaces
Once @code{gtroff} has a @dfn{filled} line, it tries to @dfn{adjust}
it.  This means it widens the spacing between words until the text
reaches the right margin (in the default adjustment mode).  Extra spaces
between words are preserved, but spaces at the end of lines are ignored.
Spaces at the front of a line cause a @dfn{break} (breaks are
explained in @ref{Implicit Line Breaks}).

@xref{Manipulating Filling and Adjusting}.

@c ---------------------------------------------------------------------

@node Hyphenation, Sentences, Filling and Adjusting, Text
@subsection Hyphenation
@cindex hyphenation

Since the odds are not great for finding a set of words, for every
output line, which fit nicely on a line without inserting excessive
amounts of space between words, @code{gtroff} hyphenates words so
that it can justify lines without inserting too much space between
words.  It uses an internal hyphenation algorithm (a simplified version
of the algorithm used within @TeX{}) to indicate which words can be
hyphenated and how to do so.  When a word is hyphenated, the first part
of the word is added to the current filled line being output (with
an attached hyphen), and the other portion is added to the next
line to be filled.

@xref{Manipulating Hyphenation}.

@c ---------------------------------------------------------------------

@node Sentences, Tab Stops, Hyphenation, Text
@subsection Sentences
@cindex sentences

Although it is often debated, some typesetting rules say there should be
different amounts of space after various punctuation marks.  For
example, the @cite{Chicago typsetting manual} says that a period at the
end of a sentence should have twice as much space following it as would
a comma or a period as part of an abbreviation.

@c XXX exact citation of Chicago manual

@cindex sentence space
@cindex space between sentences
@cindex french-spacing
@code{gtroff} does this by flagging certain characters (normally
@samp{!}, @samp{?}, and @samp{.}) as @dfn{end-of-sentence} characters.
When @code{gtroff} encounters one of these characters at the end of a
line, it appends a normal space followed by a @dfn{sentence space} in
the formatted output.  (This justifies one of the conventions mentioned
in @ref{Input Conventions}.)

@cindex transparent characters
@cindex character, transparent
@cindex @code{dg} glyph, at end of sentence
@cindex @code{rq} glyph, at end of sentence
@cindex @code{"}, at end of sentence
@cindex @code{'}, at end of sentence
@cindex @code{)}, at end of sentence
@cindex @code{]}, at end of sentence
@cindex @code{*}, at end of sentence
In addition, the following characters and symbols are treated
transparently while handling end-of-sentence characters: @samp{"},
@samp{'}, @samp{)}, @samp{]}, @samp{*}, @code{\[dg]}, and @code{\[rq]}.

See the @code{cflags} request in @ref{Using Symbols}, for more details.

@cindex @code{\&}, at end of sentence
To prevent the insertion of extra space after an end-of-sentence
character (at the end of a line), append @code{\&}.

@c ---------------------------------------------------------------------

@node Tab Stops, Implicit Line Breaks, Sentences, Text
@subsection Tab Stops
@cindex tab stops
@cindex stops, tabulator
@cindex tab character
@cindex character, tabulator

@cindex @acronym{EBCDIC} encoding
@cindex encoding, @acronym{EBCDIC}
@code{gtroff} translates @dfn{tabulator characters}, also called
@dfn{tabs} (normally code point @acronym{ASCII} @code{0x09} or
@acronym{EBCDIC} @code{0x05}), in the input into movements to the next
tabulator stop.  These tab stops are initially located every half inch
across the page.  Using this, simple tables can be made easily.
However, it can often be deceptive as the appearance (and width) of the
text on a terminal and the results from @code{gtroff} can vary greatly.

Also, a possible sticking point is that lines beginning with tab
characters are still filled, again producing unexpected results.
For example, the following input

@multitable {12345678} {12345678} {12345678} {12345678}
@item
@tab 1 @tab 2 @tab 3
@item
@tab   @tab 4 @tab 5
@end multitable

@noindent
produces

@multitable {12345678} {12345678} {12345678} {12345678} {12345678} {12345678} {12345678}
@item
@tab 1 @tab 2 @tab 3 @tab   @tab 4 @tab 5
@end multitable

@xref{Tabs and Fields}.

@c ---------------------------------------------------------------------

@node Implicit Line Breaks,  , Tab Stops, Text
@subsection Implicit Line Breaks
@cindex implicit line breaks
@cindex implicit breaks of lines
@cindex line, implicit breaks
@cindex break, implicit
@cindex line break

An important concept in @code{gtroff} is the @dfn{break}.  When a break
occurs, @code{gtroff} outputs the partially filled line
(unjustified), and resumes collecting and filling text on the next output
line.

@cindex blank line
@cindex empty line
@cindex line, blank
@cindex blank line macro (@code{blm})
There are several ways to cause a break in @code{gtroff}.  A blank
line not only causes a break, but it also outputs a one-line vertical
space (effectively a blank line).  Note that this behaviour can be
modified with the blank line macro request @code{blm}.
@xref{Blank Line Traps}.

@cindex fill mode
@cindex mode, fill
A line that begins with a space causes a break and the space is
output at the beginning of the next line.  Note that this space isn't
adjusted, even in fill mode.

The end of file also causes a break -- otherwise the last line of
the document may vanish!

Certain requests also cause breaks, implicitly or explicitly.  This is
discussed in @ref{Manipulating Filling and Adjusting}.


@c =====================================================================

@node Input Conventions, Measurements, Text, gtroff Reference
@section Input Conventions
@cindex input conventions
@cindex conventions for input

Since @code{gtroff} does filling automatically, it is traditional in
@code{groff} not to try and type things in as nicely formatted
paragraphs.  These are some conventions commonly used when typing
@code{gtroff} text:

@itemize @bullet
@item
Break lines after punctuation, particularly at the end of a sentence
and in other logical places.  Keep separate phrases on lines by
themselves, as entire phrases are often added or deleted when editing.

@item
Try to keep lines less than 40-60@w{ }characters, to allow space for
inserting more text.

@item
Do not try to do any formatting in a @acronym{WYSIWYG} manner (i.e.,
don't try using spaces to get proper indentation).
@end itemize


@c =====================================================================

@node Measurements, Expressions, Input Conventions, gtroff Reference
@section Measurements
@cindex measurements

@cindex units of measurement
@cindex basic unit (@code{u})
@cindex machine unit (@code{u})
@cindex measurement unit
@cindex @code{u} unit
@cindex unit, @code{u}
@code{gtroff} (like many other programs) requires numeric parameters to
specify various measurements.  Most numeric parameters@footnote{those
that specify vertical or horizontal motion or a type size} may have a
@dfn{measurement unit} attached.  These units are specified as a single
character which immediately follows the number or expression.  Each of
these units are understood, by @code{gtroff}, to be a multiple of its
@dfn{basic unit}.  So, whenever a different measurement unit is
specified @code{gtroff} converts this into its @dfn{basic units}.  This
basic unit, represented by a @samp{u}, is a device dependent measurement
which is quite small, ranging from 1/75@dmn{th} to 1/72000@dmn{th} of an
inch.  The values may be given as fractional numbers; however,
fractional basic units are always rounded to integers.

Some of the measurement units are completely independent of any of the
current settings (e.g.@: type size) of @code{gtroff}.

@table @code
@item i
@cindex inch unit (@code{i})
@cindex @code{i} unit
@cindex unit, @code{i}
Inches.  An antiquated measurement unit still in use in certain
backwards countries with incredibly low-cost computer equipment.  One
inch is equal to@w{ }2.54@dmn{cm}.

@item c
@cindex centimeter unit (@code{c})
@cindex @code{c} unit
@cindex unit, @code{c}
Centimeters.  One centimeter is equal to@w{ }0.3937@dmn{in}.

@item p
@cindex point unit (@code{p})
@cindex @code{p} unit
@cindex unit, @code{p}
Points.  This is a typesetter's measurement used for measure type size.
It is 72@w{ }points to an inch.

@item P
@cindex pica unit (@code{P})
@cindex @code{P} unit
@cindex unit, @code{P}
Pica.  Another typesetting measurement.  6@w{ }Picas to an inch (and
12@w{ }points to a pica).

@item s
@itemx z
@cindex @code{s} unit
@cindex unit, @code{s}
@cindex @code{z} unit
@cindex unit, @code{z}
@xref{Fractional Type Sizes}, for a discussion of these units.

@item f
@cindex @code{f} unit
@cindex unit, @code{f}
Fractions. Value is 65536.
@xref{Colors}, for usage.
@end table

The other measurements understood by @code{gtroff} depend on
settings currently in effect in @code{gtroff}.  These are very useful
for specifying measurements which should look proper with any size of
text.

@table @code
@item m
@cindex em unit (@code{m})
@cindex @code{m} unit
@cindex unit, @code{m}
Ems.  This unit is equal to the current font size in points.  So called
because it is @emph{approximately} the width of the letter@w{ }@samp{m}
in the current font.

@item n
@cindex en unit (@code{n})
@cindex @code{n} unit
@cindex unit, @code{n}
Ens.  In @code{groff}, this is half of an em.

@item v
@cindex vertical space unit (@code{v})
@cindex space, vertical, unit (@code{v})
@cindex @code{v} unit
@cindex unit, @code{v}
Vertical space.  This is equivalent to the current line spacing.
@xref{Sizes}, for more information about this.

@item M
@cindex @code{M} unit
@cindex unit, @code{M}
100ths of an em.
@end table

@menu
* Default Units::
@end menu

@c ---------------------------------------------------------------------

@node Default Units,  , Measurements, Measurements
@subsection Default Units
@cindex default units
@cindex units, default

Many requests take a default unit.  While this can be helpful at times,
it can cause strange errors in some expressions.  For example, the line
length request expects em units.  Here are several attempts to get a
line length of 3.5@w{ }inches and their results:

@Example
3.5i      @result{}   3.5i
7/2       @result{}   0i
7/2i      @result{}   0i
(7 / 2)u  @result{}   0i
7i/2      @result{}   0.1i
7i/2u     @result{}   3.5i
@endExample

@noindent
Everything is converted to basic units first.  In the above example it
is assumed that 1@dmn{i} equals@w{ }240@dmn{u}, and 1@dmn{m} equals@w{
}10@dmn{p} (thus 1@dmn{m} equals@w{ }33@dmn{u}).  The value 7@dmn{i}/2
is first handled as 7@dmn{i}/2@dmn{m}, then converted to
1680@dmn{u}/66@dmn{u} which is 25@dmn{u}, and this is approximately
0.1@dmn{i}.  As can be seen, a scaling indicator after a closing
parenthesis is simply ignored.

@cindex measurements, specifying safely
Thus, the safest way to specify measurements is to always
attach a scaling indicator.  If you want to multiply or divide by a
certain scalar value, use @samp{u} as the unit for that value.


@c =====================================================================

@node Expressions, Identifiers, Measurements, gtroff Reference
@section Expressions
@cindex expressions

@code{gtroff} has most arithmetic operators common to other languages:

@itemize @bullet
@item
@cindex arithmetic operators
@cindex operators, arithmetic
@opindex +
@opindex -
@opindex /
@opindex *
@opindex %
Arithmetic: @samp{+} (addition), @samp{-} (subtraction), @samp{/}
(division), @samp{*} (multiplication), @samp{%} (modulo).

@code{gtroff} only provides integer arithmetic.  The internal type used
for computing results is @samp{int}, which is usually a 32@dmn{bit}
signed integer.

@item
@cindex comparison operators
@cindex operators, comparison
@opindex <
@opindex >
@opindex >=
@opindex <=
@opindex =
@opindex ==
Comparison: @samp{<} (less than), @samp{>} (greater than), @samp{<=}
(less than or equal), @samp{>=} (greater than or equal), @samp{=}
(equal), @samp{==} (the same as @samp{=}).

@item
@cindex logical operators
@cindex operators, logical
@opindex &
@ifnotinfo
@opindex :
@end ifnotinfo
@ifinfo
@opindex @r{<colon>}
@end ifinfo
Logical: @samp{&} (logical and), @samp{:} (logical or).

@item
@cindex unary operators
@cindex operators, unary
@opindex -
@opindex +
@opindex !
@cindex @code{if} request, and the @samp{!} operator
@cindex @code{while} request, and the @samp{!} operator
Unary operators: @samp{-} (negating, i.e.@: changing the sign), @samp{+}
(just for completeness; does nothing in expressions), @samp{!} (logical
not; this works only within @code{if} and @code{while} requests).  See
below for the use of unary operators in motion requests.

@item
@cindex extremum operators (@code{>?}, @code{<?})
@cindex operators, extremum (@code{>?}, @code{<?})
@opindex >?
@opindex <?
Extrema: @samp{>?} (maximum), @samp{<?} (minimum).

Example:

@Example
.nr x 5
.nr y 3
.nr z (\n[x] >? \n[y])
@endExample

@noindent
The register@w{ }@code{z} now contains@w{ }5.

@item
@cindex scaling operator
@cindex operator, scaling
Scaling: @code{(@var{c};@var{e})}.  Evaluate@w{ }@var{e} using@w{ }@var{c}
as the default scaling indicator.  If @var{c} is missing, ignore scaling
indicators in the evaluation of@w{ }@var{e}.
@end itemize

@cindex parentheses
@cindex order of evaluation in expressions
@cindex expression, order of evaluation
@opindex (
@opindex )
Parentheses may be used as in any other language.  However, in
@code{gtroff} they are necessary to ensure order of evaluation.
@code{gtroff} has no operator precedence; expressions are evaluated left
to right.  This means that @code{gtroff} evaluates @samp{3+5*4} as if it were
parenthesized like @samp{(3+5)*4}, not as @samp{3+(5*4)}, as might be
expected.

@cindex @code{+}, and page motion
@cindex @code{-}, and page motion
@cindex motion operators
@cindex operators, motion
For many requests which cause a motion on the page, the unary operators
@samp{+} and @samp{-} work differently if leading an expression.  They
then indicate a motion relative to the current position (down or up,
respectively).

@cindex @code{|}, and page motion
@cindex absolute position operator (@code{|})
@cindex position, absolute, operator (@code{|})
Similarly, a leading @samp{|} operator indicates an absolute position.
For vertical movements, it specifies the distance from the top of the
page; for horizontal movements, it gives the distance from the beginning
of the @emph{input} line.

@cindex @code{bp} request, using @code{+} and@w{ }@code{-}
@cindex @code{in} request, using @code{+} and@w{ }@code{-}
@cindex @code{ll} request, using @code{+} and@w{ }@code{-}
@cindex @code{lt} request, using @code{+} and@w{ }@code{-}
@cindex @code{nm} request, using @code{+} and@w{ }@code{-}
@cindex @code{nr} request, using @code{+} and@w{ }@code{-}
@cindex @code{pl} request, using @code{+} and@w{ }@code{-}
@cindex @code{pn} request, using @code{+} and@w{ }@code{-}
@cindex @code{po} request, using @code{+} and@w{ }@code{-}
@cindex @code{ps} request, using @code{+} and@w{ }@code{-}
@cindex @code{pvs} request, using @code{+} and@w{ }@code{-}
@cindex @code{rt} request, using @code{+} and@w{ }@code{-}
@cindex @code{ti} request, using @code{+} and@w{ }@code{-}
@cindex @code{\H}, using @code{+} and@w{ }@code{-}
@cindex @code{\R}, using @code{+} and@w{ }@code{-}
@cindex @code{\s}, using @code{+} and@w{ }@code{-}
@samp{+} and @samp{-} are also treated differently by the following
requests and escapes: @code{bp}, @code{in}, @code{ll}, @code{lt},
@code{nm}, @code{nr}, @code{pl}, @code{pn}, @code{po}, @code{ps},
@code{pvs}, @code{rt}, @code{ti}, @code{\H}, @code{\R}, and @code{\s}.
Here, leading plus and minus signs indicate increments and decrements.

@xref{Setting Registers}, for some examples.

@Defesc {\\B, ', anything, '}
@cindex numeric expression, valid
@cindex valid numeric expression
Return@w{ }1 if @var{anything} is a valid numeric expression;
or@w{ }0 if @var{anything} is empty or not a valid numeric expression.
@endDefesc

@cindex space characters, in expressions
@cindex expressions, and space characters
Due to the way arguments are parsed, spaces are not allowed in
expressions, unless the entire expression is surrounded by parentheses.

@xref{Request Arguments}, and @ref{Conditionals and Loops}.


@c =====================================================================

@node Identifiers, Embedded Commands, Expressions, gtroff Reference
@section Identifiers
@cindex identifiers

Like any other language, @code{gtroff} has rules for properly formed
@dfn{identifiers}.  In @code{gtroff}, an identifier can be made up of
almost any printable character, with the exception of the following
characters:

@itemize @bullet
@item
@cindex whitespace characters
@cindex newline character
@cindex character, whitespace
Whitespace characters (spaces, tabs, and newlines).

@item
@cindex character, backspace
@cindex backspace character
@cindex @acronym{EBCDIC} encoding of backspace
Backspace (@acronym{ASCII}@w{ }@code{0x08} or @acronym{EBCDIC}@w{
}@code{0x16}) and character code @code{0x01}.

@item
@cindex invalid input characters
@cindex input characters, invalid
@cindex characters, invalid input
@cindex Unicode
The following input characters are invalid and are ignored if
@code{groff} runs on a machine based on @acronym{ASCII}, causing a
warning message of type @samp{input} (see @ref{Debugging}, for more
details): @code{0x00}, @code{0x0B}, @code{0x0D}-@code{0x1F},
@code{0x80}-@code{0x9F}.

And here are the invalid input characters if @code{groff} runs on an
@acronym{EBCDIC} host: @code{0x00}, @code{0x08}, @code{0x09},
@code{0x0B}, @code{0x0D}-@code{0x14}, @code{0x17}-@code{0x1F},
@code{0x30}-@code{0x3F}.

Currently, some of these reserved codepoints are used internally, thus
making it non-trivial to extend @code{gtroff} to cover Unicode or other
character sets and encodings which use characters of these ranges.

Note that invalid characters are removed before parsing; an
identifier @code{foo}, followed by an invalid character, followed by
@code{bar} is treated as @code{foobar}.
@end itemize

For example, any of the following is valid.

@Example
br
PP
(l
end-list
@@_
@endExample

@cindex @code{]}, as part of an identifier
@noindent
Note that identifiers longer than two characters with a closing bracket
(@samp{]}) in its name can't be accessed with escape sequences which
expect an identifier as a parameter.  For example, @samp{\[foo]]}
accesses the glyph @samp{foo}, followed by @samp{]}, whereas
@samp{\C'foo]'} really asks for glyph @samp{foo]}.

@cindex @code{refer}, and macro names starting with @code{[} or @code{]}
@cindex @code{[}, macro names starting with, and @code{refer}
@cindex @code{]}, macro names starting with, and @code{refer}
@cindex macro names, starting with @code{[} or @code{]}, and @code{refer}
To avoid problems with the @code{refer} preprocessor, macro names
should not start with @samp{[} or @samp{]}.  Due to backwards
compatibility, everything after @samp{.[} and @samp{.]} is handled as
a special argument to @code{refer}.  For example, @samp{.[foo} makes
@code{refer} to start a reference, using @samp{foo} as a parameter.

@Defesc {\\A, ', ident, '}
Test whether an identifier @var{ident} is valid in @code{gtroff}.  It
expands to the character@w{ }1 or@w{ }0 according to whether its
argument (usually delimited by quotes) is or is not acceptable as the
name of a string, macro, diversion, number register, environment, or
font.  It returns@w{ }0 if no argument is given.  This is useful for
looking up user input in some sort of associative table.

@Example
\A'end-list'
    @result{} 1
@endExample
@endDefesc

@xref{Escapes}, for details on parameter delimiting characters.

Identifiers in @code{gtroff} can be any length, but, in some contexts,
@code{gtroff} needs to be told where identifiers end and text begins
(and in different ways depending on their length):

@itemize @bullet
@item
Single character.

@cindex @code{(}, starting a two-character identifier
@item
Two characters.  Must be prefixed with @samp{(} in some situations.

@cindex @code{[}, starting an identifier
@cindex @code{]}, ending an identifier
@item
Arbitrary length (@code{gtroff} only).  Must be bracketed with @samp{[}
and@w{ }@samp{]} in some situations.  Any length identifier can be put
in brackets.
@end itemize

@cindex undefined identifiers
@cindex identifiers, undefined
Unlike many other programming languages, undefined identifiers are
silently ignored or expanded to nothing.
When @code{gtroff} finds an undefined identifier, it emits a
warning, doing the following:

@itemize @bullet
@item
If the identifier is a string, macro, or diversion,
@code{gtroff} defines it as empty.

@item
If the identifier is a number register, @code{gtroff}
defines it with a value of@w{ }0.
@end itemize

@xref{Warnings}., @ref{Interpolating Registers}, and @ref{Strings}.

Note that macros, strings, and diversions share the same name space.

@Example
.de xxx
.  nop foo
..
.
.di xxx
bar
.br
.di
.
.xxx
    @result{} bar
@endExample

@noindent
As can be seen in the previous example, @code{gtroff} reuses the
identifier @samp{xxx}, changing it from a macro to a diversion.
No warning is emitted!  The contents of the first macro definition is
lost.

@xref{Interpolating Registers}, and @ref{Strings}.


@c =====================================================================

@node Embedded Commands, Registers, Identifiers, gtroff Reference
@section Embedded Commands
@cindex embedded commands
@cindex commands, embedded

Most documents need more functionality beyond filling, adjusting and
implicit line breaking.  In order to gain further functionality,
@code{gtroff} allows commands to be embedded into the text, in two ways.

The first is a @dfn{request} which takes up an entire line, and does
some large-scale operation (e.g.@: break lines, start new pages).

The other is an @dfn{escape} which can be usually embedded anywhere
in the text; most requests can accept it even as an argument.
Escapes generally do more minor operations like sub- and superscripts,
print a symbol, etc.

@menu
* Requests::
* Macros::
* Escapes::
@end menu

@c ---------------------------------------------------------------------

@node Requests, Macros, Embedded Commands, Embedded Commands
@subsection Requests
@cindex requests

@cindex control character (@code{.})
@cindex character, control (@code{.})
@cindex no-break control character (@code{'})
@cindex character, no-break control (@code{'})
@cindex control character, no-break (@code{'})
A request line begins with a control character, which is either a single
quote (@samp{'}, the @dfn{no-break control character}) or a period
(@samp{.}, the normal @dfn{control character}).  These can be changed;
see @ref{Character Translations}, for details.  After this there may be
optional tabs or spaces followed by an identifier which is the name of
the request.  This may be followed by any number of space-separated
arguments (@emph{no} tabs here).

@cindex structuring source code of documents or macro packages
@cindex documents, structuring the source code
@cindex macro packages, structuring the source code
Since a control character followed by whitespace only is ignored, it
is common practice to use this feature for structuring the source code
of documents or macro packages.

@Example
.de foo
.  tm This is foo.
..
.
.
.de bar
.  tm This is bar.
..
@endExample

@cindex blank line
@cindex blank line macro (@code{blm})
Another possibility is to use the blank line macro request @code{blm}
by assigning an empty macro to it.

@Example
.de do-nothing
..
.blm do-nothing  \" activate blank line macro

.de foo
.  tm This is foo.
..


.de bar
.  tm This is bar.
..

.blm             \" deactivate blank line macro
@endExample

@xref{Blank Line Traps}.

@cindex zero width space character (@code{\&})
@cindex character, zero width space (@code{\&})
@cindex space character, zero width (@code{\&})
@cindex @code{\&}, escaping control characters
To begin a line with a control character without it being interpreted,
precede it with @code{\&}.  This represents a zero width space, which
means it does not affect the output.

In most cases the period is used as a control character.  Several
requests cause a break implicitly; using the single quote control
character prevents this.

@menu
* Request Arguments::
@end menu

@node Request Arguments,  , Requests, Requests
@subsubsection Request Arguments
@cindex request arguments
@cindex arguments to requests

Arguments to requests (and macros) are processed much like the shell:
The line is split into arguments according to
spaces.@footnote{Plan@w{ }9's @code{troff} implementation also allows
tabs for argument separation -- @code{gtroff} intentionally doesn't
support this.}  An argument which is intended to contain spaces can
either be enclosed in double quotes, or have the spaces @dfn{escaped}
with backslashes.

Here are a few examples:

@Example
.uh The Mouse Problem
.uh "The Mouse Problem"
.uh The\ Mouse\ Problem
@endExample

@cindex @code{\~}, difference to @code{\@key{SP}}
@cindex @code{\@key{SP}}, difference to @code{\~}
@noindent
The first line is the @code{uh} macro being called with 3 arguments,
@samp{The}, @samp{Mouse}, and @samp{Problem}.  The latter two have the
same effect of calling the @code{uh} macro with one argument, @samp{The
Mouse Problem}.@footnote{The last solution, i.e., using escaped spaces,
is ``classical'' in the sense that it can be found in most @code{troff}
documents.  Nevertheless, it is not optimal in all situations, since
@w{@samp{\ }} inserts a fixed-width, non-breaking space character which
can't stretch.  @code{gtroff} provides a different command @code{\~} to
insert a stretchable, non-breaking space.}

@cindex @code{"}, in a macro argument
@cindex double quote, in a macro argument
A double quote which isn't preceded by a space doesn't start a macro
argument.  If not closing a string, it is printed literally.

For example,

@Example
.xxx a" "b c" "de"fg"
@endExample

@noindent
has the arguments @samp{a"}, @w{@samp{b c}}, @samp{de}, and @samp{fg"}.
Don't rely on this obscure behaviour!

There are two possibilities to get a double quote reliably.

@itemize @bullet
@item
Enclose the whole argument with double quotes and use two consecutive double
quotes to represent a single one.  This traditional solution has the
disadvantage that double quotes don't survive argument expansion again if
called in compatibility mode (using the @option{-C} option of @code{groff}):

@Example
.de xx
.  tm xx: `\\$1' `\\$2' `\\$3'
.
.  yy "\\$1" "\\$2" "\\$3"
..
.de yy
.  tm yy: `\\$1' `\\$2' `\\$3'
..
.xx A "test with ""quotes""" .
    @result{} xx: `A' `test with "quotes"' `.'
    @result{} yy: `A' `test with ' `quotes""'
@endExample

@noindent
If not in compatibility mode, you get the expected result

@Example
xx: `A' `test with "quotes"' `.'
yy: `A' `test with "quotes"' `.'
@endExample

@noindent
since @code{gtroff} preserves the input level.

@item
Use the double quote glyph @code{\(dq}.  This works with and without
compatibility mode enabled since @code{gtroff} doesn't convert @code{\(dq}
back to a double quote input character.

Not that this method won't work with @acronym{UNIX} @code{troff} in general
since the glyph `dq' isn't defined normally.
@end itemize

@cindex @code{ds} request, and double quotes
Double quotes in the @code{ds} request are handled differently.
@xref{Strings}, for more details.

@c ---------------------------------------------------------------------

@node Macros, Escapes, Requests, Embedded Commands
@subsection Macros
@cindex macros

@code{gtroff} has a @dfn{macro} facility for defining a series of lines
which can be invoked by name.  They are called in the same manner as
requests -- arguments also may be passed in the same manner.

@xref{Writing Macros}, and @ref{Request Arguments}.

@c ---------------------------------------------------------------------

@node Escapes,  , Macros, Embedded Commands
@subsection Escapes
@cindex escapes

Escapes may occur anywhere in the input to @code{gtroff}.  They usually
begin with a backslash and are followed by a single character which
indicates the function to be performed.  The escape character can be
changed; see @ref{Character Translations}.

Escape sequences which require an identifier as a parameter accept three
possible syntax forms.

@itemize @bullet
@item
The next single character is the identifier.

@cindex @code{(}, starting a two-character identifier
@item
If this single character is an opening parenthesis, take the following
two characters as the identifier.  Note that there is no closing
parenthesis after the identifier.

@cindex @code{[}, starting an identifier
@cindex @code{]}, ending an identifier
@item
If this single character is an opening bracket, take all characters
until a closing bracket as the identifier.
@end itemize

@noindent
Examples:

@Example
\fB
\n(XX
\*[TeX]
@endExample

@cindex @code{'}, delimiting arguments
@cindex argument delimiting characters
@cindex characters, argument delimiting
@cindex delimiting characters for arguments
Other escapes may require several arguments and/or some special format.
In such cases the argument is traditionally enclosed in single quotes
(and quotes are always used in this manual for the definitions of escape
sequences).  The enclosed text is then processed according to what that
escape expects.  Example:

@Example
\l'1.5i\(bu'
@endExample

@cindex @code{\o}, possible quote characters
@cindex @code{\b}, possible quote characters
@cindex @code{\X}, possible quote characters
Note that the quote character can be replaced with any other character
which does not occur in the argument (even a newline or a space
character) in the following escapes: @code{\o}, @code{\b}, and
@code{\X}.  This makes e.g.

@Example
A caf
\o
e\'


in Paris
  @result{} A caf@'e in Paris
@endExample

@noindent
possible, but it is better not to use this feature to avoid confusion.

@cindex @code{\%}, used as delimiter
@cindex @code{\@key{SP}}, used as delimiter
@cindex @code{\|}, used as delimiter
@cindex @code{\^}, used as delimiter
@cindex @code{\@{}, used as delimiter
@cindex @code{\@}}, used as delimiter
@cindex @code{\'}, used as delimiter
@cindex @code{\`}, used as delimiter
@cindex @code{\-}, used as delimiter
@cindex @code{\_}, used as delimiter
@cindex @code{\!}, used as delimiter
@cindex @code{\?}, used as delimiter
@cindex @code{\@@}, used as delimiter
@cindex @code{\)}, used as delimiter
@cindex @code{\/}, used as delimiter
@cindex @code{\,}, used as delimiter
@cindex @code{\&}, used as delimiter
@ifnotinfo
@cindex @code{\:}, used as delimiter
@end ifnotinfo
@ifinfo
@cindex @code{\@r{<colon>}}, used as delimiter
@end ifinfo
@cindex @code{\~}, used as delimiter
@cindex @code{\0}, used as delimiter
@cindex @code{\a}, used as delimiter
@cindex @code{\c}, used as delimiter
@cindex @code{\d}, used as delimiter
@cindex @code{\e}, used as delimiter
@cindex @code{\E}, used as delimiter
@cindex @code{\p}, used as delimiter
@cindex @code{\r}, used as delimiter
@cindex @code{\t}, used as delimiter
@cindex @code{\u}, used as delimiter
The following escapes sequences (which are handled similarly to
characters since they don't take a parameter) are also allowed as
delimiters: @code{\%}, @w{@samp{\ }}, @code{\|}, @code{\^}, @code{\@{},
@code{\@}}, @code{\'}, @code{\`}, @code{\-}, @code{\_}, @code{\!},
@code{\?}, @code{\@@}, @code{\)}, @code{\/}, @code{\,}, @code{\&},
@code{\:}, @code{\~}, @code{\0}, @code{\a}, @code{\c}, @code{\d},
@code{\e}, @code{\E}, @code{\p}, @code{\r}, @code{\t}, and @code{\u}.
Again, don't use these if possible.

@cindex @code{\A}, allowed delimiters
@cindex @code{\B}, allowed delimiters
@cindex @code{\Z}, allowed delimiters
@cindex @code{\C}, allowed delimiters
@cindex @code{\w}, allowed delimiters
No newline characters as delimiters are allowed in the following
escapes: @code{\A}, @code{\B}, @code{\Z}, @code{\C}, and @code{\w}.

@cindex @code{\D}, allowed delimiters
@cindex @code{\h}, allowed delimiters
@cindex @code{\H}, allowed delimiters
@cindex @code{\l}, allowed delimiters
@cindex @code{\L}, allowed delimiters
@cindex @code{\N}, allowed delimiters
@cindex @code{\R}, allowed delimiters
@cindex @code{\s}, allowed delimiters
@cindex @code{\S}, allowed delimiters
@cindex @code{\v}, allowed delimiters
@cindex @code{\x}, allowed delimiters
Finally, the escapes @code{\D}, @code{\h}, @code{\H}, @code{\l},
@code{\L}, @code{\N}, @code{\R}, @code{\s}, @code{\S}, @code{\v},
and @code{\x} can't use the following characters as delimiters:

@itemize @bullet
@item
@cindex numbers, and delimiters
@cindex digits, and delimiters
The digits @code{0}-@code{9}.

@item
@cindex operators, as delimiters
@cindex @code{+}, as delimiter
@cindex @code{-}, as delimiter
@cindex @code{/}, as delimiter
@cindex @code{*}, as delimiter
@cindex @code{%}, as delimiter
@cindex @code{<}, as delimiter
@cindex @code{>}, as delimiter
@cindex @code{=}, as delimiter
@cindex @code{&}, as delimiter
@ifnotinfo
@cindex @code{:}, as delimiter
@end ifnotinfo
@ifinfo
@cindex <colon>, as delimiter
@end ifinfo
@cindex @code{(}, as delimiter
@cindex @code{)}, as delimiter
@cindex @code{.}, as delimiter
The (single-character) operators @samp{+-/*%<>=&:().}.

@item
@cindex space character
@cindex character, space
@cindex tab character
@cindex character, tab
@cindex newline character
@cindex character, newline
The space, tab, and newline characters.

@item
@cindex @code{\%}, used as delimiter
@ifnotinfo
@cindex @code{\:}, used as delimiter
@end ifnotinfo
@ifinfo
@cindex @code{\@r{<colon>}}, used as delimiter
@end ifinfo
@cindex @code{\@{}, used as delimiter
@cindex @code{\@}}, used as delimiter
@cindex @code{\'}, used as delimiter
@cindex @code{\`}, used as delimiter
@cindex @code{\-}, used as delimiter
@cindex @code{\_}, used as delimiter
@cindex @code{\!}, used as delimiter
@cindex @code{\@@}, used as delimiter
@cindex @code{\/}, used as delimiter
@cindex @code{\c}, used as delimiter
@cindex @code{\e}, used as delimiter
@cindex @code{\p}, used as delimiter
All escape sequences except @code{\%}, @code{\:}, @code{\@{}, @code{\@}},
@code{\'}, @code{\`}, @code{\-}, @code{\_}, @code{\!}, @code{\@@},
@code{\/}, @code{\c}, @code{\e}, and @code{\p}.
@end itemize

@cindex printing backslash (@code{\\}, @code{\e}, @code{\E}, @code{\[rs]})
@cindex backslash, printing (@code{\\}, @code{\e}, @code{\E}, @code{\[rs]})
To have a backslash (actually, the current escape character) appear in the
output several escapes are defined: @code{\\}, @code{\e} or @code{\E}.
These are very similar, and only differ with respect to being used in
macros or diversions.  @xref{Character Translations}, for an exact
description of those escapes.

@xref{Implementation Differences}, @ref{Copy-in Mode}, and @ref{Diversions},
@ref{Identifiers}, for more information.

@menu
* Comments::
@end menu

@node Comments,  , Escapes, Escapes
@subsubsection Comments
@cindex comments

Probably one of the most@footnote{Unfortunately, this is a lie.  But
hopefully future @code{gtroff} hackers will believe it @code{:-)}}
common forms of escapes is the comment.

@Defesc {\\", , , }
Start a comment.  Everything to the end of the input line is ignored.

This may sound simple, but it can be tricky to keep the comments from
interfering with the appearance of the final output.

@cindex @code{ds}, @code{ds1} requests, and comments
@cindex @code{as}, @code{as1} requests, and comments
If the escape is to the right of some text or a request, that portion
of the line is ignored, but the space leading up to it is noticed by
@code{gtroff}.  This only affects the @code{ds} and @code{as}
request and its variants.

@cindex tabs, before comments
@cindex comments, lining up with tabs
One possibly irritating idiosyncracy is that tabs must not be used to
line up comments.  Tabs are not treated as whitespace between the
request and macro arguments.

@cindex undefined request
@cindex request, undefined
A comment on a line by itself is treated as a blank line, because
after eliminating the comment, that is all that remains:

@Example
Test
\" comment
Test
@endExample

@noindent
produces

@Example
Test

Test
@endExample

To avoid this, it is common to start the line with @code{.\"} which
causes the line to be treated as an undefined request and thus ignored
completely.

@cindex @code{'}, as a comment
Another commenting scheme seen sometimes is three consecutive single
quotes (@code{'''}) at the beginning of a line.  This works, but
@code{gtroff} gives a warning about an undefined macro (namely
@code{''}), which is harmless, but irritating.
@endDefesc

@Defesc {\\#, , , }
To avoid all this, @code{gtroff} has a new comment mechanism using the
@code{\#} escape.  This escape works the same as @code{\"} except that
the newline is also ignored:

@Example
Test
\# comment
Test
@endExample

@noindent
produces

@Example
Test Test
@endExample

@noindent
as expected.
@endDefesc

@Defreq {ig, yy}
Ignore all input until @code{gtroff} encounters the macro named
@code{.}@var{yy} on a line by itself (or @code{..} if @var{yy} is not
specified).  This is useful for commenting out large blocks of text:

@Example
text text text...
.ig
This is part of a large block
of text that has been
temporarily(?) commented out.

We can restore it simply by removing
the .ig request and the ".." at the
end of the block.
..
More text text text...
@endExample

@noindent
produces

@Example
text text text@dots{}  More text text text@dots{}
@endExample

@noindent
Note that the commented-out block of text does not
cause a break.

The input is read in copy-mode; auto-incremented registers @emph{are}
affected (@pxref{Auto-increment}).
@endDefreq


@c =====================================================================

@node Registers, Manipulating Filling and Adjusting, Embedded Commands, gtroff Reference
@section Registers
@cindex registers

Numeric variables in @code{gtroff} are called @dfn{registers}.  There
are a number of built-in registers, supplying anything from the date to
details of formatting parameters.

@xref{Identifiers}, for details on register identifiers.

@menu
* Setting Registers::
* Interpolating Registers::
* Auto-increment::
* Assigning Formats::
* Built-in Registers::
@end menu

@c ---------------------------------------------------------------------

@node Setting Registers, Interpolating Registers, Registers, Registers
@subsection Setting Registers
@cindex setting registers (@code{nr}, @code{\R})
@cindex registers, setting (@code{nr}, @code{\R})

Define or set registers using the @code{nr} request or the
@code{\R} escape.

@DefreqList {nr, ident value}
@DefescListEnd {\\R, ', ident value, '}
Set number register @var{ident} to @var{value}.  If @var{ident}
doesn't exist, @code{gtroff} creates it.

The argument to @code{\R} usually has to be enclosed in quotes.
@xref{Escapes}, for details on parameter delimiting characters.

The @code{\R} escape doesn't produce an input token in @code{gtroff};
with other words, it vanishes completely after @code{gtroff} has
processed it.
@endDefreq

For example, the following two lines are equivalent:

@Example
.nr a (((17 + (3 * 4))) % 4)
\R'a (((17 + (3 * 4))) % 4)'
    @result{} 1
@endExample

Both @code{nr} and @code{\R} have two additional special forms to
increment or decrement a register.

@DefreqList {nr, ident @t{+}@Var{value}}
@DefreqItem {nr, ident @t{-}@Var{value}}
@DefescItem {\\R, ', ident @t{+}@Var{value}, '}
@DefescListEnd {\\R, ', ident @t{-}@Var{value}, '}
Increment (decrement) register @var{ident} by @var{value}.

@Example
.nr a 1
.nr a +1
\na
    @result{} 2
@endExample

@cindex negating register values
To assign the negated value of a register to another register, some care
must be taken to get the desired result:

@Example
.nr a 7
.nr b 3
.nr a -\nb
\na
    @result{} 4
.nr a (-\nb)
\na
    @result{} -3
@endExample

@noindent
The surrounding parentheses prevent the interpretation of the minus sign
as a decrementing operator.  An alternative is to start the assignment
with a @samp{0}:

@Example
.nr a 7
.nr b -3
.nr a \nb
\na
    @result{} 4
.nr a 0\nb
\na
    @result{} -3
@endExample
@endDefreq

@Defreq {rr, ident}
@cindex removing number register (@code{rr})
@cindex number register, removing (@code{rr})
@cindex register, removing (@code{rr})
Remove number register @var{ident}.  If @var{ident} doesn't exist, the
request is ignored.
@endDefreq

@Defreq {rnn, ident1 ident2}
@cindex renaming number register (@code{rnn})
@cindex number register, renaming (@code{rnn})
@cindex register, renaming (@code{rnn})
Rename number register @var{ident1} to @var{ident2}.  If either
@var{ident1} or @var{ident2} doesn't exist, the request is ignored.
@endDefreq

@Defreq {aln, ident1 ident2}
@cindex alias, number register, creating (@code{aln})
@cindex creating alias, for number register (@code{aln})
@cindex number register, creating alias (@code{aln})
@cindex register, creating alias (@code{aln})
Create an alias @var{ident1} for a number register @var{ident2}.  The
new name and the old name are exactly equivalent.  If @var{ident1} is
undefined, a warning of type @samp{reg} is generated, and the request
is ignored.  @xref{Debugging}, for information about warnings.
@endDefreq

@c ---------------------------------------------------------------------

@node Interpolating Registers, Auto-increment, Setting Registers, Registers
@subsection Interpolating Registers
@cindex interpolating registers (@code{\n})
@cindex registers, interpolating (@code{\n})

Numeric registers can be accessed via the @code{\n} escape.

