* doc/groff.texinfo: More info on conditionals.

[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 %**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.)


@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   glindex: glyph names
@c   opindex: operators
@c
@c tindex and cindex are merged.

@defcodeindex rq
@defcodeindex es
@defcodeindex ma
@defcodeindex st
@defcodeindex gl
@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 definition of requests

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

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

@macro endDefreq
@end deffn
@end macro


@c definition of escapes

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

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

@macro endDefesc
@end deffn
@end macro


@c definition of registers

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

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

@macro endDefreg
@end deffn
@end macro


@c definition of macros

@macro Defmac{name, arg}
@maindex \name\
@defmac @t{.\name\} \arg\
@end macro

@macro Defmacx{name, arg}
@maindex \name\
@defmacx @t{.\name\} \arg\
@end macro

@macro endDefmac
@end defmac
@end macro


@c definition of strings

@macro Defstr{name, arg}
@stindex \name\
@deffn String @t{\name\} \arg\
@end macro

@macro Defstrx{name, arg}
@stindex \name\
@deffnx String @t{\name\} \arg\
@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 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 This is true for texinfo 4.0.

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

@iftex
@macro lparen
@@lparen
@end macro
@macro rparen
@@rparen
@end macro
@macro lbrack
@@lbrack
@end macro
@macro rbrack
@@rbrack
@end macro
@end iftex


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


@c XXX comment all examples


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


@smallbook


@iftex
@finalout
@end iftex


@ifinfo
This Info file documents GNU troff version 1.16.

Published by the Free Software Foundation
59 Temple Place, Suite 330
Boston, MA  02111-1307  USA

Copyright (C) 1994-2000 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission notice
identical to this one except for the removal of this paragraph (this
paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation
approved by the Foundation.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU General Public License'' may be
included in a translation approved by the Free Software Foundation
instead of in the original English.
@end ifinfo


@titlepage
@title groff
@subtitle The GNU implementation of @code{troff}
@subtitle Edition 1.16
@subtitle Spring 2000
@author by Trent A.@w{ }Fisher
@author and Werner Lemberg

@c Include the Distribution inside the titlepage environment so
@c that headings are turned off.  Headings on and off do not work.

@page
@vskip 0pt plus 1filll
Copyright @copyright@w{ }1994-2000 Free Software Foundation,@w{ }Inc.
@sp 2
Version 1.16 of @code{groff}, @*
Spring 2000
@sp 2
Published by the Free Software Foundation @*
59 Temple Place, Suite 330 @*
Boston, MA  02111-1307  USA


Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU General Public License'' may be
included in a translation approved by the Free Software Foundation
instead of in the original English.

Cover art by Etienne Suvasa.
@end titlepage
@page



@node Top, Copying, (dir), (dir)

@ifinfo
This Info file documents groff version 1.16, the GNU implementation of
the troff typesetting system.

This is an in-progress document; contributions, comments, or
contributions are welcome.  Send them to bug-groff@@gnu.org.
@end ifinfo

@menu
* Copying::
* Introduction::
* Invoking groff::
* Tutorial for Macro Users::
* Macro Packages::
* gtroff Reference::
* Preprocessors::
* Output Devices::
* File formats::
* Installation::
* 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



@node Copying, Introduction, Top, Top
@cindex copying
@unnumbered GNU GENERAL PUBLIC LICENSE
@center Version 2, June 1991

@display
Copyright @copyright{}@w{ }1989, 1991 Free Software Foundation, Inc.
59@w{ }Temple Place, Suite@w{ }330, Boston, MA@w{ }02111, USA

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

The licenses for most software are designed to take away your freedom to
share and change it.  By contrast, the GNU General Public License is
intended to guarantee your freedom to share and change free software --
to make sure the software is free for all its users.  This General
Public License applies to most of the Free Software Foundation's
software and to any other program whose authors commit to using it.
(Some other Free Software Foundation software is covered by the GNU
Library General Public License instead.)  You can apply it to your
programs, too.