@DefescList {\\n, , i, }
@DefescItem {\\n, @lparen{}, id, }
@DefescListEnd {\\n, @lbrack{}, ident, @rbrack}
@cindex nested assignments
@cindex assignments, nested
@cindex indirect assignments
@cindex assignments, indirect
Interpolate number register with name @var{ident} (one-character name@w{
}@var{i}, two-character name @var{id}). This means that the value of the
register is expanded in-place while @code{gtroff} is parsing the input line.
Nested assignments (also called indirect assignments) are possible.

@Example
.nr a 5
.nr as \na+\na
\n(as
    @result{} 10
@endExample

@Example
.nr a1 5
.nr ab 6
.ds str b
.ds num 1
\n[a\n[num]]
    @result{} 5
\n[a\*[str]]
    @result{} 6
@endExample
@endDefesc

@c ---------------------------------------------------------------------

@node Auto-increment, Assigning Formats, Interpolating Registers, Registers
@subsection Auto-increment
@cindex auto-increment
@cindex increment, automatic

Number registers can also be auto-incremented and auto-decremented.
The increment or decrement value can be specified with a third
argument to the @code{nr} request or @code{\R} escape.

@Defreq {nr, ident value incr}
@cindex @code{\R}, difference to @code{nr}
Set number register @var{ident} to @var{value}; the increment for
auto-incrementing is set to @var{incr}.  Note that the @code{\R}
escape doesn't support this notation.
@endDefreq

To activate auto-incrementing, the escape @code{\n} has a special
syntax form.

@DefescList {\\n, +, i, }
@DefescItem {\\n, -, i, }
@DefescItem {\\n, @lparen{}+, id, }
@DefescItem {\\n, @lparen{}-, id, }
@DefescItem {\\n, +@lparen{}, id, }
@DefescItem {\\n, -@lparen{}, id, }
@DefescItem {\\n, @lbrack{}+, ident, @rbrack{}}
@DefescItem {\\n, @lbrack{}-, ident, @rbrack{}}
@DefescItem {\\n, +@lbrack{}, ident, @rbrack{}}
@DefescListEnd {\\n, -@lbrack{}, ident, @rbrack{}}
Before interpolating, increment or decrement @var{ident}
(one-character name@w{ }@var{i}, two-character name @var{id}) by the
auto-increment value as specified with the @code{nr} request (or the
@code{\R} escape).  If no auto-increment value has been specified,
these syntax forms are identical to @code{\n}.
@endDefesc

For example,

@Example
.nr a 0 1
.nr xx 0 5
.nr foo 0 -2
\n+a, \n+a, \n+a, \n+a, \n+a
.br
\n-(xx, \n-(xx, \n-(xx, \n-(xx, \n-(xx
.br
\n+[foo], \n+[foo], \n+[foo], \n+[foo], \n+[foo]
@endExample

@noindent
produces

@Example
1, 2, 3, 4, 5
-5, -10, -15, -20, -25
-2, -4, -6, -8, -10
@endExample

@cindex increment value without changing the register
@cindex value, incrementing without changing the register
To change the increment value without changing the value of a register
(@var{a} in the example), the following can be used:

@Example
.nr a \na 10
@endExample

@c ---------------------------------------------------------------------

@node Assigning Formats, Built-in Registers, Auto-increment, Registers
@subsection Assigning Formats
@cindex assigning formats (@code{af})
@cindex formats, assigning (@code{af})

When a register is used in the text of an input file (as opposed to
part of an expression), it is textually replaced (or interpolated)
with a representation of that number.  This output format can be
changed to a variety of formats (numbers, Roman numerals, etc.).  This
is done using the @code{af} request.

@Defreq {af, ident format}
Change the output format of a number register.  The first argument
@var{ident} is the name of the number register to be changed, and the
second argument @var{format} is the output format.  The following
output formats are available:

@table @code
@item 1
Decimal arabic numbers.  This is the default format: 0, 1, 2, 3,@w{
}@enddots{}

@item 0@dots{}0
Decimal numbers with as many digits as specified.  So, @samp{00} would
result in printing numbers as 01, 02, 03,@w{ }@enddots{}

In fact, any digit instead of zero will do; @code{gtroff} only counts
how many digits are specified.  As a consequence, @code{af}'s default
format @samp{1} could be specified as @samp{0} also (and exactly this is
returned by the @code{\g} escape, see below).

@item I
@cindex Roman numerals
@cindex numerals, Roman
Upper-case Roman numerals: 0, I, II, III, IV,@w{ }@enddots{}

@item i
Lower-case Roman numerals: 0, i, ii, iii, iv,@w{ }@enddots{}

@item A
Upper-case letters: 0, A, B, C, @dots{},@w{ }Z, AA, AB,@w{ }@enddots{}

@item a
Lower-case letters: 0, a, b, c, @dots{},@w{ }z, aa, ab,@w{ }@enddots{}
@end table

Omitting the number register format causes a warning of type
@samp{missing}.  @xref{Debugging}, for more details.  Specifying a
nonexistent format causes an error.

The following example produces @samp{10, X, j, 010}:

@Example
.nr a 10
.af a 1           \" the default format
\na,
.af a I
\na,
.af a a
\na,
.af a 001
\na
@endExample

@cindex Roman numerals, maximum and minimum
@cindex maximum values of Roman numerals
@cindex minimum values of Roman numerals
The largest number representable for the @samp{i} and @samp{I} formats
is 39999 (or @minus{}39999); @acronym{UNIX} @code{troff} uses @samp{z}
and @samp{w} to represent 10000 and 5000 in Roman numerals, and so does
@code{gtroff}.  Currently, the correct glyphs of Roman numeral five
thousand and Roman numeral ten thousand (Unicode code points
@code{U+2182} and @code{U+2181}, respectively) are not available.

If @var{ident} doesn't exist, it is created.

@cindex read-only register, changing format
@cindex changing format, and read-only registers
Changing the output format of a read-only register causes an error.  It
is necessary to first copy the register's value to a writeable register,
then apply the @code{af} request to this other register.
@endDefreq

@DefescList {\\g, , i, }
@DefescItem {\\g, @lparen{}, id, }
@DefescListEnd {\\g, @lbrack{}, ident, @rbrack{}}
@cindex format of register (@code{\g})
@cindex register, format (@code{\g})
Return the current format of the specified register @var{ident}
(one-character name@w{ }@var{i}, two-character name @var{id}).  For
example, @samp{\ga} after the previous example would produce the
string @samp{000}.  If the register hasn't been defined yet, nothing
is returned.
@endDefesc

@c ---------------------------------------------------------------------

@node Built-in Registers,  , Assigning Formats, Registers
@subsection Built-in Registers
@cindex built-in registers
@cindex registers, built-in

The following lists some built-in registers which are not described
elsewhere in this manual.  Any register which begins with a @samp{.} is
read-only.  A complete listing of all built-in registers can be found in
@ref{Register Index}.

@table @code
@item .F
@cindex current input file name register (@code{.F})
@cindex input file name, current, register (@code{.F})
@vindex .F
This string-valued register returns the current input file name.

@item .H
@cindex horizontal resolution register (@code{.H})
@cindex resolution, horizontal, register (@code{.H})
@vindex .H
Horizontal resolution in basic units.

@item .V
@cindex vertical resolution register (@code{.V})
@cindex resolution, vertical, register (@code{.V})
@vindex .V
Vertical resolution in basic units.

@item seconds
@cindex seconds, current time (@code{seconds})
@cindex time, current, seconds (@code{seconds})
@cindex current time, seconds (@code{seconds})
@vindex seconds
The number of seconds after the minute, normally in the range@w{ }0
to@w{ }59, but can be up to@w{ }61 to allow for leap seconds.  Initialized
at start-up of @code{gtroff}.

@item minutes
@cindex minutes, current time (@code{minutes})
@cindex time, current, minutes (@code{minutes})
@cindex current time, minutes (@code{minutes})
@vindex minutes
The number of minutes after the hour, in the range@w{ }0 to@w{ }59.
Initialized at start-up of @code{gtroff}.

@item hours
@cindex hours, current time (@code{hours})
@cindex time, current, hours (@code{hours})
@cindex current time, hours (@code{hours})
@vindex hours
The number of hours past midnight, in the range@w{ }0 to@w{ }23.
Initialized at start-up of @code{gtroff}.

@item dw
@cindex day of the week register (@code{dw})
@cindex date, day of the week register (@code{dw})
@vindex dw
Day of the week (1-7).

@item dy
@cindex day of the month register (@code{dy})
@cindex date, day of the month register (@code{dy})
@vindex dy
Day of the month (1-31).

@item mo
@cindex month of the year register (@code{mo})
@cindex date, month of the year register (@code{mo})
@vindex mo
Current month (1-12).

@item year
@cindex date, year register (@code{year}, @code{yr})
@cindex year, current, register (@code{year}, @code{yr})
@vindex year
The current year.

@item yr
@vindex yr
The current year minus@w{ }1900.  Unfortunately, the documentation of
@acronym{UNIX} Version@w{ }7's @code{troff} had a year@w{ }2000 bug: It
incorrectly claimed that @code{yr} contains the last two digits of the
year.  That claim has never been true of either @acronym{AT&T}
@code{troff} or GNU @code{troff}.  Old @code{troff} input that looks
like this:

@Example
'\" The following line stopped working after 1999
This document was formatted in 19\n(yr.
@endExample

@noindent
can be corrected as follows:

@Example
This document was formatted in \n[year].
@endExample

@noindent
or, to be portable to older @code{troff} versions, as follows:

@Example
.nr y4 1900+\n(yr
This document was formatted in \n(y4.
@endExample

@item .c
@vindex .c
@itemx c.
@vindex c.
@cindex input line number register (@code{.c}, @code{c.})
@cindex line number, input, register (@code{.c}, @code{c.})
The current @emph{input} line number.  Register @samp{.c} is read-only,
whereas @samp{c.} (a @code{gtroff} extension) is writable also,
affecting both @samp{.c} and @samp{c.}.

@item ln
@vindex ln
@cindex output line number register (@code{ln})
@cindex line number, output, register (@code{ln})
The current @emph{output} line number after a call to the @code{nm}
request to activate line numbering.

@xref{Miscellaneous}, for more information about line numbering.

@item .x
@vindex .x
@cindex major version number register (@code{.x})
@cindex version number, major, register (@code{.x})
The major version number.  For example, if the version number is@w{
}1.03 then @code{.x} contains@w{ }@samp{1}.

@item .y
@vindex .y
@cindex minor version number register (@code{.y})
@cindex version number, minor, register (@code{.y})
The minor version number.  For example, if the version number is@w{
}1.03 then @code{.y} contains@w{ }@samp{03}.

@item .Y
@vindex .Y
@cindex revision number register (@code{.Y})
The revision number of @code{groff}.

@item $$
@vindex $$
@cindex process ID of @code{gtroff} register (@code{$$})
@cindex @code{gtroff}, process ID register (@code{$$})
The process ID of @code{gtroff}.

@item .g
@vindex .g
@cindex @code{gtroff}, identification register (@code{.g})
@cindex GNU-specific register (@code{.g})
Always@w{ }1.  Macros should use this to determine whether they are
running under GNU @code{troff}.

@item .A
@vindex .A
@cindex @acronym{ASCII} approximation output register (@code{.A})
If the command line option @option{-a} is used to produce an
@acronym{ASCII} approximation of the output, this is set to@w{ }1, zero
otherwise.  @xref{Groff Options}.

@item .P
@vindex .P
This register is set to@w{ }1 (and to@w{ }0 otherwise) if the current
page is actually being printed, i.e., if the @option{-o} option is being
used to only print selected pages.  @xref{Groff Options}, for more
information.

@item .T
@vindex .T
If @code{gtroff} is called with the @option{-T} command line option, the
number register @code{.T} is set to@w{ }1, and zero otherwise.
@xref{Groff Options}.

@stindex .T
@cindex output device name string register (@code{.T})
Additionally, @code{gtroff} predefines a single read-write string
register @code{.T} which contains the current output device (for
example, @samp{latin1} or @samp{ps}).
@end table


@c =====================================================================

@node Manipulating Filling and Adjusting, Manipulating Hyphenation, Registers, gtroff Reference
@section Manipulating Filling and Adjusting
@cindex manipulating filling and adjusting
@cindex filling and adjusting, manipulating
@cindex adjusting and filling, manipulating
@cindex justifying text
@cindex text, justifying

@cindex break
@cindex line break
@cindex @code{bp} request, causing implicit linebreak
@cindex @code{ce} request, causing implicit linebreak
@cindex @code{cf} request, causing implicit linebreak
@cindex @code{fi} request, causing implicit linebreak
@cindex @code{fl} request, causing implicit linebreak
@cindex @code{in} request, causing implicit linebreak
@cindex @code{nf} request, causing implicit linebreak
@cindex @code{rj} request, causing implicit linebreak
@cindex @code{sp} request, causing implicit linebreak
@cindex @code{ti} request, causing implicit linebreak
@cindex @code{trf} request, causing implicit linebreak
Various ways of causing @dfn{breaks} were given in @ref{Implicit Line
Breaks}.  The @code{br} request likewise causes a break.  Several
other requests also cause breaks, but implicitly.  These are
@code{bp}, @code{ce}, @code{cf}, @code{fi}, @code{fl}, @code{in},
@code{nf}, @code{rj}, @code{sp}, @code{ti}, and @code{trf}.

@Defreq {br, }
Break the current line, i.e., the input collected so far is emitted
without adjustment.

If the no-break control character is used, @code{gtroff} suppresses
the break:

@Example
a
'br
b
    @result{} a b
@endExample
@endDefreq

Initially, @code{gtroff} fills and adjusts text to both margins.
Filling can be disabled via the @code{nf} request and re-enabled with
the @code{fi} request.

@DefreqList {fi, }
@DefregListEnd {.u}
@cindex fill mode (@code{fi})
@cindex mode, fill (@code{fi})
Activate fill mode (which is the default).  This request implicitly
enables adjusting; it also inserts a break in the text currently being
filled.  The read-only number register @code{.u} is set to@w{ }1.

The fill mode status is associated with the current environment
(@pxref{Environments}).

See @ref{Line Control}, for interaction with the @code{\c} escape.
@endDefreq

@Defreq {nf, }
@cindex no-fill mode (@code{nf})
@cindex mode, no-fill (@code{nf})
Activate no-fill mode.  Input lines are output as-is, retaining line
breaks and ignoring the current line length.  This command implicitly
disables adjusting; it also causes a break.  The number register
@code{.u} is set to@w{ }0.

The fill mode status is associated with the current environment
(@pxref{Environments}).

See @ref{Line Control}, for interaction with the @code{\c} escape.
@endDefreq

@DefreqList {ad, [@Var{mode}]}
@DefregListEnd {.j}
Set adjusting mode.

Activation and deactivation of adjusting is done implicitly with
calls to the @code{fi} or @code{nf} requests.

@var{mode} can have one of the following values:

@table @code
@item l
@cindex ragged-right
Adjust text to the left margin.  This produces what is traditionally
called ragged-right text.

@item r
@cindex ragged-left
Adjust text to the right margin, producing ragged-left text.

@item c
@cindex centered text
@cindex @code{ce} request, difference to @samp{.ad@w{ }c}
Center filled text.  This is different to the @code{ce} request which
only centers text without filling.

@item b
@itemx n
Justify to both margins.  This is the default used by @code{gtroff}.
@end table

With no argument, @code{gtroff} adjusts lines in the same way it did
before adjusting was deactivated (with a call to @code{na}, for
example).

@Example
text
.ad r
text
.ad c
text
.na
text
.ad  \" back to centering
text
@endExample

@cindex adjustment mode register (@code{.j})
The current adjustment mode is available in the read-only number
register @code{.j}; it can be stored and subsequently used to set
adjustment.

The adjustment mode status is associated with the current environment
(@pxref{Environments}).
@endDefreq

@Defreq {na, }
Disable adjusting.  This request won't change the current adjustment
mode: A subsequent call to @code{ad} uses the previous adjustment
setting.

The adjustment mode status is associated with the current environment
(@pxref{Environments}).
@endDefreq

@DefreqList {brp, }
@DefescListEnd {\\p, , , }
Adjust the current line and cause a break.

In most cases this produces very ugly results since @code{gtroff}
doesn't have a sophisticated paragraph building algorithm (as @TeX{}
have, for example); instead, @code{gtroff} fills and adjusts a paragraph
line by line:

@Example
  This is an uninteresting sentence.
  This is an uninteresting sentence.\p
  This is an uninteresting sentence.
@endExample

@noindent
is formatted as

@Example
  This is  an uninteresting  sentence.   This  is an
  uninteresting                            sentence.
  This is an uninteresting sentence.
@endExample
@endDefreq

@DefreqList {ss, word_space_size [@Var{sentence_space_size}]}
@DefregItem {.ss}
@DefregListEnd {.sss}
@cindex word space size register (@code{.ss})
@cindex size of word space register (@code{.ss})
@cindex space between words register (@code{.ss})
@cindex sentence space size register (@code{.sss})
@cindex size of sentence space register (@code{.sss})
@cindex space between sentences register (@code{.sss})
Change the minimum size of a space between filled words.  It takes its
units as one twelfth of the space width parameter for the current
font.  Initially both the @var{word_space_size} and
@var{sentence_space_size} are@w{ }12.

@cindex fill mode
@cindex mode, fill
If two arguments are given to the @code{ss} request, the second
argument sets the sentence space size.  If the second argument is not
given, sentence space size is set to @var{word_space_size}.  The
sentence space size is used in two circumstances: If the end of a
sentence occurs at the end of a line in fill mode, then both an
inter-word space and a sentence space are added; if two spaces follow
the end of a sentence in the middle of a line, then the second space
is a sentence space.  If a second argument is never given to the
@code{ss} request, the behaviour of @acronym{UNIX} @code{troff} is the
same as that exhibited by GNU @code{troff}.  In GNU @code{troff}, as
in @acronym{UNIX} @code{troff}, a sentence should always be followed
by either a newline or two spaces.

The read-only number registers @code{.ss} and @code{.sss} hold the
values of the parameters set by the first and second arguments of the
@code{ss} request.

The word space and sentence space values are associated with the current
environment (@pxref{Environments}).

Contrary to @acronym{AT&T} @code{troff}, this request is @emph{not}
ignored if a TTY output device is used; the given values are then
rounded down to a multiple of@w{ }12 (@pxref{Implementation Differences}).

The request is ignored if there is no parameter.
@endDefreq

@DefreqList {ce, [@Var{nnn}]}
@DefregListEnd {.ce}
@cindex centering lines (@code{ce})
@cindex lines, centering (@code{ce})
Center text.  While the @w{@samp{.ad c}} request also centers text,
it fills the text as well.  @code{ce} does not fill the
text it affects.  This request causes a break.  The number of lines
still to be centered is associated with the current environment
(@pxref{Environments}).

The following example demonstrates the differences.
Here the input:

@Example
.ll 4i
.ce 1000
This is a small text fragment which shows the differences
between the `.ce' and the `.ad c' request.
.ce 0

.ad c
This is a small text fragment which shows the differences
between the `.ce' and the `.ad c' request.
@endExample

@noindent
And here the result:

@Example
  This is a small text fragment which
         shows the differences
between the `.ce' and the `.ad c' request.

  This is a small text fragment which
shows the differences between the `.ce'
        and the `.ad c' request.
@endExample

With no arguments, @code{ce} centers the next line of text.  @var{nnn}
specifies the number of lines to be centered.  If the argument is zero
or negative, centering is disabled.

The basic length for centering text is the line length (as set with the
@code{ll} request) minus the indentation (as set with the @code{in}
request).  Temporary indentation is ignored.

As can be seen in the previous example, it is a common idiom to turn
on centering for a large number of lines, and to turn off centering
after text to be centered.  This is useful for any request which takes
a number of lines as an argument.

The @code{.ce} read-only number register contains the number of lines
remaining to be centered, as set by the @code{ce} request.
@endDefreq

@DefreqList {rj, [@Var{nnn}]}
@DefregListEnd {.rj}
@cindex justifying text (@code{rj})
@cindex text, justifying (@code{rj})
@cindex right-justifying (@code{rj})
Justify unfilled text to the right margin.  Arguments are identical to
the @code{ce} request.  The @code{.rj} read-only number register is
the number of lines to be right-justified as set by the @code{rj}
request.  This request causes a break.  The number of lines still to be
right-justified is associated with the current environment
(@pxref{Environments}).
@endDefreq


@c =====================================================================

@node Manipulating Hyphenation, Manipulating Spacing, Manipulating Filling and Adjusting, gtroff Reference
@section Manipulating Hyphenation
@cindex manipulating hyphenation
@cindex hyphenation, manipulating

As discussed in @ref{Hyphenation}, @code{gtroff} hyphenates words.
There are a number of ways to influence hyphenation.

@DefreqList {hy, [@Var{mode}]}
@DefregListEnd {.hy}
Enable hyphenation.  The request has an optional numeric argument,
@var{mode}, to restrict hyphenation if necessary:

@table @code
@item 1
The default argument if @var{mode} is omitted.  Hyphenate without
restrictions.  This is also the start-up value of @code{gtroff}.

@item 2
Do not hyphenate the last word on a page or column.

@item 4
Do not hyphenate the last two characters of a word.

@item 8
Do not hyphenate the first two characters of a word.
@end table

Values in the previous table are additive.  For example, the value@w{
}12 causes @code{gtroff} to neither hyphenate the last two nor the first
two characters of a word.

@cindex hyphenation restrictions register (@code{.hy})
The current hyphenation restrictions can be found in the read-only
number register @samp{.hy}.

The hyphenation mode is associated with the current environment
(@pxref{Environments}).
@endDefreq

@Defreq {nh, }
Disable hyphenation (i.e., set the hyphenation mode to zero).  Note
that the hyphenation mode of the last call to @code{hy} is not
remembered.

The hyphenation mode is associated with the current environment
(@pxref{Environments}).
@endDefreq

@DefreqList {hlm, [@Var{nnn}]}
@DefregItem {.hlm}
@DefregListEnd {.hlc}
@cindex explicit hyphen (@code{\%})
@cindex hyphen, explicit (@code{\%})
@cindex consecutive hyphenated lines (@code{hlm})
@cindex lines, consecutive hyphenated (@code{hlm})
@cindex hyphenated lines, consecutive (@code{hlm})
Set the maximum number of consecutive hyphenated lines to @var{nnn}.
If this number is negative, there is no maximum.  The default value
is@w{ }@minus{}1 if @var{nnn} is omitted.  This value is associated
with the current environment (@pxref{Environments}).  Only lines
output from a given environment count towards the maximum associated
with that environment.  Hyphens resulting from @code{\%} are counted;
explicit hyphens are not.

The current setting of @code{hlm} is available in the @code{.hlm}
read-only number register.  Also the number of immediately preceding
consecutive hyphenated lines are available in the read-only number
register @samp{.hlc}.
@endDefreq

@Defreq {hw, word1 word2 @dots{}}
Define how @var{word1}, @var{word2}, etc.@: are to be hyphenated.  The
words must be given with hyphens at the hyphenation points.  For
example:

@Example
.hw in-sa-lub-rious
@endExample

@noindent
Besides the space character, any character whose hyphenation code value
is zero can be used to separate the arguments of @code{hw} (see the
documentation for the @code{hcode} request below for more information).
In addition, this request can be used more than once.

Hyphenation exceptions specified with the @code{hw} request are
associated with the current hyphenation language; it causes an error
if there is no current hyphenation language.

This request is ignored if there is no parameter.

In old versions of @code{troff} there was a limited amount of space to
store such information; fortunately, with @code{gtroff}, this is no
longer a restriction.
@endDefreq

@DefescList {\\%, , , }
@deffnx Escape @t{\:}
@ifnotinfo
@esindex \:
@end ifnotinfo
@ifinfo
@esindex @r{<colon>}
@end ifinfo
@cindex hyphenation character (@code{\%})
@cindex character, hyphenation (@code{\%})
@cindex disabling hyphenation (@code{\%})
@cindex hyphenation, disabling (@code{\%})
To tell @code{gtroff} how to hyphenate words on the fly, use the
@code{\%} escape, also known as the @dfn{hyphenation character}.
Preceding a word with this character prevents it from being
hyphenated; putting it inside a word indicates to @code{gtroff} that
the word may be hyphenated at that point.  Note that this mechanism
only affects that one occurrence of the word; to change the
hyphenation of a word for the entire document, use the @code{hw}
request.

The @code{\:} escape inserts a zero-width break point
(that is, the word breaks but without adding a hyphen).

@Example
... check the /var/log/\:httpd/\:access_log file ...
@endExample

@cindex @code{\X}, followed by @code{\%}
@cindex @code{\Y}, followed by @code{\%}
@cindex @code{\%}, following @code{\X} or @code{\Y}
Note that @code{\X} and @code{\Y} start a word, that is, the @code{\%}
escape in (say) @w{@samp{ \X'...'\%foobar}} and
@w{@samp{ \Y'...'\%foobar}} no longer prevents hyphenation but inserts
a hyphenation point at the beginning of @samp{foobar}; most likely
this isn't what you want to do.
@endDefesc

@Defreq {hc, [@Var{char}]}
Change the hyphenation character to @var{char}.  This character then
works the same as the @code{\%} escape, and thus, no longer appears in
the output.  Without an argument, @code{hc} resets the hyphenation
character to be @code{\%} (the default) only.

The hyphenation character is associated with the current environment
(@pxref{Environments}).
@endDefreq

@DefreqList {hpf, pattern_file}
@DefreqItem {hpfa, pattern_file}
@DefreqListEnd {hpfcode, a b [c d @dots{}]}
@cindex hyphenation patterns (@code{hpf})
@cindex patterns for hyphenation (@code{hpf})
Read in a file of hyphenation patterns.  This file is searched for in
the same way as @file{@var{name}.tmac} (or @file{tmac.@var{name}}) is
searched for if the @option{-m@var{name}} option is specified.

It should have the same format as (simple) @TeX{} patterns files.
More specifically, the following scanning rules are implemented.

@itemize @bullet
@item
A percent sign starts a comment (up to the end of the line)
even if preceded by a backslash.

@item
No support for `digraphs' like @code{\$}.

@item
@code{^^@var{xx}} (@var{x} is 0-9 or a-f) and @code{^^@var{x}} (character
code of @var{x} in the range 0-127) are recognized; other use of @code{^}
causes an error.

@item
No macro expansion.

@item
@code{hpf} checks for the expression @code{\patterns@{@dots{}@}}
(possibly with whitespace before and after the braces).
Everything between the braces is taken as hyphenation patterns.
Consequently, @code{@{} and @code{@}} are not allowed in patterns.

@item
Similarly, @code{\hyphenation@{@dots{}@}} gives a list of hyphenation
exceptions.

@item
@code{\endinput} is recognized also.

@item
For backwards compatibility, if @code{\patterns} is missing,
the whole file is treated as a list of hyphenation patterns
(only recognizing the @code{%} character as the start of a comment).
@end itemize

If no @code{hpf} request is specified (either in the document or in a
macro package), @code{gtroff} won't hyphenate at all.

The @code{hpfa} request appends a file of patterns to the current list.

The @code{hpfcode} request defines mapping values for character codes in
hyphenation patterns.  @code{hpf} or @code{hpfa} then apply the mapping
(after reading the patterns) before replacing or appending them to
the current list of patterns.  Its arguments are pairs of character codes
-- integers from 0 to@w{ }255.  The request maps character code@w{ }@var{a}
to code@w{ }@var{b}, code@w{ }@var{c} to code@w{ }@var{d}, and so on.  You
can use character codes which would be invalid otherwise.

@pindex troffrc
@pindex troffrc-end
@pindex hyphen.us
The set of hyphenation patterns is associated with the current language
set by the @code{hla} request.  The @code{hpf} request is usually
invoked by the @file{troffrc} or @file{troffrc-end} file; by default,
@file{troffrc} loads hyphenation patterns for American English (in file
@file{hyphen.us}).

A second call to @code{hpf} (for the same language) will replace the
hyphenation patterns with the new ones.

Invoking @code{hpf} causes an error if there is no current hyphenation
language.
@endDefreq

@Defreq {hcode, c1 code1 c2 code2 @dots{}}
@cindex hyphenation code (@code{hcode})
@cindex code, hyphenation (@code{hcode})
Set the hyphenation code of character @var{c1} to @var{code1}, that of
@var{c2} to @var{code2}, etc.  A hyphenation code must be a single
input character (not a special character) other than a digit or a
space.  Initially each lower-case letter (@samp{a}-@samp{z}) has its
hyphenation code set to itself, and each upper-case letter
(@samp{A}-@samp{Z}) has a hyphenation code which is the lower-case
version of itself.

This request is ignored if it has no parameter.
@endDefreq

@DefreqList {hym, [@Var{length}]}
@DefregListEnd {.hym}
@cindex hyphenation margin (@code{hym})
@cindex margin for hyphenation (@code{hym})
@cindex @code{ad} request, and hyphenation margin
Set the (right) hyphenation margin to @var{length}.  If the current
adjustment mode is not @samp{b} or @samp{n}, the line is not
hyphenated if it is shorter than @var{length}.  Without an argument,
the hyphenation margin is reset to its default value, which is@w{ }0.
The default scaling indicator for this request is @samp{m}.  The
hyphenation margin is associated with the current environment
(@pxref{Environments}).

A negative argument resets the hyphenation margin to zero, emitting
a warning of type @samp{range}.

@cindex hyphenation margin register (@code{.hym})
The current hyphenation margin is available in the @code{.hym} read-only
number register.
@endDefreq

@DefreqList {hys, [@Var{hyphenation_space}]}
@DefregListEnd {.hys}
@cindex hyphenation space (@code{hys})
@cindex @code{ad} request, and hyphenation space
Set the hyphenation space to @var{hyphenation_space}.  If the current
adjustment mode is @samp{b} or @samp{n}, don't hyphenate the line
if it can be justified by adding no more than @var{hyphenation_space}
extra space to each word space.  Without argument, the hyphenation
space is set to its default value, which is@w{ }0.  The default
scaling indicator for this request is @samp{m}.  The hyphenation
space is associated with the current environment
(@pxref{Environments}).

A negative argument resets the hyphenation space to zero, emitting a
warning of type @samp{range}.

@cindex hyphenation space register (@code{.hys})
The current hyphenation space is available in the @code{.hys} read-only
number register.
@endDefreq

@Defreq {shc, [@Var{glyph}]}
@cindex soft hyphen character, setting (@code{shc})
@cindex character, soft hyphen, setting (@code{shc})
@cindex glyph, soft hyphen (@code{hy})
@cindex soft hyphen glyph (@code{hy})
@cindex @code{char} request, and soft hyphen character
@cindex @code{tr} request, and soft hyphen character
Set the @dfn{soft hyphen character} to @var{glyph}.@footnote{@dfn{Soft
hyphen character} is a misnomer since it is an output glyph.}  If the
argument is omitted, the soft hyphen character is set to the default
glyph @code{\(hy} (this is the start-up value of @code{gtroff} also).
The soft hyphen character is the glyph that is inserted when a word is
hyphenated at a line break.  If the soft hyphen character does not
exist in the font of the character immediately preceding a potential
break point, then the line is not broken at that point.  Neither
definitions (specified with the @code{char} request) nor translations
(specified with the @code{tr} request) are considered when finding the
soft hyphen character.
@endDefreq

@DefreqList {hla, language}
@DefregListEnd {.hla}
@cindex @code{hpf} request, and hyphenation language
@cindex @code{hw} request, and hyphenation language
@pindex troffrc
@pindex troffrc-end
Set the current hyphenation language to the string @var{language}.
Hyphenation exceptions specified with the @code{hw} request and
hyphenation patterns specified with the @code{hpf} and @code{hpfa}
requests are both associated with the current hyphenation language.
The @code{hla} request is usually invoked by the @file{troffrc} or the
@file{troffrc-end} files; @file{troffrc} sets the default language to
@samp{us}.

@cindex hyphenation language register (@code{.hla})
The current hyphenation language is available as a string in the
read-only number register @samp{.hla}.

@Example
.ds curr_language \n[.hla]
\*[curr_language]
    @result{} us
@endExample
@endDefreq


@c =====================================================================

@node Manipulating Spacing, Tabs and Fields, Manipulating Hyphenation, gtroff Reference
@section Manipulating Spacing
@cindex manipulating spacing
@cindex spacing, manipulating

@Defreq {sp, [@Var{distance}]}
Space downwards @var{distance}.  With no argument it advances 1@w{
}line.  A negative argument causes @code{gtroff} to move up the page
the specified distance.  If the argument is preceded by a @samp{|}
then @code{gtroff} moves that distance from the top of the page.  This
request causes a line break.  The default scaling indicator is @samp{v}.
@endDefreq

@DefreqList {ls, [@Var{nnn}]}
@DefregListEnd {.L}
@cindex double-spacing (@code{ls})
Output @w{@var{nnn}@minus{}1} blank lines after each line of text.
With no argument, @code{gtroff} uses the previous value before the
last @code{ls} call.

@Example
.ls 2    \" This causes double-spaced output
.ls 3    \" This causes triple-spaced output
.ls      \" Again double-spaced
@endExample

The line spacing is associated with the current environment
(@pxref{Environments}).

@cindex line spacing register (@code{.L})
The read-only number register @code{.L} contains the current line
spacing setting.
@endDefreq

@xref{Changing Type Sizes}, for the requests @code{vs} and @code{pvs}
as alternatives to @code{ls}.

@DefescList {\\x, ', spacing, '}
@DefregListEnd {.a}
Sometimes, extra vertical spacing is only needed occasionally, e.g.@:
to allow space for a tall construct (like an equation).  The @code{\x}
escape does this.  The escape is given a numerical argument, usually
enclosed in quotes (like @samp{\x'3p'}); the default scaling indicator
is @samp{v}.  If this number is positive extra vertical space is
inserted below the current line.  A negative number adds space above.
If this escape is used multiple times on the same line, the maximum of
the values is used.

@xref{Escapes}, for details on parameter delimiting characters.

@cindex extra post-vertical line space register (@code{.a})
The @code{.a} read-only number register contains the most recent
(nonnegative) extra vertical line space.

Using @code{\x} can be necessary in combination with the @code{\b}
escape, as the following example shows.

@Example
This is a test with the \[rs]b escape.
.br
This is a test with the \[rs]b escape.
.br
This is a test with \b'xyz'\x'-1m'\x'1m'.
.br
This is a test with the \[rs]b escape.
.br
This is a test with the \[rs]b escape.
@endExample

@noindent
produces

@Example
This is a test with the \b escape.
This is a test with the \b escape.
                    x
This is a test with y.
                    z
This is a test with the \b escape.
This is a test with the \b escape.
@endExample
@endDefesc

@DefreqList {ns, }
@DefreqItem {rs, }
@DefregListEnd {.ns}
@cindex @code{sp} request, and no-space mode
@cindex no-space mode (@code{ns})
@cindex mode, no-space (@code{ns})
@cindex blank lines, disabling
@cindex lines, blank, disabling
Enable @dfn{no-space mode}.  In this mode, spacing (either via
@code{sp} or via blank lines) is disabled.  The @code{bp} request to
advance to the next page is also disabled, except if it is accompanied
by a page number (see @ref{Page Control}, for more information).  This
mode ends when actual text is output or the @code{rs} request is
encountered which ends no-space mode.  The read-only number register
@code{.ns} is set to@w{ }1 as long as no-space mode is active.

This request is useful for macros that conditionally
insert vertical space before the text starts
(for example, a paragraph macro could insert some space
except when it is the first paragraph after a section header).
@endDefreq


@c =====================================================================

@node Tabs and Fields, Character Translations, Manipulating Spacing, gtroff Reference
@section Tabs and Fields
@cindex tabs, and fields
@cindex fields, and tabs

@cindex @acronym{EBCDIC} encoding of a tab
A tab character (@acronym{ASCII} char@w{ }9, @acronym{EBCDIC} char@w{
}5) causes a horizontal movement to the next tab stop (much
like it did on a typewriter).

@Defesc {\\t, , , }
@cindex tab character, non-interpreted (@code{\t})
@cindex character, tab, non-interpreted (@code{\t})
This escape is a non-interpreted tab character.  In copy mode
(@pxref{Copy-in Mode}), @code{\t} is the same as a real tab character.
@endDefesc

@DefreqList {ta, [@Var{n1} @Var{n2} @dots{} @Var{nn} @t{T} @Var{r1} @Var{r2} @dots{} @Var{rn}]}
@DefregListEnd {.tabs}
Change tab stop positions.  This request takes a series of tab
specifiers as arguments (optionally divided into two groups with the
letter @samp{T}) which indicate where each tab stop is to be
(overriding any previous settings).

Tab stops can be specified absolutely, i.e., as the distance from the
left margin.  For example, the following sets 6@w{ }tab stops every
one inch.

@Example
.ta 1i 2i 3i 4i 5i 6i
@endExample

Tab stops can also be specified using a leading @samp{+}
which means that the specified tab stop is set relative to
the previous tab stop.  For example, the following is equivalent to the
previous example.

@Example
.ta 1i +1i +1i +1i +1i +1i
@endExample

@code{gtroff} supports an extended syntax to specify repeat values after
the @samp{T} mark (these values are always taken as relative) -- this is
the usual way to specify tabs set at equal intervals.  The following is,
yet again, the same as the previous examples.  It does even more since
it defines an infinite number of tab stops separated by one inch.

@Example
.ta T 1i
@endExample

Now we are ready to interpret the full syntax given at the beginning:
Set tabs at positions @var{n1}, @var{n2}, @dots{}, @var{nn} and then set
tabs at @var{nn}+@var{r1}, @var{nn}+@var{r2}, @dots{}, @var{nn}+@var{rn}
and then at @var{nn}+@var{rn}+@var{r1}, @var{nn}+@var{rn}+@var{r2},
@dots{}, @var{nn}+@var{rn}+@var{rn}, and so on.

Example: @samp{4c +6c T 3c 5c 2c} is equivalent to @samp{4c 10c 13c 18c
20c 23c 28c 30c @dots{}}.

The material in each tab column (i.e., the column between two tab stops)
may be justified to the right or left or centered in the column.  This
is specified by appending @samp{R}, @samp{L}, or @samp{C} to the tab
specifier.  The default justification is @samp{L}.  Example:

@Example
.ta 1i 2iC 3iR
@endExample

Some notes:

@itemize @bullet
@item
The default unit of the @code{ta} request is @samp{m}.

@item
A tab stop is converted into a non-breakable horizontal movement which
can be neither stretched nor squeezed.  For example,

@Example
.ds foo a\tb\tc
.ta T 5i
\*[foo]
@endExample

@noindent
creates a single line which is a bit longer than 10@w{ }inches (a string
is used to show exactly where the tab characters are).  Now consider the
following:

@Example
.ds bar a\tb b\tc
.ta T 5i
\*[bar]
@endExample

@noindent
@code{gtroff} first converts the tab stops of the line into unbreakable
horizontal movements, then splits the line after the second @samp{b}
(assuming a sufficiently short line length).  Usually, this isn't what
the user wants.

@item
Superfluous tabs (i.e., tab characters which do not correspond to a tab
stop) are ignored except the first one which delimits the characters
belonging to the last tab stop for right-justifying or centering.
Consider the following example

@Example
.ds Z   foo\tbar\tfoo
.ds ZZ  foo\tbar\tfoobar
.ds ZZZ foo\tbar\tfoo\tbar
.ta 2i 4iR
\*[Z]
.br
\*[ZZ]
.br
\*[ZZZ]
.br
@endExample

@noindent
which produces the following output:

@Example
foo                 bar              foo
foo                 bar           foobar
foo                 bar              foobar
@endExample

@noindent
The first line right-justifies the second `foo' relative to the tab
stop.  The second line right-justifies `foobar'.  The third line finally
right-justifies only `foo' because of the additional tab character which
marks the end of the string belonging to the last defined tab stop.

@item
Tab stops are associated with the current environment
(@pxref{Environments}).

@item
Calling @code{ta} without an argument removes all tab stops.

@item
@cindex tab stops, for TTY output devices
The start-up value of @code{gtroff} is @w{@samp{T 0.5i}} in troff mode
and @w{@samp{T 0.8i}} in nroff mode (the latter is done with an
explicit call to the @code{ta} request in the file @file{tty.tmac}.
@end itemize

@cindex tab settings register (@code{.tabs})
The read-only number register @code{.tabs} contains a string
representation of the current tab settings suitable for use as an
argument to the @code{ta} request.

@Example
.ds tab-string \n[.tabs]
\*[tab-string]
    @result{} T120u
@endExample

@cindex @code{.S} register, Plan@w{ }9 alias for @code{.tabs}
@cindex @code{.tabs} register, Plan@w{ }9 alias (@code{.S})
The @code{troff} version of the Plan@w{ }9 operating system uses
register @code{.S} for the same purpose.
@endDefreq

@Defreq {tc, [@Var{fill-glyph}]}
@cindex tab repetition character (@code{tc})
@cindex character, tab repetition (@code{tc})
@cindex glyph, tab repetition (@code{tc})
Normally @code{gtroff} fills the space to the next tab stop with
whitespace.  This can be changed with the @code{tc} request.  With no
argument @code{gtroff} reverts to using whitespace, which is the
default.  The value of this @dfn{tab repetition character} is
associated with the current environment
(@pxref{Environments}).@footnote{@dfn{Tab repetition character} is a
misnomer since it is an output glyph.}
@endDefreq

@DefreqList {linetabs, n}
@DefregListEnd {.linetabs}
@cindex tab, line-tabs mode
@cindex line-tabs mode
@cindex mode, line-tabs
If @var{n} is missing or not zero, enable @dfn{line-tabs} mode,
or disable it otherwise (the default).
In line-tabs mode, @code{gtroff} computes tab distances
relative to the (current) output line instead of the input line.

For example, the following code:

@Example
.ds x a\t\c
.ds y b\t\c
.ds z c
.ta 1i 3i
\*x
\*y
\*z
@endExample

@noindent
in normal mode, results in the output

@Example
a         b         c
@endExample

@noindent
in line-tabs mode, the same code outputs

@Example
a         b                   c
@endExample

Line-tabs mode is associated with the current environment.
The read-only register @code{.linetabs} is set to@w{ }1 if in line-tabs
mode, and 0 in normal mode.
@endDefreq

@menu
* Leaders::
* Fields::
@end menu

@c ---------------------------------------------------------------------

@node Leaders, Fields, Tabs and Fields, Tabs and Fields
@subsection Leaders
@cindex leaders

Sometimes it may may be desirable to use the @code{tc} request to fill a
particular tab stop with a given glyph (for example dots in a table
of contents), but also normal tab stops on the rest of the line.  For
this @code{gtroff} provides an alternate tab mechanism, called
@dfn{leaders} which does just that.

@cindex leader character
A leader character (character code@w{ }1) behaves similarly to a tab
character: It moves to the next tab stop.  The only difference is that
for this movement, the fill glyph defaults to a period character and
not to space.

@Defesc {\\a, , , }
@cindex leader character, non-interpreted (@code{\a})
@cindex character, leader, non-interpreted (@code{\a})
This escape is a non-interpreted leader character.  In copy mode
(@pxref{Copy-in Mode}), @code{\a} is the same as a real leader
character.
@endDefesc

@Defreq {lc, [@Var{fill-glyph}]}
@cindex leader repetition character (@code{lc})
@cindex character, leader repetition (@code{lc})
@cindex glyph, leader repetition (@code{lc})
Declare the @dfn{leader repetition character}.@footnote{@dfn{Leader
repetition character} is a misnomer since it is an output glyph.}
Without an argument, leaders act the same as tabs (i.e., using
whitespace for filling).  @code{gtroff}'s start-up value is a dot
(@samp{.}).  The value of the leader repetition character is
associated with the current environment (@pxref{Environments}).
@endDefreq

@cindex table of contents
@cindex contents, table of
For a table of contents, to name an example, tab stops may be defined so
that the section number is one tab stop, the title is the second with
the remaining space being filled with a line of dots, and then the page
number slightly separated from the dots.

@Example
.ds entry 1.1\tFoo\a\t12
.lc .
.ta 1i 5i +.25i
\*[entry]
@endExample

@noindent
This produces

@Example
1.1  Foo..........................................  12
@endExample

@c ---------------------------------------------------------------------

@node Fields,  , Leaders, Tabs and Fields
@subsection Fields
@cindex fields

@cindex field delimiting character (@code{fc})
@cindex delimiting character, for fields (@code{fc})
@cindex character, field delimiting (@code{fc})
@cindex field padding character (@code{fc})
@cindex padding character, for fields (@code{fc})
@cindex character, field padding (@code{fc})
@dfn{Fields} are a more general way of laying out tabular data.  A field
is defined as the data between a pair of @dfn{delimiting characters}.
It contains substrings which are separated by @dfn{padding characters}.
The width of a field is the distance on the @emph{input} line from the
position where the field starts to the next tab stop.  A padding
character inserts stretchable space similar to @TeX{}'s @code{\hss}
command (thus it can even be negative) to make the sum of all substring
lengths plus the stretchable space equal to the field width.  If more
than one padding character is inserted, the available space is evenly
distributed among them.

@Defreq {fc, [@Var{delim-char} [@Var{padding-char}]]}
Define a delimiting and a padding character for fields.  If the latter
is missing, the padding character defaults to a space character.  If
there is no argument at all, the field mechanism is disabled (which is
the default).  Note that contrary to e.g.@: the tab repetition
character, delimiting and padding characters are @emph{not} associated
to the current environment (@pxref{Environments}).

Example:

@Example
.fc # ^
.ta T 3i
#foo^bar^smurf#
.br
#foo^^bar^smurf#
@endExample

@noindent
and here the result:

@Example
foo         bar          smurf
foo            bar       smurf
@endExample
@endDefreq


@c =====================================================================

@node Character Translations, Troff and Nroff Mode, Tabs and Fields, gtroff Reference
@section Character Translations
@cindex character translations
@cindex translations of characters

@cindex control character, changing (@code{cc})
@cindex character, control, changing (@code{cc})
@cindex no-break control character, changing (@code{c2})
@cindex character, no-break control, changing (@code{c2})
@cindex control character, no-break, changing (@code{c2})
The control character (@samp{.}) and the no-break control character
(@samp{'}) can be changed with the @code{cc} and @code{c2} requests,
respectively.

@Defreq {cc, [@Var{c}]}
Set the control character to@w{ }@var{c}.  With no argument the default
control character @samp{.} is restored.  The value of the control
character is associated with the current environment
(@pxref{Environments}).
@endDefreq

@Defreq {c2, [@Var{c}]}
Set the no-break control character to@w{ }@var{c}.  With no argument the
default control character @samp{'} is restored.  The value of the
no-break control character is associated with the current environment
(@pxref{Environments}).
@endDefreq

@Defreq {eo, }
@cindex disabling @code{\} (@code{eo})
@cindex @code{\}, disabling (@code{eo})
Disable the escape mechanism completely.  After executing this
request, the backslash character @samp{\} no longer starts an escape
sequence.

This request can be very helpful in writing macros since it is not
necessary then to double the escape character.  Here an example:

@Example
.\" This is a simplified version of the
.\" .BR request from the man macro package
.eo
.de BR
.  ds result \&
.  while (\n[.$] >= 2) \@{\
.    as result \fB\$1\fR\$2
.    shift 2
.  \@}
.  if \n[.$] .as result \fB\$1
\*[result]
.  ft R
..
.ec
@endExample
@endDefreq

@Defreq {ec, [@Var{c}]}
@cindex escape character, changing (@code{ec})
@cindex character, escape, changing (@code{ec})
Set the escape character to@w{ }@var{c}.  With no argument the default
escape character @samp{\} is restored.  It can be also used to
re-enable the escape mechanism after an @code{eo} request.

Note that changing the escape character globally will likely break
macro packages since @code{gtroff} has no mechanism to `intern' macros,
i.e., to convert a macro definition into an internal form which is
independent of its representation (@TeX{} has this mechanism).
If a macro is called, it is executed literally.
@endDefreq

@DefreqList {ecs, }
@DefreqListEnd {ecr, }
The @code{ecs} request saves the current escape character
in an internal register.
Use this request in combination with the @code{ec} request to
temporarily change the escape character.

The @code{ecr} request restores the escape character
saved with @code{ecs}.
Without a previous call to @code{ecs}, this request
sets the escape character to @code{\}.
@endDefreq

@DefescList {\\\\, , , }
@DefescItem {\\e, , , }
@DefescListEnd {\\E, , , }
Print the current escape character (which is the backslash character
@samp{\} by default).

@code{\\} is a `delayed' backslash; more precisely, it is the default
escape character followed by a backslash, which no longer has special
meaning due to the leading escape character.  It is @emph{not} an escape
sequence in the usual sense!  In any unknown escape sequence
@code{\@var{X}} the escape character is ignored and @var{X} is printed.
But if @var{X} is equal to the current escape character, no warning is
emitted.

As a consequence, only at top-level or in a diversion a backslash glyph is
printed; in copy-in mode, it expands to a single backslash which then
combines with the following character to an escape sequence.

The @code{\E} escape differs from @code{\e} by printing an escape
character that is not interpreted in copy mode.
Use this to define strings with escapes that work
when used in copy mode (for example, as a macro argument).
The following example defines strings to begin and end
a superscript:

@Example
.ds @{ \v'-.3m'\s'\Es[.s]*60/100'
.ds @} \s0\v'.3m'
@endExample

Another example to demonstrate the differences between the various escape
sequences, using a strange escape character, @samp{-}.

@Example
.ec -
.de xxx
--A'123'
..
.xxx
    @result{} -A'foo'
@endExample

@noindent
The result is surprising for most users, expecting @samp{1} since
@samp{foo} is a valid identifier.  What has happened?  As mentioned
above, the leading escape character makes the following character
ordinary.  Written with the default escape character the sequence
@samp{--} becomes @samp{\-} -- this is the minus sign.

If the escape character followed by itself is a valid escape sequence,
only @code{\E} yields the expected result:

@Example
.ec -
.de xxx
-EA'123'
..
.xxx
    @result{} 1
@endExample
@endDefesc

@Defesc {\\., , , }
Similar to @code{\\}, the sequence @code{\.} isn't a real escape sequence.
As before, a warning message is suppressed if the escape character is
followed by a dot, and the dot itself is printed.

@Example
.de foo
.  nop foo
.
.  de bar
.    nop bar
\\..
.
..
.foo
.bar
    @result{} foo bar
@endExample

@noindent
The first backslash is consumed while the macro is read, and the second
is swallowed while exexuting macro @code{foo}.
@endDefesc

A @dfn{translation} is a mapping of an input character to an output
glyph.  The mapping occurs at output time, i.e., the input character
gets assigned the metric information of the mapped output character
right before input tokens are converted to nodes (@pxref{Gtroff
Internals}, for more on this process).

@DefreqList {tr, @Var{a}@Var{b}@Var{c}@Var{d}@dots{}}
@DefreqListEnd {trin, @Var{a}@Var{b}@Var{c}@Var{d}@dots{}}
Translate character @var{a} to glyph@w{ }@var{b}, character @var{c} to
glyph@w{ }@var{d}, etc.  If there is an odd number of arguments, the
last one is translated to an unstretchable space (@w{@samp{\ }}).

The @code{trin} request is identical to @code{tr},
but when you unformat a diversion with @code{asciify}
it ignores the translation.
@xref{Diversions}, for details about the @code{asciify} request.

Some notes:

@itemize @bullet
@item
@cindex @code{\(}, and translations
@cindex @code{\[}, and translations
@cindex @code{\'}, and translations
@cindex @code{\`}, and translations
@cindex @code{\-}, and translations
@cindex @code{\_}, and translations
@cindex @code{\C}, and translations
@cindex @code{\N}, and translations
@cindex @code{char} request, and translations
@cindex special characters
@cindex character, special
@cindex numbered glyph (@code{\N})
@cindex glyph, numbered (@code{\N})
Special characters (@code{\(@var{xx}}, @code{\[@var{xxx}]},
@code{\C'@var{xxx}'}, @code{\'}, @code{\`}, @code{\-}, @code{\_}),
glyphs defined with the @code{char} request, and numbered glyphs
(@code{\N'@var{xxx}'}) can be translated also.

@item
@cindex @code{\e}, and translations
The @code{\e} escape can be translated also.

@item
@cindex @code{\%}, and translations
@cindex @code{\~}, and translations
Characters can be mapped onto the @code{\%} and @code{\~} escapes (but
@code{\%} and @code{\~} can't be mapped onto another glyph).

@item
@cindex backspace character, and translations
@cindex character, backspace, and translations
@cindex leader character, and translations
@cindex character, leader, and translations
@cindex newline character, and translations
@cindex character, newline, and translations
@cindex tab character, and translations
@cindex character, tab, and translations
@cindex @code{\a}, and translations
@cindex @code{\t}, and translations
The following characters can't be translated: space (with one exception,
see below), backspace, newline, leader (and @code{\a}), tab (and
@code{\t}).

@item
@cindex @code{shc} request, and translations
Translations are not considered for finding the soft hyphen character
set with the @code{shc} request.

@item
@cindex @code{\&}, and translations
The pair @samp{@var{c}\&} (this is an arbitrary character@w{
}@var{c} followed by the zero width space character) maps this
character to nothing.

@Example
.tr a\&
foo bar
    @result{} foo br
@endExample

@noindent
It is even possible to map the space character to nothing:

@Example
.tr aa \&
foo bar
    @result{} foobar
@endExample

@noindent
As shown in the example, the space character can't be the first
character/glyph pair as an argument of @code{tr}.  Additionally, it is
not possible to map the space character to any other glyph; requests
like @w{@samp{.tr aa x}} undo @w{@samp{.tr aa \&}} instead.

If justification is active, lines are justified in spite of the
`empty' space character (but there is no minimal distance, i.e.@: the
space character, between words).

@item
After an output glyph has been constructed (this happens at the
moment immediately before the glyph is appended to an output
glyph list, either by direct output, in a macro, diversion, or
string), it is no longer affected by @code{tr}.

@item
Translating character to glyphs where one of them or both are
undefined is possible also; @code{tr} does not check whether the
entities in its argument do exist.

@xref{Gtroff Internals}.

@item
@code{troff} no longer has a hard-coded dependency on @w{Latin-1};
all @code{char@var{XXX}} entities have been removed from the font
description files.  This has a notable consequence which shows up in
warnings like @code{can't find character with input code @var{XXX}}
if the @code{tr} request isn't handled properly.

Consider the following translation:

@Example
.tr @'e@'E
@endExample

@noindent
This maps input character @code{@'e} onto glyph @code{@'E}, which is
identical to glyph @code{char201}.  But this glyph intentionally
doesn't exist!  Instead, @code{\[char201]} is treated as an input
character entity and is by default mapped onto @code{\['E]}, and
@code{gtroff} doesn't handle translations of translations.

The right way to write the above translation is

@Example
.tr @'e\['E]
@endExample

@noindent
With other words, the first argument of @code{tr} should be an input
character or entity, and the second one a glyph entity.

@item
Without an argument, the @code{tr} request is ignored.
@end itemize
@endDefreq

@Defreq {trnt, @Var{a}@Var{b}@Var{c}@Var{d}@dots{}}
@cindex @code{\!}, and @code{trnt}
@code{trnt} is the same as the @code{tr} request except that the
translations do not apply to text that is transparently throughput
into a diversion with @code{\!}.  @xref{Diversions}, for more
information.

For example,

@Example
.tr ab
.di x
\!.tm a
.di
.x
@endExample

@noindent
prints @samp{b} to the standard error stream; if @code{trnt} is used
instead of @code{tr} it prints @samp{a}.
@endDefreq


@c =====================================================================

@node Troff and Nroff Mode, Line Layout, Character Translations, gtroff Reference
@section Troff and Nroff Mode
@cindex troff mode
@cindex mode, troff
@cindex nroff mode
@cindex mode, nroff

Originally, @code{nroff} and @code{troff} were two separate programs,
the former for TTY output, the latter for everything else.  With GNU
@code{troff}, both programs are merged into one executable, sending
its output to a device driver (@code{grotty} for TTY devices,
@code{grops} for @sc{PostScript}, etc.) which interprets the
intermediate output of @code{gtroff}.  For @acronym{UNIX} @code{troff}
it makes sense to talk about @dfn{Nroff mode} and @dfn{Troff mode}
since the differences are hardcoded.  For GNU @code{troff}, this
distinction is not appropriate because @code{gtroff} simply takes the
information given in the font files for a particular device without
handling requests specially if a TTY output device is used.

Usually, a macro package can be used with all output devices.
Nevertheless, it is sometimes necessary to make a distinction between
TTY and non-TTY devices: @code{gtroff} provides two built-in
conditions @samp{n} and @samp{t} for the @code{if}, @code{ie}, and
@code{while} requests to decide whether @code{gtroff} shall behave
like @code{nroff} or like @code{troff}.

@Defreq {troff, }
@pindex troffrc
@pindex troffrc-end
Make the @samp{t} built-in condition true (and the @samp{n} built-in
condition false) for @code{if}, @code{ie}, and @code{while}
conditional requests.  This is the default if @code{gtroff}
(@emph{not} @code{groff}) is started with the @option{-R} switch to
avoid loading of the start-up files @file{troffrc} and
@file{troffrc-end}.  Without @option{-R}, @code{gtroff} stays in troff
mode if the output device is not a TTY (e.g.@: `ps').
@endDefreq

@Defreq {nroff, }
@pindex tty.tmac
Make the @samp{n} built-in condition true (and the @samp{t} built-in
condition false) for @code{if}, @code{ie}, and @code{while}
conditional requests.  This is the default if @code{gtroff} uses a TTY
output device; the code for switching to nroff mode is in the file
@file{tty.tmac} which is loaded by the start-up file @code{troffrc}.
@endDefreq

@xref{Conditionals and Loops}, for more details on built-in
conditions.


@c =====================================================================

@node Line Layout, Line Control, Troff and Nroff Mode, gtroff Reference
@section Line Layout
@cindex line layout
@cindex layout, line

@cindex dimensions, line
@cindex line dimensions
The following drawing shows the dimensions which @code{gtroff} uses for
placing a line of output onto the page.  They are labeled with the
request which manipulates each dimension.

@Example
                -->| in |<--
                   |<-----------ll------------>|
              +----+----+----------------------+----+
              |    :    :                      :    |
              +----+----+----------------------+----+
           -->| po |<--
              |<--------paper width---------------->|
@endExample

@noindent
These dimensions are:

@ftable @code
@item po
@cindex left margin (@code{po})
@cindex margin, left (@code{po})
@cindex page offset (@code{po})
@cindex offset, page (@code{po})
@dfn{Page offset} -- this is the leftmost position of text on the final
output, defining the @dfn{left margin}.

@item in
@cindex indentation (@code{in})
@cindex line indentation (@code{in})
@dfn{Indentation} -- this is the distance from the left margin where
text is printed.

@item ll
@cindex line length (@code{ll})
@cindex length of line (@code{ll})
@dfn{Line length} -- this is the distance from the left margin to right
margin.
@end ftable

A simple demonstration:

@Example
.ll 3i
This is text without indentation.
The line length has been set to 3\~inch.
.in +.5i
.ll -.5i
Now the left and right margins are both increased.
.in
.ll
Calling .in and .ll without parameters restore
the previous values.
@endExample

Result:

@Example
This  is text without indenta-
tion.   The  line  length  has
been set to 3 inch.
     Now   the  left  and
     right  margins   are
     both increased.
Calling  .in  and  .ll without
parameters restore the  previ-
ous values.
@endExample

@DefreqList {po, [@Var{offset}]}
@DefreqItem {po, @t{+}@Var{offset}}
@DefreqItem {po, @t{-}@Var{offset}}
@DefregListEnd {.o}
@pindex troffrc
Set horizontal page offset to @var{offset} (or increment or decrement
the current value by @var{offset}).  Note that this request does not
cause a break, so changing the page offset in the middle of text being
filled may not yield the expected result.  The initial value is
1@dmn{i}.  For TTY output devices, it is set to 0 in the startup file
@file{troffrc}; the default scaling indicator is @samp{m} (and
not @samp{v} as incorrectly documented in the original
@acronym{UNIX} troff manual).

The current page offset can be found in the read-only number register
@samp{.o}.

If @code{po} is called without an argument, the page offset is reset to
the previous value before the last call to @code{po}.

@Example
.po 3i
\n[.o]
    @result{} 720
.po -1i
\n[.o]
    @result{} 480
.po
\n[.o]
    @result{} 720
@endExample
@endDefreq

@DefreqList {in, [@Var{indent}]}
@DefreqItem {in, @t{+}@Var{indent}}
@DefreqItem {in, @t{-}@Var{indent}}
@DefregListEnd {.i}
Set indentation to @var{indent} (or increment or decrement the
current value by @var{indent}).  This request causes a break.
Initially, there is no indentation.

If @code{in} is called without an argument, the indentation is reset to
the previous value before the last call to @code{in}.  The default
scaling indicator is @samp{m}.

The indentation is associated with the current environment
(@pxref{Environments}).

If a negative indentation value is specified (which is not allowed),
@code{gtroff} emits a warning of type @samp{range} and sets the
indentation to zero.

The effect of @code{in} is delayed until a partially collected line (if
it exists) is output.  A temporary indent value is reset to zero also.

The current indentation (as set by @code{in}) can be found in the
read-only number register @samp{.i}.
@endDefreq

@DefreqList {ti, offset}
@DefreqItem {ti, @t{+}@Var{offset}}
@DefreqItem {ti, @t{-}@Var{offset}}
@DefregListEnd {.in}
Temporarily indent the next output line by @var{offset}.  If an
increment or decrement value is specified, adjust the temporary
indentation relative to the value set by the @code{in} request.

This request causes a break; its value is associated with the current
environment (@pxref{Environments}).  The default scaling indicator
is @samp{m}.  A call of @code{ti} without an argument is ignored.

If the total indentation value is negative (which is not allowed),
@code{gtroff} emits a warning of type @samp{range} and sets the
temporary indentation to zero.  `Total indentation' is either
@var{offset} if specified as an absolute value, or the temporary plus
normal indentation, if @var{offset} is given as a relative value.

The effect of @code{ti} is delayed until a partially collected line (if
it exists) is output.

The read-only number register @code{.in} is the indentation that applies
to the current output line.

The difference between @code{.i} and @code{.in} is that the latter takes
into account whether a partially collected line still uses the old
indentation value or a temporary indentation value is active.
@endDefreq

@DefreqList {ll, [@Var{length}]}
@DefreqItem {ll, @t{+}@Var{length}}
@DefreqItem {ll, @t{-}@Var{length}}
@DefregItem {.l}
@DefregListEnd {.ll}
Set the line length to @var{length} (or increment or decrement the
current value by @var{length}).  Initially, the line length is set to
6.5@dmn{i}.  The effect of @code{ll} is delayed until a partially
collected line (if it exists) is output.  The default scaling
indicator is @samp{m}.

If @code{ll} is called without an argument, the line length is reset to
the previous value before the last call to @code{ll}.  If a negative
line length is specified (which is not allowed), @code{gtroff} emits a
warning of type @samp{range} and sets the line length to zero.

The line length is associated with the current environment
(@pxref{Environments}).

@cindex line length register (@code{.l})
The current line length (as set by @code{ll}) can be found in the
read-only number register @samp{.l}.  The read-only number register
@code{.ll} is the line length that applies to the current output line.

Similar to @code{.i} and @code{.in}, the difference between @code{.l}
and @code{.ll} is that the latter takes into account whether a partially
collected line still uses the old line length value.
@endDefreq


@c =====================================================================

@node Line Control, Page Layout, Line Layout, gtroff Reference
@section Line Control
@cindex line control
@cindex control, line

It is important to understand how @code{gtroff} handles input and output
lines.

Many escapes use positioning relative to the input line.  For example,
this

@Example
This is a \h'|1.2i'test.

This is a
\h'|1.2i'test.
@endExample

@noindent
produces

@Example
This is a   test.

This is a             test.
@endExample

The main usage of this feature is to define macros which act exactly
at the place where called.

@Example
.\" A simple macro to underline a word
.de underline
.  nop \\$1\l'|0\[ul]'
..
@endExample

@noindent
In the above example, @samp{|0} specifies a negative distance from the
current position (at the end of the just emitted argument @code{\$1}) back
to the beginning of the input line.  Thus, the @samp{\l} escape draws a
line from right to left.

@cindex input line continuation (@code{\})
@cindex line, input, continuation (@code{\})
@cindex continuation, input line (@code{\})
@cindex output line, continuation (@code{\c})
@cindex line, output, continuation (@code{\c})
@cindex continuation, output line (@code{\c})
@cindex interrupted line
@cindex line, interrupted
@code{gtroff} makes a difference between input and output line
continuation; the latter is also called @dfn{interrupting} a line.

@DefescList {\\@key{RET}, , ,}
@DefescItem {\\c, , ,}
@DefregListEnd{.int}
Continue a line.  @code{\@key{RET}} (this is a backslash at the end
of a line immediately followed by a newline) works on the input level,
suppressing the effects of the following newline in the input.

@Example
This is a \
.test
    @result{} This is a .test
@endExample

The @samp{|} operator is also affected.

@cindex @code{\R}, after @code{\c}
@code{\c} works on the output level.  Anything after this escape on the
same line is ignored, except @code{\R} which works as usual.  Anything
before @code{\c} on the same line will be appended to the current partial
output line.  The next non-command line after an interrupted line counts
as a new input line.

The visual results depend on whether no-fill mode is active.

@itemize @bullet
@item
@cindex @code{\c}, and no-fill mode
@cindex no-fill mode, and @code{\c}
@cindex mode, no-fill, and @code{\c}
If no-fill mode is active (using the @code{nf} request), the next input
text line after @code{\c} will be handled as a continuation of the same
input text line.

@Example
.nf
This is a \c
test.
    @result{} This is a test.
@endExample

@item
@cindex @code{\c}, and fill mode
@cindex fill mode, and @code{\c}
@cindex mode, fill, and @code{\c}
If fill mode is active (using the @code{fi} request), a word interrupted
with @code{\c} will be continued with the text on the next input text line,
without an intervening space.

@Example
This is a te\c
st.
    @result{} This is a test.
@endExample
@end itemize

Note that an intervening control line which causes a break is stronger
than @code{\c}, flushing out the current partial line in the usual way.

@cindex interrupted line register (@code{.int})
The @code{.int} register contains a positive value
if the last output line was interrupted with @code{\c}; this is
associated with the current environment (@pxref{Environments}).

@endDefesc

@c =====================================================================

@node Page Layout, Page Control, Line Control, gtroff Reference
@section Page Layout
@cindex page layout
@cindex layout, page

@code{gtroff} provides some very primitive operations for controlling
page layout.

@DefreqList {pl, [@Var{length}]}
@DefreqItem {pl, @t{+}@Var{length}}
@DefreqItem {pl, @t{-}@Var{length}}
@DefregListEnd {.p}
@cindex page length (@code{pl})
@cindex length of page (@code{pl})
Set the @dfn{page length} to @var{length} (or increment or decrement
the current value by @var{length}).  This is the length of the
physical output page.  The default scaling indicator is @samp{v}.

@cindex page length register (@code{.p})
The current setting can be found in the read-only number register
@samp{.p}.

@cindex top margin
@cindex margin, top
@cindex bottom margin
@cindex margin, bottom
Note that this only specifies the size of the page, not the top and
bottom margins.  Those are not set by @code{gtroff} directly.
@xref{Traps}, for further information on how to do this.

Negative @code{pl} values are possible also, but not very useful: No
trap is sprung, and each line is output on a single page (thus
suppressing all vertical spacing).

If no argument or an invalid argument is given, @code{pl} sets the page
length to 11@dmn{i}.
@endDefreq

@cindex headers
@cindex footers
@cindex titles
@code{gtroff} provides several operations which help in setting up top
and bottom titles (or headers and footers).

@Defreq {tl, @t{'}@Var{left}@t{'}@Var{center}@t{'}@Var{right}@t{'}}
@cindex title line (@code{tl})
@cindex three-part title (@code{tl})
@cindex page number character (@code{%})
Print a @dfn{title line}.  It consists of three parts: a left
justified portion, a centered portion, and a right justified portion.
The argument separator @samp{'} can be replaced with any character not
occurring in the title line.  The @samp{%} character is replaced with
the current page number.  This character can be changed with the
@code{pc} request (see below).

Without argument, @code{tl} is ignored.

Some notes:

@itemize @bullet
@item
A title line is not restricted to the top or bottom of a page.

@item
@code{tl} prints the title line immediately, ignoring a partially filled
line (which stays untouched).

@item
It is not an error to omit closing delimiters.  For example,
@w{@samp{.tl /foo}} is equivalent to @w{@samp{.tl /foo///}}: It prints a
title line with the left justified word @samp{foo}; the centered and
right justfied parts are empty.

@item
Any modifications to the current environment within @code{tl} (e.g.@:
changing the font or font size) are undone after processing @code{tl}.

@item
@code{tl} accepts the same parameter delimiting characters as the
@code{\A} escape; see @ref{Escapes}.
@end itemize
@endDefreq

@DefreqList {lt, [@Var{length}]}
@DefreqItem {lt, @t{+}@Var{length}}
@DefreqItem {lt, @t{-}@Var{length}}
@DefregListEnd {.lt}
@cindex length of title line (@code{lt})
@cindex title line, length (@code{lt})
@cindex title line length register (@code{.lt})
The title line is printed using its own line length, which is
specified (or incremented or decremented) with the @code{lt} request.
Initially, the title line length is set to 6.5@dmn{i}.  If a negative
line length is specified (which is not allowed), @code{gtroff} emits a
warning of type @samp{range} and sets the title line length to zero.
The default scaling indicator is @samp{m}.  If @code{lt} is called
without an argument, the title length is reset to the previous value
before the last call to @code{lt}.

The current setting of this is available in the @code{.lt} read-only
number register; it is associated with the current environment
(@pxref{Environments}).

@endDefreq

@DefreqList {pn, page}
@DefreqItem {pn, @t{+}@Var{page}}
@DefreqItem {pn, @t{-}@Var{page}}
@DefregListEnd {.pn}
@cindex page number (@code{pn})
@cindex number, page (@code{pn})
Change (increase or decrease) the page number of the @emph{next} page.
The only argument is the page number; the request is ignored without a
parameter.

The read-only number register @code{.pn} contains the number of the next
page: either the value set by a @code{pn} request, or the number of the
current page plus@w{ }1.
@endDefreq

@Defreg {%}
@cindex page number register (@code{%})
A read-write register holding the current page number.
@endDefreg

@Defreq {pc, [@Var{char}]}
@cindex changing the page number character (@code{pc})
@cindex page number character, changing (@code{pc})
@vindex %
Change the page number character (used by the @code{tl} request) to a
different character.  With no argument, this mechanism is disabled.
Note that this doesn't affect the number register@w{ }@code{%}.
@endDefreq

@xref{Traps}.


@c =====================================================================

@node Page Control, Fonts, Page Layout, gtroff Reference
@section Page Control
@cindex page control
@cindex control, page

@DefreqList {bp, [@Var{page}]}
@DefreqItem {bp, @t{+}@Var{page}}
@DefreqListEnd {bp, @t{-}@Var{page}}
@cindex new page (@code{bp})
@cindex page, new (@code{bp})
Stop processing the current page and move to the next page.  This
request causes a break.  It can also take an argument to set
(increase, decrease) the page number of the next page.  The only
difference between @code{bp} and @code{pn} is that @code{pn} does not
cause a break or actually eject a page.

@Example
.de newpage                         \" define macro
'bp                                 \" begin page
'sp .5i                             \" vertical space
.tl 'left top'center top'right top' \" title
'sp .3i                             \" vertical space
..                                  \" end macro
@endExample

@cindex @code{bp} request, and top-level diversion
@cindex top-level diversion, and @code{bp}
@cindex diversion, top-level, and @code{bp}
@code{bp} has no effect if not called within the top-level diversion
(@pxref{Diversions}).
@endDefreq

@Defreq {ne, [@Var{space}]}
@cindex orphan lines, preventing with @code{ne}
@cindex conditional page break (@code{ne})
@cindex page break, conditional (@code{ne})
It is often necessary to force a certain amount of space before a new
page occurs.  This is most useful to make sure that there is not a
single @dfn{orphan} line left at the bottom of a page.  The @code{ne}
request ensures that there is a certain distance, specified by the
first argument, before the next page is triggered (see @ref{Traps},
for further information).  The default scaling indicator for @code{ne}
is @samp{v}; the default value of @var{space} is@w{ }1@dmn{v} if no
argument is given.

For example, to make sure that no fewer than 2@w{ }lines get orphaned,
do the following before each paragraph:

@Example
.ne 2
text text text
@endExample

@code{ne} will then automatically cause a page break if there is space
for one line only.
@endDefreq

@DefreqList {sv, [@Var{space}]}
@DefreqListEnd {os, }
@cindex @code{ne} request, comparison with @code{sv}
@code{sv} is similar to the @code{ne} request; it reserves the
specified amount of vertical space.  If the desired amount of space
exists before the next trap (or the bottom page boundary if no trap is
set), the space is output immediately (ignoring a partially filled line
which stays untouched).  If there is not enough space, it is stored for
later output via the @code{os} request.  The default value is@w{ }1@dmn{v}
if no argument is given; the default scaling indicator is @samp{v}.

@cindex @code{sv} request, and no-space mode
@cindex @code{os} request, and no-space mode
Both @code{sv} and @code{os} ignore no-space mode.  While the @code{sv}
request allows negative values for @var{space}, @code{os} will ignore
them.
@endDefreq

@Defreg {nl}
This register contains the current vertical position.  If the vertical
position is zero and the top of page transition hasn't happened yet,
@code{nl} is set to negative value.  @code{gtroff} itself does this at
the very beginning of a document before anything has been printed, but
the main usage is to plant a header trap on a page if this page has
already started.

Consider the following:

@Example
.de xxx
.  sp
.  tl ''Header''
.  sp
..
.
First page.
.bp
.wh 0 xxx
.nr nl (-1)
Second page.
@endExample

@noindent
Result:

@Example
First page.

...

                             Header

Second page.

...
@endExample

@noindent
Without resetting @code{nl} to a negative value, the just planted trap
would be active beginning with the @emph{next} page, not the current
one.

@xref{Diversions}, for a comparison with the @code{.h} and @code{.d}
registers.
@endDefreg

@c =====================================================================

@node Fonts, Sizes, Page Control, gtroff Reference
@section Fonts
@cindex fonts

@code{gtroff} can switch fonts at any point in the text.

The basic set of fonts is @samp{R}, @samp{I}, @samp{B}, and @samp{BI}.
These are Times Roman, Italic, Bold, and Bold Italic.  For non-TTY
devices, there is also at least one symbol font which contains various
special symbols (Greek, mathematics).

@menu
* Changing Fonts::
* Font Families::
* Font Positions::
* Using Symbols::
* Special Fonts::
* Artificial Fonts::
* Ligatures and Kerning::
@end menu

@c ---------------------------------------------------------------------

@node Changing Fonts, Font Families, Fonts, Fonts
@subsection Changing Fonts
@cindex fonts

@DefreqList {ft, [@Var{font}]}
@DefescItem {\\f, , f, }
@DefescItem {\\f, @lparen{}, fn, }
@DefescListEnd {\\f, @lbrack{}, font, @rbrack}
@cindex changing fonts (@code{ft}, @code{\f})
@cindex fonts, changing (@code{ft}, @code{\f})
@cindex @code{sty} request, and changing fonts
@cindex @code{fam} request, and changing fonts
@cindex @code{\F}, and changing fonts
@kindex styles
@kindex family
@pindex DESC
The @code{ft} request and the @code{\f} escape change the current font
to @var{font} (one-character name@w{ }@var{f}, two-character name
@var{fn}).

If @var{font} is a style name (as set with the @code{sty} request or
with the @code{styles} command in the @file{DESC} file), use it within
the current font family (as set with the @code{fam} request, @code{\F}
escape, or with the @code{family} command in the @file{DESC} file).

@cindex previous font (@code{ft}, @code{\f[]}, @code{\fP})
@cindex font, previous (@code{ft}, @code{\f[]}, @code{\fP})
With no argument or using @samp{P} as an argument, @code{.ft} switches
to the previous font.  Use @code{\f[]} to do this with the escape.  The
old syntax forms @code{\fP} or @code{\f[P]} are also supported.

Fonts are generally specified as upper-case strings, which are usually
1@w{ }to 4 characters representing an abbreviation or acronym of the
font name.  This is no limitation, just a convention.

The example below produces two identical lines.

@Example
eggs, bacon,
.ft B
spam
.ft
and sausage.

eggs, bacon, \fBspam\fP and sausage.
@endExample

Note that @code{\f} doesn't produce an input token in @code{gtroff}.
As a consequence, it can be used in requests like @code{mc} (which
expects a single character as an argument) to change the font on
the fly:

@Example
.mc \f[I]x\f[]
@endExample

@xref{Font Positions}, for an alternative syntax.
@endDefreq

@Defreq {ftr, f [@Var{g}]}
@cindex @code{ft} request, and font translations
@cindex @code{ul} request, and font translations
@cindex @code{bd} request, and font translations
@cindex @code{\f}, and font translations
@cindex @code{cs} request, and font translations
@cindex @code{tkf} request, and font translations
@cindex @code{special} request, and font translations
@cindex @code{fspecial} request, and font translations
@cindex @code{fp} request, and font translations
@cindex @code{sty} request, and font translations
Translate font@w{ }@var{f} to font@w{ }@var{g}.  Whenever a font named@w{
}@var{f} is referred to in a @code{\f} escape sequence, or in the
@code{ft}, @code{ul}, @code{bd}, @code{cs}, @code{tkf},
@code{special}, @code{fspecial}, @code{fp}, or @code{sty} requests,
font@w{ }@var{g} is used.  If @var{g} is missing or equal to@w{ }@var{f}
the translation is undone.
@endDefreq

@c ---------------------------------------------------------------------

@node Font Families, Font Positions, Changing Fonts, Fonts
@subsection Font Families
@cindex font families
@cindex families, font
@cindex font styles
@cindex styles, font

Due to the variety of fonts available, @code{gtroff} has added the
concept of @dfn{font families} and @dfn{font styles}.  The fonts are
specified as the concatenation of the font family and style.  Specifying
a font without the family part causes @code{gtroff} to use that style of
the current family.

@cindex PostScript fonts
@cindex fonts, PostScript
Currently, fonts for the devices @option{-Tps}, @option{-Tdvi}, and
@option{-Tlbp} are set up to this mechanism.
By default, @code{gtroff} uses the Times family with the four styles
@samp{R}, @samp{I}, @samp{B}, and @samp{BI}.

This way, it is possible to use the basic four fonts and to select a
different font family on the command line (@pxref{Groff Options}).

@DefreqList {fam, [@Var{family}]}
@DefregItem {.fam}
@DefescItem {\\F, , f, }
@DefescItem {\\F, @lparen{}, fm, }
@DefescItem {\\F, @lbrack{}, family, @rbrack}
@DefregListEnd {.fn}
@cindex changing font family (@code{fam}, @code{\F})
@cindex font family, changing (@code{fam}, @code{\F})
Switch font family to @var{family} (one-character name@w{ }@var{f},
two-character name @var{fm}).  If no argument is given, switch
back to the previous font family.  Use @code{\F[]} to do this with the
escape.  Note that @code{\FP} doesn't work; it selects font family
@samp{P} instead.

The value at start-up is @samp{T}.
The current font family is available in the read-only number register
@samp{.fam} (this is a string-valued register); it is associated with
the current environment.