When we speak of free software, we are referring to freedom, not price.
Our General Public Licenses are designed to make sure that you have the
freedom to distribute copies of free software (and charge for this
service if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone
to deny you these rights or to ask you to surrender the rights.  These
restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis
or for a fee, you must give the recipients all the rights that you have.
You must make sure that they, too, receive or can get the source code.
And you must show them these terms so they know their rights.

We protect your rights with two steps: (1)@w{ }copyright the software,
and (2)@w{ }offer you this license which gives you legal permission to
copy, distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

Finally, any free program is threatened constantly by software patents.
We wish to avoid the danger that redistributors of a free program will
individually obtain patent licenses, in effect making the program
proprietary.  To prevent this, we have made it clear that any patent
must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and
modification follow.

@iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate 0
@item
This License applies to any program or other work which contains a
notice placed by the copyright holder saying it may be distributed under
the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it, either
verbatim or with modifications and/or translated into another language.
(Hereinafter, translation is included without limitation in the term
``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of running
the Program is not restricted, and the output from the Program is
covered only if its contents constitute a work based on the Program
(independent of having been made by running the Program).  Whether that
is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's source code
as you receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice and
disclaimer of warranty; keep intact all the notices that refer to this
License and to the absence of any warranty; and give any other
recipients of the Program a copy of this License along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion of it,
thus forming a work based on the Program, and copy and distribute such
modifications or work under the terms of Section@w{ }1 above, provided
that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices stating
that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in whole or
in part contains or is derived from the Program or any part thereof, to
be licensed as a whole at no charge to all third parties under the terms
of this License.

@item
If the modified program normally reads commands interactively when run,
you must cause it, when started running for such interactive use in the
most ordinary way, to print or display an announcement including an
appropriate copyright notice and a notice that there is no warranty (or
else, saying that you provide a warranty) and that users may
redistribute the program under these conditions, and telling the user
how to view a copy of this License.  (Exception: if the Program itself
is interactive but does not normally print such an announcement, your
work based on the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program, and
can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based on
the Program, the distribution of the whole must be on the terms of this
License, whose permissions for other licensees extend to the entire
whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of a
storage or distribution medium does not bring the other work under the
scope of this License.

@item
You may copy and distribute the Program (or a work based on it, under
Section@w{ }2) in object code or executable form under the terms of
Sections@w{ }1 and@w{ }2 above provided that you also do one of the
following:

@enumerate a
@item
Accompany it with the complete corresponding machine-readable source
code, which must be distributed under the terms of Sections@w{ }1 and@w{
}2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three years, to
give any third party, for a charge no more than your cost of physically
performing source distribution, a complete machine-readable copy of the
corresponding source code, to be distributed under the terms of
Sections@w{ }1 and@w{ }2 above on a medium customarily used for software
interchange; or,

@item
Accompany it with the information you received as to the offer to
distribute corresponding source code.  (This alternative is allowed only
for noncommercial distribution and only if you received the program in
object code or executable form with such an offer, in accord with
Subsection@w{ }b above.)
@end enumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to control
compilation and installation of the executable.  However, as a special
exception, the source code distributed need not include anything that is
normally distributed (in either source or binary form) with the major
components (compiler, kernel, and so on) of the operating system on
which the executable runs, unless that component itself accompanies the
executable.

If distribution of executable or object code is made by offering access
to copy from a designated place, then offering equivalent access to copy
the source code from the same place counts as distribution of the source
code, even though third parties are not compelled to copy the source
along with the object code.

@item
You may not copy, modify, sublicense, or distribute the Program except
as expressly provided under this License.  Any attempt otherwise to
copy, modify, sublicense or distribute the Program is void, and will
automatically terminate your rights under this License.  However,
parties who have received copies, or rights, from you under this License
will not have their licenses terminated so long as such parties remain
in full compliance.

@item
You are not required to accept this License, since you have not signed
it.  However, nothing else grants you permission to modify or distribute
the Program or its derivative works.  These actions are prohibited by
law if you do not accept this License.  Therefore, by modifying or
distributing the Program (or any work based on the Program), you
indicate your acceptance of this License to do so, and all its terms and
conditions for copying, distributing or modifying the Program or works
based on it.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further restrictions
on the recipients' exercise of the rights granted herein.  You are not
responsible for enforcing compliance by third parties to this License.