@Example
spam,
.fam H    \" helvetica family
spam,     \" used font is family H + style R = HR
.ft B     \" family H + style B = font HB
spam,
.fam T    \" times family
spam,     \" used font is family T + style B = TB
.ft AR    \" font AR (not a style)
baked beans,
.ft R     \" family T + style R = font TR
and spam.
@endExample

Note that @code{\F} doesn't produce an input token in @code{gtroff}.
As a consequence, it can be used in requests like @code{mc} (which
expects a single character as an argument) to change the font family on
the fly:

@Example
.mc \F[P]x\F[]
@endExample

The @samp{.fn} register contains the current @dfn{real font name}
of the current font.
This is a string-valued register.
If the current font is a style, the value of @code{\n[.fn]}
is the proper concatenation of family and style name.
@endDefreq

@Defreq {sty, n style}
@cindex changing font style (@code{sty})
@cindex font style, changing (@code{sty})
@cindex @code{cs} request, and font styles
@cindex @code{bd} request, and font styles
@cindex @code{tkf} request, and font styles
@cindex @code{uf} request, and font styles
@cindex @code{fspecial} request, and font styles
Associate @var{style} with font position@w{ }@var{n}.  A font position
can be associated either with a font or with a style.  The current
font is the index of a font position and so is also either a font or a
style.  If it is a style, the font that is actually used is the font
which name is the concatenation of the name of the current
family and the name of the current style.  For example, if the current
font is@w{ }1 and font position@w{ }1 is associated with style
@samp{R} and the current font family is @samp{T}, then font
@samp{TR} will be used.  If the current font is not a style, then the
current family is ignored.  If the requests @code{cs}, @code{bd},
@code{tkf}, @code{uf}, or @code{fspecial} are applied to a style,
they will instead be applied to the member of the current family
corresponding to that style.

@var{n}@w{ }must be a non-negative integer value.

@pindex DESC
@kindex styles
The default family can be set with the @option{-f} option
(@pxref{Groff Options}).  The @code{styles} command in the @file{DESC}
file controls which font positions (if any) are initially associated
with styles rather than fonts.  For example, the default setting for
@sc{PostScript} fonts

@Example
styles R I B BI
@endExample

@noindent
is equivalent to

@Example
.sty 1 R
.sty 2 I
.sty 3 B
.sty 4 BI
@endExample

@code{fam} and @code{\F} always check whether the current font position
is valid; this can give surprising results if the current font position is
associated with a style.

In the following example, we want to access the @sc{PostScript} font
@code{FooBar} from the font family @code{Foo}:

@Example
.sty \n[.fp] Bar
.fam Foo
    @result{} warning: can't find font `FooR'
@endExample

@noindent
The default font position at start-up is@w{ }1; for the
@sc{PostScript} device, this is associated with style @samp{R}, so
@code{gtroff} tries to open @code{FooR}.

A solution to this problem is to use a dummy font like the following:

@Example
.fp 0 dummy TR    \" set up dummy font at position 0
.sty \n[.fp] Bar  \" register style `Bar'
.ft 0             \" switch to font at position 0
.fam Foo          \" activate family `Foo'
.ft Bar           \" switch to font `FooBar'
@endExample

@xref{Font Positions}.
@endDefreq

@c ---------------------------------------------------------------------

@node Font Positions, Using Symbols, Font Families, Fonts
@subsection Font Positions
@cindex font positions
@cindex positions, font

For the sake of old phototypesetters and compatibility with old versions
of @code{troff}, @code{gtroff} has the concept of font @dfn{positions},
on which various fonts are mounted.

@DefreqList {fp, pos font [@Var{external-name}]}
@DefregItem {.f}
@DefregListEnd {.fp}
@cindex mounting font (@code{fp})
@cindex font, mounting (@code{fp})
Mount font @var{font} at position @var{pos} (which must be a
non-negative integer).  This numeric position can then be referred to
with font changing commands.  When @code{gtroff} starts it is using
font position@w{ }1 (which must exist; position@w{ }0 is unused
usually at start-up).

@cindex font position register (@code{.f})
The current font in use, as a font position, is available in the
read-only number register @samp{.f}.  This can be useful to remember the
current font for later recall.  It is associated with the current
environment (@pxref{Environments}).

@Example
.nr save-font \n[.f]
.ft B
... text text text ...
.ft \n[save-font]
@endExample

@cindex next free font position register (@code{.fp})
The number of the next free font position is available in the read-only
number register @samp{.fp}.  This is useful when mounting a new font,
like so:

@Example
.fp \n[.fp] NEATOFONT
@endExample

@pindex DESC@r{, and font mounting}
Fonts not listed in the @file{DESC} file are automatically mounted on
the next available font position when they are referenced.  If a font
is to be mounted explicitly with the @code{fp} request on an unused
font position, it should be mounted on the first unused font position,
which can be found in the @code{.fp} register.  Although @code{gtroff}
does not enforce this strictly, it is not allowed to mount a font at a
position whose number is much greater (approx.@: 1000 positions) than
that of any currently used position.

The @code{fp} request has an optional third argument.  This argument
gives the external name of the font, which is used for finding the font
description file.  The second argument gives the internal name of the
font which is used to refer to the font in @code{gtroff} after it has
been mounted.  If there is no third argument then the internal name is
used as the external name.  This feature makes it possible to use
fonts with long names in compatibility mode.
@endDefreq

Both the @code{ft} request and the @code{\f} escape have alternative
syntax forms to access font positions.

@DefreqList {ft, nnn}
@DefescItem {\\f, , n, }
@DefescItem {\\f, @lparen{}, nn, }
@DefescListEnd {\\f, @lbrack{}, nnn, @rbrack}
@cindex changing font position (@code{\f})
@cindex font position, changing (@code{\f})
@cindex @code{sty} request, and font positions
@cindex @code{fam} request, and font positions
@cindex @code{\F}, and font positions
@kindex styles
@kindex family
@pindex DESC
Change the current font position to @var{nnn} (one-digit position@w{
}@var{n}, two-digit position @var{nn}), which must be a non-negative
integer.

If @var{nnn} is associated with a style (as set with the @code{sty}
request or with the @code{styles} command in the @file{DESC} file), use
it within the current font family (as set with the @code{fam} request,
the @code{\F} escape, or with the @code{family} command in the @file{DESC}
file).

@Example
this is font 1
.ft 2
this is font 2
.ft                   \" switch back to font 1
.ft 3
this is font 3
.ft
this is font 1 again
@endExample

@xref{Changing Fonts}, for the standard syntax form.
@endDefreq

@c ---------------------------------------------------------------------

@node Using Symbols, Special Fonts, Font Positions, Fonts
@subsection Using Symbols
@cindex using symbols
@cindex symbols, using

@cindex glyph
@cindex character
@cindex ligature
A @dfn{glyph} is a graphical representation of a @dfn{character}.
While a character is an abstract entity containing semantic
information, a glyph is something which can be actually seen on screen
or paper.  It is possible that a character has multiple glyph
representation forms (for example, the character `A' can be either
written in a roman or an italic font, yielding two different glyphs);
sometimes more than one character maps to a single glyph (this is a
@dfn{ligature} -- the most common is `fi').

@cindex symbol
@cindex special fonts
@kindex fonts
@pindex DESC
A @dfn{symbol} is simply a named glyph.  Within @code{gtroff}, all
glyph names of a particular font are defined in its font file.  If the
user requests a glyph not available in this font, @code{gtroff} looks
up an ordered list of @dfn{special fonts}.  By default, the
@sc{PostScript} output device supports the two special fonts @samp{SS}
(slanted symbols) and @samp{S} (symbols) (the former is looked up
before the latter).  Other output devices use different names for
special fonts.  Fonts mounted with the @code{fonts} keyword in the
@file{DESC} file are globally available.  To install additional
special fonts locally (i.e.@: for a particular font), use the
@code{fspecial} request.

In summary, @code{gtroff} tries the following to find a given symbol:

@itemize @bullet
@item
If the symbol has been defined with the @code{char} request, use it.
This hides a symbol with the same name in the current font.

@item
Check the current font.

@item
If the symbol has been defined with the @code{fchar} request, use it.

@item
Check all fonts given with the @code{fspecial} request, in the order
of appearance in @code{fspecial} calls.

@item
Check all fonts given with the @code{special} request, in the order
of appearance in @code{special} calls (inclusively the special fonts
defined in the @file{DESC} file, which come first).

@item
As a last resort, consult all fonts loaded up to now (in the order they
have been called the first time) for special fonts and check them.
@end itemize

@xref{Font Files}, and @ref{Special Fonts}, for more details.

@DefescList {\\, @lparen{}, nm, }
@DefescListEnd {\\, @lbrack{}, name, @rbrack}
Insert a symbol @var{name} (two-character name @var{nm}).  There is no
special syntax for one-character names -- the natural form
@samp{\@var{n}} would collide with escapes.@footnote{Note that a
one-character symbol is not the same as an input character, i.e., the
character @code{a} is not the same as @code{\[a]}.  By default,
@code{groff} defines only a single one-character symbol, @code{\[-]};
it is usually accessed as @code{\-}.  On the other hand, @code{gtroff}
has the special feature that @code{\[char@var{XXX}]} is the same as the
input character with character code @var{XXX}.  For example,
@code{\[char97]} is identical to the letter @code{a} if @acronym{ASCII}
encoding is active.}

If @var{name} is undefined, a warning of type @samp{char} is generated,
and the escape is ignored.  @xref{Debugging}, for information about
warnings.

@cindex list of available glyphs (@cite{groff_char(7)} man page)
@cindex available glyphs, list (@cite{groff_char(7)} man page)
@cindex glyphs, available, list (@cite{groff_char(7)} man page)
The list of available symbols is device dependent; see the
@cite{groff_char(7)} man page for a complete list for the given output
device.  For example, say

@Example
man -Tdvi groff_char > groff_char.dvi
@endExample

@noindent
for a list using the default DVI fonts (not all versions of the
@code{man} program support the @option{-T} option).  If you want to
use an additional macro package to change the used fonts, @code{groff}
must be called directly:

@Example
groff -Tdvi -mec -man groff_char.7 > groff_char.dvi
@endExample

@c XXX list of common symbols
@endDefesc

@Defesc {\\C, ', xxx, '}
@cindex named character (@code{\C})
@cindex character, named (@code{\C})
Typeset the glyph named @var{xxx}.@footnote{@code{\C} is actually a
misnomer since it accesses an output glyph.}  Normally it is more
convenient to use @code{\[@var{xxx}]}, but @code{\C} has the advantage
that it is compatible with newer versions of @acronym{AT&T}
@code{troff} and is available in compatibility mode.
@endDefesc

@Defesc {\\N, ', n, '}
@cindex numbered glyph (@code{\N})
@cindex glyph, numbered (@code{\N})
@cindex @code{char} request, used with @code{\N}
@cindex Unicode
Typeset the glyph with code@w{ }@var{n} in the current font
(@code{n}@w{ }is @strong{not} the input character code).  The
number @var{n}@w{ }can be any non-negative integer.  Most devices
only have glyphs with codes between 0 and@w{ }255; the Unicode
output device uses codes in the range 0--65535.  If the current
font does not contain a glyph with that code, special fonts are
@emph{not} searched.  The @code{\N} escape sequence can be
conveniently used in conjunction with the @code{char} request:

@Example
.char \[phone] \f[ZD]\N'37'
@endExample

@noindent
@pindex DESC
@cindex unnamed glyphs
@cindex glyphs, unnamed
The code of each glyph is given in the fourth column in the font
description file after the @code{charset} command.  It is possible to
include unnamed glyphs in the font description file by using a
name of @samp{---}; the @code{\N} escape sequence is the only way to
use these.
@endDefesc

Some escape sequences directly map onto special glyphs.

@Defesc {\\', , , }
This is a backslash followed by the apostrophe character, @acronym{ASCII}
character @code{0x27} (@acronym{EBCDIC} character @code{0x7D}).  The same
as @code{\[aa]}, the acute accent.
@endDefesc

@Defesc {\\`, , , }
This is a backslash followed by @acronym{ASCII} character @code{0x60}
(@acronym{EBCDIC} character @code{0x79} usually).  The same as
@code{\[ga]}, the grave accent.
@endDefesc

@Defesc {\\-, , , }
This is the same as @code{\[-]}, the minus sign in the current font.
@endDefesc

@Defreq {cflags, n c1 c2 @dots{}}
@cindex glyph properties (@code{cflags})
@cindex character properties (@code{cflags})
@cindex properties of glyphs (@code{cflags})
@cindex properties of characters (@code{cflags})
Input characters and symbols have certain properties associated
with it.@footnote{Note that the output glyphs themselves don't have
such properties.  For @code{gtroff}, a glyph is a numbered box with
a given width, depth, and height, nothing else.  All manipulations
with the @code{cflags} request work on the input level.}  These
properties can be modified with the @code{cflags} request.  The
first argument is the sum of the desired flags and the remaining
arguments are the characters or symbols to have those properties.
It is possible to omit the spaces between the characters or symbols.

@table @code
@item 1
@cindex end-of-sentence characters
@cindex characters, end-of-sentence
The character ends sentences (initially characters @samp{.?!} have this
property).

@item 2
@cindex hyphenating characters
@cindex characters, hyphenation
Lines can be broken before the character (initially no characters have
this property).

@item 4
@cindex @code{hy} glyph, and @code{cflags}
@cindex @code{em} glyph, and @code{cflags}
Lines can be broken after the character (initially the character
@samp{-} and the symbols @samp{\(hy} and @samp{\(em} have this property).

@item 8
@cindex overlapping characters
@cindex characters, overlapping
@cindex @code{ul} glyph, and @code{cflags}
@cindex @code{rn} glyph, and @code{cflags}
@cindex @code{ru} glyph, and @code{cflags}
The character overlaps horizontally (initially the symbols
@samp{\(ul\(rn\(ru} have this property).

@item 16
@cindex @code{br} glyph, and @code{cflags}
The character overlaps vertically (initially symbol @samp{\(br} has
this property).

@item 32
@cindex transparent characters
@cindex character, transparent
@cindex @code{"}, at end of sentence
@cindex @code{'}, at end of sentence
@cindex @code{)}, at end of sentence
@cindex @code{]}, at end of sentence
@cindex @code{*}, at end of sentence
@cindex @code{dg} glyph, at end of sentence
@cindex @code{rq} glyph, at end of sentence
An end-of-sentence character followed by any number of characters with
this property is treated as the end of a sentence if followed by a
newline or two spaces; in other words the character is
@dfn{transparent} for the purposes of end-of-sentence recognition --
this is the same as having a zero space factor in @TeX{} (initially
characters @samp{"')]*} and the symbols @samp{\(dg\(rq} have this
property).
@end table
@endDefreq

@DefreqList {char, g [@Var{string}]}
@DefreqListEnd {fchar, g [@Var{string}]}
@cindex defining character (@code{char})
@cindex character, defining (@code{char})
@cindex creating new characters (@code{char})
@cindex defining symbol (@code{char})
@cindex symbol, defining (@code{char})
@cindex defining glyph (@code{char})
@cindex glyph, defining (@code{char})
@cindex escape character, while defining glyph
@cindex character, escape, while defining glyph
@cindex @code{tr} request, and glyph definitions
@cindex @code{cp} request, and glyph definitions
@cindex @code{rc} request, and glyph definitions
@cindex @code{lc} request, and glyph definitions
@cindex @code{\l}, and glyph definitions
@cindex @code{\L}, and glyph definitions
@cindex @code{\&}, and glyph definitions
@cindex @code{\e}, and glyph definitions
@cindex @code{hcode} request, and glyph definitions
Define a new glyph@w{ }@var{g} to be @var{string} (which can be
empty).@footnote{@code{char} is a misnomer since an output glyph is
defined.}  Every time glyph@w{ }@var{g} needs to be printed,
@var{string} is processed in a temporary environment and the result is
wrapped up into a single object.  Compatibility mode is turned off and
the escape character is set to @samp{\} while @var{string} is being
processed.  Any emboldening, constant spacing or track kerning is
applied to this object rather than to individual characters in
@var{string}.

A glyph defined by this request can be used just
like a normal glyph provided by the output device.  In particular,
other characters can be translated to it with the @code{tr} or
@code{trin} requests; it can be made the leader character by the
@code{lc} request; repeated patterns can be drawn with the glyph
using the @code{\l} and @code{\L} escape sequences; words containing
the glyph can be hyphenated correctly if the @code{hcode} request
is used to give the glyph's symbol a hyphenation code.

There is a special anti-recursion feature: Use of @code{g} within
the glyph's definition is handled like normal characters and symbols
not defined with @code{char}.

Note that the @code{tr} and @code{trin} requests take precedence if
@code{char} accesses the same symbol.

@Example
.tr XY
X
    @result{} Y
.char X Z
X
    @result{} Y
.tr XX
X
    @result{} Z
@endExample

The @code{fchar} request defines a fallback glyph:
@code{gtroff} only checks for glyphs defined with @code{fchar}
if it cannot find the glyph in the current font.
@code{gtroff} carries out this test before checking special fonts.
@endDefreq

@Defreq {rchar, c1 c2 @dots{}}
@cindex removing glyph definition (@code{rchar})
@cindex glyph, removing definition (@code{rchar})
Remove the definitions of glyphs @var{c1}, @var{c2},@w{
}@enddots{} This undoes the effect of a @code{char} or @code{fchar}
request.

It is possible to omit the whitespace between arguments.
@endDefreq

@xref{Special Characters}.

@c ---------------------------------------------------------------------

@node Special Fonts, Artificial Fonts, Using Symbols, Fonts
@subsection Special Fonts
@cindex special fonts
@cindex fonts, special

Special fonts are those that @code{gtroff} searches
when it cannot find the requested glyph in the current font.
The Symbol font is usually a special font.

@code{gtroff} provides the following two requests to add more special
fonts.  @xref{Using Symbols}, for a detailed description of the glyph
searching mechanism in @code{gtroff}.

Usually, only non-TTY devices have special fonts.

@DefreqList {special, s1 s2 @dots{}}
@DefreqListEnd {fspecial, f s1 s2 @dots{}}
@kindex fonts
@pindex DESC
Use the @code{special} request to define special fonts.  They are
appended to the list of global special fonts in the given order.
The first entries in this list are the fonts defined with the
@code{fonts} command in the @file{DESC} file which are marked as
special in the corresponding font description files.

Use the @code{fspecial} request to designate special fonts
only when font@w{ }@var{f} font is active.  They are appended to the
list of special fonts for @var{f} in the given order.  Initially, this
list is empty.
@endDefreq

@c ---------------------------------------------------------------------

@node Artificial Fonts, Ligatures and Kerning, Special Fonts, Fonts
@subsection Artificial Fonts
@cindex artificial fonts
@cindex fonts, artificial

There are a number of requests and escapes for artificially creating
fonts.  These are largely vestiges of the days when output devices
did not have a wide variety of fonts, and when @code{nroff} and
@code{troff} were separate programs.  Most of them are no longer
necessary in GNU @code{troff}.  Nevertheless, they are supported.

@DefescList {\\H, ', height, '}
@DefescItem {\\H, ', @t{+}@Var{height}, '}
@DefescListEnd {\\H, ', @t{-}@Var{height}, '}
@cindex changing the font height (@code{\H})
@cindex font height, changing (@code{\H})
@cindex height, font, changing (@code{\H})
Change (increment, decrement) the height of the current font, but not
the width.  If @var{height} is zero, restore the original height.
Default scaling indicator is @samp{z}.

Currently, only the @option{-Tps} device supports this feature.

Note that @code{\H} doesn't produce an input token in @code{gtroff}.
As a consequence, it can be used in requests like @code{mc} (which
expects a single character as an argument) to change the font on
the fly:

@Example
.mc \H'+5z'x\H'0'
@endExample

In compatibility mode, @code{gtroff} behaves differently:  If an
increment or decrement is used, it is always taken relative to the
current point size and not relative to the previously selected font
height.  Thus,

@Example
.cp 1
\H'+5'test \H'+5'test
@endExample

@noindent
prints the word @samp{test} twice with the same font height (five
points larger than the current font size).
@endDefesc

@DefescList {\\S, ', slant, '}
@cindex changing the font slant (@code{\S})
@cindex font slant, changing (@code{\S})
@cindex slant, font, changing (@code{\S})
Slant the current font by @var{slant} degrees.  Positive values slant
to the right.

Currently, only the @option{-Tps} device supports this feature.

Note that @code{\S} doesn't produce an input token in @code{gtroff}.
As a consequence, it can be used in requests like @code{mc} (which
expects a single character as an argument) to change the font on
the fly:

@Example
.mc \S'20'x\S'0'
@endExample

This request is incorrectly documented in the original @acronym{UNIX}
troff manual; the slant is always set to an absolute value.
@endDefesc

@Defreq {ul, [@Var{lines}]}
@cindex underlining (@code{ul})
The @code{ul} request normally underlines subsequent lines if a TTY
output device is used.  Otherwise, the lines are printed in italics
(only the term `underlined' is used in the following).  The single
argument is the number of input lines to be underlined; with no
argument, the next line is underlined.  If @var{lines} is zero or
negative, stop the effects of @code{ul} (if it was active).  Requests
and empty lines do not count for computing the number of underlined
input lines, even if they produce some output like @code{tl}.  Lines
inserted by macros (e.g.@: invoked by a trap) do count.

At the beginning of @code{ul}, the current font is stored and the
underline font is activated.  Within the span of a @code{ul} request,
it is possible to change fonts, but after the last line affected by
@code{ul} the saved font is restored.

This number of lines still to be underlined is associated with the
current environment (@pxref{Environments}).  The underline font can be
changed with the @code{uf} request.

@c XXX @xref should be changed to grotty

@c @xref{Troff and Nroff Mode}, for a discussion how underlining is
@c implemented in for TTY output devices, and which problems can arise.

The @code{ul} request does not underline spaces.
@endDefreq

@Defreq {cu, [@Var{lines}]}
@cindex continuous underlining (@code{cu})
@cindex underlining, continuous (@code{cu})
The @code{cu} request is similar to @code{ul} but underlines spaces as
well (if a TTY output device is used).
@endDefreq

@Defreq {uf, font}
@cindex underline font (@code{uf})
@cindex font for underlining (@code{uf})
Set the underline font (globally) used by @code{ul} and @code{cu}.  By
default, this is the font at position@w{ }2.  @var{font} can be either
a non-negative font position or the name of a font.
@endDefreq

@DefreqList {bd, font [@Var{offset}]}
@DefreqItem {bd, font1 font2 [@Var{offset}]}
@DefregListEnd {.b}
@cindex imitating bold face (@code{bd})
@cindex bold face, imitating (@code{bd})
Artificially create a bold font by printing each glyph twice,
slightly offset.

Two syntax forms are available.

@itemize @bullet
@item
Imitate a bold font unconditionally.  The first argument specifies the
font to embolden, and the second is the number of basic units, minus
one, by which the two glyphs are offset.  If the second argument is
missing, emboldening is turned off.

@var{font} can be either a non-negative font position or the name of a
font.

@var{offset} is available in the @code{.b} read-only register if a
special font is active; in the @code{bd} request, its default unit is
@samp{u}.

@cindex @code{fspecial} request, and imitating bold
@kindex special
@cindex embolding of special fonts
@cindex special fonts, emboldening
@item
Imitate a bold form conditionally.  Embolden @var{font1} by
@var{offset} only if font @var{font2} is the current font.  This
command can be issued repeatedly to set up different emboldening
values for different current fonts.  If the second argument is
missing, emboldening is turned off for this particular current font.

This affects special fonts only (either set up with the @code{special}
command in font files or with the @code{fspecial} request).
@end itemize
@endDefreq

@Defreq {cs, font [@Var{width} [@Var{em-size}]]}
@cindex constant glyph space mode (@code{cs})
@cindex mode for constant glyph space (@code{cs})
@cindex glyph, constant space
@cindex @code{ps} request, and constant glyph space mode
Switch to and from @dfn{constant glyph space mode}.  If activated, the
width of every glyph is @math{@var{width}/36} ems.  The em size is
given absolutely by @var{em-size}; if this argument is missing, the em
value is taken from the current font size (as set with the @code{ps}
request) when the font is effectively in use.  Without second and
third argument, constant glyph space mode is deactivated.

Default scaling indicator for @var{em-size} is @samp{z}; @var{width} is
an integer.
@endDefreq

@c ---------------------------------------------------------------------

@node Ligatures and Kerning,  , Artificial Fonts, Fonts
@subsection Ligatures and Kerning
@cindex ligatures and kerning
@cindex kerning and ligatures

Ligatures are groups of characters that are run together, i.e, producing
a single glyph.  For example, the letters `f' and `i' can form a
ligature `fi' as in the word `file'.  This produces a cleaner look
(albeit subtle) to the printed output.  Usually, ligatures are not
available in fonts for TTY output devices.

Most @sc{PostScript} fonts support the fi and fl ligatures.  The C/A/T
typesetter that was the target of @acronym{AT&T} @code{troff} also
supported `ff', `ffi', and `ffl' ligatures.  Advanced typesetters or
`expert' fonts may include ligatures for `ft' and `ct', although GNU
@code{troff} does not support these (yet).

@DefreqList {lg, [@Var{flag}]}
@DefregListEnd {.lg}
@cindex activating ligatures (@code{lg})
@cindex ligatures, activating (@code{lg})
@cindex ligatures enabled register (@code{.lg})
Switch the ligature mechanism on or off; if the parameter is non-zero
or missing, ligatures are enabled, otherwise disabled.  Default is on.
The current ligature mode can be found in the read-only number register
@code{.lg} (set to 1 or@w{ }2 if ligatures are enabled, 0@w{ }otherwise).

Setting the ligature mode to@w{ }2 enables the two-character ligatures
(fi, fl, and ff) and disables the three-character ligatures (ffi and
ffl).
@endDefreq

@dfn{Pairwise kerning} is another subtle typesetting mechanism that
modifies the distance between a glyph pair to improve readability.
In most cases (but not always) the distance is decreased.
@ifnotinfo
For example, compare the combination of the letters `V' and `A'.  With
kerning, `VA' is printed.  Without kerning it appears as `V@w{}A'.
@end ifnotinfo
Typewriter-like fonts and fonts for terminals where all glyphs
have the same width don't use kerning.

@DefreqList {kern, [@Var{flag}]}
@DefregListEnd {.kern}
@cindex activating kerning (@code{kern})
@cindex kerning, activating (@code{kern})
@cindex kerning enabled register (@code{.kern})
Switch kerning on or off.  If the parameter is non-zero or missing,
enable pairwise kerning, otherwise disable it.  The read-only number
register @code{.kern} is set to@w{ }1 if pairwise kerning is enabled,
0@w{ }otherwise.

@cindex zero width space character (@code{\&})
@cindex character, zero width space (@code{\&})
@cindex space character, zero width (@code{\&})
If the font description file contains pairwise kerning information,
glyphs from that font are kerned.  Kerning between two glyphs
can be inhibited by placing @code{\&} between them: @samp{V\&A}.

@xref{Font File Format}.
@endDefreq

@cindex track kerning
@cindex kerning, track
@dfn{Track kerning} expands or reduces the space between glyphs.
This can be handy, for example, if you need to squeeze a long word
onto a single line or spread some text to fill a narrow column.  It
must be used with great care since it is usually considered bad
typography if the reader notices the effect.

@Defreq {tkf, f s1 n1 s2 n2}
@cindex activating track kerning (@code{tkf})
@cindex track kerning, activating (@code{tkf})
Enable track kerning for font@w{ }@var{f}.  If the current font is@w{
}@var{f} the width of every glyph is increased by an amount
between @var{n1} and @var{n2} (@var{n1}, @var{n2} can be negative); if
the current point size is less than or equal to @var{s1} the width is
increased by @var{n1}; if it is greater than or equal to @var{s2} the
width is increased by @var{n2}; if the point size is greater than or
equal to @var{s1} and less than or equal to @var{s2} the increase in
width is a linear function of the point size.

The default scaling indicator is @samp{z} for @var{s1} and @var{s2},
@samp{p} for @var{n1} and @var{n2}.

Note that the track kerning amount is added even to the rightmost glyph
in a line; for large values it is thus recommended to increase the line
length by the same amount to compensate it.
@endDefreq

Sometimes, when typesetting letters of different fonts, more or less
space at such boundaries are needed.  There are two escapes to help
with this.

@Defesc {\\/, , , }
@cindex italic correction (@code{\/})
@cindex correction, italic (@code{\/})
@cindex correction between italic and roman glyph (@code{\/}, @code{\,})
@cindex roman glyph, correction after italic glyph (@code{\/})
@cindex italic glyph, correction before roman glyph (@code{\/})
@cindex glyph, italic correction (@code{\/})
Increase the width of the preceding glyph so that the spacing
between that glyph and the following glyph is correct if the
following glyph is a roman glyph.  For example, if an
italic@w{ }@code{f} is immediately followed by a roman right
parenthesis, then in many fonts the top right portion of the@w{ }@code{f}
overlaps the top left of the right parenthesis.  Use this escape
sequence whenever an italic glyph is immediately followed by a
roman glyph without any intervening space.  This small amount of
space is also called @dfn{italic correction}.

@iftex
@example
@group
\f[I]f\f[R])
    @result{} {@it f}@r{)}
\f[I]f\/\f[R])
    @result{} @i{f}@r{)}
@end group
@end example
@end iftex
@endDefesc

@Defesc {\\\,, , , }
@cindex left italic correction (@code{\,})
@cindex correction, left italic (@code{\,})
@cindex glyph, left italic correction (@code{\,})
@cindex roman glyph, correction before italic glyph (@code{\,})
@cindex italic glyph, correction after roman glyph (@code{\,})
Modify the spacing of the following glyph so that the spacing
between that glyph and the preceding glyph is correct if the
preceding glyph is a roman glyph.  Use this escape sequence
whenever a roman glyph is immediately followed by an italic
glyph without any intervening space.  In analogy to above, this
space could be called @dfn{left italic correction}, but this term
isn't used widely.

@iftex
@example
@group
q\f[I]f
    @result{} @r{q}@i{f}
q\,\f[I]f
    @result{} @r{q}@math{@ptexcomma}@i{f}
@end group
@end example
@end iftex
@endDefesc

@Defesc {\\&, , , }
Insert a zero-width character, which is invisible.  Its intended use
is to stop interaction of a character with its surrounding.

@itemize @bullet
@item
It prevents the insertion of extra space after an end-of-sentence
character.

@Example
Test.
Test.
    @result{} Test.  Test.
Test.\&
Test.
    @result{} Test. Test.
@endExample

@item
It prevents interpretation of a control character at the beginning of
an input line.

@Example
.Test
    @result{} warning: `Test' not defined
\&.Test
    @result{} .Test
@endExample

@item
It prevents kerning between two glyphs.

@ifnotinfo
@example
@group
VA
    @result{} @r{VA}
V\&A
    @result{} @r{V@w{}A}
@end group
@end example
@end ifnotinfo

@item
It is needed to map an arbitrary character to nothing in the @code{tr}
request (@pxref{Character Translations}).
@end itemize
@endDefesc

@Defesc {\\), , , }
This escape is similar to @code{\&} except that it behaves like a
character declared with the @code{cflags} request to be transparent
for the purposes of an end-of-sentence character.

Its main usage is in macro definitions to protect against arguments
starting with a control character.

@Example
.de xxx
\)\\$1
..
.de yyy
\&\\$1
..
This is a test.\c
.xxx '
This is a test.
    @result{}This is a test.'  This is a test.
This is a test.\c
.yyy '
This is a test.
    @result{}This is a test.' This is a test.
@endExample
@endDefesc


@c =====================================================================

@node Sizes, Strings, Fonts, gtroff Reference
@section Sizes
@cindex sizes

@cindex baseline
@cindex type size
@cindex size of type
@cindex vertical spacing
@cindex spacing, vertical
@code{gtroff} uses two dimensions with each line of text, type size
and vertical spacing.  The @dfn{type size} is approximately the height
of the tallest glyph.@footnote{This is usually the parenthesis.
Note that in most cases the real dimensions of the glyphs in a font
are @emph{not} related to its type size!  For example, the standard
@sc{PostScript} font families `Times Roman', `Helvetica', and
`Courier' can't be used together at 10@dmn{pt}; to get acceptable
output, the size of `Helvetica' has to be reduced by one point, and
the size of `Courier' must be increased by one point.}  @dfn{Vertical
spacing} is the amount of space @code{gtroff} allows for a line of
text; normally, this is about 20%@w{ }larger than the current type
size.  Ratios smaller than this can result in hard-to-read text;
larger than this, it spreads the text out more vertically (useful for
term papers).  By default, @code{gtroff} uses 10@w{ }point type on
12@w{ }point spacing.

@cindex leading
The difference between type size and vertical spacing is known, by
typesetters, as @dfn{leading} (this is pronounced `ledding').

@menu
* Changing Type Sizes::
* Fractional Type Sizes::
@end menu

@c ---------------------------------------------------------------------

@node Changing Type Sizes, Fractional Type Sizes, Sizes, Sizes
@subsection Changing Type Sizes

@DefreqList {ps, [@Var{size}]}
@DefreqItem {ps, @t{+}@Var{size}}
@DefreqItem {ps, @t{-}@Var{size}}
@DefescItem {\\s, , size, }
@DefregListEnd {.s}
@cindex changing type sizes (@code{ps}, @code{\s})
@cindex type sizes, changing (@code{ps}, @code{\s})
@cindex point sizes, changing (@code{ps}, @code{\s})
Use the @code{ps} request or the @code{\s} escape to change (increase,
decrease) the type size (in points).  Specify @var{size} as either an
absolute point size, or as a relative change from the current size.
The size@w{ }0, or no argument, goes back to the previous size.

Default scaling indicator of @code{size} is @samp{z}.  If @code{size}
is zero or negative, it is set to 1@dmn{u}.

@cindex type size registers (@code{.s}, @code{.ps})
@cindex point size registers (@code{.s}, @code{.ps})
The read-only number register @code{.s} returns the point size in
points as a decimal fraction.  This is a string.  To get the point
size in scaled points, use the @code{.ps} register instead.

@code{.s} is associated with the current environment
(@pxref{Environments}).

@Example
snap, snap,
.ps +2
grin, grin,
.ps +2
wink, wink, \s+2nudge, nudge,\s+8 say no more!
.ps 10
@endExample

The @code{\s} escape may be called in a variety of ways.  Much like
other escapes there must be a way to determine where the argument ends
and the text begins.  Any of the following forms are valid:

@table @code
@item \s@var{n}
Set the point size to @var{n}@w{ }points.  @var{n}@w{ }must be either
0 or in the range 4 to@w{ }39.

@item \s+@var{n}
@itemx \s-@var{n}
Increase or decrease the point size by @var{n}@w{ }points.  @var{n}@w{
}must be exactly one digit.

@item \s(@var{nn}
Set the point size to @var{nn}@w{ }points.  @var{nn} must be exactly
two digits.

@item \s+(@var{nn}
@itemx \s-(@var{nn}
@itemx \s(+@var{nn}
@itemx \s(-@var{nn}
Increase or decrease the point size by @var{nn}@w{ }points.  @var{nn}
must be exactly two digits.
@end table

Note that @code{\s} doesn't produce an input token in @code{gtroff}.
As a consequence, it can be used in requests like @code{mc} (which
expects a single character as an argument) to change the font on
the fly:

@Example
.mc \s[20]x\s[0]
@endExample

@xref{Fractional Type Sizes}, for yet another syntactical form of
using the @code{\s} escape.
@endDefreq

@Defreq {sizes, s1 s2 @dots{} sn [0]}
Some devices may only have certain permissible sizes, in which case
@code{gtroff} rounds to the nearest permissible size.
The @file{DESC} file specifies which sizes are permissible for the device.

Use the @code{sizes} request to change the permissible sizes
for the current output device.
Arguments are in scaled points;
the @code{sizescale} line in the
@file{DESC} file for the output device
provides the scaling factor.
For example, if the scaling factor is 1000,
then the value 12000 is 12@w{ }points.

Each argument can be a single point size (such as @samp{12000}),
or a range of sizes (such as @samp{4000-72000}).
You can optionally end the list with a zero.
@endDefreq

@DefreqList {vs, [@Var{space}]}
@DefreqItem {vs, @t{+}@Var{space}}
@DefreqItem {vs, @t{-}@Var{space}}
@DefregListEnd {.v}
@cindex changing vertical line spacing (@code{vs})
@cindex vertical line spacing, changing (@code{vs})
@cindex vertical line spacing register (@code{.v})
Change (increase, decrease) the vertical spacing by @var{space}.  The
default scaling indicator is @samp{p}.

If @code{vs} is called without an argument, the vertical spacing is
reset to the previous value before the last call to @code{vs}.

@cindex @code{.V} register, and @code{vs}
@code{gtroff} creates a warning of type @samp{range} if @var{space} is
zero or negative; the vertical spacing is then set to the vertical
resolution (as given in the @code{.V} register).

The read-only number register @code{.v} contains the current vertical
spacing; it is associated with the current environment
(@pxref{Environments}).
@endDefreq

@cindex vertical line spacing, effective value
The effective vertical line spacing consists of four components.

@itemize @bullet
@item
The vertical line spacing as set with the @code{vs} request.

@cindex post-vertical line spacing
@cindex line spacing, post-vertical (@code{pvs})
@item
The @dfn{post-vertical line spacing} as set with the @code{pvs} request.
This is vertical space which will be added after a line has been
output.

@cindex extra pre-vertical line space (@code{\x})
@cindex line space, extra pre-vertical (@code{\x})
@item
The @dfn{extra pre-vertical line space} as set with the @code{\x} request,
using a negative value.  This is vertical space which will be added once
before the current line has been output.

@cindex extra post-vertical line space (@code{\x})
@cindex line space, extra post-vertical (@code{\x})
@item
The @dfn{extra post-vertical line space} as set with the @code{\x} request,
using a positive value.  This is vertical space which will be added once
after the current line has been output.
@end itemize

@cindex double-spacing (@code{vs}, @code{pvs})
It is usually better to use @code{vs} or @code{pvs} instead of @code{ls}
to produce double-spaced documents: @code{vs} and @code{pvs} have a finer
granularity for the inserted vertical space compared to @code{ls};
furthermore, certain preprocessors assume single-spacing.

@xref{Manipulating Spacing}, for more details on the @code{\x} escape
and the @code{ls} request.

@DefreqList {pvs, [@Var{space}]}
@DefreqItem {pvs, @t{+}@Var{space}}
@DefreqItem {pvs, @t{-}@Var{space}}
@DefregListEnd {.pvs}
@cindex @code{ls} request, alternative to (@code{pvs})
@cindex post-vertical line spacing, changing (@code{pvs})
@cindex post-vertical line spacing register (@code{.pvs})
Change (increase, decrease) the post-vertical spacing by
@var{space}.  The default scaling indicator is @samp{p}.

If @code{pvs} is called without an argument, the post-vertical spacing is
reset to the previous value before the last call to @code{pvs}.

@code{gtroff} creates a warning of type @samp{range} if @var{space} is
zero or negative; the vertical spacing is then set to zero.

The read-only number register @code{.pvs} contains the current
post-vertical spacing; it is associated with the current environment
(@pxref{Environments}).
@endDefreq


@c ---------------------------------------------------------------------

@node Fractional Type Sizes,  , Changing Type Sizes, Sizes
@subsection Fractional Type Sizes
@cindex fractional type sizes
@cindex fractional point sizes
@cindex type sizes, fractional
@cindex point sizes, fractional
@cindex sizes, fractional

@cindex @code{s} unit
@cindex unit, @code{s}
@cindex @code{z} unit
@cindex unit, @code{z}
@cindex @code{ps} request, with fractional type sizes
@cindex @code{cs} request, with fractional type sizes
@cindex @code{tkf} request, with fractional type sizes
@cindex @code{\H}, with fractional type sizes
@cindex @code{\s}, with fractional type sizes
A @dfn{scaled point} is equal to @math{1/@var{sizescale}} points,
where @var{sizescale} is specified in the @file{DESC} file (1@w{ }by
default).  There is a new scale indicator @samp{z} which has the
effect of multiplying by @var{sizescale}.  Requests and escape
sequences in @code{gtroff} interpret arguments that represent a point
size as being in units of scaled points, but they evaluate each such
argument using a default scale indicator of @samp{z}.  Arguments
treated in this way are the argument to the @code{ps} request, the
third argument to the @code{cs} request, the second and fourth
arguments to the @code{tkf} request, the argument to the @code{\H}
escape sequence, and those variants of the @code{\s} escape sequence
that take a numeric expression as their argument (see below).

For example, suppose @var{sizescale} is@w{ }1000; then a scaled point
is equivalent to a millipoint; the request @w{@samp{.ps 10.25}} is
equivalent to @w{@samp{.ps 10.25z}} and thus sets the point size to
10250@w{ }scaled points, which is equal to 10.25@w{ }points.

@code{gtroff} disallows the use of the @samp{z} scale indicator in
instances where it would make no sense, such as a numeric
expression whose default scale indicator was neither @samp{u} nor
@samp{z}.  Similarly it would make
no sense to use a scaling indicator other than @samp{z} or @samp{u} in a
numeric expression whose default scale indicator was @samp{z}, and so
@code{gtroff} disallows this as well.

There is also new scale indicator @samp{s} which multiplies by the
number of units in a scaled point.  So, for example, @samp{\n[.ps]s} is
equal to @samp{1m}.  Be sure not to confuse the @samp{s} and @samp{z}
scale indicators.

@Defreg {.ps}
A read-only number register returning the point size in scaled points.

@code{.ps} is associated with the current environment
(@pxref{Environments}).
@endDefreg

@DefregList {.psr}
@DefregListEnd {.sr}
@cindex last-requested point size registers (@code{.psr}, @code{.sr})
@cindex point size registers, last-requested (@code{.psr}, @code{.sr})
@cindex @code{.ps} register, in comparison with @code{.psr}
@cindex @code{.s} register, in comparison with @code{.sr}
The last-requested point size in scaled points is contained in the
@code{.psr} read-only number register.  The last requested point size
in points as a decimal fraction can be found in @code{.sr}.  This is a
string-valued read-only number register.

Note that the requested point sizes are device-independent, whereas
the values returned by the @code{.ps} and @code{.s} registers are not.
For example, if a point size of 11@dmn{pt} is requested, and a
@code{sizes} request (or a @code{sizescale} line in a @file{DESC} file)
specifies 10.95@dmn{pt} instead, this value is actually used.

Both registers are associated with the current environment
(@pxref{Environments}).
@endDefreg

The @code{\s} escape has the following syntax for working with
fractional type sizes:

@table @code
@item \s[@var{n}]
@itemx \s'@var{n}'
Set the point size to @var{n}@w{ }scaled points; @var{n}@w{ }is a numeric
expression with a default scale indicator of @samp{z}.

@item \s[+@var{n}]
@itemx \s[-@var{n}]
@itemx \s+[@var{n}]
@itemx \s-[@var{n}]
@itemx \s'+@var{n}'
@itemx \s'-@var{n}'
@itemx \s+'@var{n}'
@itemx \s-'@var{n}'
Increase or or decrease the point size by @var{n}@w{ }scaled points;
@var{n}@w{ }is a numeric expression with a default scale indicator of
@samp{z}.
@end table

@xref{Font Files}.


@c =====================================================================

@node Strings, Conditionals and Loops, Sizes, gtroff Reference
@section Strings
@cindex strings

@code{gtroff} has string variables, which are entirely for user
convenience (i.e.@: there are no built-in strings exept @code{.T}, but
even this is a read-write string variable).

@DefreqList {ds, name [@Var{string}]}
@DefreqItem {ds1, name [@Var{string}]}
@DefescItem {\\*, , n, }
@DefescItem {\\*, @lparen{}, nm, }
@DefescListEnd {\\*, @lbrack{}, name, @rbrack{}}
@cindex string interpolation (@code{\*})
@cindex string expansion (@code{\*})
@cindex interpolation of strings (@code{\*})
@cindex expansion of strings (@code{\*})
Define and access a string variable @var{name} (one-character name@w{
}@var{n}, two-character name @var{nm}).  If @var{name} already exists,
@code{ds} overwrites the previous definition.

Example:

@Example
.ds UX \s-1UNIX\s0\u\s-3tm\s0\d
.
The \*(UX Operating System
@endExample

The @code{\*} escape @dfn{interpolates} (expands in-place) a
previously-defined string variable.  To be more precise, the stored
string is pushed onto the input stack which is then parsed by
@code{gtroff}.  Similar to number registers, it is possible to nest
strings, i.e. string variables can be called within string variables.

If the string named by the @code{\*} escape does not exist, it is
defined as empty, and a warning of type @samp{mac} is emitted (see
@ref{Debugging}, for more details).

@cindex comments, with @code{ds}
@cindex @code{ds} request, and comments
@strong{Caution:} Unlike other requests, the second argument to the
@code{ds} request takes up the entire line including trailing spaces.
This means that comments on a line with such a request can introduce
unwanted space into a string.

@Example
.ds UX \s-1UNIX\s0\u\s-3tm\s0\d \" UNIX trademark
@endExample

@noindent
Instead the comment should be put on another line or have the comment
escape adjacent with the end of the string.

@Example
.ds UX \s-1UNIX\s0\u\s-3tm\s0\d\"  UNIX trademark
@endExample

@cindex trailing quotes
@cindex quotes, trailing
@cindex leading spaces with @code{ds}
@cindex spaces with @code{ds}
@cindex @code{ds} request, and leading spaces
To produce leading space the string can be started with a double
quote.  No trailing quote is needed; in fact, any trailing quote is
included in your string.

@Example
.ds sign "           Yours in a white wine sauce,
@endExample

@cindex multi-line strings
@cindex strings, multi-line
@cindex newline character, in strings, escaping
@cindex escaping newline characters, in strings
Strings are not limited to a single line of text.  A string can span
several lines by escaping the newlines with a backslash.  The
resulting string is stored @emph{without} the newlines.

@Example
.ds foo lots and lots \
of text are on these \
next several lines
@endExample

It is not possible to have real newlines in a string.  To put a single
double quote character into a string, use two consecutive double quote
characters.

The @code{ds1} request turns off compatibility mode
while interpreting a string.  To be more precise, a @dfn{compatibility
save} input token is inserted at the beginning of  the string, and a
@dfn{compatibility restore} input token at the end.

@Example
.nr xxx 12345
.ds aa The value of xxx is \\n[xxx].
.ds1 bb The value of xxx ix \\n[xxx].
.
.cp 1
.
\*(aa
    @result{} warning: number register `[' not defined
    @result{} The value of xxx is 0xxx].
\*(bb
    @result{} The value of xxx ix 12345.
@endExample

@cindex name space, common, of macros, diversions, and strings
@cindex common name space of macros, diversions, and strings
@cindex macros, shared name space with strings and diversions
@cindex strings, shared name space with macros and diversions
@cindex diversions, shared name space with macros and strings
Strings, macros, and diversions (and boxes) share the same name space.
Internally, even the same mechanism is used to store them.  This has
some interesting consequences.  For example, it is possible to call a
macro with string syntax and vice versa.

@Example
.de xxx
a funny test.
..
This is \*[xxx]
    @result{} This is a funny test.

.ds yyy a funny test
This is
.yyy
    @result{} This is a funny test.
@endExample

Diversions and boxes can be also called with string syntax.  It is not
possible to pass arguments to a macro if called with @code{\*}.

Another consequence is that you can copy one-line diversions or boxes
to a string.

@Example
.di xxx
a \fItest\fR
.br
.di
.ds yyy This is \*[xxx]\c
\*[yyy].
    @result{} @r{This is a }@i{test}.
@endExample

@noindent
As the previous example shows, it is possible to store formatted
output in strings.  The @code{\c} escape prevents the insertion of an
additional blank line in the output.

Copying diversions longer than a single output line produces
unexpected results.

@Example
.di xxx
a funny
.br
test
.br
.di
.ds yyy This is \*[xxx]\c
\*[yyy].
    @result{} test This is a funny.
@endExample

Usually, it is not predictable whether a diversion contains one or
more output lines, so this mechanism should be avoided.  With
@acronym{UNIX} @code{troff}, this was the only solution to strip off a
final newline from a diversion.  Another disadvantage is that the
spaces in the copied string are already formatted, making them
unstretchable.  This can cause ugly results.

@cindex stripping final newline in diversions
@cindex diversion, stripping final newline
@cindex final newline, stripping in diversions
@cindex newline, final, stripping in diversions
@cindex horizontal space, unformatting
@cindex space, horizontal, unformatting
@cindex unformatting horizontal space
A clean solution to this problem is available in GNU @code{troff},
using the requests @code{chop} to remove the final newline of a
diversion, and @code{unformat} to make the horizontal spaces
stretchable again.

@Example
.box xxx
a funny
.br
test
.br
.box
.chop xxx
.unformat xxx
This is \*[xxx].
    @result{} This is a funny test.
@endExample

@xref{Gtroff Internals}, for more information.
@endDefreq

@DefreqList {as, name [@Var{string}]}
@DefreqListEnd {as1, name [@Var{string}]}
@cindex appending to a string (@code{as})
@cindex string, appending (@code{as})
The @code{as} request is similar to @code{ds} but appends @var{string}
to the string stored as @var{name} instead of redefining it.  If
@var{name} doesn't exist yet, it is created.

@Example
.as sign " with shallots, onions and garlic,
@endExample

The @code{as1} request is similar to @code{as}, but compatibility mode
is switched off while the appended string is interpreted.  To be more
precise, a @dfn{compatibility save} input token is inserted at the
beginning of the appended string, and a @dfn{compatibility restore}
input token at the end.
@endDefreq

Rudimentary string manipulation routines are given with the next two
requests.

@Defreq {substring, str n1 [@Var{n2}]}
@cindex substring (@code{substring})
Replace the string named @var{str} with the substring
defined by the indices @var{n1} and@w{ }@var{n2}.  The first character
in the string has index one.  If @var{n2} is omitted, it is taken to
be equal to the string's length.  If the index value @var{n1} or
@var{n2} is negative or zero, it is counted from the end of the
string, going backwards: The last character has index@w{ }0, the
character before the last character has index@w{ }@minus{}1, etc.

@Example
.ds xxx abcdefgh
.substring xxx 2 -3
\*[xxx]
    @result{} bcde
@endExample
@endDefreq

@Defreq {length, reg str}
@cindex length of a string (@code{length})
@cindex string, length of (@code{length})
Compute the number of characters of @var{str} and return it in the
number register @var{reg}.  If @var{reg} doesn't exist, it is created.
@code{str} is read in copy mode.

@Example
.ds xxx abcd\h'3i'efgh
.length yyy \n[xxx]
\n[yyy]
    @result{} 14
@endExample
@endDefreq

@Defreq {rn, xx yy}
@cindex renaming request (@code{rn})
@cindex request, renaming (@code{rn})
@cindex renaming macro (@code{rn})
@cindex macro, renaming (@code{rn})
@cindex renaming string (@code{rn})
@cindex string, renaming (@code{rn})
@cindex renaming diversion (@code{rn})
@cindex diversion, renaming (@code{rn})
Rename the request, macro, diversion, or string @var{xx} to @var{yy}.
@endDefreq

@Defreq {rm, xx}
@cindex removing request (@code{rm})
@cindex request, removing (@code{rm})
@cindex removing macro (@code{rm})
@cindex macro, removing (@code{rm})
@cindex removing string (@code{rm})
@cindex string, removing (@code{rm})
@cindex removing diversion (@code{rm})
@cindex diversion, removing (@code{rm})
Remove the request, macro, diversion, or string @var{xx}.  @code{gtroff}
treats subsequent invocations as if the object had never been defined.
@endDefreq

@Defreq {als, new old}
@cindex alias, string, creating (@code{als})
@cindex alias, macro, creating (@code{als})
@cindex alias, diversion, creating (@code{als})
@cindex creating alias, for string (@code{als})
@cindex creating alias, for macro (@code{als})
@cindex creating alias, for diversion (@code{als})
@cindex string, creating alias (@code{als})
@cindex macro, creating alias (@code{als})
@cindex diversion, creating alias (@code{als})
Create an alias named @var{new} for the request, string, macro, or
diversion object named @var{old}.  The new name and the old name are
exactly equivalent (it is similar to a hard rather than a soft
link). If @var{old} is undefined, @code{gtroff} generates a warning of
type @samp{mac} and ignores the request.
@endDefreq

@Defreq {chop, xx}
Remove (chop) the last character from the macro, string, or diversion
named @var{xx}. This is useful for removing the newline from the end
of diversions that are to be interpolated as strings.  This command
can be used repeatedly; see @ref{Gtroff Internals}, for details on
nodes inserted additionally by @code{gtroff}.
@endDefreq

@xref{Identifiers}, and @ref{Comments}.


@c =====================================================================

@node Conditionals and Loops, Writing Macros, Strings, gtroff Reference
@section Conditionals and Loops
@cindex conditionals and loops
@cindex loops and conditionals

@menu
* Operators in Conditionals::
* if-else::
* while::
@end menu

@c ---------------------------------------------------------------------

@node Operators in Conditionals, if-else, Conditionals and Loops, Conditionals and Loops
@subsection Operators in Conditionals

@cindex @code{if} request, operators to use with
@cindex @code{while} request, operators to use with
In @code{if} and @code{while} requests, there are several more
operators available:

@table @code
@item e
@itemx o
True if the current page is even or odd numbered (respectively).

@item n
True if the document is being processed in nroff mode (i.e., the
@code{.nroff} command has been issued).

@item t
True if the document is being processed in troff mode (i.e., the
@code{.troff} command has been issued).

@item v
Always false.  This condition is for compatibility with other
@code{troff} versions only.

@item '@var{xxx}'@var{yyy}'
True if the string @var{xxx} is equal to the string @var{yyy}.  Other
characters can be used in place of the single quotes; the same set of
delimiters as for the @code{\D} escape is used (@pxref{Escapes}).
@code{gtroff} formats the strings before being compared:

@Example
.ie "|"\fR|\fP" \
true
.el \
false
    @result{} true
@endExample

@noindent
The resulting motions, glyph sizes, and fonts have to
match,@footnote{The created output nodes must be identical.
@xref{Gtroff Internals}.} and not the individual motion, size, and
font requests.  In the previous example, @samp{|} and @samp{\fR|\fP}
both result in a roman @samp{|} glyph with the same point size and
at the same location on the page, so the strings are equal.  If
@samp{.ft@w{ }I} had been added before the @samp{.ie}, the result
would be ``false'' because (the first) @samp{|} produces an italic
@samp{|} rather than a roman one.

@item r @var{xxx}
True if there is a number register named @var{xxx}.

@item d @var{xxx}
True if there is a string, macro, diversion, or request named @var{xxx}.

@item m @var{xxx}
True if there is a color named @var{xxx}.

@item c @var{g}
True if there is a glyph @var{g} available@footnote{The name of this
conditional operator is a misnomer since it tests names of output
glyphs.}; @var{g} is either an @acronym{ASCII} character or a special
character (@code{\(@var{gg}} or @code{\[@var{ggg}]}); the condition
is also true if @var{g} has been defined by the @code{char} request.
@end table

Note that these operators can't be combined with other operators like
@samp{:} or @samp{&}; only a leading @samp{!} (without whitespace
between the exclamation mark and the operator) can be used to negate
the result.

@Example
.nr xxx 1
.ie !r xxx \
true
.el \
false
    @result{} false
@endExample

A whitespace after @samp{!} always evaluates to zero (this bizarre
behaviour is due to compatibility with @acronym{UNIX} @code{troff}).

@Example
.nr xxx 1
.ie ! r xxx \
true
.el \
false
    @result{} r xxx true
@endExample

It is possible to omit the whitespace before the argument to the
@samp{r}, @samp{d}, and @samp{c} operators.

@xref{Expressions}.

@c ---------------------------------------------------------------------

@node if-else, while, Operators in Conditionals, Conditionals and Loops
@subsection if-else
@cindex if-else

@code{gtroff} has if-then-else constructs like other languages, although
the formatting can be painful.

@Defreq {if, expr anything}
Evaluate the expression @var{expr}, and executes @var{anything} (the
remainder of the line) if @var{expr} evaluates to non-zero (true).
@var{anything} is interpreted as though it was on a line by itself
(except that leading spaces are swallowed).  @xref{Expressions}, for
more info.

@Example
.nr xxx 1
.nr yyy 2
.if ((\n[xxx] == 1) & (\n[yyy] == 2)) true
    @result{} true
@endExample
@endDefreq

@Defreq{nop, anything}
Executes @var{anything}.
This is similar to @code{.if@w{ }1}.
@endDefreq

@DefreqList {ie, expr anything}
@DefreqListEnd {el, anything}
Use the @code{ie} and @code{el} requests to write an if-then-else.
The first request is the `if' part and the latter is the `else' part.

@Example
.ie n .ls 2 \" double-spacing in nroff
.el   .ls 1 \" single-spacing in troff
@endExample
@endDefreq

@c there is a bug in makeinfo <= 4.1a: you can't have `@{' as an argument
@c to @deffn
@c
@c and in 4.2 you still can't use @{ in macros.

@c @DefescList {\@{, , , }
@c @DefescListEnd {\@}, , , }
@deffn Escape @t{\@{}
@deffnx Escape @t{\@}}
@esindex \@{
@esindex \@}
@cindex begin of conditional block (@code{\@{})
@cindex end of conditional block (@code{\@}})
@cindex conditional block, begin (@code{\@{})
@cindex conditional block, end (@code{\@}})
@cindex block, conditional, begin (@code{\@{})
@cindex block, condititional, end (@code{\@}})
In many cases, an if (or if-else) construct needs to execute more than
one request.  This can be done using the @code{\@{} and @code{\@}}
escapes.  The following example shows the possible ways to use these
escapes (note the position of the opening and closing braces).

@Example
.ie t \@{\
.    ds lq ``
.    ds rq ''
.\@}
.el \
.\@{\
.    ds lq "
.    ds rq "\@}
@endExample
@c @endDefesc
@end deffn

@xref{Expressions}.

@c ---------------------------------------------------------------------

@node while,  , if-else, Conditionals and Loops
@subsection while
@cindex while

@code{gtroff} provides a looping construct using the @code{while}
request, which is used much like the @code{if} (and related) requests.

@Defreq {while, expr anything}
Evaluate the expression @var{expr}, and repeatedly execute
@var{anything} (the remainder of the line) until @var{expr} evaluates
to@w{ }0.

@Example
.nr a 0 1
.while (\na < 9) \@{\
\n+a,
.\@}
\n+a
    @result{} 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
@endExample

Some remarks.

@cindex @code{de} request, and @code{while}
@itemize @bullet
@item
The body of a @code{while} request is treated like the body of a
@code{de} request: @code{gtroff} temporarily stores it in a macro
which is deleted after the loop has been exited.  It can considerably
slow down a macro if the body of the @code{while} request (within the
macro) is large.  Each time the macro is executed, the @code{while}
body is parsed and stored again as a temporary macro.

@Example
.de xxx
.  nr num 10
.  while (\\n[num] > 0) \@{\
.    \" many lines of code
.    nr num -1
.  \@}
..
@endExample

@cindex recursive macros
@cindex macros, recursive
@noindent
The traditional and ofter better solution (@acronym{UNIX} @code{troff}
doesn't have the @code{while} request) is to use a recursive macro
instead which is parsed only once during its definition.

@Example
.de yyy
.  if (\\n[num] > 0) \@{\
.    \" many lines of code
.    nr num -1
.    yyy
.  \@}
..
.
.de xxx
.  nr num 10
.  yyy
..
@endExample

@noindent
Note that the number of available recursion levels is set to@w{ }1000
(this is a compile-time constant value of @code{gtroff}).

@item
The closing brace of a @code{while} body must end a line.

@Example
.if 1 \@{\
.  nr a 0 1
.  while (\n[a] < 10) \@{\
.    nop \n+[a]
.\@}\@}
    @result{} unbalanced \@{ \@}
@endExample
@end itemize
@endDefreq

@Defreq {break, }
@cindex @code{while} request, confusing with @code{br}
@cindex @code{break} request, in a @code{while} loop
@cindex @code{continue} request, in a @code{while} loop
Break out of a @code{while} loop.  Be sure not to confuse this with
the @code{br} request (causing a line break).
@endDefreq

@Defreq {continue, }
Finish the current iteration of a @code{while} loop, immediately
restarting the next iteration.
@endDefreq

@xref{Expressions}.


@c =====================================================================

@node Writing Macros, Page Motions, Conditionals and Loops, gtroff Reference
@section Writing Macros
@cindex writing macros
@cindex macros, writing

A @dfn{macro} is a collection of text and embedded commands which can
be invoked multiple times.  Use macros to define common operations.

@DefreqList {de, name [@Var{end}]}
@DefreqItem {de1, name [@Var{end}]}
@DefreqListEnd {dei, name [@Var{end}]}
Define a new macro named @var{name}.  @code{gtroff} copies subsequent
lines (starting with the next one) into an internal buffer until it
encounters the line @samp{..} (two dots).  The optional second
argument to @code{de} changes this to a macro to @samp{.@var{end}}.

There can be whitespace after the first dot in the line containing the
ending token (either @samp{.} or macro @samp{@var{end}}).

Here a small example macro called @samp{P} which causes a break and
inserts some vertical space.  It could be used to separate paragraphs.

@Example
.de P
.  br
.  sp .8v
..
@endExample

The following example defines a macro within another.  Remember that
expansion must be protected twice; once for reading the macro and
once for executing.

@Example
\# a dummy macro to avoid a warning
.de end
..
.
.de foo
.  de bar end
.    nop \f[B]Hallo \\\\$1!\f[]
.  end
..
.
.foo
.bar Joe
    @result{} @b{Hallo Joe!}
@endExample

@noindent
Since @code{\f} has no expansion, it isn't necessary to protect its
backslash.  Had we defined another macro within @code{bar} which takes
a parameter, eight backslashes would be necessary before @samp{$1}.

The @code{de1} request turns off compatibility mode
while executing the macro.  On entry, the current compatibility mode
is saved and restored at exit.

@Example
.nr xxx 12345
.
.de aa
The value of xxx is \\n[xxx].
..
.de1 bb
The value of xxx ix \\n[xxx].
..
.
.cp 1
.
.aa
    @result{} warning: number register \e' not defined
    @result{} The value of xxx is 0xxx].
.bb
    @result{} The value of xxx ix 12345.
@endExample

The @code{dei} request defines a macro indirectly.
That is, it expands strings whose names
are @var{name} or @var{end} before performing the append.

This:

@Example
.ds xx aa
.ds yy bb
.dei xx yy
@endExample

@noindent
is equivalent to:

@Example
.de aa bb
@endExample

@pindex trace.tmac
Using @file{trace.tmac}, you can trace calls to @code{de} and @code{de1}.

Note that macro identifiers are shared with identifiers for strings and
diversions.
@endDefreq

@DefreqList {am, xx}
@DefreqItem {am1, xx}
@DefreqListEnd {ami, xx yy}
@cindex appending to a macro (@code{am})
@cindex macro, appending (@code{am})
Works similarly to @code{de} except it appends onto the macro named
@var{xx}.  So, to make the previously defined @samp{P} macro actually
do indented instead of block paragraphs, add the necessary code to the
existing macro like this:

@Example
.am P
.ti +5n
..
@endExample

The @code{am1} request turns off compatibility mode
while executing the appended macro piece.  To be more precise, a
@dfn{compatibility save} input token is inserted at the beginning of
the appended code, and a @dfn{compatibility restore} input token at
the end.

The @code{ami} request appends indirectly,
meaning that @code{gtroff} expands strings whose names
are @var{xx} or @var{yy} before performing the append.

@pindex trace.tmac
Using @file{trace.tmac}, you can trace calls to @code{am} and @code{am1}.
@endDefreq

@xref{Strings}, for the @code{als} request to rename a macro.

The @code{de}, @code{am}, @code{di}, @code{da}, @code{ds}, and
@code{as} requests (together with its variants) only create a new object
if the name of the macro, diversion or string diversion is currently
undefined or if it is defined to be a request; normally they modify the
value of an existing object.

@Defreq {return, }
Exit a macro, immediately returning to the caller.
@endDefreq

@menu
* Copy-in Mode::
* Parameters::
@end menu

@c ---------------------------------------------------------------------

@node Copy-in Mode, Parameters, Writing Macros, Writing Macros
@subsection Copy-in Mode
@cindex copy-in mode
@cindex mode, copy-in

@cindex @code{\n}, when reading text for a macro
@cindex @code{\$}, when reading text for a macro
@cindex @code{\*}, when reading text for a macro
@cindex @code{\\}, when reading text for a macro
@cindex \@key{RET}, when reading text for a macro
When @code{gtroff} reads in the text for a macro or diversion, it copies
the text (including request lines, but excluding escapes) into an
internal buffer.  Escapes are converted into an internal form,
except for @code{\n}, @code{\$}, @code{\*}, @code{\\} and
@code{\@key{RET}} which are evaluated and inserted into the text where
the escape was located.  This is known as @dfn{copy-in} mode or
@dfn{copy} mode.

What this means is that you can specify when these escapes are to be
evaluated (either at copy-in time or at the time of use) by insulating
the escapes with an extra backslash.  Compare this to the @code{\def}
and @code{\edef} commands in @TeX{}.

The following example prints the numbers 20 and@w{ }10:

@Example
.nr x 20
.de y
.nr x 10
\&\nx
\&\\nx
..
.y
@endExample

@c ---------------------------------------------------------------------

@node Parameters,  , Copy-in Mode, Writing Macros
@subsection Parameters
@cindex parameters

The arguments to a macro can be examined using a variety of escapes.

@Defreg {.$}
@cindex number of arguments register (@code{.$})
The number of arguments in a macro.  This is a read-only number register.
@endDefreg

Any individual argument can be retrieved with one of the following
escapes:

@DefescList {\\$, , n, }
@DefescItem {\\$, @lparen{}, nn, }
@DefescListEnd {\\$, @lbrack{}, nnn, @rbrack{}}
@cindex copy-in mode, and macro arguments
@cindex macro, arguments (@code{\$})
@cindex arguments, macro (@code{\$})
Retrieve the @var{n}@dmn{th}, @var{nn}@dmn{th} or @var{nnn}@dmn{th}
argument.  As usual, the first form only accepts a single number
(larger than zero), the second a two-digit number (larger or equal
to@w{ }10), and the third any positive integer value (larger
than zero).  Macros can have an unlimited number of arguments.  Note
that due to copy-in mode, use two backslashes on these in actual use to
prevent interpolation until the macro is actually invoked.
@endDefesc

@Defreq {shift, [@Var{n}]}
Shift the arguments 1@w{ }position, or as
many positions as specified by its argument.  After executing this
request, argument@w{ }@var{i} becomes argument @math{@var{i}-@var{n}};
arguments 1 to@w{ }@var{n} are no longer available.  Shifting by
negative amounts is currently undefined.
@endDefreq

@DefescList {\\$*, , , }
@DefescListEnd {\\$@@, , , }
In some cases it is convenient to use all of the arguments at once (for
example, to pass the arguments along to another macro).  The @code{\$*}
escape concatenates all the arguments separated by spaces.  A
similar escape is @code{\$@@}, which concatenates all the
arguments with each surrounded by double quotes, and separated by
spaces.  If not in compatibility mode, the input level of double quotes
is preserved (see @ref{Request Arguments}).
@endDefesc

@Defesc {\\$0, , , }
@cindex macro name register (@code{\$0})
@cindex @code{als} request, and @code{\$0}
The name used to invoke the current macro.
The @code{als} request can make a macro have more than one name.

@Example
.de generic-macro
.  ...
.  if \\n[error] \@{\
.    tm \\$0: Houston, we have a problem.
.    return
.  \@}
..
.
.als foo generic-macro
.als bar generic-macro
@endExample
@endDefesc

@xref{Request Arguments}.


@c =====================================================================

@node Page Motions, Drawing Requests, Writing Macros, gtroff Reference
@section Page Motions
@cindex page motions
@cindex motions, page

@xref{Manipulating Spacing}, for a discussion of the main request for
vertical motion, @code{sp}.

@DefreqList {mk, [@Var{reg}]}
@DefreqListEnd {rt, [@Var{dist}]}
@cindex marking vertical page location (@code{mk})
@cindex page location, vertical, marking (@code{mk})
@cindex location, vertical, page, marking (@code{mk})
@cindex vertical page location, marking (@code{mk})
@cindex returning to marked vertical page location (@code{rt})
@cindex page location, vertical, returning to marked (@code{rt})
@cindex location, vertical, page, returning to marked (@code{rt})
@cindex vertical page location, returning to marked (@code{rt})
The request @code{mk} can be used to mark a location on a page, for
movement to later.  This request takes a register name as an argument
in which to store the current page location.  With no argument it
stores the location in an internal register.  The results of this can
be used later by the @code{rt} or the @code{sp} request (or the
@code{\v} escape).

The @code{rt} request returns @emph{upwards} to the location marked
with the last @code{mk} request.  If used with an argument, return to
a position which distance from the top of the page is @var{dist} (no
previous call to @code{mk} is necessary in this case).  Default scaling
indicator is @samp{v}.

Here a primitive solution for a two-column macro.

@Example
.nr column-length 1.5i
.nr column-gap 4m
.nr bottom-margin 1m
.
@endExample
@Example
.de 2c
.  br
.  mk
.  ll \\n[column-length]u
.  wh -\\n[bottom-margin]u 2c-trap
.  nr right-side 0
..
.
@endExample
@Example
.de 2c-trap
.  ie \\n[right-side] \@{\.    nr right-side 0
.    po -(\\n[column-length]u + \\n[column-gap]u)
.    \" remove trap
.    wh -\\n[bottom-margin]u
.  \@}
.  el \@{\
.    \" switch to right side
.    nr right-side 1
.    po +(\\n[column-length]u + \\n[column-gap]u)
.    rt
.  \@}
..
.
@endExample
@Example
.pl 1.5i
.ll 4i
This is a small test which shows how the
rt request works in combination with mk.

.2c
Starting here, text is typeset in two columns.
Note that this implementation isn't robust
and thus not suited for a real two-column
macro.
@endExample

Result:

@Example
This is a small test which shows how the
rt request works in combination with mk.

Starting  here,    isn't    robust
text is typeset    and   thus  not
in two columns.    suited  for   a
Note that  this    real two-column
implementation     macro.
@endExample
@endDefreq

The following escapes give fine control of movements about the page.

@Defesc {\\v, ', e, '}
@cindex vertical motion (@code{\v})
@cindex motion, vertical (@code{\v})
Move vertically, usually from the current location on the page (if no
absolute position operator @samp{|} is used).  The
argument@w{ }@var{e} specifies the distance to move; positive is
downwards and negative upwards.  The default scaling indicator for this
escape is @samp{v}.  Beware, however, that @code{gtroff} continues text
processing at the point where the motion ends, so you should always
balance motions to avoid interference with text processing.

@code{\v} doesn't trigger a trap.  This can be quite useful; for example,
consider a page bottom trap macro which prints a marker in the margin to
indicate continuation of a footnote or something similar.
@endDefesc

There are some special-case escapes for vertical motion.

@Defesc {\\r, , , }
Move upwards@w{ }1@dmn{v}.
@endDefesc

@Defesc {\\u, , , }
Move upwards@w{ }.5@dmn{v}.
@endDefesc

@Defesc {\\d, , , }
Move down@w{ }.5@dmn{v}.
@endDefesc

@Defesc {\\h, ', e, '}
@cindex inserting horizontal space (@code{\h})
@cindex horizontal space (@code{\h})
@cindex space, horizontal (@code{\h})
@cindex horizontal motion (@code{\h})
@cindex motion, horizontal (@code{\h})
Move horizontally, usually from the current location (if no absolute
position operator @samp{|} is used).  The expression@w{ }@var{e}
indicates how far to move: positive is rightwards and negative
leftwards.  The default scaling indicator for this escape is @samp{m}.
@endDefesc

There are a number of special-case escapes for horizontal motion.

@Defesc {\\@key{SP}, , , }
@cindex space, unbreakable
@cindex unbreakable space
An unbreakable and unpaddable (i.e.@: not expanded during filling)
space.  (Note: This is a backslash followed by a space.)
@endDefesc

@Defesc {\\~, , , }
An unbreakable space that stretches like a normal inter-word space
when a line is adjusted.
@endDefesc

@Defesc {\\|, , , }
A 1/6@dmn{th} em space.  Ignored for TTY output devices (rounded to
zero).
@endDefesc

@Defesc {\\^, , , }
A 1/12@dmn{th} em space.  Ignored for TTY output devices (rounded to
zero).
@endDefesc

@Defesc {\\0, , , }
@cindex space, width of a digit (@code{\0})
@cindex digit width space (@code{\0})
A space the size of a digit.
@endDefesc

The following string sets the @TeX{} logo:

@Example
.ds TeX T\h'-.1667m'\v'.224m'E\v'-.224m'\h'-.125m'X
@endExample

@DefescList {\\w, ', text, '}
@DefregItem {st}
@DefregItem {sb}
@DefregItem {rst}
@DefregItem {rsb}
@DefregItem {ct}
@DefregItem {ssc}
@DefregListEnd {skw}
@cindex width escape (@code{\w})
Return the width of the specified @var{text} in basic units.
This allows horizontal movement based on the width of some
arbitrary text (e.g.@: given as an argument to a macro).

@Example
The length of the string `abc' is \w'abc'u.
    @result{} The length of the string `abc' is 72u.
@endExample

Font changes may occur in @var{text} which don't affect current
settings.

After use, @code{\w} sets several registers:

@table @code
@item st
@itemx sb
The highest and lowest point of the baseline, respectively, in @var{text}.

@item rst
@itemx rsb
Like the @code{st} and @code{sb} registers, but takes account of the
heights and depths of glyphs.  With other words, this gives the
highest and lowest point of @var{text}.

@item ct
Defines the kinds of glyphs occurring in @var{text}:

@table @asis
@item 0
only short glyphs, no descenders or tall glyphs.

@item 1
at least one descender.

@item 2
at least one tall glyph.

@item 3
at least one each of a descender and a tall glyph.
@end table

@item ssc
The amount of horizontal space (possibly negative) that should be added
to the last glyph before a subscript.

@item skw
How far to right of the center of the last glyph in the @code{\w}
argument, the center of an accent from a roman font should be placed
over that glyph.
@end table
@endDefesc

@DefescList {\\k, , p, }
@DefescItem {\\k, @lparen{}, ps, }
@DefescListEnd {\\k, @lbrack{}, position, @rbrack}
@cindex saving horizontal input line position (@code{\k})
@cindex horizontal input line position, saving (@code{\k})
@cindex input line position, horizontal, saving (@code{\k})
@cindex position, horizontal input line, saving (@code{\k})
@cindex line, input, horizontal position, saving (@code{\k})
Store the current horizontal position in the @emph{input} line in
number register with name @var{position} (one-character name@w{ }@var{p},
two-character name @var{ps}).  Use this, for example, to return to the
beginning of a string for highlighting or other decoration.
@endDefesc

@Defreg {hp}
@cindex horizontal input line position register (@code{hp})
@cindex input line, horizontal position, register (@code{hp})
@cindex position, horizontal, in input line, register (@code{hp})
@cindex line, input, horizontal position, register (@code{hp})
The current horizontal position at the input line.
@endDefreg

@Defreg {.k}
@cindex horizontal output line position register (@code{.k})
@cindex output line, horizontal position, register (@code{.k})
@cindex position, horizontal, in output line, register (@code{.k})
@cindex line, output, horizontal position, register (@code{.k})
A read-only number register containing the current horizontal output
position.
@endDefreg

@Defesc {\\o, ', @Var{a}@Var{b}@Var{c}, '}
@cindex overstriking glyphs (@code{\o})
@cindex glyphs, overstriking (@code{\o})
Overstrike glyphs @var{a}, @var{b}, @var{c}, @dots{}; the glyphs
are centered, and the resulting spacing is the largest width of the
affected glyphs.
@endDefesc

@Defesc {\\z, , g, , }
@cindex zero-width printing (@code{\z}, @code{\Z})
@cindex printing, zero-width (@code{\z}, @code{\Z})
Print glyph @var{g} with zero width, i.e., without spacing.  Use
this to overstrike glyphs left-aligned.
@endDefesc

@Defesc {\\Z, ', anything, '}
@cindex zero-width printing (@code{\z}, @code{\Z})
@cindex printing, zero-width (@code{\z}, @code{\Z})
Print @var{anything}, then restore the horizontal and vertical position.
The argument may not contain tabs or leaders.

The following is an example of a strike-through macro:

@Example
.de ST
.nr ww \w'\\$1'
\Z@@\v'-.25m'\l'\\n[ww]u'@@\\$1
..
.
This is
.ST "a test"
an actual emergency!
@endExample
@endDefesc


@c =====================================================================

@node Drawing Requests, Traps, Page Motions, gtroff Reference
@section Drawing Requests
@cindex drawing requests
@cindex requests for drawing

@code{gtroff} provides a number of ways to draw lines and other figures
on the page.  Used in combination with the page motion commands (see
@ref{Page Motions}, for more info), a wide variety of figures can be
drawn.  However, for complex drawings these operations can be quite
cumbersome, and it may be wise to use graphic preprocessors like
@code{gpic} or @code{ggrn}.  @xref{gpic}, and @ref{ggrn}, for more
information.

All drawing is done via escapes.

@DefescList {\\l, ', @Var{l}, '}
@DefescListEnd {\\l, ', @Var{l}@Var{g}, '}
@cindex drawing horizontal lines (@code{\l})
@cindex horizontal line, drawing (@code{\l})
@cindex line, horizontal, drawing (@code{\l})
Draw a line horizontally.  @var{l} is the length of the line to be
drawn.  If it is positive, start the line at the current location and
draw to the right; its end point is the new current location.  Negative
values are handled differently: The line starts at the current location
and draws to the left, but the current location doesn't move.

@var{l} can also be specified absolutely (i.e.@: with a leading
@samp{|}) which draws back to the beginning of the input line.
Default scaling indicator is @samp{m}.

@cindex underscore glyph (@code{\[ru]})
@cindex glyph, underscore (@code{\[ru]})
@cindex line drawing glyph
@cindex glyph, for line drawing
The optional second parameter@w{ }@var{g} is a glyph to draw the line
with.  If this second argument is not specified, @code{gtroff} uses
the underscore glyph, @code{\[ru]}.

@cindex zero width space character (@code{\&})
@cindex character, zero width space (@code{\&})
@cindex space character, zero width (@code{\&})
To separate the two arguments (to prevent @code{gtroff} from
interpreting a drawing glyph as a scaling indicator if the glyph is
represented by a single character) use @code{\&}.

Here a small useful example:

@Example
.de box
\[br]\\$*\[br]\l'|0\[rn]'\l'|0\[ul]'
..
@endExample

@noindent
Note that this works by outputting a box rule (a vertical line), then
the text given as an argument and then another box rule.  Finally, the
line drawing escapes both draw from the current location to the
beginning of the @emph{input} line -- this works because the line
length is negative, not moving the current point.
@endDefesc

@DefescList {\\L, ', @Var{l}, '}
@DefescListEnd {\\L, ', @Var{l}@Var{g}, '}
@cindex drawing vertical lines (@code{\L})
@cindex vertical line drawing (@code{\L})
@cindex line, vertical, drawing (@code{\L})
@cindex line drawing glyph
@cindex glyph for line drawing
@cindex box rule glyph (@code{\[br]})
@cindex glyph, box rule (@code{\[br]})
Draw vertical lines.  Its parameters are
similar to the @code{\l} escape, except that the default scaling
indicator is @samp{v}.  The movement is downwards for positive values,
and upwards for negative values.  The default glyph is the box rule
glyph, @code{\[br]}.  As with the vertical motion escapes, text
processing blindly continues where the line ends.

@Example
This is a \L'3v'test.
@endExample

@noindent
Here the result, produced with @code{grotty}.

@Example
This is a
          |
          |
          |test.
@endExample
@endDefesc

@Defesc {\\D, ', command arg @dots{}, '}
The @code{\D} escape provides a variety of drawing functions.
Note that on character devices, only vertical and horizontal lines are
supported within @code{grotty}; other devices may only support a subset
of the available drawing functions.

The default scaling indicator for all subcommands of @code{\D} is
@samp{m} for horizontal distances and @samp{v} for vertical ones.
Exceptions are @w{@code{\D'f @dots{}'}} and @w{@code{\D't @dots{}'}}
which use @code{u} as the default.

@table @code
@item \D'l @var{dx} @var{dy}'
@cindex line, drawing (@w{@code{\D'l @dots{}'}})
@cindex drawing a line (@w{@code{\D'l @dots{}'}})
Draw a line from the current location to the relative point specified by
(@var{dx},@var{dy}).