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent license
would not permit royalty-free redistribution of the Program by all those
who receive copies directly or indirectly through you, then the only way
you could satisfy both it and this License would be to refrain entirely
from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is implemented
by public license practices.  Many people have made generous
contributions to the wide range of software distributed through that
system in reliance on consistent application of that system; it is up to
the author/donor to decide if he or she is willing to distribute
software through any other system and a licensee cannot impose that
choice.

This section is intended to make thoroughly clear what is believed to be
a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in certain
countries either by patents or by copyrighted interfaces, the original
copyright holder who places the Program under this License may add an
explicit geographical distribution limitation excluding those countries,
so that distribution is permitted only in or among countries not thus
excluded.  In such case, this License incorporates the limitation as if
written in the body of this License.

@item
The Free Software Foundation may publish revised and/or new versions of
the General Public License from time to time.  Such new versions will be
similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and ``any
later version'', you have the option of following the terms and
conditions either of that version or of any later version published by
the Free Software Foundation.  If the Program does not specify a version
number of this License, you may choose any version ever published by the
Free Software Foundation.

@item
If you wish to incorporate parts of the Program into other free programs
whose distribution conditions are different, write to the author to ask
for permission.  For software which is copyrighted by the Free Software
Foundation, write to the Free Software Foundation; we sometimes make
exceptions for this.  Our decision will be guided by the two goals of
preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@iftex
@heading NO WARRANTY
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR
THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW@.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER
EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE@.
THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH
YOU@.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
NECESSARY SERVICING, REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR
DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL
DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM
(INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED
INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF
THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR
OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo


@page
@unnumberedsec How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

To do so, attach the following notices to the program.  It is safest to
attach them to the start of each source file to most effectively convey
the exclusion of warranty; and each file should have at least the
``copyright'' line and a pointer to where the full notice is found.

@smallexample
@var{one line to give the program's name and an idea of what it does.}
Copyright (C) 19@var{yy} @var{name of author}

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.

This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE@.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
@end smallexample

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

@smallexample
Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type
`show w'.  This is free software, and you are welcome to redistribute
it under certain conditions; type `show c' for details.
@end smallexample

The hypothetical commands @samp{show@w{ }w} and @samp{show@w{ }c} should
show the appropriate parts of the General Public License.  Of course,
the commands you use may be called something other than @samp{show@w{
}w} and @samp{show@w{ }c}; they could even be mouse-clicks or menu items
-- whatever suits your program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the program, if
necessary.  Here is a sample; alter the names:

@smallexample
@group
Yoyodyne, Inc., hereby disclaims all copyright interest
in the program `Gnomovision' (which makes passes at compilers)
written by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end group
@end smallexample

This General Public License does not permit incorporating your program
into proprietary programs.  If your program is a subroutine library, you
may consider it more useful to permit linking proprietary applications
with the library.  If this is what you want to do, use the GNU Library
General Public License instead of this License.



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

@node Introduction, Invoking groff, Copying, 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}
@cindex @code{rf}
@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 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 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}
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 an auto accident.

@pindex ditroff
@cindex @code{ditroff}
So, Brian Kernighan took on the task of rewriting @code{troff}.  The
newly rewritten version produced a 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 character 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 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 Linux or the Hurd, which
don't contain proprietary software, 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: @code{groff} is never replaced by @code{roff}.

@menu
* Groff Options::
* Environment::
* 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 [ -abeghilpstvzCEGNRSUVXZ ] [ -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 [ -abivzCERU ] [ -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 @option{-}.
A filename of @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 -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 X100
For a 100@dmn{dpi} X11 previewer.

@item ascii
For typewriter-like devices.

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

@item utf8
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 cp1047
@cindex IBM cp1047
For typewriter-like devices which use the @acronym{EBCDIC} encoding IBM
cp1047.

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

@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}
@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

@vindex .T
@stindex .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
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
Unsafe mode.  Reverts to the old unsafe behaviour.

@item -a
@vindex .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 traditional Unix
@code{troff}.

@item -d@var{cs}
@itemx -d@var{name}=s
Define @var{c} or @var{name} to be a string @var{s}.  @var{c} 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}} (and searches in the same directories).

@c XXX document local and system macro dirs

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

@item -o@var{list}
@vindex .P
Output only pages in @var{list}, which is a comma-separated list of page
ranges; @samp{@var{n}} means print page @var{n}, @samp{@var{m}-@var{n}}
means print every page between @var{m} and @var{n}, @samp{-@var{n}}
means print every page up to @var{n}, @samp{@var{n}-} means print every
page beginning with @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{cn}
@itemx -r@var{name}=@var{n}
Set number register @var{c} or @var{name} to the value @var{n}.  @var{c}
must be a one-letter name; @var{name} can be of arbitrary length.
@var{n} 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.

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


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

@node Environment, Invocation Examples, 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, environment variable
If this is set to @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}.

@c XXX document default values

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

@c XXX document local and system macro dirs

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

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

@item GROFF_TMPDIR
@tindex GROFF_TMPDIR, environment variable
@tindex TMPDIR, 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 Invocation Examples,  , Environment, 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{-ms}, @option{-mdoc},
@option{-mdoc-old}, @option{-p}, @option{-R}, @option{-g}, @option{-G},
@option{-s}, and @option{-t}.

A special file name @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,...
@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

@rqindex ls
@cindex double spacing
@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}}.

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{white space} (blank lines or
spaces).

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

@cindex blank line
@cindex empty line
@cindex line, empty
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}).

@rqindex ce
@cindex centering lines
@cindex lines, centering
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.

@rqindex br
@cindex line break
@cindex break
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
@group
one   This is  the first paragraph.  Notice  how the first
      line of  the resulting  paragraph lines up  with the
      other lines in the paragraph.
@end group
@group
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.
@end group
@end example

A variation of this is a bulleted list.

@c XXX description

@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 headers and footers
(or @dfn{titles}) on each page.  Some packages allow for different ones
on the even and odd pages (for material printed in a book form).

The titles are called 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

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
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.

@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 the 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, @code{.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 @file{man}
@cindex manual 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 [ -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 -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

@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}]}
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}]}
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}]}
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}]}
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

@Defmac {LP}
@Defmacx {PP}
@Defmacx {P}
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).  Finally, the current
left margin is restored.
@endDefmac

@Defmac {IP, [@Var{designator}] [@Var{nnn}]}
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@dmn{en} indentation, write

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

@cindex hanging indentation, in manual pages
@Defmac {HP, [@Var{nnn}]}
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

@cindex left margin, how to move, in manual pages
@Defmac {RS, [@Var{nnn}]}
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}]}
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

@maindex SH
@maindex SS
@maindex TP
@maindex LP
@maindex PP
@maindex P
@maindex IP
@maindex HP
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}.

@maindex RS
@maindex RE
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 fonts in manual pages
@cindex @code{man}, how to set fonts

The standard font is roman; the default text size is 10@w{ }point.

@Defmac {SM, [@Var{text}]}
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

@cindex boldface, in manual pages
@Defmac {SB, [@Var{text}]}
Set the text on the same line or the text on the next line in boldface
font, one point size smaller than the default font.
@endDefmac

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

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

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

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

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

@Defmac {R, [@Var{text}]}
Set @var{text} in roman font.  If no text is present on the line where
the macro is called, then the text of the next line appears in roman.
This is the default font to which text is returned at the end of
processing of the other macros.
@endDefmac

@Defmac {B, [@Var{text}]}
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

@cindex italic, in manual pages
@Defmac {I, [@Var{text}]}
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 @file{man}, default indentation
@cindex default indentation, @file{man}
The default indentation is 7.2@dmn{n} for all output devices except for
@code{grohtml} which ignores indentation.