The following example is a macro for creating a box around a text string;
for simplicity, the box margin is taken as a fixed value, 0.2@dmn{m}.

@Example
.de BOX
.  nr @@wd \w'\\$1'
\h'.2m'\
\h'-.2m'\v'(.2m - \\n[rsb]u)'\
\D'l 0 -(\\n[rst]u - \\n[rsb]u + .4m)'\
\D'l (\\n[@@wd]u + .4m) 0'\
\D'l 0 (\\n[rst]u - \\n[rsb]u + .4m)'\
\D'l -(\\n[@@wd]u + .4m) 0'\
\h'.2m'\v'-(.2m - \\n[rsb]u)'\
\\$1\
\h'.2m'
..
@endExample

@noindent
First, the width of the string is stored in register @code{@@wd}.  Then,
four lines are drawn to form a box, properly offset by the box margin.
The registers @code{rst} and @code{rsb} are set by the @code{\w} escape,
containing the largest height and depth of the whole string.

@item \D'c @var{d}'
@cindex circle, drawing (@w{@code{\D'c @dots{}'}})
@cindex drawing a circle (@w{@code{\D'c @dots{}'}})
Draw a circle with a diameter of@w{ }@var{d} with the leftmost point at the
current position.

@item \D'C @var{d}'
@cindex circle, solid, drawing (@w{@code{\D'C @dots{}'}})
@cindex drawing a solid circle (@w{@code{\D'C @dots{}'}})
@cindex solid circle, drawing (@w{@code{\D'C @dots{}'}})
Draw a solid circle with the same parameters as an outlined circle.  No
outline is drawn.

@item \D'e @var{x} @var{y}'
@cindex drawing an ellipse (@w{@code{\D'e @dots{}'}})
@cindex ellipse, drawing (@w{@code{\D'e @dots{}'}})
Draw an ellipse with a horizontal diameter of @var{x} and a vertical
diameter of @var{y} with the leftmost point at the current position.

@item \D'E @var{x} @var{y}'
@cindex ellipse, solid, drawing (@w{@code{\D'E @dots{}'}})
@cindex drawing a solid ellipse (@w{@code{\D'E @dots{}'}})
@cindex solid ellipse, drawing (@w{@code{\D'E @dots{}'}})
Draw a solid ellipse with the same parameters as an outlined ellipse.
No outline is drawn.

@item \D'a @var{dx1} @var{dy1} @var{dx2} @var{dy2}'
@cindex arc, drawing (@w{@code{\D'a @dots{}'}})
@cindex drawing an arc (@w{@code{\D'a @dots{}'}})
Draw an arc clockwise from the current location through the two
specified relative locations (@var{dx1},@var{dy1}) and
(@var{dx2},@var{dy2}).  The coordinates of the first point are relative
to the current position, and the coordinates of the second point are
relative to the first point.

@item \D'~ @var{dx1} @var{dy1} @var{dx2} @var{dy2} @dots{}'
@cindex drawing a spline (@w{@code{\D'~ @dots{}'}})
@cindex spline, drawing (@w{@code{\D'~ @dots{}'}})
Draw a spline from the current location to the relative point
(@var{dx1},@var{dy1}) and then to (@var{dx2},@var{dy2}), and so on.

@item \D'f @var{n}'
@cindex gray shading (@w{@code{\D'f @dots{}'}})
@cindex shading filled objects (@w{@code{\D'f @dots{}'}})
Set the shade of gray to be used for filling solid objects to@w{
}@var{n}; @var{n}@w{ }must be an integer between 0 and@w{ }1000, where 0
corresponds solid white and 1000 to solid black, and values in between
correspond to intermediate shades of gray.  This applies only to solid
circles, solid ellipses, and solid polygons.  By default, a level of
1000 is used.

@item \D'p @var{dx1} @var{dy1} @var{dx2} @var{dy2} @dots{}'
@cindex drawing a polygon (@w{@code{\D'p @dots{}'}})
@cindex polygon, drawing (@w{@code{\D'p @dots{}'}})
Draw a polygon from the current location to the relative position
(@var{dx1},@var{dy1}) and then to (@var{dx2},@var{dy2}) and so on.
When the specified data points are exhausted, a line is drawn back
to the starting point.

@item \D'P @var{dx1} @var{dy1} @var{dx2} @var{dy2} @dots{}'
@cindex polygon, solid, drawing (@w{@code{\D'P @dots{}'}})
@cindex drawing a solid polygon (@w{@code{\D'P @dots{}'}})
@cindex solid polygon, drawing (@w{@code{\D'P @dots{}'}})
Draw a solid polygon with the same parameters as an outlined polygon.
No outline is drawn.

Here a better variant of the box macro to fill the box with some color.
Note that the box must be drawn before the text since colors in
@code{gtroff} are not transparent; the filled polygon would hide the
text completely.

@Example
.de BOX
.  nr @@wd \w'\\$1'
\h'.2m'\
\h'-.2m'\v'(.2m - \\n[rsb]u)'\
\M[lightcyan]\
\D'P 0 -(\\n[rst]u - \\n[rsb]u + .4m) \
     (\\n[@@wd]u + .4m) 0 \
     0 (\\n[rst]u - \\n[rsb]u + .4m) \
     -(\\n[@@wd]u + .4m) 0'\
\h'.2m'\v'-(.2m - \\n[rsb]u)'\
\M[]\
\\$1\
\h'.2m'
..
@endExample

@item \D't @var{n}'
@cindex line thickness (@w{@code{\D't @dots{}'}})
@cindex thickness of lines (@w{@code{\D't @dots{}'}})
Set the current line thickness to @var{n}@w{ }machine units.  A value of
zero selects the smallest available line thickness.  A negative value
makes the line thickness proportional to the current point size (this is
the default behaviour of @acronym{AT&T} @code{troff}).
@end table
@endDefesc

@xref{Graphics Commands}.

@Defesc {\\b, ', string, '}
@cindex pile, glyph (@code{\b})
@cindex glyph pile (@code{\b})
@cindex stacking glyphs (@code{\b})
@dfn{Pile} a sequence of glyphs vertically, and center it vertically
on the current line.  Use it to build large brackets and braces.

Here an example how to create a large opening brace:

@Example
\b'\[lt]\[bv]\[lk]\[bv]\[lb]'
@endExample

@cindex @code{\b}, limitations
@cindex limitations of @code{\b} escape
The first glyph is on the top, the last glyph in @var{string} is
at the bottom.  Note that @code{gtroff} separates the glyphs
vertically by 1@dmn{m}, and the whole object is centered 0.5@dmn{m}
above the current baseline; the largest glyph width is used as the
width for the whole object.  This rather unflexible positioning
algorithm doesn't work with @option{-Tdvi} since the bracket pieces vary
in height for this device.  Instead, use the @code{eqn} preprocessor.

@xref{Manipulating Spacing}, how to adjust the vertical spacing with
the @code{\x} escape.
@endDefesc


@c =====================================================================

@node Traps, Diversions, Drawing Requests, gtroff Reference
@section Traps
@cindex traps

@dfn{Traps} are locations, which, when reached, call a specified
macro.  These traps can occur at a given location on the page, at a
given location in the current diversion, at a blank line,
after a certain number of input lines, or at the end of input.

@cindex planting a trap
@cindex trap, planting
Setting a trap is also called @dfn{planting}.
@cindex trap, springing
@cindex springing a trap
It is also said that a trap is @dfn{sprung} if the associated macro
is executed.

@menu
* Page Location Traps::
* Diversion Traps::
* Input Line Traps::
* Blank Line Traps::
* End-of-input Traps::
@end menu

@c ---------------------------------------------------------------------

@node Page Location Traps, Diversion Traps, Traps, Traps
@subsection Page Location Traps
@cindex page location traps
@cindex traps, page location

@dfn{Page location traps} perform an action when @code{gtroff}
reaches or passes a certain vertical location on the page.  Page
location traps have a variety of purposes, including:

@itemize
@item
setting headers and footers

@item
setting body text in multiple columns

@item
setting footnotes
@end itemize

@DefreqList {vpt, flag}
@DefregListEnd {.vpt}
@cindex enabling vertical position traps (@code{vpt})
@cindex vertical position traps, enabling (@code{vpt})
@cindex vertical position trap enable register (@code{.vpt})
Enable vertical position traps if @var{flag} is non-zero, or disables
them otherwise.  Vertical position traps are traps set by the @code{wh}
or @code{dt} requests.  Traps set by the @code{it} request are not
vertical position traps.  The parameter that controls whether vertical
position traps are enabled is global.  Initially vertical position traps
are enabled.  The current setting of this is available in the
@code{.vpt} read-only number register.
@endDefreq

@Defreq {wh, dist [@Var{macro}]}
Set a page location trap.  Positive values for @var{dist} set
the trap relative to the top of the page; negative values set
the trap relative to the bottom of the page.  Default scaling
indicator is @samp{v}.

@var{macro} is the name of the macro to execute when the
trap is sprung.  If @var{macro} is missing, remove the first trap
(if any) at @var{dist}.

@cindex page headers
@cindex page footers
@cindex headers
@cindex footers
The following is a simple example of how many macro packages
set headers and footers.

@Example
.de hd                \" Page header
'  sp .5i
.  tl 'Title''date'
'  sp .3i
..
.
.de fo                \" Page footer
'  sp 1v
.  tl ''%''
'  bp
..
.
.wh 0   hd            \" trap at top of the page
.wh -1i fo            \" trap one inch from bottom
@endExample

A trap at or below the bottom of the page is ignored; it can be made
active by either moving it up or increasing the page length so that the
trap is on the page.

It is possible to have more than one trap at the same location; to do so,
the traps must be defined at different locations, then moved together with
the @code{ch} request; otherwise the second trap would replace the first
one.  Earlier defined traps hide later defined traps if moved to the same
position (the many empty lines caused by the @code{bp} request are omitted):

@Example
.de a
.  nop a
..
.de b
.  nop b
..
.de c
.  nop c
..
.
.wh 1i a
.wh 2i b
.wh 3i c
.bp
    @result{} a b c
@endExample
@Example
.ch b 1i
.ch c 1i
.bp
    @result{} a
@endExample
@Example
.ch a 0.5i
.bp
    @result{} a b
@endExample
@endDefreq

@Defreg {.t}
@cindex distance to next trap register (@code{.t})
@cindex trap, distance, register (@code{.t})
A read-only number register holding the distance to the next trap.

If there are no traps between the current position and the bottom of the
page, it contains the distance to the page bottom.  In a diversion, the
distance to the page bottom is infinite (the returned value is the biggest
integer which can be represented in @code{groff}) if there are no diversion
traps.
@endDefreg

@Defreq {ch, macro dist}
@cindex changing trap location (@code{ch})
@cindex trap, changing location (@code{ch})
Change the location of a trap.
The first argument is the name of the macro to be invoked at
the trap, and the second argument is the new location for the trap
(note that the parameters are specified the opposite of the @code{wh}
request).  This is useful for building up footnotes in a diversion to
allow more space at the bottom of the page for them.

Default scaling indicator for @var{dist} is @samp{v}.  If @var{dist}
is missing, the trap is removed.

@c XXX

@ignore
@Example
... (simplified) footnote example ...
@endExample
@end ignore
@endDefreq

@Defreg {.ne}
The read-only number register @code{.ne} contains the amount of space
that was needed in the last @code{ne} request that caused a trap to be
sprung.  Useful in conjunction with the @code{.trunc} register.
@xref{Page Control}, for more information.
@endDefreg

@Defreg {.trunc}
@cindex @code{ne} request, and the @code{.trunc} register
@cindex truncated vertical space register (@code{.trunc})
A read-only register containing the amount of vertical space truncated
by the most recently sprung vertical position trap, or, if the trap was
sprung by an @code{ne} request, minus the amount of vertical motion
produced by the @code{ne} request.  In other words, at the point a trap
is sprung, it represents the difference of what the vertical position
would have been but for the trap, and what the vertical position
actually is.
@endDefreg

@c ---------------------------------------------------------------------

@node Diversion Traps, Input Line Traps, Page Location Traps, Traps
@subsection Diversion Traps
@cindex diversion traps
@cindex traps, diversion

@Defreq {dt, dist macro}
@cindex @code{.t} register, and diversions
@cindex setting diversion trap (@code{dt})
@cindex diversion trap, setting (@code{dt})
@cindex trap, diversion, setting (@code{dt})
Set a trap @emph{within} a diversion.
@var{dist} is the location of the trap
(identical to the @code{.wh} request; default scaling indicator is
@samp{v}) and @var{macro} is the name of the macro to be invoked.  The
number register @code{.t} still works within diversions.
@xref{Diversions}, for more information.
@endDefreq

@c ---------------------------------------------------------------------

@node Input Line Traps, Blank Line Traps, Diversion Traps, Traps
@subsection Input Line Traps
@cindex input line traps
@cindex traps, input line

@DefreqList {it, n macro}
@DefreqItem {itc, n macro}
@cindex setting input line trap (@code{it})
@cindex input line trap, setting (@code{it})
@cindex trap, input line, setting (@code{it})
Set an input line trap.
@var{n}@w{ }is the number of lines of input which may be read before
springing the trap, @var{macro} is the macro to be invoked.
Request lines are not counted as input lines.

For example, one possible use is to have a macro which prints the
next @var{n}@w{ }lines in a bold font.

@Example
.de B
.  it \\$1 B-end
.  ft B
..
.
.de B-end
.  ft R
..
@endExample

@cindex input line traps and interrupted lines (@code{itc})
@cindex interrupted lines and input line traps (@code{itc})
@cindex traps, input line, and interrupted lines (@code{itc})
@cindex lines, interrupted, and input line traps (@code{itc})
The @code{itc} request is identical,
except that a line interrupted with @code{\c}
counts as one input line.

Both requests are associated with the current environment
(@pxref{Environments}); switching to another environment disables the
current input trap, and going back reactivates it, restoring the number
of already processed lines.
@endDefreq

@c ---------------------------------------------------------------------

@node Blank Line Traps, End-of-input Traps, Input Line Traps, Traps
@subsection Blank Line Traps
@cindex blank line traps
@cindex traps, blank line

@Defreq {blm, macro}
@cindex blank line macro (@code{blm})
Set a blank line trap.
@code{gtroff} executes @var{macro} when it encounters a blank line in
the input file.
@endDefreq

@c ---------------------------------------------------------------------

@node End-of-input Traps,  , Blank Line Traps, Traps
@subsection End-of-input Traps
@cindex end-of-input traps
@cindex traps, end-of-input

@Defreq {em, macro}
@cindex setting end-of-input trap (@code{em})
@cindex end-of-input trap, setting (@code{em})
@cindex trap, end-of-input, setting (@code{em})
@cindex end-of-input macro (@code{em})
@cindex macro, end-of-input (@code{em})
Set a trap at the end of input.  @var{macro} is executed after the
last line of the input file has been processed.

For example, if the document had to have a section at the bottom of the
last page for someone to approve it, the @code{em} request could be
used.

@Example
.de approval
.  ne 5v
.  sp |(\\n[.t] - 6v)
.  in +4i
.  lc _
.  br
Approved:\t\a
.  sp
Date:\t\t\a
..
.
.em approval
@endExample
@endDefreq


@c =====================================================================

@node Diversions, Environments, Traps, gtroff Reference
@section Diversions
@cindex diversions

In @code{gtroff} it is possible to @dfn{divert} text into a named
storage area.  Due to the similarity to defining macros it is sometimes
said to be stored in a macro.  This is used for saving text for output
at a later time, which is useful for keeping blocks of text on the same
page, footnotes, tables of contents, and indices.

@cindex top-level diversion
@cindex diversion, top-level
For orthogonality it is said that @code{gtroff} is in the @dfn{top-level
diversion} if no diversion is active (i.e., the data is diverted to the
output device).

@DefreqList {di, macro}
@DefreqListEnd {da, macro}
@cindex beginning diversion (@code{di})
@cindex diversion, beginning (@code{di})
@cindex ending diversion (@code{di})
@cindex diversion, ending (@code{di})
@cindex appending to a diversion (@code{da})
@cindex diversion, appending (@code{da})
Begin a diversion.  Like the @code{de}
request, it takes an argument of a macro name to divert subsequent text
into.  The @code{da} macro appends to an existing diversion.

@code{di} or @code{da} without an argument ends the diversion.
@endDefreq

@DefreqList {box, macro}
@DefreqListEnd {boxa, macro}
Begin (or appends to) a diversion like the
@code{di} and @code{da} requests.
The difference is that @code{box} and @code{boxa}
do not include a partially-filled line in the diversion.

Compare this:

@Example
Before the box.
.box xxx
In the box.
.br
.box
After the box.
.br
    @result{} Before the box.  After the box.
.xxx
    @result{} In the box.
@endExample

@noindent
with this:

@Example
Before the diversion.
.di yyy
In the diversion.
.br
.di
After the diversion.
.br
    @result{} After the diversion.
.yyy
    @result{} Before the diversion.  In the diversion.
@endExample

@code{box} or @code{boxa} without an argument ends the diversion.
@endDefreq

@DefregList {.z}
@DefregListEnd {.d}
@cindex @code{nl} register, and @code{.d}
@cindex nested diversions
@cindex diversion, nested
@cindex diversion name register (@code{.z})
@cindex vertical position in diversion register (@code{.d})
@cindex position, vertical, in diversion, register (@code{.d})
@cindex diversion, vertical position in, register (@code{.d})
Diversions may be nested.  The read-only number register @code{.z}
contains the name of the current diversion (this is a string-valued
register).  The read-only number register @code{.d} contains the current
vertical place in the diversion.  If not in a diversion it is the same
as the register @code{nl}.
@endDefreg

@Defreg {.h}
@cindex high-water mark register (@code{.h})
@cindex mark, high-water, register (@code{.h})
@cindex position of lowest text line (@code{.h})
@cindex text line, position of lowest (@code{.h})
The @dfn{high-water mark} on the current page.  It corresponds to the
text baseline of the lowest line on the page.  This is a read-only
register.

@Example
.tm .h==\n[.h], nl==\n[nl]
    @result{} .h==0, nl==-1
This is a test.
.br
.sp 2
.tm .h==\n[.h], nl==\n[nl]
    @result{} .h==40, nl==120
@endExample

@cindex @code{.h} register, difference to @code{nl}
@cindex @code{nl} register, difference to @code{.h}
@noindent
As can be seen in the previous example, empty lines are not considered
in the return value of the @code{.h} register.
@endDefreg

@DefregList {dn}
@DefregListEnd {dl}
After completing a diversion, the read-write number registers @code{dn}
and @code{dl} contain the vertical and horizontal size of the diversion.

@Example
.\" Center text both horizontally & vertically
.
.\" Enclose macro definitions in .eo and .ec
.\" to avoid the doubling of the backslash
.eo
.\" macro .(c starts centering mode
.de (c
.  br
.  ev (c
.  evc 0
.  in 0
.  nf
.  di @@c
..
@endExample
@Example
.\" macro .)c terminates centering mode
.de )c
.  br
.  ev
.  di
.  nr @@s (((\n[.t]u - \n[dn]u) / 2u) - 1v)
.  sp \n[@@s]u
.  ce 1000
.  @@c
.  ce 0
.  sp \n[@@s]u
.  br
.  fi
.  rr @@s
.  rm @@s
.  rm @@c
..
.\" End of macro definitions, restore escape mechanism
.ec
@endExample
@endDefreg

@DefescList {\\!, , , }
@DefescListEnd {\\?, , @Var{anything}, \\?}
@cindex transparent output (@code{\!}, @code{\?})
@cindex output, transparent (@code{\!}, @code{\?})
Prevent requests, macros, and escapes from being
interpreted when read into a diversion.  This takes the given text
and @dfn{transparently} embeds it into the diversion.  This is useful for
macros which shouldn't be invoked until the diverted text is actually
output.

The @code{\!} escape transparently embeds text up to
and including the end of the line.
The @code{\?} escape transparently embeds text until the next
occurrence of the @code{\?} escape.  For example:

@Example
\?@var{anything}\?
@endExample

@noindent
@var{anything} may not contain newlines; use @code{\!}  to embed
newlines in a diversion.  The escape sequence @code{\?} is also
recognized in copy mode and turned into a single internal code; it is
this code that terminates @var{anything}.  Thus the following example
prints@w{ }4.

@Example
.nr x 1
.nf
.di d
\?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
.di
.nr x 2
.di e
.d
.di
.nr x 3
.di f
.e
.di
.nr x 4
.f
@endExample

Both escapes read the data in copy mode.

@cindex @code{\!}, in top-level diversion
@cindex top-level diversion, and @code{\!}
@cindex diversion, top-level, and @code{\!}
If @code{\!} is used in the top-level diversion, its argument is
directly embedded into the @code{gtroff} intermediate output.  This can
be used for example to control a postprocessor which processes the data
before it is sent to the device driver.

@cindex @code{\?}, in top-level diversion
@cindex top-level diversion, and @code{\?}
@cindex diversion, top-level, and @code{\?}
The @code{\?} escape used in the top-level diversion produces no output
at all; its argument is simply ignored.
@endDefesc

@cindex @code{\!}, and @code{output}
@cindex @code{output} request, and @code{\!}
@Defreq {output, string}
Emit @var{string} directly to the @code{gtroff} intermediate output
(subject to copy-mode interpretation);  this is similar to @code{\!} used
at the top level.  An initial double quote in @var{string} is stripped off
to allow initial blanks.

This request can't be used before the first page has started -- if you get
an error, simply insert @code{.br} before the @code{output} request.

Without argument, @code{output} is ignored.

Use with caution!  It is normally only needed for mark-up used by a
postprocessor which does something with the output before sending it to
the output device, filtering out @code{string} again.
@endDefreq

@Defreq {asciify, div}
@cindex unformatting diversions (@code{asciify})
@cindex diversion, unformatting (@code{asciify})
@cindex @code{trin} request, and @code{asciify}
@dfn{Unformat} the diversion specified by @var{div}
in such a way that @acronym{ASCII} characters, characters translated with
the @code{trin} request, space characters, and some escape sequences that
were formatted and diverted are treated like ordinary input
characters when the diversion is reread.  It can be also used for gross
hacks; for example, the following sets register@w{ }@code{n} to@w{ }1.

@Example
.tr @@.
.di x
@@nr n 1
.br
.di
.tr @@@@
.asciify x
.x
@endExample

@xref{Copy-in Mode}.
@endDefreq

@Defreq {unformat, div}
Like @code{asciify}, unformat the specified diversion.
However, @code{unformat} only unformats spaces and tabs
between words.
Unformatted tabs are treated as input tokens,
and spaces are stretchable again.

The vertical size of lines is not preserved; glyph information (font,
font size, space width, etc.)@: is retained.
@endDefreq


@c =====================================================================

@node Environments, Suppressing output, Diversions, gtroff Reference
@section Environments
@cindex environments

It happens frequently that some text should be printed in a certain
format regardless of what may be in effect at the time, for example, in
a trap invoked macro to print headers and footers.  To solve this
@code{gtroff} processes text in @dfn{environments}.  An
environment contains most of the parameters that control text
processing.  It is possible to switch amongst these environments; by
default @code{gtroff} processes text in environment@w{ }0.  The
following is the information kept in an environment.

@itemize @bullet
@item
font parameters (size, family, style, glyph height and slant, space
and sentence space size)

@item
page parameters (line length, title length, vertical spacing,
line spacing, indentation, line numbering, centering, right-justifying,
underlining, hyphenation data)

@item
fill and adjust mode

@item
tab stops, tab and leader characters, escape character,
no-break and hyphen indicators, margin character data

@item
partially collected lines

@item
input traps

@item
drawing and fill colours
@end itemize

These environments may be given arbitrary names (see @ref{Identifiers},
for more info).  Old versions of @code{troff} only had environments
named @samp{0}, @samp{1}, and @samp{2}.

@DefreqList {ev, [@Var{env}]}
@DefregListEnd {.ev}
@cindex switching environments (@code{ev})
@cindex environment, switching (@code{ev})
@cindex environment number/name register (@code{.ev})
Switch to another environment.  The argument @var{env} is the name of
the environment to switch to.  With no argument, @code{gtroff} switches
back to the previous environment.  There is no limit on the number of
named environments; they are created the first time that they are
referenced.  The @code{.ev} read-only register contains the name or
number of the current environment.  This is a string-valued register.

Note that a call to @code{ev} (with argument) pushes the previously
active environment onto a stack.  If, say, environments @samp{foo},
@samp{bar}, and @samp{zap} are called (in that order), the first
@code{ev} request without parameter switches back to environment
@samp{bar} (which is popped off the stack), and a second call
switches back to environment @samp{foo}.

Here is an example:

@Example
.ev footnote-env
.fam N
.ps 6
.vs 8
.ll -.5i
.ev

...

.ev footnote-env
\(dg Note the large, friendly letters.
.ev
@endExample
@endDefreq

@Defreq {evc, env}
@cindex copying environment (@code{evc})
@cindex environment, copying (@code{evc})
Copy the environment @var{env} into the current environment.

The following environment data is not copied:

@itemize @bullet
@item
Partially filled lines.

@item
The status whether the previous line was interrupted.

@item
The number of lines still to center, or to right-justify, or to underline
(with or without underlined spaces); they are set to zero.

@item
The status whether a temporary indent is active.

@item
Input traps and its associated data.

@item
Line numbering mode is disabled; it can be reactivated with
@w{@samp{.nm +0}}.

@item
The number of consecutive hyphenated lines (set to zero).
@end itemize
@endDefreq

@DefregList {.cht}
@DefregItem {.cdp}
@DefregListEnd {.csk}
@cindex environment, last glyph
The @code{\n[.cht]} register contains the
maximum extent (above the baseline)
of the last glyph added to the current environment.

The @code{\n[.cdp]} register contains the
maximum extent (below the baseline)
of the last glyph added to the current environment.

The @code{\n[.csk]} register contains the
@dfn{skew} (how far to the right of the glyph's center
that @code{gtroff} shold place an accent)
of the last glyph added to the current environment.
@endDefreg


@c =====================================================================

@node Suppressing output, Colors, Environments, gtroff Reference
@section Suppressing output

@Defesc {\\O, , num, }
@cindex suppressing output (@code{\O})
@cindex output, suppressing (@code{\O})
Disable or enable output depending on the value of @var{num}:

@table @samp
@item \O0
Disable any glyphs from being emitted to the device driver, provided that
the escape occurs at the outer level (see @code{\O[3]} and @code{\O[4]}).
Motion is not suppressed so effectively @code{\O[0]} means @emph{pen up}.

@item \O1
Enable output of glyphs, provided that the escape occurs at the outer
level.
@end table

@vindex opminx
@vindex opminy
@vindex opmaxx
@vindex opmaxy
@code{\O0} and @code{\O1} also reset the four registers @samp{opminx},
@samp{opminy}, @samp{opmaxx}, and @samp{opmaxy} to @minus{}1.
@xref{Register Index}.  These four registers mark the top left and
bottom right hand corners of a box which encompasses all written glyphs.

For example the input text:

@Example
Hello \O[0]world \O[1]this is a test.
@endExample

@noindent
produces the following output:

@Example
Hello       this is a test.
@endExample

@table @samp
@item \O2
Provided that the escape occurs at the outer level, enable output of
glyphs and also write out to @code{stderr} the page number and four
registers encompassing the glyphs previously written since the last call
to @code{\O}.

@item \O3
Begin a nesting level.  At start-up, @code{gtroff} is at outer level.

@item \O4
End a nesting level.

@item \O[5@var{P}@var{filename}]
This escape is @code{grohtml} specific.  Provided that this escape
occurs at the outer nesting level write the @code{filename} to
@code{stderr}.  The position of the image, @var{P}, must be specified
and must be one of @code{l}, @code{r}, @code{c}, or@w{ }@code{i} (left,
right, centered, inline).  @var{filename} will be associated with the
production of the next inline image.
@end table
@endDefesc

@c =====================================================================

@node Colors, I/O, Suppressing output, gtroff Reference
@section Colors
@cindex colors

@DefreqList {color, [@Var{n}]}
@DefregListEnd {.color}
If @var{n} is missing or non-zero, activate colors (this is the default);
otherwise, turn it off.

The read-only number register @code{.color} is@w{ }1 if colors are active,
0@w{ }otherwise.

Internally, @code{color} sets a global flag; it does not produce a token.
Similar to the @code{cp} request, you should use it at the beginning of
your document to control color output.

Colors can be also turned off with the @option{-c} command line option.
@endDefreq

@Defreq {defcolor, ident scheme color_components}
Define color with name @var{ident}.  @var{scheme} can be one of  the
following values: @code{rgb} (three components), @code{cym} (three
components), @code{cmyk} (four components), and @code{gray} or
@code{grey} (one component).

@cindex default color
@cindex color, default
Color components can be given either as a hexadecimal string or as
positive decimal integers in the range 0--65535.  A hexadecimal string
contains all color components concatenated.  It must start with either
@code{#} or @code{##}; the former specifies hex values in the range
0--255 (which are internally multiplied by@w{ }257), the latter in the
range 0--65535.  Examples: @code{#FFC0CB} (pink), @code{##ffff0000ffff}
(magenta).  The default color name @c{default} can't be redefined; its
value is device-specific (usually black).  It is possible that the
default color for @code{\m} and @code{\M} is not identical.

@cindex @code{f} unit, and colors
@cindex unit, @code{f}, and colors
A new scaling indicator@w{ }@code{f} has been introduced which multiplies
its value by 65536; this makes it convenient to specify color components
as fractions in the range 0 to@w{ }1 (1f equals 65536u).  Example:

@Example
.defcolor darkgreen rgb 0.1f 0.5f 0.2f
@endExample

Note that @code{f} is the default scaling indicator for the
@code{defcolor} request, thus the above statement is equivalent to

@Example
.defcolor darkgreen rgb 0.1 0.5 0.2
@endExample
@endDefreq

@DefescList {\\m, , c, }
@DefescItem {\\m, @lparen{}, co, }
@DefescListEnd {\\m, @lbrack{}, color, @rbrack}
Set drawing color.  The following example shows how to turn the next four
words red.

@Example
\m[red]these are in red\m[] and these words are in black.
@endExample

The escape @code{\m[]} returns to the previous color.

The drawing color is associated with the current environment
(@pxref{Environments}).
@endDefesc

@DefescList {\\M, , c, }
@DefescItem {\\M, @lparen{}, co, }
@DefescListEnd {\\M, @lbrack{}, color, @rbrack}
Set background color for filled objects drawn with the
@code{\D'@dots{}'} commands.

A red ellipse can be created with the following code:

@Example
\M[red]\h'0.5i'\D'E 2i 1i'\M[]
@endExample

The escape @code{\M[]} returns to the previous fill color.

The fill color is associated with the current environment
(@pxref{Environments}).
@endDefesc


@c =====================================================================

@node I/O, Postprocessor Access, Colors, gtroff Reference
@section I/O
@cindex i/o
@cindex input and output requests
@cindex requests for input and output
@cindex output and input requests

@code{gtroff} has several requests for including files:

@Defreq {so, file}
@cindex including a file (@code{so})
@cindex file, inclusion (@code{so})
Read in the specified @var{file} and
includes it in place of the @code{so} request.  This is quite useful for
large documents, e.g.@: keeping each chapter in a separate file.
@xref{gsoelim}, for more information.

Since @code{gtroff} replaces the @code{so} request with the contents
of @code{file}, it makes a difference whether the data is terminated with
a newline or not: Assuming that file @file{xxx} contains the word
@samp{foo} without a final newline, this

@Example
This is
.so xxx
bar
@endExample

@noindent
yields @samp{This is foobar}.
@endDefreq

@Defreq {pso, command}
Read the standard output from the specified @var{command}
and includes it in place of the @code{pso} request.

@cindex safer mode
@cindex mode, safer
@cindex unsafe mode
@cindex mode, unsafe
This request causes an error if used in safer mode (which is the default).
Use @code{groff}'s or @code{troff}'s @option{-U} option to activate unsafe
mode.

The comment regarding a final newline for the @code{so} request is valid
for @code{pso} also.
@endDefreq

@Defreq {mso, file}
Identical to the @code{so} request except that @code{gtroff} searches for
the specified @var{file} in the same directories as macro files for the
the @option{-m} command line option.  If the file name to be included
has the form @file{@var{name}.tmac} and it isn't found, @code{mso} tries
to include @file{tmac.@var{name}} and vice versa.
@endDefreq

@DefreqList {trf, file}
@DefreqListEnd {cf, file}
@cindex transparent output (@code{cf}, @code{trf})
@cindex output, transparent (@code{cf}, @code{trf})
Transparently output the contents of @var{file}.  Each line is output
as if it were preceded by @code{\!}; however, the lines are not subject
to copy mode interpretation.  If the file does not end with a newline,
then a newline is added (@code{trf} only).  For example, to define a
macro@w{ }@code{x} containing the contents of file@w{ }@file{f}, use

@Example
.di x
.trf f
.di
@endExample

Both @code{trf} and @code{cf}, when used in a diversion,
embeds an object in the diversion which, when reread, causes the
contents of @var{file} to be transparently copied through to the
output.  In @acronym{UNIX} @code{troff}, the contents of @var{file}
is immediately copied through to the output regardless of whether there
is a current diversion; this behaviour is so anomalous that it must be
considered a bug.

@cindex @code{trf} request, and invalid characters
@cindex characters, invalid for @code{trf} request
@cindex invalid characters for @code{trf} request
While @code{cf} copies the contents of @var{file} completely unprocessed,
@code{trf} disallows characters such as NUL that are not valid
@code{gtroff} input characters (@pxref{Identifiers}).

Both requests cause a line break.
@endDefreq

@Defreq {nx, [@Var{file}]}
@cindex processing next file (@code{nx})
@cindex file, processing next (@code{nx})
@cindex next file, processing (@code{nx})
Force @code{gtroff} to continue processing of
the file specified as an argument.  If no argument is given, immediately
jump to the end of file.
@endDefreq

@Defreq {rd, [@Var{prompt} [@Var{arg1} @Var{arg2} @dots{}]]}
@cindex reading from standard input (@code{rd})
@cindex standard input, reading from (@code{rd})
@cindex input, standard, reading from (@code{rd})
Read from standard input, and include what is read as though it
were part of the input file.  Text is read until a blank line
is encountered.

If standard input is a TTY input device (keyboard), write @var{prompt}
to standard error, followed by a colon (or send BEL for a beep if no
argument is given).

Arguments after @var{prompt} are available for the input.  For example,
the line

@Example
.rd data foo bar
@endExample

with the input @w{@samp{This is \$2.}} prints

@Example
This is bar.
@endExample
@endDefreq

@cindex form letters
@cindex letters, form
Using the @code{nx} and @code{rd} requests,
it is easy to set up form letters.  The form
letter template is constructed like this, putting the following lines
into a file called @file{repeat.let}:

@Example
.ce
\*(td
.sp 2
.nf
.rd
.sp
.rd
.fi
Body of letter.
.bp
.nx repeat.let
@endExample

@cindex @code{ex} request, used with @code{nx} and @code{rd}
@noindent
When this is run, a file containing the following lines should be
redirected in.  Note that requests included in this file are executed
as though they were part of the form letter.  The last block of input
is the @code{ex} request which tells groff to stop processing.  If
this was not there, groff would not know when to stop.

@Example
Trent A. Fisher
708 NW 19th Av., #202
Portland, OR  97209

Dear Trent,

Len Adollar
4315 Sierra Vista
San Diego, CA  92103

Dear Mr. Adollar,

.ex
@endExample

@Defreq {pi, pipe}
Pipe the output of @code{gtroff} to the shell command(s)
specified by @var{pipe}.  This request must occur before
@code{gtroff} has a chance to print anything.

@cindex safer mode
@cindex mode, safer
@cindex unsafe mode
@cindex mode, unsafe
@code{pi} causes an error if used in safer mode (which is the default).
Use @code{groff}'s or @code{troff}'s @option{-U} option to activate unsafe
mode.

Multiple calls to @code{pi} are allowed, acting as a chain.  For example,

@Example
.pi foo
.pi bar
...
@endExample

is the same as @w{@samp{.pi foo | bar}}.

@cindex @code{groff}, and @code{pi} request
@cindex @code{pi} request, and @code{groff}
Note that the intermediate output format of @code{gtroff} is piped to
the specified commands.  Consequently, calling @code{groff} without the
@option{-Z} option normally causes a fatal error.
@endDefreq

@DefreqList {sy, cmds}
@DefregListEnd {systat}
Execute the shell command(s) specified by @var{cmds}.  The output is not
saved anyplace, so it is up to the user to do so.

@cindex safer mode
@cindex mode, safer
@cindex unsafe mode
@cindex mode, unsafe
This request causes an error if used in safer mode (which is the default).
Use @code{groff}'s or @code{troff}'s @option{-U} option to activate unsafe
mode.

For example, the following code fragment introduces the current time into a
document:

@cindex time, current
@cindex current time
@pindex perl
@Example
.sy perl -e 'printf ".nr H %d\\n.nr M %d\\n.nr S %d\\n",\
             (localtime(time))[2,1,0]' > /tmp/x\n[$$]
.so /tmp/x\n[$$]
.sy rm /tmp/x\n[$$]
\nH:\nM:\nS
@endExample

@noindent
Note that this works by having the @code{perl} script (run by @code{sy})
print out the @code{nr} requests which set the number registers
@code{H}, @code{M}, and @code{S}, and then reads those commands in with
the @code{so} request.

For most practical purposes, the number registers @code{seconds},
@code{minutes}, and @code{hours} which are initialized at start-up of
@code{gtroff} should be sufficient.  Use the @code{af} request to get a
formatted output:

@Example
.af hours 00
.af minutes 00
.af seconds 00
\n[hours]:\n[minutes]:\n[seconds]
@endExample

@cindex @code{system()} return value register (@code{systat})
The @code{systat} read-write number register contains the return value
of the @code{system()} function executed by the last @code{sy} request.
@endDefreq

@DefreqList {open, stream file}
@DefreqListEnd {opena, stream file}
@cindex opening file (@code{open})
@cindex file, opening (@code{open})
@cindex appending to a file (@code{opena})
@cindex file, appending to (@code{opena})
Open the specified @var{file} for writing and
associates the specified @var{stream} with it.

The @code{opena} request is like @code{open}, but if the file exists,
append to it instead of truncating it.

@cindex safer mode
@cindex mode, safer
@cindex unsafe mode
@cindex mode, unsafe
Both @code{open} and @code{opena} cause an error if used in safer mode
(which is the default).  Use @code{groff}'s or @code{troff}'s @option{-U}
option to activate unsafe mode.
@endDefreq

@DefreqList {write, stream data}
@DefreqListEnd {writec, stream data}
@cindex copy-in mode, and @code{write} requests
@cindex mode, copy-in, and @code{write} requests
@cindex writing to file (@code{write})
@cindex file, writing to (@code{write})
Write to the file associated with the specified @var{stream}.
The stream must previously have
been the subject of an open request.  The remainder of the line is
interpreted as the @code{ds} request reads its second argument: A
leading @samp{"} is stripped, and it is read in copy-in mode.

The @code{writec} request is like @code{write}, but only
@code{write} appends a newline to the data.
@endDefreq

@Defreq {writem, stream xx}
@cindex @code{asciify} request, and @code{writem}
Write the contents of the macro or string @var{xx}
to the file associated with the specified @var{stream}.

@var{xx} is read in copy mode, i.e., already formatted elements are
ignored.  Consequently, diversions must be unformatted with the
@code{asciify} request before calling @code{writem}.  Usually, this
means a loss of information.
@endDefreq

@Defreq {close, stream}
@cindex closing file (@code{close})
@cindex file, closing (@code{close})
Close the specified @var{stream};
the stream is no longer an acceptable argument to the
@code{write} request.

Here a simple macro to write an index entry.

@Example
.open idx test.idx
.
.de IX
.  write idx \\n[%] \\$*
..
.
.IX test entry
.
.close idx
@endExample
@endDefreq

@DefescList {\\V, , e, }
@DefescItem {\\V, @lparen{}, ev, }
@DefescListEnd {\\V, @lbrack{}, env, @rbrack}
Interpolate the contents of the specified environment variable
@var{env} (one-character name@w{ }@var{e}, two-character name @var{ev})
as returned by the function @code{getenv}.  @code{\V} is interpreted
in copy-in mode.
@endDefesc


@c =====================================================================

@node Postprocessor Access, Miscellaneous, I/O, gtroff Reference
@section Postprocessor Access
@cindex postprocessor access
@cindex access of postprocessor

There are two escapes which give information directly to the
postprocessor.  This is particularly useful for embedding
@sc{PostScript} into the final document.

@Defesc {\\X, ', xxx, '}
Embeds its argument into the @code{gtroff}
output preceded with @w{@samp{x X}}.

@cindex @code{\&}, in @code{\X}
@cindex @code{\)}, in @code{\X}
@cindex @code{\%}, in @code{\X}
@ifnotinfo
@cindex @code{\:}, in @code{\X}
@end ifnotinfo
@ifinfo
@cindex @code{\@r{<colon>}}, in @code{\X}
@end ifinfo
The escapes @code{\&}, @code{\)}, @code{\%}, and @code{\:} are ignored
within @code{\X}, @w{@samp{\ }} and @code{\~} are converted to single
space characters.  All other escapes (except @code{\\} which produces a
backslash) cause an error.

@kindex use_charnames_in_special
@pindex DESC@r{, and @code{use_charnames_in_special}}
@cindex @code{\X}, and special characters
If the @samp{use_charnames_in_special} keyword is set in the @file{DESC}
file, special characters no longer cause an error; the name @var{xx} is
represented as @samp{\(@var{xx})} in the @w{@samp{x X}} output command.
Additionally, the backslash is represented as @code{\\}.

This mechanism is currently used by @code{grohtml} only.
@endDefesc

@DefescList {\\Y, , n, }
@DefescItem {\\Y, @lparen{}, nm, }
@DefescListEnd {\\Y, @lbrack{}, name, @rbrack}
This is approximately equivalent to @samp{\X'\*[@var{name}]'}
(one-character name@w{ }@var{n}, two-character name @var{nm}).
However, the contents of the string or macro @var{name} are not
interpreted; also it is permitted for @var{name} to have been defined
as a macro and thus contain newlines (it is not permitted for the
argument to @code{\X} to contain newlines).  The inclusion of
newlines requires an extension to the @acronym{UNIX} @code{troff}
output format, and confuses drivers that do not know about this
extension (@pxref{Device Control Commands}).
@endDefesc

@xref{Output Devices}.


@c =====================================================================

@node Miscellaneous, Gtroff Internals, Postprocessor Access, gtroff Reference
@section Miscellaneous

This section documents parts of @code{gtroff} which cannot (yet) be
categorized elsewhere in this manual.

@Defreq {nm, [@Var{start} [@Var{inc} [@Var{space} [@Var{indent}]]]]}
@cindex printing line numbers (@code{nm})
@cindex line numbers, printing (@code{nm})
@cindex numbers, line, printing (@code{nm})
Print line numbers.
@var{start} is the line number of the @emph{next}
output line.  @var{inc} indicates which line numbers are printed.
For example, the value@w{ }5 means to emit only line numbers which
are multiples of@w{ }5; this defaults to@w{ }1.  @var{space} is the
space to be left between the number and the text; this defaults to
one digit space.  The fourth argument is the indentation of the line
numbers, defaulting to zero.  Both @var{space} and @var{indent} are
given as multiples of digit spaces; they can be negative also.
Without any arguments, line numbers are turned off.

@code{gtroff} reserves three digit spaces for the line number (which is
printed right-justified) plus the amount given by @var{indent}; the
output lines are concatenated to the line numbers, separated by
@var{space}, and @emph{without} reducing the line length.  Depending
on the value of the horizontal page offset (as set with the
@code{po} request), line numbers which are longer than the reserved
space stick out to the left, or the whole line is moved to the right.

Parameters corresponding to missing arguments are not changed; any
non-digit argument (to be more precise, any argument starting with a
character valid as a delimiter for identifiers) is also treated as
missing.

If line numbering has been disabled with a call to @code{nm} without
an argument, it can be reactivated with @samp{.nm +0}, using the
previously active line numbering parameters.

The parameters of @code{nm} are associated with the current environment
(@pxref{Environments}).  The current output line number is available
in the number register @code{ln}.

@Example
.po 1m
.ll 2i
This test shows how line numbering works with groff.
.nm 999
This test shows how line numbering works with groff.
.br
.nm xxx 3 2
.ll -\w'0'u
This test shows how line numbering works with groff.
.nn 2
This test shows how line numbering works with groff.
@endExample

@noindent
And here the result:

@Example
 This  test shows how
 line numbering works
 999 with   groff.   This
1000 test shows how  line
1001 numbering works with
1002 groff.
      This test shows how
      line      numbering
 works  with  groff.
 This test shows how
1005  line      numbering
      works with groff.
@endExample
@endDefreq

@Defreq {nn, [@Var{skip}]}
Temporarily turn off line numbering.  The argument is the number
of lines not to be numbered; this defaults to@w{ }1.
@endDefreq

@Defreq {mc, glyph [@Var{dist}]}
@cindex margin glyph (@code{mc})
@cindex glyph, for margins (@code{mc})
Print a @dfn{margin character} to the right of the
text.@footnote{@dfn{Margin character} is a misnomer since it is an
output glyph.}  The first argument is the glyph to be
printed.  The second argument is the distance away from the right
margin.  If missing, the previously set value is used; default is
10@dmn{pt}).  For text lines that are too long (that is, longer than
the text length plus @var{dist}), the margin character is directly
appended to the lines.

With no arguments the margin character is turned off.
If this occurs before a break, no margin character is printed.

@cindex @code{tl} request, and @code{mc}
For empty lines and lines produced by the @code{tl} request no margin
character is emitted.

The margin character is associated with the current environment
(@pxref{Environments}).

@pindex nrchbar
@pindex changebar
This is quite useful for indicating text that has changed, and, in fact,
there are programs available for doing this (they are called
@code{nrchbar} and @code{changebar} and can be found in any
@samp{comp.sources.unix} archive.

@Example
.ll 3i
.mc |
This paragraph is highlighted with a margin
character.
.sp
Note that vertical space isn't marked.
.br
\&
.br
But we can fake it with `\&'.
@endExample

Result:

@Example
This  paragraph is highlighted |
with a margin character.       |

Note that vertical space isn't |
marked.                        |
                               |
But we can fake it with `\&'.  |
@endExample
@endDefreq

@DefreqList {psbb, filename}
@DefregItem {llx}
@DefregItem {lly}
@DefregItem {urx}
@DefregListEnd {ury}
@cindex PostScript, bounding box
@cindex bounding box
Retrieve the bounding box of the PostScript image
found in @var{filename}.
The file must conform to
Adobe's @dfn{Document Structuring Conventions} (DSC);
the command searches for a @code{%%BoundingBox} comment
and extracts the bounding box values into the number registers
@code{llx}, @code{lly}, @code{urx}, and @code{ury}.
If an error occurs (for example, @code{psbb} cannot find
the @code{%%BoundingBox} comment),
it sets the four number registers to zero.
@endDefreq


@c =====================================================================

@node Gtroff Internals, Debugging, Miscellaneous, gtroff Reference
@section @code{gtroff} Internals

@cindex input token
@cindex token, input
@cindex output node
@cindex node, output
@code{gtroff} processes input in three steps.  One or more input
characters are converted to an @dfn{input token}.@footnote{Except the
escapes @code{\f}, @code{\F}, @code{\H}, @code{\R}, @code{\s}, and
@code{\S} which are processed immediately if not in copy-in mode.}
Then, one or more input tokens are converted to an @dfn{output node}.
Finally, output nodes are converted to the intermediate output language
understood by all output devices.

Actually, before step one happens, @code{gtroff} converts certain
escape sequences into reserved input characters (not accessible by
the user); such reserved characters are used for other internal
processing also -- this is the very reason why not all characters
are valid input.  @xref{Identifiers}, for more on this topic.

For example, the input string @samp{fi\[:u]} is converted into a
character token @samp{f}, a character token @samp{i}, and a special
token @samp{:u} (representing u@w{ }umlaut).  Later on, the character
tokens @samp{f} and @samp{i} are merged to a single output node
representing the ligature glyph @samp{fi} (provided the current font
has a glyph for this ligature); the same happens with @samp{:u}.  All
output glyph nodes are `processed' which means that they are invariably
associated with a given font, font size, advance width, etc.  During
the formatting process, @code{gtroff} itself adds various nodes to
control the data flow.

Macros, diversions, and strings collect elements in two chained lists:
a list of input tokens which have been passed unprocessed, and a list
of output nodes.  Consider the following the diversion.

@Example
.di xxx
a
\!b
c
.br
.di
@endExample

@noindent
It contains these elements.

@multitable {@i{vertical size node}} {token list} {element number}
@item node list               @tab token list @tab element number

@item @i{line start node}     @tab ---        @tab 1
@item @i{glyph node @code{a}} @tab ---        @tab 2
@item @i{word space node}     @tab ---        @tab 3
@item ---                     @tab @code{b}   @tab 4
@item ---                     @tab @code{\n}  @tab 5
@item @i{glyph node @code{c}} @tab ---        @tab 6
@item @i{vertical size node}  @tab ---        @tab 7
@item @i{vertical size node}  @tab ---        @tab 8
@item ---                     @tab @code{\n}  @tab 9
@end multitable

@cindex @code{\v}, internal representation
@noindent
Elements 1, 7, and@w{ }8 are inserted by @code{gtroff}; the latter two
(which are always present) specify the vertical extent of the last
line, possibly modified by @code{\x}.  The @code{br} request finishes
the current partial line, inserting a newline input token which is
subsequently converted to a space when the diversion is reread.  Note
that the word space node has a fixed width which isn't stretchable
anymore.  To convert horizontal space nodes back to input tokens, use
the @code{unformat} request.

Macros only contain elements in the token list (and the node list is
empty); diversions and strings can contain elements in both lists.

Some requests like @code{tr} or @code{cflags} work on glyph
identifiers only; this means that the associated glyph can be changed
without destroying this association.  This can be very helpful for
substituting glyphs.  In the following example, we assume that
glyph @samp{foo} isn't available by default, so we provide a
substitution using the @code{fchar} request and map it to input
character @samp{x}.

@Example
.fchar \[foo] foo
.tr x \[foo]
@endExample

@noindent
Now let us assume that we install an additional special font
@samp{bar} which has glyph @samp{foo}.

@Example
.special bar
.rchar \[foo]
@endExample

@noindent
Since glyphs defined with @code{fchar} are searched before glyphs
in special fonts, we must call @code{rchar} to remove the definition
of the fallback glyph.  Anyway, the translation is still active;
@samp{x} now maps to the real glyph @samp{foo}.


@c =====================================================================

@node Debugging, Implementation Differences, Gtroff Internals, gtroff Reference
@section Debugging
@cindex debugging

@code{gtroff} is not easy to debug, but there are some useful features
and strategies for debugging.

@Defreq {lf, line filename}
@pindex soelim
@cindex multi-file documents
@cindex documents, multi-file
@cindex setting input line number (@code{lf})
@cindex input line number, setting (@code{lf})
@cindex number, input line, setting (@code{lf})
Change the line number and the file name @code{gtroff} shall use for
error and warning messages.  @var{line} is the input line number of the
@emph{next} line.

Without argument, the request is ignored.

This is a debugging aid for documents which are split into many files,
then put together with @code{soelim} and other preprocessors.  Usually,
it isn't invoked manually.
@endDefreq

@DefreqList {tm, string}
@DefreqItem {tm1, string}
@DefreqListEnd {tmc, string}
@cindex printing to stderr (@code{tm}, @code{tm1}, @code{tmc})
@cindex stderr, printing to (@code{tm}, @code{tm1}, @code{tmc})
Send @var{string} to the standard error output;
this is very useful for printing debugging messages among other things.

@var{string} is read in copy mode.

The @code{tm} request ignores leading spaces of @var{string}; @code{tm1}
handles its argument similar to the @code{ds} request: a leading double
quote in @var{string} is stripped to allow initial blanks.

The @code{tmc} request is similar to @code{tm1} but does
not append a newline (as is done in @code{tm} and @code{tm1}).
@endDefreq

@Defreq {ab, [@Var{string}]}
@cindex aborting (@code{ab})
Similar to the @code{tm} request, except that
it causes @code{gtroff} to stop processing.  With no argument it
prints @samp{User Abort.} to standard error.
@endDefreq

@Defreq {ex, }
@cindex @code{ex} request, use in debugging
@cindex exiting (@code{ex})
The @code{ex} request also causes @code{gtroff} to stop processing;
see also @ref{I/O}.
@endDefreq

When doing something involved it is useful to leave the debugging
statements in the code and have them turned on by a command line flag.

@Example
.if \n(DB .tm debugging output
@endExample

@noindent
To activate these statements say

@Example
groff -rDB=1 file
@endExample

If it is known in advance that there will be many errors and no useful
output, @code{gtroff} can be forced to suppress formatted output with
the @option{-z} flag.

@Defreq {pm, }
@cindex dumping symbol table (@code{pm})
@cindex symbol table, dumping (@code{pm})
Print the entire symbol table on @code{stderr}.  Names of all defined
macros, strings, and diversions are print together with their size in
bytes.  Since @code{gtroff} sometimes adds nodes by itself, the
returned size can be larger than expected.

This request differs from @acronym{UNIX} @code{troff}: @code{gtroff}
reports the sizes of diversions, ignores an additional argument to
print only the total of the sizes, and the size isn't returned in
blocks of 128 characters.
@endDefreq

@Defreq {pnr, }
@cindex dumping number registers (@code{pnr})
@cindex number registers, dumping (@code{pnr})
Print the names and contents of all
currently defined number registers on @code{stderr}.
@endDefreq

@Defreq {ptr, }
@cindex dumping traps (@code{ptr})
@cindex traps, dumping (@code{ptr})
Print the names and positions of all traps
(not including input line traps and diversion traps) on @code{stderr}.
Empty slots in the page trap list are printed as well, because they can
affect the priority of subsequently planted traps.
@endDefreq

@Defreq {fl, }
@cindex flush output (@code{fl})
@cindex output, flush (@code{fl})
@cindex interactive use of @code{gtroff}
@cindex @code{gtroff}, interactive use
Instruct @code{gtroff} to flush its output immediately.  The intent
is for interactive use, but this behaviour is currently not
implemented in @code{gtroff}.  Contrary to @acronym{UNIX} @code{troff},
TTY output is sent to a device driver also (@code{grotty}), making it
non-trivial to communicate interactively.

This request causes a line break.
@endDefreq

@Defreq {backtrace, }
@cindex backtrace of input stack (@code{backtrace})
@cindex input stack, backtrace (@code{backtrace})
Print a backtrace of the input stack to the standard error stream.

Consider the following in file @file{test}:

@Example
.de xxx
.  backtrace
..
.de yyy
.  xxx
..
.
.yyy
@endExample

@noindent
On execution, @code{gtroff} prints the following:

@Example
test:2: backtrace: macro `xxx'
test:5: backtrace: macro `yyy'
test:8: backtrace: file `test'
@endExample

The option @option{-b} of @code{gtroff} internally calls a variant of
this request on each error and warning.
@endDefreq

@Defreg {slimit}
@cindex input stack, setting limit
Use the @code{slimit} number register
to set the maximum number of objects on the input stack.
If @code{slimit} is less than or equal to@w{ }0,
there is no limit set.
With no limit, a buggy recursive macro can exhaust virtual memory.

The default value is 1000; this is a compile-time constant.
@endDefreg

@Defreq {warnscale, si}
Set the scaling indicator used in warnings to @var{si}.  Valid values for
@var{si} are @samp{u}, @samp{i}, @samp{c}, @samp{p}, and @samp{P}.  At
startup, it is set to @samp{i}.
@endDefreq

@Defreq {spreadwarn, [@Var{limit}]}
Make @code{gtroff} emit a warning if the additional space inserted for
each space between words in an output line is larger or equal to
@var{limit}.  A negative value is changed to zero; no argument toggles the
warning on and off without changing @var{limit}.  The default scaling
indicator is @samp{m}.  At startup, @code{spreadwarn} is deactivated, and
@var{limit} is set to 3@dmn{m}.

For example,

@Example
.spreadwarn 0.2m
@endExample

@noindent
will cause a warning if @code{gtroff} must add 0.2@dmn{m} or more for each
interword space in a line.

This request is active only if text is justified to both margins (using
@w{@samp{.ad b}}).
@endDefreq

@cindex warnings
@code{gtroff} has command line options for printing out more warnings
(@option{-w}) and for printing backtraces (@option{-b}) when a warning
or an error occurs.  The most verbose level of warnings is @option{-ww}.

@DefreqList {warn, [@Var{flags}]}
@DefregListEnd {.warn}
@cindex level of warnings (@code{warn})
@cindex warnings, level (@code{warn})
Control the level of warnings checked for.  The @var{flags} are the sum
of the numbers associated with each warning that is to be enabled; all
other warnings are disabled.  The number associated with each warning is
listed below.  For example, @w{@code{.warn 0}} disables all warnings,
and @w{@code{.warn 1}} disables all warnings except that about missing
glyphs.  If no argument is given, all warnings are enabled.

The read-only number register @code{.warn} contains the current warning
level.
@endDefreq

@menu
* Warnings::
@end menu

@c ---------------------------------------------------------------------

@node Warnings,  , Debugging, Debugging
@subsection Warnings
@cindex warnings

The warnings that can be given to @code{gtroff} are divided into the
following categories.  The name associated with each warning is used by
the @option{-w} and @option{-W} options; the number is used by the
@code{warn} request and by the @code{.warn} register.

@table @samp
@item char
@itemx 1
Non-existent glyphs.@footnote{@code{char} is a misnomer since it reports
missing glyphs -- there aren't missing input characters, only invalid
ones.}  This is enabled by default.

@item number
@itemx 2
Invalid numeric expressions.  This is enabled by default.
@xref{Expressions}.

@item break
@itemx 4
@cindex fill mode
@cindex mode, fill
In fill mode, lines which could not be broken so that their length was
less than the line length.  This is enabled by default.

@item delim
@itemx 8
Missing or mismatched closing delimiters.

@item el
@itemx 16
@cindex @code{ie} request, and warnings
@cindex @code{el} request, and warnings
Use of the @code{el} request with no matching @code{ie} request.
@xref{if-else}.

@item scale
@itemx 32
Meaningless scaling indicators.

@item range
@itemx 64
Out of range arguments.

@item syntax
@itemx 128
Dubious syntax in numeric expressions.

@item di
@itemx 256
@cindex @code{di} request, and warnings
@cindex @code{da} request, and warnings
Use of @code{di} or @code{da} without an argument when there is no
current diversion.

@item mac
@itemx 512
@cindex @code{de}, @code{de1}, @code{dei} requests, and warnings
@cindex @code{am}, @code{am1}, @code{ami} requests, and warnings
@cindex @code{ds}, @code{ds1} requests, and warnings
@cindex @code{as}, @code{as1} requests, and warnings
@cindex @code{di} request, and warnings
@cindex @code{da} request, and warnings
@cindex @code{box}, @code{boxa} requests, and warnings
@cindex @code{\*}, and warnings
Use of undefined strings, macros and diversions.  When an undefined
string, macro, or diversion is used, that string is automatically
defined as empty.  So, in most cases, at most one warning is given
for each name.

@item reg
@itemx 1024
@cindex @code{nr} request, and warnings
@cindex @code{\R}, and warnings
@cindex @code{\n}, and warnings
Use of undefined number registers.  When an undefined number register is
used, that register is automatically defined to have a value of@w{ }0.
So, in most cases, at most one warning is given for use of a particular
name.

@item tab
@itemx 2048
@cindex @code{\t}, and warnings
Use of a tab character where a number was expected.

@item right-brace
@itemx 4096
@cindex @code{\@}}, and warnings
Use of @code{\@}} where a number was expected.

@item missing
@itemx 8192
Requests that are missing non-optional arguments.

@item input
@itemx 16384
Invalid input characters.

@item escape
@itemx 32768
Unrecognized escape sequences.  When an unrecognized escape sequence
@code{\@var{X}} is encountered, the escape character is ignored, and
@var{X} is printed.

@item space
@itemx 65536
@cindex compatibility mode
Missing space between a request or macro and its argument.  This warning
is given when an undefined name longer than two characters is
encountered, and the first two characters of the name make a defined
name.  The request or macro is not invoked.  When this warning is
given, no macro is automatically defined.  This is enabled by default.
This warning never occurs in compatibility mode.

@item font
@itemx 131072
Non-existent fonts.  This is enabled by default.

@item ig
@itemx 262144
Invalid escapes in text ignored with the @code{ig} request.  These are
conditions that are errors when they do not occur in ignored text.

@item color
@itemx 524288
Color related warnings.

@item all
All warnings except @samp{di}, @samp{mac} and @samp{reg}.  It is
intended that this covers all warnings that are useful with traditional
macro packages.

@item w
All warnings.
@end table


@c =====================================================================

@node Implementation Differences,  , Debugging, gtroff Reference
@section Implementation Differences
@cindex implementation differences
@cindex differences in implementation
@cindex incompatibilities with @acronym{AT&T} @code{troff}
@cindex compatibility mode
@cindex mode, compatibility

GNU @code{troff} has a number of features which cause incompatibilities
with documents written with old versions of @code{troff}.

@cindex long names
@cindex names, long
Long names cause some incompatibilities.  @acronym{UNIX} @code{troff}
interprets

@Example
.dsabcd
@endExample

@cindex @code{\*}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\n}, incompatibilities with @acronym{AT&T} @code{troff}
@noindent
as defining a string @samp{ab} with contents @samp{cd}.  Normally, GNU
@code{troff} interprets this as a call of a macro named
@code{dsabcd}.  Also @acronym{UNIX} @code{troff} interprets
@code{\*[} or @code{\n[} as references to a string or number register
called @samp{[}.  In GNU @code{troff}, however, this is normally
interpreted as the start of a long name.  In compatibility mode GNU
@code{troff} interprets long names in the traditional way
(which means that they are not recognized as names).

@DefreqList {cp, [@Var{n}]}
@DefreqItem {do, cmd}
@DefregListEnd {.C}
If @var{n} is missing or non-zero, turn on compatibility mode;
otherwise, turn it off.

The read-only number register @code{.C} is@w{ }1 if compatibility mode is
on, 0@w{ }otherwise.

Compatibility mode can be also turned on with the @option{-C} command line
option.

The @code{do} request turns off compatibility mode
while executing its arguments as a @code{gtroff} command.

@Example
.do fam T
@endExample

@noindent
executes the @code{fam} request when compatibility mode
is enabled.

@code{gtroff} restores the previous compatibility setting
before interpreting any files sourced by the @var{cmd}.
@endDefreq

@cindex input level in delimited arguments
@cindex delimited arguments, incompatibilities with @acronym{AT&T} @code{troff}
Two other features are controlled by @option{-C}.  If not in
compatibility mode, GNU @code{troff} preserves the input level in
delimited arguments:

@Example
.ds xx '
\w'abc\*(xxdef'
@endExample

@noindent
In compatibility mode, the string @samp{72def'} is returned; without
@option{-C} the resulting string is @samp{168} (assuming a TTY output
device).

@cindex @code{\f}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\H}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\R}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\s}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\S}, incompatibilities with @acronym{AT&T} @code{troff}
Finally, the escapes @code{\f}, @code{\H}, @code{\R}, @code{\s}, and
@code{\S} are transparent for recognizing the beginning of a line only
in compatibility mode (this is a rather obscure feature).  For example,
the code

@Example
.de xx
Hallo!
..
\fB.xx\fP
@endExample

prints @samp{Hallo!} in bold face if in compatibility mode, and
@samp{.xx} in bold face otherwise.

@cindex @code{\A}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\|}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\^}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\&}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\@{}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\@}}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\@key{SP}}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\'}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\`}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\-}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\_}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\!}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\%}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\c}, incompatibilities with @acronym{AT&T} @code{troff}
GNU @code{troff} does not allow the use of the escape sequences
@code{\|}, @code{\^}, @code{\&}, @code{\@{}, @code{\@}},
@code{\@key{SP}}, @code{\'}, @code{\`}, @code{\-}, @code{\_}, @code{\!},
@code{\%}, and @code{\c} in names of strings, macros, diversions, number
registers, fonts or environments; @acronym{UNIX} @code{troff} does.  The
@code{\A} escape sequence (@pxref{Identifiers}) may be helpful in
avoiding use of these escape sequences in names.

@cindex fractional point sizes
@cindex fractional type sizes
@cindex point sizes, fractional
@cindex type sizes, fractional
@cindex sizes, fractional
@cindex @code{ps} request, incompatibilities with @acronym{AT&T} @code{troff}
Fractional point sizes cause one noteworthy incompatibility.  In
@acronym{UNIX} @code{troff} the @code{ps} request ignores scale
indicators and thus

@Example
.ps 10u
@endExample

@noindent
sets the point size to 10@w{ }points, whereas in GNU @code{troff} it
sets the point size to 10@w{ }scaled points.  @xref{Fractional Type
Sizes}, for more information.

@cindex @code{bd} request, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{cs} request, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{tr} request, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{fp} request, incompatibilities with @acronym{AT&T} @code{troff}
@cindex input characters and output glyphs, compatibility with @acronym{AT&T} @code{troff}
@cindex output glyphs, and input characters,compatibility with @acronym{AT&T} @code{troff}
@cindex characters, input, and output glyphs, compatibility with @acronym{AT&T} @code{troff}
@cindex glyphs, output, and input characters, compatibility with @acronym{AT&T} @code{troff}
In GNU @code{troff} there is a fundamental difference between
(unformatted) input characters and (formatted) output glyphs.
Everything that affects how a glyph is output is stored
with the glyph node; once a glyph node has been constructed it is
unaffected by any subsequent requests that are executed, including
@code{bd}, @code{cs}, @code{tkf}, @code{tr}, or @code{fp} requests.
Normally glyphs are constructed from input characters at the
moment immediately before the glyph is added to the current output
line.  Macros, diversions and strings are all, in fact, the same type of
object; they contain lists of input characters and glyph nodes in
any combination.  A glyph node does not behave like an input
character for the purposes of macro processing; it does not inherit any
of the special properties that the input character from which it was
constructed might have had.  For example,

@Example
.di x
\\\\
.br
.di
.x
@endExample

@cindex printing backslash (@code{\\}, @code{\e}, @code{\E}, @code{\[rs]})
@cindex backslash, printing (@code{\\}, @code{\e}, @code{\E}, @code{\[rs]})
@cindex @code{\e}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\!}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex @code{\?}, incompatibilities with @acronym{AT&T} @code{troff}
@cindex transparent output, incompatibilities with @acronym{AT&T} @code{troff}
@cindex output, transparent, incompatibilities with @acronym{AT&T} @code{troff}
@noindent
prints @samp{\\} in GNU @code{troff}; each pair of input backslashes
is turned into one output backslash and the resulting output backslashes
are not interpreted as escape characters when they are reread.
@acronym{UNIX} @code{troff} would interpret them as escape characters
when they were reread and would end up printing one @samp{\}.  The
correct way to obtain a printable backslash is to use the @code{\e}
escape sequence: This always prints a single instance of the current
escape character, regardless of whether or not it is used in a
diversion; it also works in both GNU @code{troff} and @acronym{UNIX}
@code{troff}.@footnote{To be completely independent of the current
escape character, use @code{\(rs} which represents a reverse solidus
(backslash) glyph.}  To store, for some reason, an escape sequence in a
diversion that will be interpreted when the diversion is reread, either
use the traditional @code{\!} transparent output facility, or, if this
is unsuitable, the new @code{\?} escape sequence.

@xref{Diversions}, and @ref{Gtroff Internals}, for more information.



@c =====================================================================
@c =====================================================================

@node Preprocessors, Output Devices, gtroff Reference, Top
@chapter Preprocessors
@cindex preprocessors

This chapter describes all preprocessors that come with @code{groff} or
which are freely available.

@menu
* geqn::
* gtbl::
* gpic::
* ggrn::
* grap::
* grefer::
* gsoelim::
@end menu


@c =====================================================================

@node geqn, gtbl, Preprocessors, Preprocessors
@section @code{geqn}
@cindex @code{eqn}, the program
@cindex @code{geqn}, the program

@c XXX

@menu
* Invoking geqn::
@end menu

@c ---------------------------------------------------------------------

@node Invoking geqn,  , geqn, geqn
@subsection Invoking @code{geqn}
@cindex invoking @code{geqn}
@cindex @code{geqn}, invoking

@c XXX


@c =====================================================================

@node gtbl, gpic, geqn, Preprocessors
@section @code{gtbl}
@cindex @code{tbl}, the program
@cindex @code{gtbl}, the program

@c XXX

@menu
* Invoking gtbl::
@end menu

@c ---------------------------------------------------------------------

@node Invoking gtbl,  , gtbl, gtbl
@subsection Invoking @code{gtbl}
@cindex invoking @code{gtbl}
@cindex @code{gtbl}, invoking

@c XXX


@c =====================================================================

@node gpic, ggrn, gtbl, Preprocessors
@section @code{gpic}
@cindex @code{pic}, the program
@cindex @code{gpic}, the program

@c XXX

@menu
* Invoking gpic::
@end menu

@c ---------------------------------------------------------------------

@node Invoking gpic,  , gpic, gpic
@subsection Invoking @code{gpic}
@cindex invoking @code{gpic}
@cindex @code{gpic}, invoking

@c XXX


@c =====================================================================

@node ggrn, grap, gpic, Preprocessors
@section @code{ggrn}
@cindex @code{grn}, the program
@cindex @code{ggrn}, the program

@c XXX

@menu
* Invoking ggrn::
@end menu

@c ---------------------------------------------------------------------

@node Invoking ggrn,  , ggrn, ggrn
@subsection Invoking @code{ggrn}
@cindex invoking @code{ggrn}
@cindex @code{ggrn}, invoking

@c XXX


@c =====================================================================

@node grap, grefer, ggrn, Preprocessors
@section @code{grap}
@cindex @code{grap}, the program

A free implementation of @code{grap}, written by Ted Faber,
is available as an extra package from the following address:

@display
@url{http://www.lunabase.org/~faber/Vault/software/grap/}
@end display


@c =====================================================================

@node grefer, gsoelim, grap, Preprocessors
@section @code{grefer}
@cindex @code{refer}, the program
@cindex @code{grefer}, the program

@c XXX

@menu
* Invoking grefer::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grefer,  , grefer, grefer
@subsection Invoking @code{grefer}
@cindex invoking @code{grefer}
@cindex @code{grefer}, invoking

@c XXX


@c =====================================================================

@node gsoelim,  , grefer, Preprocessors
@section @code{gsoelim}
@cindex @code{soelim}, the program
@cindex @code{gsoelim}, the program

@c XXX

@menu
* Invoking gsoelim::
@end menu

@c ---------------------------------------------------------------------

@node Invoking gsoelim,  , gsoelim, gsoelim
@subsection Invoking @code{gsoelim}
@cindex invoking @code{gsoelim}
@cindex @code{gsoelim}, invoking

@c XXX



@c =====================================================================
@c =====================================================================

@node Output Devices, File formats, Preprocessors, Top
@chapter Output Devices
@cindex output devices
@cindex devices for output

@c XXX

@menu
* Special Characters::
* grotty::
* grops::
* grodvi::
* grolj4::
* grolbp::
* grohtml::
* gxditview::
@end menu


@c =====================================================================

@node Special Characters, grotty, Output Devices, Output Devices
@section Special Characters
@cindex special characters
@cindex characters, special

@c XXX

@xref{Font Files}.


@c =====================================================================

@node grotty, grops, Special Characters, Output Devices
@section @code{grotty}
@cindex @code{grotty}, the program

@c XXX

@menu
* Invoking grotty::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grotty,  , grotty, grotty
@subsection Invoking @code{grotty}
@cindex invoking @code{grotty}
@cindex @code{grotty}, invoking

@c XXX

@c The following is no longer true; fix and extend it.

@c @pindex less
@c @cindex Teletype
@c @cindex ISO 6249 SGR
@c @cindex terminal control sequences
@c @cindex control sequences, for terminals
@c For TTY output devices, underlining is done by emitting sequences of
@c @samp{_} and @samp{\b} (the backspace character) before the actual
@c character.  Literally, this is printing an underline character, then
@c moving back one character position, and printing the actual character
@c at the same position as the underline character (similar to a
@c typewriter).  Usually, a modern terminal can't interpret this (and the
@c original Teletype machines for which this sequence was appropriate are
@c no longer in use).  You need a pager program like @code{less} which
@c translates this into ISO 6429 SGR sequences to control terminals.


@c =====================================================================

@node grops, grodvi, grotty, Output Devices
@section @code{grops}
@cindex @code{grops}, the program

@c XXX

@menu
* Invoking grops::
* Embedding PostScript::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grops, Embedding PostScript, grops, grops
@subsection Invoking @code{grops}
@cindex invoking @code{grops}
@cindex @code{grops}, invoking

@c XXX

@c ---------------------------------------------------------------------

@node Embedding PostScript,  , Invoking grops, grops
@subsection Embedding @sc{PostScript}
@cindex embedding PostScript
@cindex PostScript, embedding

@c XXX


@c =====================================================================

@node grodvi, grolj4, grops, Output Devices
@section @code{grodvi}
@cindex @code{grodvi}, the program

@c XXX

@menu
* Invoking grodvi::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grodvi,  , grodvi, grodvi
@subsection Invoking @code{grodvi}
@cindex invoking @code{grodvi}
@cindex @code{grodvi}, invoking

@c XXX


@c =====================================================================

@node grolj4, grolbp, grodvi, Output Devices
@section @code{grolj4}
@cindex @code{grolj4}, the program

@c XXX

@menu
* Invoking grolj4::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grolj4,  , grolj4, grolj4
@subsection Invoking @code{grolj4}
@cindex invoking @code{grolj4}
@cindex @code{grolj4}, invoking

@c XXX


@c =====================================================================

@node grolbp, grohtml, grolj4, Output Devices
@section @code{grolbp}
@cindex @code{grolbp}, the program

@c XXX

@menu
* Invoking grolbp::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grolbp,  , grolbp, grolbp
@subsection Invoking @code{grolbp}
@cindex invoking @code{grolbp}
@cindex @code{grolbp}, invoking

@c XXX


@c =====================================================================

@node grohtml, gxditview, grolbp, Output Devices
@section @code{grohtml}
@cindex @code{grohtml}, the program

@c XXX

@menu
* Invoking grohtml::
* grohtml specific registers and strings::
@end menu

@c ---------------------------------------------------------------------

@node Invoking grohtml, grohtml specific registers and strings, grohtml, grohtml
@subsection Invoking @code{grohtml}
@cindex invoking @code{grohtml}
@cindex @code{grohtml}, invoking

@c XXX

@c ---------------------------------------------------------------------

@node grohtml specific registers and strings,  , Invoking grohtml, grohtml
@subsection @code{grohtml} specific registers and strings
@cindex registers specific to @code{grohtml}
@cindex strings specific to @code{grohtml}
@cindex @code{grohtml}, registers and strings

@DefmpregList {ps4html, grohtml}
@DefstrListEnd {www-image-template, grohtml}
The registers @code{ps4html} and @code{www-image-template} are defined
by the @code{pre-grohtml} preprocessor.  @code{pre-grohtml} reads in
the @code{troff} input, marks up the inline equations and passes the
result firstly to

@Example
troff -Tps -rps4html=1 -dwww-image-template=@var{template}
@endExample

@noindent
and secondly to

@Example
troff -Thtml
@endExample

The PostScript device is used to create all the image files, and the
register @code{ps4html} enables the macro sets to ignore floating
keeps, footers, and headings.

The register @code{www-image-template} is set to the user specified
template name or the default name.
@endDefmpreg


@c =====================================================================

@node gxditview,  , grohtml, Output Devices
@section @code{gxditview}
@cindex @code{gxditview}, the program

@c XXX

@menu
* Invoking gxditview::
@end menu

@c ---------------------------------------------------------------------

@node Invoking gxditview,  , gxditview, gxditview
@subsection Invoking @code{gxditview}
@cindex invoking @code{gxditview}
@cindex @code{gxditview}, invoking

@c XXX
@c X11's xditview



@c =====================================================================
@c =====================================================================

@node File formats, Installation, Output Devices, Top
@chapter File formats
@cindex file formats
@cindex formats, file

All files read and written by @code{gtroff} are text files.  The
following two sections describe their format.

@menu
* gtroff Output::
* Font Files::
@end menu


@c =====================================================================

@node gtroff Output, Font Files, File formats, File formats
@section @code{gtroff} Output
@cindex @code{gtroff}, output
@cindex output, @code{gtroff}

This section describes the intermediate output format of GNU
@code{troff}.  This output is produced by a run of @code{gtroff}
before it is fed into a device postprocessor program.

As @code{groff} is a wrapper program around @code{gtroff} that
automatically calls a postprocessor, this output does not show up
normally.  This is why it is called @dfn{intermediate}.
@code{groff} provides the option @option{-Z} to inhibit postprocessing,
such that the produced intermediate output is sent to standard output
just like calling @code{gtroff} manually.

@cindex troff output
@cindex output, troff
@cindex intermediate output
@cindex output, intermediate
Here, the term @dfn{troff output} describes what is output by
@code{gtroff}, while @dfn{intermediate output} refers to the language
that is accepted by the parser that prepares this output for the
postprocessors.  This parser is smarter on whitespace and implements
obsolete elements for compatibility, otherwise both formats are the
same.@footnote{The parser and postprocessor for intermediate output
can be found in the file@*
@file{@var{groff-source-dir}/src/libs/libdriver/input.cc}.}

The main purpose of the intermediate output concept is to facilitate
the development of postprocessors by providing a common programming
interface for all devices.  It has a language of its own that is
completely different from the @code{gtroff} language.  While the
@code{gtroff} language is a high-level programming language for text
processing, the intermediate output language is a kind of low-level
assembler language by specifying all positions on the page for writing
and drawing.

The intermediate output produced by @code{gtroff} is fairly readable,
while output from @acronym{AT&T} @code{troff} is rather hard to
understand because of strange habits that are still supported, but not
used any longer by @code{gtroff}.

@menu
* Language Concepts::
* Command Reference::
* Intermediate Output Examples::
* Output Language Compatibility::
@end menu

@c ---------------------------------------------------------------------

@node Language Concepts, Command Reference, gtroff Output, gtroff Output
@subsection Language Concepts

During the run of @code{gtroff}, the input data is cracked down to the
information on what has to be printed at what position on the intended
device.  So the language of the intermediate output format can be quite
small.  Its only elements are commands with and without arguments.
In this section, the term @dfn{command} always refers to the intermediate
output language, and never to the @code{gtroff} language used for document
formatting.  There are commands for positioning and text writing, for drawing, and
for device controlling.

@menu
* Separation::
* Argument Units::
* Document Parts::
@end menu

@node Separation, Argument Units, Language Concepts, Language Concepts
@subsubsection Separation

@acronym{AT&T} @code{troff} output has strange requirements on whitespace.
The @code{gtroff} output parser, however, is smart about whitespace by
making it maximally optional.  The whitespace characters, i.e., the
tab, space, and newline characters, always have a syntactical meaning.
They are never printable because spacing within the output is always
done by positioning commands.

Any sequence of space or tab characters is treated as a single
@dfn{syntactical space}.  It separates commands and arguments, but is
only required when there would occur a clashing between the command code
and the arguments without the space.  Most often, this happens when
variable-length command names, arguments, argument lists, or command
clusters meet.  Commands and arguments with a known, fixed length need
not be separated by syntactical space.

A line break is a syntactical element, too.  Every command argument can be
followed by whitespace, a comment, or a newline character.  Thus a
@dfn{syntactical line break} is defined to consist of optional
syntactical space that is optionally followed by a comment, and a
newline character.

The normal commands, those for positioning and text, consist of a
single letter taking a fixed number of arguments.  For historical reasons,
the parser allows to stack such commands on the same line, but
fortunately, in @code{gtroff}'s intermediate output, every command with
at least one argument is followed by a line break, thus providing
excellent readability.

The other commands -- those for drawing and device controlling --
have a more complicated structure; some recognize long command names,
and some take a variable number of arguments.  So all @samp{D} and
@samp{x} commands were designed to request a syntactical line break
after their last argument.  Only one command, @w{@samp{x X}},
has an argument that can stretch over several lines; all other
commands must have all of their arguments on the same line as the
command, i.e., the arguments may not be splitted by a line break.

Empty lines (these are lines containing only space and/or a comment), can
occur everywhere.  They are just ignored.