@maindex TH
@cindex tab stops, in manual pages
@Defmac {DT}
Set tabs every 0.5@w{ }inches.  Since this macro is always called
during a @code{TH} request, it makes sense to call it only if the tab
positions have been changed.
@endDefmac

@cindex empty space before a paragraph, in manual pages
@Defmac {PD, [@Var{nnn}]}
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

@maindex SH
@maindex SS
@maindex TP
@maindex LP
@maindex PP
@maindex P
@maindex IP
@maindex HP
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}
Switch back to the default font size.
@endDefstr

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

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

@glindex lq
@glindex rq
@Defstr {lq}
@Defstrx {rq}
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 @file{mdoc}

@c XXX documentation


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

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

@c XXX documentation


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

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

@c XXX documentation


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

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

@c XXX documentation



@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::
* Page Layout::
* Page Control::
* Fonts::
* Sizes::
* Strings::
* Conditionals and Loops::
* Writing Macros::
* Page Motions::
* Drawing Requests::
* Traps::
* Diversions::
* Environments::
* Suppressing output::
* I/O::
* Postprocessor Access::
* Miscellaneous::
* Gtroff Internals::
* Debugging::
* Implementation Differences::
* Summary::
@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
@glindex dg
@glindex rq
@cindex "
@cindex '
@cindex )
@cindex ]
@cindex *
In addition, the following characters or glyphs 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.

@esindex \&
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
@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
@rqindex 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}.

@c XXX xref for blm

@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 units
@cindex machine units
@cindex measurement units
@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
@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
@cindex @code{c} unit
@cindex unit, @code{c}
Centimeters.  One centimeter is equal to@w{ }0.3937@dmn{in}.

@item p
@cindex points
@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
@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.
@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
@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
@cindex @code{n} unit
@cindex unit, @code{n}
Ens.  This is half of an em.

@item v
@cindex vertical space
@cindex space, vertical
@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
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}.

@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:

@c XXX more details; examples

@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 &
@opindex :
Logical: @samp{&} (logical and), @samp{:} (logical or).

@item
@cindex unary operators
@cindex operators, unary
@opindex -
@opindex +
@opindex !
@rqindex if
@rqindex while
@cindex @code{if}, and the @samp{!} operator
@cindex @code{while}, 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
@cindex operators, extremum
@opindex >?
@opindex <?
Extrema: @samp{>?} (maximum), @samp{<?} (minimum).  For example,
@samp{5>?3} yields@w{ }@samp{5}.

@c XXX add examples

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

@opindex +@r{, and page motion}
@opindex -@r{, and page motion}
@opindex |@r{, and page motion}
@cindex motion operators
@cindex operators, motion
For many requests which cause a motion on the page, the unary operators
work differently.  The @samp{+} and @samp{-} operators then indicate a
motion relative to the current position (down or up, respectively), and
the @samp{|} operator indicates an absolute position on the page or
input line.
@c XXX xref
@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{rt}, @code{ti}, @code{\R}, and @code{\s}.  Here the plus and minus
signs indicate increments and decrements.

@c XXX add more xref
@xref{Setting Registers}.

@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

@rqindex ]
@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]}.

@c XXX xref

@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.

@c XXX add xrefs above

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):

@rqindex (
@rqindex [
@rqindex ]
@itemize @bullet
@item
Single character.

@item
Two characters.  Must be prefixed with @samp{(} in some situations.

@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 indentifiers, 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 then:

@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}.

@c XXX info about common identifier pool for strings and macros.

@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 embedded anywhere in the text,
or even as an argument to a request.
@c XXX (Not always?)
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
@cindex character, control
@cindex no-break control character
@cindex character, no-break control
@cindex control character, no-break
@rqindex '
@rqindex .
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, strucuring 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
@rqindex 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

@c XXX xref to blm

@cindex zero width space character
@cindex character, zero width space
@cindex space character, zero width
@esindex \&
@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.  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

@esindex \~
@esindex \@key{SP}
@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{"}, as a macro argument
@cindex double quote, as 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"}.

@rqindex ds
Duoble 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}.

@rqindex (
@rqindex [
@rqindex ]
Escape sequences which require an identifier as a parameter accept three
possible syntax forms.

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

@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

@rqindex '
@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

@esindex \o
@esindex \b
@esindex \X
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.

@esindex \%
@esindex \@key{SP}
@esindex \|
@esindex \^
@esindex \@{
@esindex \@}
@esindex \'
@esindex \`
@esindex \-
@esindex \_
@esindex \!
@esindex \?
@esindex \@@
@esindex \)
@esindex \/
@esindex \,
@esindex \&
@esindex \~
@esindex \0
@esindex \a
@esindex \c
@esindex \d
@esindex \e
@esindex \E
@esindex \p
@esindex \r
@esindex \t
@esindex \u
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{\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.

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

@esindex \D
@esindex \h
@esindex \H
@esindex \l
@esindex \L
@esindex \N
@esindex \R
@esindex \s
@esindex \S
@esindex \v
@esindex \x
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
@cindex digits
The digits @code{0}-@code{9}.

@item
@cindex operators
@opindex +
@opindex -
@opindex /
@opindex *
@opindex %
@opindex <
@opindex >
@opindex =
@opindex &
@opindex :
@opindex (
@opindex )
@opindex .
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
@esindex \%
@esindex \@{
@esindex \@}
@esindex \'
@esindex \`
@esindex \-
@esindex \_
@esindex \!
@esindex \@@
@esindex \/
@esindex \c
@esindex \e
@esindex \p
All escape sequences except @code{\%}, @code{\@{}, @code{\@}},
@code{\'}, @code{\`}, @code{\-}, @code{\_}, @code{\!}, @code{\@@},
@code{\/}, @code{\c}, @code{\e}, and @code{\p}.
@end itemize

@esindex \\
@esindex \e
@esindex \E
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{Copy-in Mode}, and @ref{Diversions}, for
more information.

@c XXX explanation of \E

@xref{Identifiers}, and @ref{Character Translations}.

@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.

@rqindex ds
@rqindex as
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.

@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 white space 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.

@rqindex '
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
@cindex registers, setting

Define or set registers using the @code{nr} request or the
@code{\R} escape.

@Defreq {nr, ident value}
@Defescx {\\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.
@endDefreq

For example, the following two lines are equivalent:

@Example
.nr a 1
\R'a 1'
@endExample

Both @code{nr} and @code{\R} have two additional special forms to
increment or decrement a register.

@Defreq {nr, ident @t{+}@Var{value}}
@Defreqx {nr, ident @t{-}@Var{value}}
@Defescx {\\R, ', ident @t{+}@Var{value}, '}
@Defescx {\\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}
Remove number register @var{ident}.  If @var{ident} doesn't exist, the
request is ignored.
@endDefreq

@Defreq {rnn, ident1 ident2}
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}
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
@cindex registers, interpolating

Numeric registers can be accessed via the @code{\n} escape.

@cindex nested assignments
@cindex assignments, nested
@cindex indirect assignments
@cindex assignments, indirect
@Defesc {\\n, , i, }
@Defescx {\\n, @lparen{}, id, }
@Defescx {\\n, @lbrack{}, ident, @rbrack}
Interpolate number register with name @var{ident} (one-character name
@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.

@esindex \R
@Defreq {nr, ident value incr}
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.

@Defesc {\\n, +, i, }
@Defescx {\\n, -, i, }
@Defescx {\\n, @lparen{}+, id, }
@Defescx {\\n, @lparen{}-, id, }
@Defescx {\\n, +@lparen{}, id, }
@Defescx {\\n, -@lparen{}, id, }
@Defescx {\\n, @lbrack{}+, ident, @rbrack{}}
@Defescx {\\n, @lbrack{}-, ident, @rbrack{}}
@Defescx {\\n, +@lbrack{}, ident, @rbrack{}}
@Defescx {\\n, -@lbrack{}, ident, @rbrack{}}
Before interpolating, increment or decrement @var{ident}
(one-character name @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
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
@cindex formats, assigning

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, read-only register
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

@cindex format of register
@cindex register, format
@Defesc {\\g, , i, }
@Defescx {\\g, @lparen{}, id, }
@Defescx {\\g, @lbrack{}, ident, @rbrack{}}
Return the current format of the specified register @var{ident}
(one-character name @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 .H
@cindex horizontal resolution register
@cindex resolution, horizontal, register
@vindex .H
Horizontal resolution in basic units.

@item .V
@cindex vertical resolution register
@cindex resolution, vertical, register
@vindex .V
Vertical resolution in basic units.