@node Argument Units, Document Parts, Separation, Language Concepts
@subsubsection Argument Units

Some commands take integer arguments that are assumed to represent
values in a measurement unit, but the letter for the corresponding
scale indicator is not written with the output command arguments.
Most commands assume the scale indicator @samp{u}, the basic unit of
the device, some use @samp{z}, the scaled point unit of the device,
while others, such as the color commands, expect plain integers.

Note that single characters can have the eighth bit set, as can the
names of fonts and special characters.  The names of characters and
fonts can be of arbitrary length.  A character that is to be printed
will always be in the current font.

A string argument is always terminated by the next whitespace
character (space, tab, or newline); an embedded @samp{#} character is
regarded as part of the argument, not as the beginning of a comment
command.  An integer argument is already terminated by the next
non-digit character, which then is regarded as the first character of
the next argument or command.

@node Document Parts,  , Argument Units, Language Concepts
@subsubsection Document Parts

A correct intermediate output document consists of two parts, the
@dfn{prologue} and the @dfn{body}.

The task of the prologue is to set the general device parameters
using three exactly specified commands.  @code{gtroff}'s prologue
is guaranteed to consist of the following three lines (in that order):

@Example
x T @var{device}
x res @var{n} @var{h} @var{v}
x init
@endExample

@noindent
with the arguments set as outlined in @ref{Device Control Commands}.
Note that the parser for the intermediate output format is able to
swallow additional whitespace and comments as well even in the
prologue.

The body is the main section for processing the document data.
Syntactically, it is a sequence of any commands different from the
ones used in the prologue.  Processing is terminated as soon as the
first @w{@samp{x stop}} command is encountered; the last line of any
@code{gtroff} intermediate output always contains such a command.

Semantically, the body is page oriented.  A new page is started by a
@samp{p} command.  Positioning, writing, and drawing commands are
always done within the current page, so they cannot occur before the
first @samp{p} command.  Absolute positioning (by the @samp{H} and
@samp{V} commands) is done relative to the current page; all other
positioning is done relative to the current location within this page.

@c ---------------------------------------------------------------------

@node Command Reference, Intermediate Output Examples, Language Concepts, gtroff Output
@subsection Command Reference

This section describes all intermediate output commands, both from
@acronym{AT&T} @code{troff} as well as the @code{gtroff} extensions.

@menu
* Comment Command::
* Simple Commands::
* Graphics Commands::
* Device Control Commands::
* Obsolete Command::
@end menu

@node Comment Command, Simple Commands, Command Reference, Command Reference
@subsubsection Comment Command

@table @code
@item #@var{anything}@angles{end of line}
A comment.  Ignore any characters from the @samp{#} character up to
the next newline character.

This command is the only possibility for commenting in the intermediate
output.  Each comment can be preceded by arbitrary syntactical space;
every command can be terminated by a comment.
@end table

@node Simple Commands, Graphics Commands, Comment Command, Command Reference
@subsubsection Simple Commands

The commands in this subsection have a command code consisting of a
single character, taking a fixed number of arguments.  Most of them
are commands for positioning and text writing.  These commands are
smart about whitespace.  Optionally, syntactical space can be inserted
before, after, and between the command letter and its arguments.
All of these commands are stackable, i.e., they can be preceded by
other simple commands or followed by arbitrary other commands on the
same line.  A separating syntactical space is only necessary when two
integer arguments would clash or if the preceding argument ends with a
string argument.

@table @code
@ignore
.if (\n[@USE_ENV_STACK] == 1) \{\
.command {
Open a new environment by copying the actual device configuration data
to the environment stack.
.
The current environment is setup by the device specification and
manipulated by the setting commands.
.
.
.command }
Close the actual environment (opened by a preceding
.BR { \~command)
and restore the previous environment from the environment
stack as the actual device configuration data.
.
\}              \" endif @USE_ENV_STACK
@end ignore

@item C @var{xxx}@angles{whitespace}
Print a special character named @var{xxx}.  The trailing
syntactical space or line break is necessary to allow glyph names
of arbitrary length.  The glyph is printed at the current print
position; the glyph's size is read from the font file.  The print
position is not changed.

@item c @var{g}
Print glyph@w{ }@var{g} at the current print position;@footnote{@samp{c}
is actually a misnomer since it outputs a glyph.} the glyph's size is
read from the font file.  The print position is not changed.

@item f @var{n}
Set font to font number@w{ }@var{n} (a non-negative integer).

@item H @var{n}
Move right to the absolute vertical position@w{ }@var{n} (a
non-negative integer in basic units @samp{u} relative to left edge
of current page.

@item h @var{n}
Move @var{n} (a non-negative integer) basic units @samp{u} horizontally
to the right.  The original @acronym{UNIX} troff manual allows negative
values for @var{n} also, but @code{gtroff} doesn't use this.

@item m @var{color-scheme} @r{[}@var{component} @dots{}@r{]}
Set the color for text (glyphs), line drawing, and the outline of
graphic objects using different color schemes; the analoguous command
for the filling color of graphic objects is @samp{DF}.  The color
components are specified as integer arguments between 0 and 65536.
The number of color components and their meaning vary for the
different color schemes.  These commands are generated by
@code{gtroff}'s escape sequence @code{\m}.  No position changing.
These commands are a @code{gtroff} extension.

@table @code
@item mc @var{cyan} @var{magenta} @var{yellow}
Set color using the CMY color scheme, having the 3@w{ }color components
@var{cyan}, @var{magenta}, and @var{yellow}.

@item md
Set color to the default color value (black in most cases).
No component arguments.

@item mg @var{gray}
Set color to the shade of gray given by the argument, an integer
between 0 (black) and 65536 (white).

@item mk @var{cyan} @var{magenta} @var{yellow} @var{black}
Set color using the CMYK color scheme, having the 4@w{ }color components
@var{cyan}, @var{magenta}, @var{yellow}, and @var{black}.

@item mr @var{red} @var{green} @var{blue}
Set color using the RGB color scheme, having the 3@w{ }color components
@var{red}, @var{green}, and @var{blue}.

@end table

@item N @var{n}
Print glyph with index@w{ }@var{n} (a non-negative integer) of the
current font.  This command is a @code{gtroff} extension.

@item n @var{b} @var{a}
Inform the device about a line break, but no positioning is done by
this command.  In @acronym{AT&T} @code{troff}, the integer arguments
@var{b} and@w{ }@var{a} informed about the space before and after the
current line to make the intermediate output more human readable
without performing any action.  In groff, they are just ignored, but
they must be provided for compatibility reasons.

@item p @var{n}
Begin a new page in the outprint.  The page number is set
to@w{ }@var{n}.  This page is completely independent of pages formerly
processed even if those have the same page number.  The vertical
position on the outprint is automatically set to@w{ }0.  All
positioning, writing, and drawing is always done relative to a page,
so a @samp{p} command must be issued before any of these commands.

@item s @var{n}
Set point size to @var{n}@w{ }scaled points (this is unit @samp{z}).
@acronym{AT&T} @code{troff} used the unit points (@samp{p}) instead.
@xref{Output Language Compatibility}.

@item t @var{xxx}@angles{whitespace}
@itemx t @var{xxx} @var{dummy-arg}@angles{whitespace}
Print a word, i.e., a sequence of characters @var{xxx} representing
output glyphs which names are single characters, terminated by
a space character or a line break; an optional second integer argument
is ignored (this allows the formatter to generate an even number of
arguments).  The first glyph should be printed at the current
position, the current horizontal position should then be increased by
the width of the first glyph, and so on for each glyph.
The widths of the glyphs are read from the font file, scaled for the
current point size, and rounded to a multiple of the horizontal
resolution.  Special characters cannot be printed using this command
(use the @samp{C} command for special characters).  This command is a
@code{gtroff} extension; it is only used for devices whose @file{DESC}
file contains the @code{tcommand} keyword (@pxref{DESC File Format}).

@item u @var{n} @var{xxx}@angles{whitespace}
Print word with track kerning.  This is the same as the @samp{t}
command except that after printing each glyph, the current
horizontal position is increased by the sum of the width of that
glyph and@w{ }@var{n} (an integer in basic units @samp{u}).
This command is a @code{gtroff} extension; it is only used for devices
whose @file{DESC} file contains the @code{tcommand} keyword
(@pxref{DESC File Format}).

@item V @var{n}
Move down to the absolute vertical position@w{ }@var{n} (a
non-negative integer in basic units @samp{u}) relative to upper edge
of current page.

@item v @var{n}
Move @var{n}@w{ }basic units @samp{u} down (@var{n} is a non-negative
integer).  The original @acronym{UNIX} troff manual allows negative
values for @var{n} also, but @code{gtroff} doesn't use this.

@item w
Informs about a paddable white space to increase readability.
The spacing itself must be performed explicitly by a move command.

@end table

@node Graphics Commands, Device Control Commands, Simple Commands, Command Reference
@subsubsection Graphics Commands

Each graphics or drawing command in the intermediate output starts
with the letter @samp{D}, followed by one or two characters that
specify a subcommand; this is followed by a fixed or variable number
of integer arguments that are separated by a single space character.
A @samp{D} command may not be followed by another command on the same line
(apart from a comment), so each @samp{D} command is terminated by a
syntactical line break.

@code{gtroff} output follows the classical spacing rules (no space
between command and subcommand, all arguments are preceded by a
single space character), but the parser allows optional space between
the command letters and makes the space before the first argument
optional.  As usual, each space can be any sequence of tab and space
characters.

Some graphics commands can take a variable number of arguments.
In this case, they are integers representing a size measured in basic
units @samp{u}.  The arguments called @var{h1}, @var{h2}, @dots{},
@var{hn} stand for horizontal distances where positive means right,
negative left.  The arguments called @var{v1}, @var{v2}, @dots{},
@var{vn} stand for vertical distances where positive means down,
negative up.  All these distances are offsets relative to the current
location.

Unless indicated otherwise, each graphics command directly corresponds
to a similar @code{gtroff} @code{\D} escape sequence.  @xref{Drawing
Requests}.

Unknown @samp{D} commands are assumed to be device-specific.
Its arguments are parsed as strings; the whole information is then
sent to the postprocessor.

In the following command reference, the syntax element
@angles{line break} means a syntactical line break as defined above.

@table @code
@item D~ @var{h1} @var{v1} @var{h2} @var{v2} @dots{} @var{hn} @var{vn}@angles{line break}
Draw B-spline from current position to offset (@var{h1},@var{v1}),
then to offset (@var{h2},@var{v2}), if given, etc.@: up to
(@var{hn},@var{vn}).  This command takes a variable number of argument
pairs; the current position is moved to the terminal point of the drawn
curve.

@item Da @var{h1} @var{v1} @var{h2} @var{v2}@angles{line break}
Draw arc from current position to
(@var{h1},@var{v1})@math{+}(@var{h2},@var{v2}) with center at
(@var{h1},@var{v1}); then move the current position to the final point
of the arc.

@item DC @var{d}@angles{line break}
@itemx DC @var{d} @var{dummy-arg}@angles{line break}
Draw a solid circle using the current fill color with
diameter@w{ }@var{d} (integer in basic units @samp{u}) with leftmost
point at the current position; then move the current position to the
rightmost point of the circle.  An optional second integer argument is
ignored (this allows the formatter to generate an even number of
arguments).  This command is a @code{gtroff} extension.

@item Dc @var{d}@angles{line break}
Draw circle line with diameter@w{ }@var{d} (integer in basic units
@samp{u}) with leftmost point at the current position; then move the
current position to the rightmost point of the circle.

@item DE @var{h} @var{v}@angles{line break}
Draw a solid ellipse in the current fill color with a horizontal
diameter of@w{ }@var{h} and a vertical diameter of@w{ }@var{v} (both
integers in basic units @samp{u}) with the leftmost point at the
current position; then move to the rightmost point of the ellipse.
This command is a @code{gtroff} extension.

@item De @var{h} @var{v}@angles{line break}
Draw an outlined ellipse with a horizontal diameter of@w{ }@var{h}
and a vertical diameter of@w{ }@var{v} (both integers in basic units
@samp{u}) with the leftmost point at current position; then move to
the rightmost point of the ellipse.

@item DF @var{color-scheme} @r{[}@var{component} @dots{}@r{]}@angles{line break}
Set fill color for solid drawing objects using different color
schemes; the analoguous command for setting the color of text, line
graphics, and the outline of graphic objects is @samp{m}.
The color components are specified as integer arguments between 0 and
65536.  The number of color components and their meaning vary for the
different color schemes.  These commands are generated by @code{gtroff}'s
escape sequences @w{@code{\D'F @dots{}'}} and @code{\M} (with no other
corresponding graphics commands).  No position changing.  This command
is a @code{gtroff} extension.

@table @code
@item DFc @var{cyan} @var{magenta} @var{yellow}@angles{line break}
Set fill color for solid drawing objects using the CMY color scheme,
having the 3@w{ }color components @var{cyan}, @var{magenta}, and
@var{yellow}.

@item DFd@angles{line break}
Set fill color for solid drawing objects to the default fill color value
(black in most cases).  No component arguments.

@item DFg @var{gray}@angles{line break}
Set fill color for solid drawing objects to the shade of gray given by
the argument, an integer between 0 (black) and 65536 (white).

@item DFk @var{cyan} @var{magenta} @var{yellow} @var{black}@angles{line break}
Set fill color for solid drawing objects using the CMYK color scheme,
having the 4@w{ }color components @var{cyan}, @var{magenta}, @var{yellow},
and @var{black}.

@item DFr @var{red} @var{green} @var{blue}@angles{line break}
Set fill color for solid drawing objects using the RGB color scheme,
having the 3@w{ }color components @var{red}, @var{green}, and @var{blue}.

@end table

@item Df @var{n}@angles{line break}
The argument@w{ }@var{n} must be an integer in the range @math{-32767}
to 32767.

@table @asis
@item @math{0 @LE @var{n} @LE 1000}
Set the color for filling solid drawing objects to a shade of gray,
where 0 corresponds to solid white, 1000 (the default) to solid black,
and values in between to intermediate shades of gray; this is
obsoleted by command @samp{DFg}.

@item @math{@var{n} @LT 0} or @math{@var{n} @LT 1000}
Set the filling color to the color that is currently being used for
the text and the outline, see command @samp{m}.  For example, the
command sequence

@Example
mg 0 0 65536
Df -1
@endExample

@noindent
sets all colors to blue.

@end table

@noindent
No position changing.  This command is a @code{gtroff} extension.

@item Dl @var{h} @var{v}@angles{line break}
Draw line from current position to offset (@var{h},@var{v}) (integers
in basic units @samp{u}); then set current position to the end of the
drawn line.

@item Dp @var{h1} @var{v1} @var{h2} @var{v2} @dots{} @var{hn} @var{vn}@angles{line break}
Draw a polygon line from current position to offset (@var{h1},@var{v1}),
from there to offset (@var{h2},@var{v2}), etc.@: up to offset
(@var{hn},@var{vn}), and from there back to the starting position.
For historical reasons, the position is changed by adding the sum of
all arguments with odd index to the actual horizontal position and the
even ones to the vertical position.  Although this doesn't make sense
it is kept for compatibility.
@ignore
As the polygon is closed, the end of drawing is the starting point, so
the position doesn't change.
@end ignore
This command is a @code{gtroff} extension.

@item Dp @var{h1} @var{v1} @var{h2} @var{v2} @dots{} @var{hn} @var{vn}@angles{line break}
Draw a solid polygon in the current fill color rather than an outlined
polygon, using the same arguments and positioning as the corresponding
@samp{Dp} command.
@ignore
No position changing.
@end ignore
This command is a @code{gtroff} extension.

@item Dt @var{n}@angles{line break}
Set the current line thickness to@w{ }@var{n} (an integer in basic
units @samp{u}) if @math{@var{n}>0}; if @math{@var{n}=0} select the
smallest available line thickness; if @math{@var{n}<0} set the line
thickness proportional to the point size (this is the default before
the first @samp{Dt} command was specified).  For historical reasons,
the horizontal position is changed by adding the argument to the actual
horizontal position, while the vertical position is not changed.
Although this doesn't make sense it is kept for compatibility.
@ignore
No position changing.
@end ignore
This command is a @code{gtroff} extension.

@end table

@node Device Control Commands, Obsolete Command, Graphics Commands, Command Reference
@subsubsection Device Control Commands

Each device control command starts with the letter @samp{x},
followed by a space character (optional or arbitrary space or tab in
@code{gtroff}) and a subcommand letter or word; each argument (if any)
must be preceded by a syntactical space.  All @samp{x} commands are
terminated by a syntactical line break; no device control command can
be followed by another command on the same line (except a comment).

The subcommand is basically a single letter, but to increase
readability, it can be written as a word, i.e., an arbitrary sequence
of characters terminated by the next tab, space, or newline character.
All characters of the subcommand word but the first are simply ignored.
For example, @code{gtroff} outputs the initialization command
@w{@samp{x i}} as @w{@samp{x init}} and the resolution command
@w{@samp{x r}} as @w{@samp{x res}}.

In the following, the syntax element @angles{line break} means a
syntactical line break (@pxref{Separation}).

@table @code
@item xF @var{name}@angles{line break}
The @samp{F} stands for @var{Filename}.

Use @var{name} as the intended name for the current file in error
reports.  This is useful for remembering the original file name when
@code{gtroff} uses an internal piping mechanism.  The input file is
not changed by this command.  This command is a @code{gtroff} extension.

@item xf @var{n} @var{s}@angles{line break}
The @samp{f} stands for @var{font}.

Mount font position@w{ }@var{n} (a non-negative integer) with font
named@w{ }@var{s} (a text word).  @xref{Font Positions}.

@item xH @var{n}@angles{line break}
The @samp{H} stands for @var{Height}.

Set glyph height to@w{ }@var{n} (a positive integer in scaled
points @samp{z}).  @acronym{AT&T} @code{troff} uses the unit points
(@samp{p}) instead.  @xref{Output Language Compatibility}.

@item xi@angles{line break}
The @samp{i} stands for @var{init}.

Initialize device.  This is the third command of the prologue.

@item xp@angles{line break}
The @samp{p} stands for @var{pause}.

Parsed but ignored.  The original @acronym{UNIX} troff manual writes

@display
pause device, can be restarted
@end display

@item xr @var{n} @var{h} @var{v}@angles{line break}
The @samp{r} stands for @var{resolution}.

Resolution is@w{ }@var{n}, while @var{h} is the minimal horizontal
motion, and @var{v} the minimal vertical motion possible with this
device; all arguments are positive integers in basic units @samp{u}
per inch.  This is the second command of the prologue.

@item xS @var{n}@angles{line break}
The @samp{S} stands for @var{Slant}.

Set slant to@w{ }@var{n} (an integer in basic units @samp{u}).

@item xs@angles{line break}
The @samp{s} stands for @var{stop}.

Terminates the processing of the current file; issued as the last
command of any intermediate troff output.

@item xt@angles{line break}
The @samp{t} stands for @var{trailer}.

Generate trailer information, if any.  In @var{gtroff}, this is
actually just ignored.

@item xT @var{xxx}@angles{line break}
The @samp{T} stands for @var{Typesetter}.

Set name of device to word @var{xxx}, a sequence of characters ended
by the next white space character.  The possible device names coincide
with those from the groff @option{-T} option.  This is the first
command of the prologue.

@item xu @var{n}@angles{line break}
The @samp{u} stands for @var{underline}.

Configure underlining of spaces.  If @var{n} is@w{ }1, start
underlining of spaces; if @var{n} is@w{ }0, stop underlining of spaces.
This is needed for the @code{cu} request in nroff mode and is ignored
otherwise.  This command is a @code{gtroff} extension.

@item xX @var{anything}@angles{line break}
The @samp{x} stands for @var{X-escape}.

Send string @var{anything} uninterpreted to the device.  If the line
following this command starts with a @samp{+} character this line is
interpreted as a continuation line in the following sense.  The
@samp{+} is ignored, but a newline character is sent instead to the
device, the rest of the line is sent uninterpreted.  The same applies
to all following lines until the first character of a line is not a
@samp{+} character.  This command is generated by the @code{gtroff}
escape sequence @code{\X}.  The line-continuing feature is a
@code{gtroff} extension.

@end table

@node Obsolete Command,  , Device Control Commands, Command Reference
@subsubsection Obsolete Command
In @acronym{AT&T} @code{troff} output, the writing of a single
glyph is mostly done by a very strange command that combines a
horizontal move and a single character giving the glyph name.  It
doesn't have a command code, but is represented by a 3-character
argument consisting of exactly 2@w{ }digits and a character.

@table @asis
@item @var{dd}@var{g}
Move right @var{dd} (exactly two decimal digits) basic units @samp{u},
then print glyph@w{ }@var{g} (represented as a single character).

In @code{gtroff}, arbitrary syntactical space around and within this
command is allowed to be added.  Only when a preceding command on the
same line ends with an argument of variable length a separating space
is obligatory.  In @acronym{AT&T} @code{troff}, large clusters of these
and other commands are used, mostly without spaces; this made such output
almost unreadable.

@end table

For modern high-resolution devices, this command does not make sense
because the width of the glyphs can become much larger than two
decimal digits.  In @code{gtroff}, this is only used for the devices
@code{X75}, @code{X75-12}, @code{X100}, and @code{X100-12}.  For other
devices, the commands @samp{t} and @samp{u} provide a better
functionality.

@c ---------------------------------------------------------------------

@node Intermediate Output Examples, Output Language Compatibility, Command Reference, gtroff Output
@subsection Intermediate Output Examples

This section presents the intermediate output generated from the same
input for three different devices.  The input is the sentence
@samp{hell world} fed into @code{gtroff} on the command line.

@table @asis
@item High-resolution device @code{ps}

This is the standard output of @code{gtroff} if no @option{-T} option
is given.

@example
@group
shell> echo "hell world" | groff -Z -T ps

x T ps
x res 72000 1 1
x init
@end group
p1
x font 5 TR
f5
s10000
V12000
H72000
thell
wh2500
tw
H96620
torld
n12000 0
@group
x trailer
V792000
x stop
@end group
@end example

@noindent
This output can be fed into @code{grops} to get its representation as
a PostScript file.

@item Low-resolution device @code{latin1}

This is similar to the high-resolution device except that the
positioning is done at a minor scale.  Some comments (lines starting
with @samp{#}) were added for clarification; they were not generated
by the formatter.

@example
@group
shell> echo "hell world" | groff -Z -T latin1

# prologue
x T latin1
x res 240 24 40
x init
@end group
# begin a new page
p1
# font setup
x font 1 R
f1
s10
# initial positioning on the page
V40
H0
# write text `hell'
thell
# inform about space, and issue a horizontal jump
wh24
# write text `world'
tworld
# announce line break, but do nothing because ...
n40 0
@group
# ... the end of the document has been reached
x trailer
V2640
x stop
@end group
@end example

@noindent
This output can be fed into @code{grotty} to get a formatted text
document.

@item @acronym{AT&T} @code{troff} output
Since a computer monitor has a very low resolution compared to modern
printers the intermediate output for the X@w{ }Window devices can use
the jump-and-write command with its 2-digit displacements.

@example
@group
shell> echo "hell world" | groff -Z -T X100

x T X100
x res 100 1 1
x init
@end group
p1
x font 5 TR
f5
s10
V16
H100
# write text with jump-and-write commands
ch07e07l03lw06w11o07r05l03dh7
n16 0
@group
x trailer
V1100
x stop
@end group
@end example

@noindent
This output can be fed into @code{xditview} or @code{gxditview}
for displaying in@w{ }X.

Due to the obsolete jump-and-write command, the text clusters in the
@acronym{AT&T} @code{troff} output are almost unreadable.

@end table

@c ---------------------------------------------------------------------

@node Output Language Compatibility,  , Intermediate Output Examples, gtroff Output
@subsection Output Language Compatibility

The intermediate output language of @acronym{AT&T} @code{troff}
was first documented in the @acronym{UNIX} troff manual, with later
additions documented in @cite{A Typesetter-indenpendent TROFF},
written by Brian Kernighan.

The @code{gtroff} intermediate output format is compatible with this
specification except for the following features.

@itemize @bullet
@item
The classical quasi device independence is not yet implemented.

@item
The old hardware was very different from what we use today.  So the
@code{groff} devices are also fundamentally different from the ones in
@acronym{AT&T} @code{troff}.  For example, the @acronym{AT&T}
PostScript device is called @code{post} and has a resolution of only
720 units per inch, suitable for printers 20 years ago, while
@code{groff}'s @code{ps} device has a resolution of
72000 units per inch.  Maybe, by implementing some rescaling
mechanism similar to the classical quasi device independence,
@code{groff} could emulate @acronym{AT&T}'s @code{post} device.

@item
The B-spline command @samp{D~} is correctly handled by the
intermediate output parser, but the drawing routines aren't
implemented in some of the postprocessor programs.

@item
The argument of the commands @samp{s} and @w{@samp{x H}} has the
implicit unit scaled point @samp{z} in @code{gtroff}, while
@acronym{AT&T} @code{troff} has point (@samp{p}).  This isn't an
incompatibility but a compatible extension, for both units coincide
for all devices without a @code{sizescale} parameter in the @file{DESC}
file, including all postprocessors from @acronym{AT&T} and
@code{groff}'s text devices.  The few @code{groff} devices with
a @code{sizescale} parameter either do not exist for @acronym{AT&T}
@code{troff}, have a different name, or seem to have a different
resolution.  So conflicts are very unlikely.

@item
The position changing after the commands @samp{Dp}, @samp{DP}, and
@samp{Dt} is illogical, but as old versions of @code{gtroff} used this
feature it is kept for compatibility reasons.

@ignore
Temporarily, there existed some confusion on the positioning after the
@samp{D} commands that are groff extensions.  This has been clarified
by establishing the classical rule for all groff drawing commands:

@itemize
@item
The position after a graphic object has been drawn is at its end;
for circles and ellipses, the `end' is at the right side.

@item
From this, the positionings specified for the drawing commands above
follow quite naturally.
@end itemize
@end ignore

@end itemize


@c =====================================================================

@node Font Files,  , gtroff Output, File formats
@section Font Files
@cindex font files
@cindex files, font

The @code{gtroff} font format is roughly a superset of the
@code{ditroff} font format (as used in later versions of @acronym{AT&T}
@code{troff} and its descendants).  Unlike the @code{ditroff} font
format, there is no associated binary format; all files are text
files.@footnote{Plan@w{ }9 @code{troff} has also abandoned the binary
format.}  The font files for device @var{name} are stored in a directory
@file{dev@var{name}}.  There are two types of file: a device description
file called @file{DESC} and for each font@w{ }@var{f} a font file
called@w{ }@file{@var{f}}.

@menu
* DESC File Format::
* Font File Format::
@end menu

@c ---------------------------------------------------------------------

@node DESC File Format, Font File Format, Font Files, Font Files
@subsection @file{DESC} File Format
@cindex @file{DESC} file, format
@cindex font description file, format
@cindex format of font description file
@pindex DESC@r{ file format}

The @file{DESC} file can contain the following types of line.  Except
for the @code{charset} keyword which must comes last (if at all), the
order of the lines is not important.

@table @code
@item res @var{n}
@kindex res
There are @var{n}@w{ }machine units per inch.

@item hor @var{n}
@kindex hor
The horizontal resolution is @var{n}@w{ }machine units.

@item vert @var{n}
@kindex vert
The vertical resolution is @var{n}@w{ }machine units.

@item sizescale @var{n}
@kindex sizescale
The scale factor for point sizes.  By default this has a value of@w{ }1.
One scaled point is equal to one point/@var{n}.  The arguments to the
@code{unitwidth} and @code{sizes} commands are given in scaled points.
@xref{Fractional Type Sizes}, for more information.

@item unitwidth @var{n}
@kindex unitwidth
Quantities in the font files are given in machine units for fonts whose
point size is @var{n}@w{ }scaled points.

@item prepro @var{program}
@kindex prepro
Call @var{program} as a preprocessor.  Currently, this keyword is used
by @code{groff} with option @option{-Thtml} only.

@item postpro @var{program}
@kindex postpro
Call @var{program} as a postprocessor.  For example, the line

@Example
postpro grodvi
@endExample

@noindent
in the file @file{devdvi/DESC} makes @code{groff} call @code{grodvi}
if option @option{-Tdvi} is given (and @option{-Z} isn't used).

@item tcommand
@kindex tcommand
This means that the postprocessor can handle the @samp{t} and @samp{u}
intermediate output commands.

@item sizes @var{s1} @var{s2} @dots{} @var{sn} 0
@kindex sizes
This means that the device has fonts at @var{s1}, @var{s2}, @dots{}
@var{sn} scaled points.  The list of sizes must be terminated by@w{ }0
(this is digit zero).  Each @var{si} can also be a range of sizes
@var{m}-@var{n}.  The list can extend over more than one line.

@item styles @var{S1} @var{S2} @dots{} @var{Sm}
@kindex styles
The first @var{m}@w{ }font positions are associated with styles
@var{S1} @dots{} @var{Sm}.

@item fonts @var{n} @var{F1} @var{F2} @var{F3} @dots{} @var{Fn}
@kindex fonts
Fonts @var{F1} @dots{} @var{Fn} are mounted in the font positions
@var{m}+1, @dots{}, @var{m}+@var{n} where @var{m} is the number of
styles.  This command may extend over more than one line.  A font name
of@w{ }0 means no font is mounted on the corresponding font position.

@item family @var{fam}
@kindex family
The default font family is @var{fam}.

@item use_charnames_in_special
@kindex use_charnames_in_special
This command indicates that @code{gtroff} should encode special
characters inside special commands.  Currently, this is only used
by the @acronym{HTML} output device.  @xref{Postprocessor Access}.

@item pass_filenames
@kindex pass_filenames
Tell @code{gtroff} to emit the name of the source file currently
being processed.  This is achieved by the intermediate output command
@samp{F}.  Currently, this is only used by the @acronym{HTML} output
device.

@item print @var{program}
@kindex print
Use @var{program} as a spooler program for printing.  If omitted,
the @option{-l} and @option{-L} options of @code{groff} are ignored.

@item charset
@kindex charset
This line and everything following in the file are ignored.  It is
allowed for the sake of backwards compatibility.
@end table

The @code{res}, @code{unitwidth}, @code{fonts}, and @code{sizes} lines
are mandatory.  Other commands are ignored by @code{gtroff} but may be
used by postprocessors to store arbitrary information about the device
in the @file{DESC} file.

@kindex spare1
@kindex spare2
@kindex biggestfont
Here a list of obsolete keywords which are recognized by @code{groff}
but completely ignored: @code{spare1}, @code{spare2},
@code{biggestfont}.


@c ---------------------------------------------------------------------

@node Font File Format,  , DESC File Format, Font Files
@subsection Font File Format
@cindex font file, format
@cindex font description file, format
@cindex format of font files
@cindex format of font description files

A @dfn{font file}, also (and probably better) called a @dfn{font
description file}, has two sections.  The first section is a sequence
of lines each containing a sequence of blank delimited words; the first
word in the line is a key, and subsequent words give a value for that
key.

@table @code
@item name @var{f}
@kindex name
The name of the font is@w{ }@var{f}.

@item spacewidth @var{n}
@kindex spacewidth
The normal width of a space is@w{ }@var{n}.

@item slant @var{n}
@kindex slant
The glyphs of the font have a slant of @var{n}@w{ }degrees.
(Positive means forward.)

@item ligatures @var{lig1} @var{lig2} @dots{} @var{lign} [0]
@kindex ligatures
Glyphs @var{lig1}, @var{lig2}, @dots{}, @var{lign} are ligatures;
possible ligatures are @samp{ff}, @samp{fi}, @samp{fl}, @samp{ffi} and
@samp{ffl}.  For backwards compatibility, the list of ligatures may be
terminated with a@w{ }0.  The list of ligatures may not extend over more
than one line.

@item special
@cindex special fonts
@kindex special
The font is @dfn{special}; this means that when a glyph is requested
that is not present in the current font, it is searched for in any
special fonts that are mounted.
@end table

Other commands are ignored by @code{gtroff} but may be used by
postprocessors to store arbitrary information about the font in the font
file.

@cindex comments in font files
@cindex font files, comments
@kindex #
The first section can contain comments which start with the @samp{#}
character and extend to the end of a line.

The second section contains one or two subsections.  It must contain a
@code{charset} subsection and it may also contain a @code{kernpairs}
subsection.  These subsections can appear in any order.  Each
subsection starts with a word on a line by itself.

@kindex charset
The word @code{charset} starts the character set
subsection.@footnote{This keyword is misnamed since it starts a list
of ordered glyphs, not characters.}  The @code{charset} line is
followed by a sequence of lines.  Each line gives information for one
glyph.  A line comprises a number of fields separated by blanks or
tabs.  The format is

@quotation
@var{name} @var{metrics} @var{type} @var{code}
[@var{entity-name}] [@code{--} @var{comment}]
@end quotation

@cindex 8-bit input
@cindex input, 8-bit
@cindex accessing unnamed glyphs with @code{\N}
@cindex unnamed glyphs, accessing with @code{\N}
@cindex characters, unnamed, accessing with @code{\N}
@cindex glyphs, unnamed, accessing with @code{\N}
@kindex ---
@noindent
@var{name} identifies the glyph name@footnote{The distinction between
input, characters, and output, glyphs, is not clearly separated in the
terminology of @code{groff}; for example, the @code{char} request
should be called @code{glyph} since it defines an output entity.}:
If @var{name} is a single character@w{ }@var{c} then it corresponds
to the @code{gtroff} input character@w{ }@var{c}; if it is of the form
@samp{\@var{c}} where @var{c} is a single character, then it
corresponds to the special character @code{\[@var{c}]}; otherwise it
corresponds to the special character @samp{\[@var{name}]}.  If it
is exactly two characters @var{xx} it can be entered as
@samp{\(@var{xx}}.  Note that single-letter special characters can't
be accessed as @samp{\@var{c}}; the only exception is @samp{\-} which
is identical to @code{\[-]}.

@code{gtroff} supports 8-bit input characters; however some utilities
have difficulties with eight-bit characters.  For this reason, there is
a convention that the entity name @samp{char@var{n}} is equivalent to
the single input character whose code is@w{ }@var{n}.  For example,
@samp{char163} would be equivalent to the character with code@w{ }163
which is the pounds sterling sign in the @w{ISO Latin-1} character set.
You shouldn't use @samp{char@var{n}} entities in font description files
since they are related to input, not output.  Otherwise, you get
hard-coded connections between input and output encoding which
prevents use of different (input) character sets.

The name @samp{---} is special and indicates that the glyph is
unnamed; such glyphs can only be used by means of the @code{\N}
escape sequence in @code{gtroff}.

The @var{type} field gives the glyph type:

@table @code
@item 1
the glyph has a descender, for example, @samp{p};

@item 2
the glyph has an ascender, for example, @samp{b};

@item 3
the glyph has both an ascender and a descender, for example, @samp{(}.
@end table

The @var{code} field gives the code which the postprocessor uses to
print the glyph.  The glyph can also be input to @code{gtroff}
using this code by means of the @code{\N} escape sequence.  @var{code}
can be any integer.  If it starts with @samp{0} it is interpreted as
octal; if it starts with @samp{0x} or @samp{0X} it is interpreted as
hexadecimal.

The @var{entity-name} field gives an @acronym{ASCII} string
identifying the glyph which the postprocessor uses to print the
@code{gtroff} glyph @var{name}.  This field is optional and has been
introduced so that the @acronym{HTML} device driver can encode its
character set.  For example, the glyph @samp{\[Po]} is
represented as @samp{&pound;} in @acronym{HTML} 4.0.

Anything on the line after the @var{entity-name} field resp.@: after
@samp{--} will be ignored.

The @var{metrics} field has the form:

@display
@group
@var{width}[@code{,}@var{height}[@code{,}@var{depth}[@code{,}@var{italic-correction}
  [@code{,}@var{left-italic-correction}[@code{,}@var{subscript-correction}]]]]]
@end group
@end display

@noindent
There must not be any spaces between these subfields (it has been split
here into two lines for better legibility only).  Missing subfields are
assumed to be@w{ }0.  The subfields are all decimal integers.  Since
there is no associated binary format, these values are not required to
fit into a variable of type @samp{char} as they are in @code{ditroff}.
The @var{width} subfield gives the width of the glyph.  The @var{height}
subfield gives the height of the glyph (upwards is positive); if a
glyph does not extend above the baseline, it should be given a zero
height, rather than a negative height.  The @var{depth} subfield gives
the depth of the glyph, that is, the distance from the baseline to the
lowest point below the baseline to which the glyph extends (downwards is
positive); if a glyph does not extend below the baseline, it should be
given a zero depth, rather than a negative depth.  The
@var{italic-correction} subfield gives the amount of space that should
be added after the glyph when it is immediately to be followed by a
glyph from a roman font.  The @var{left-italic-correction} subfield
gives the amount of space that should be added before the glyph when it
is immediately to be preceded by a glyph from a roman font.  The
@var{subscript-correction} gives the amount of space that should be
added after a glyph before adding a subscript.  This should be less
than the italic correction.

A line in the @code{charset} section can also have the format

@Example
@var{name} "
@endExample

@noindent
This indicates that @var{name} is just another name for the glyph
mentioned in the preceding line.

@kindex kernpairs
The word @code{kernpairs} starts the kernpairs section.  This contains a
sequence of lines of the form:

@Example
@var{c1} @var{c2} @var{n}
@endExample

@noindent
This means that when glyph @var{c1} appears next to glyph @var{c2}
the space between them should be increased by@w{ }@var{n}.  Most
entries in the kernpairs section have a negative value for@w{ }@var{n}.



@c =====================================================================
@c =====================================================================

@node Installation, Copying This Manual, File formats, Top
@chapter Installation
@cindex installation

@c XXX



@c =====================================================================
@c =====================================================================

@node Copying This Manual, Request Index, Installation, Top
@appendix Copying This Manual

@menu
* GNU Free Documentation License::  License for copying this manual.
@end menu

@include fdl.texi



@c =====================================================================
@c =====================================================================

@node Request Index, Escape Index, Copying This Manual, Top
@appendix Request Index

Requests appear without the leading control character (normally either
@samp{.} or @samp{'}).

@printindex rq



@c =====================================================================
@c =====================================================================

@node Escape Index, Operator Index, Request Index, Top
@appendix Escape Index

Any escape sequence @code{\@var{X}} with @var{X} not in the list below
emits a warning, printing glyph @var{X}.

@printindex es



@c =====================================================================
@c =====================================================================

@node Operator Index, Register Index, Escape Index, Top
@appendix Operator Index

@printindex op



@c =====================================================================
@c =====================================================================

@node Register Index, Macro Index, Operator Index, Top
@appendix Register Index

The macro package or program a specific register belongs to is appended in
brackets.

A register name@w{ }@code{x} consisting of exactly one character can be
accessed as @samp{\nx}.  A register name @code{xx} consisting of exactly
two characters can be accessed as @samp{\n(xx}.  Register names @code{xxx}
of any length can be accessed as @samp{\n[xxx]}.

@printindex vr



@c =====================================================================
@c =====================================================================

@node Macro Index, String Index, Register Index, Top
@appendix Macro Index

The macro package a specific macro belongs to is appended in brackets.
They appear without the leading control character (normally @samp{.}).

@printindex ma



@c =====================================================================
@c =====================================================================

@node String Index, Glyph Name Index, Macro Index, Top
@appendix String Index

The macro package or program a specific string belongs to is appended in
brackets.

A string name@w{ }@code{x} consisting of exactly one character can be
accessed as @samp{\*x}.  A string name @code{xx} consisting of exactly
two characters can be accessed as @samp{\*(xx}.  String names @code{xxx}
of any length can be accessed as @samp{\*[xxx]}.


@printindex st



@c =====================================================================
@c =====================================================================

@node Glyph Name Index, Font File Keyword Index, String Index, Top
@appendix Glyph Name Index

A glyph name @code{xx} consisting of exactly two characters can be
accessed as @samp{\(xx}.  Glyph names @code{xxx} of any length can be
accessed as @samp{\[xxx]}.

@c XXX



@c =====================================================================
@c =====================================================================

@node Font File Keyword Index, Program and File Index, Glyph Name Index, Top
@appendix Font File Keyword Index

@printindex ky



@c =====================================================================
@c =====================================================================

@node Program and File Index, Concept Index, Font File Keyword Index, Top
@appendix Program and File Index

@printindex pg



@c =====================================================================
@c =====================================================================

@node Concept Index,  , Program and File Index, Top
@appendix Concept Index

@printindex cp


@bye