@item dw
@cindex day of the week register
@cindex date, day of the week register
@vindex dw
Day of the week (1-7).

@item dy
@cindex day of the month register
@cindex date, day of the month register
@vindex dy
Day of the month (1-31).

@item mo
@cindex month of the year register
@cindex date, month of the year register
@vindex mo
Current month (1-12).

@item year
@cindex date, year register
@cindex year, current, register
@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 traditional @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
@cindex line number, input, register
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
@rqindex nm
@cindex output line number register
@cindex line number, output, register
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
@cindex version number, major, register
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
@cindex version number, minor, register
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
The revision number of @code{groff}.

@item .g
@vindex .g
@cindex @code{gtroff} identification register
@cindex GNU-specific register
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
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 register
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
@rqindex bp
@rqindex ce
@rqindex cf
@rqindex fi
@rqindex fl
@rqindex in
@rqindex nf
@rqindex rj
@rqindex sp
@rqindex ti
@rqindex trf
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.

@cindex fill mode
@cindex mode, fill
@Defreq {fi, }
@Defregx {.u}
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}).
@endDefreq

@cindex no-fill mode
@cindex mode, no-fill
@Defreq {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}).
@endDefreq

@Defreq {ad, [@Var{mode}]}
@Defregx {.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
@rqindex ce
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 current adjustment mode register
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

@Defesc {\\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
@endDefesc

@cindex word space size
@cindex size of word space
@cindex space between words
@cindex sentence space size
@cindex size of sentence space
@cindex space between sentences
@Defreq {ss, word_space_size [@Var{sentence_space_size}]}
@Defregx {.ss}
@Defregx {.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 traditional Unix @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.

@c XXX xref implementation differences

The request is ignored if there is no parameter.
@endDefreq

@cindex centering lines
@cindex lines, centering
@Defreq {ce, [@Var{nnn}]}
@Defregx {.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 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.

@rqindex ll
@rqindex in
@rqindex ti
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

@cindex justifying text
@cindex text, justifying
@cindex right-justifying
@Defreq {rj, [@Var{nnn}]}
@Defregx {.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.
@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.

@Defreq {hy, [@Var{mode}]}
@Defregx {.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
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

@esindex \%
@cindex explicit hyphens
@cindex hyphen, explicit
@cindex consecutive hyphenated lines
@cindex lines, consecutive hyphenated
@cindex hyphenated lines, consecutive
@Defreq {hlm, [@Var{nnn}]}
@Defregx {.hlm}
@Defregx {.hlc}
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

@cindex hyphenation character
@cindex character, hyphenation
@cindex disabling hyphenation
@cindex hyphenation, disabling
@Defesc {\\%, , , }
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.
@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

@cindex hyphenation patterns
@cindex patterns for hyphenation
@Defreq {hpf, pattern_file}
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 the argument to the @code{\patterns}
primitive in @TeX{} (without using @TeX{}'s macro expansion); the
letters appearing in this file are interpreted as hyphenation codes.  A
@samp{%} character in the patterns file introduces a comment that
continues to the end of the line.

If no @code{hpf} request is specified (either in the document or in a
macro package), @code{gtroff} won't hyphenate at all.

@rqindex hla
@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}).

Invoking @code{hpf} causes an error if there is no current hyphenation
language.
@endDefreq

@cindex hyphenation code
@cindex code, hyphenation
@Defreq {hcode, c1 code1 c2 code2 @dots{}}
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 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

@cindex hyphenation margin
@cindex margin for hyphenation
@rqindex ad
@Defreq {hym, [@Var{length}]}
@Defregx {.hym}
Set the (right) hyphenation margin to @var{length}.  If the current
adjustment mode is not @samp{b} or@w{ }@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@w{ }@code{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 current hyphenation margin register
The current hyphenation margin is available in the @code{.hym} read-only
number register.
@endDefreq

@cindex hyphenation space
@rqindex ad
@Defreq {hys, [@Var{hyphenation_space}]}
@Defregx {.hys}
Set the hyphenation space to @var{hyphenation_space}.  If the current
adjustment mode is @samp{b} or@w{ }@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@w{ }@code{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 current hyphenation space register
The current hyphenation space is available in the @code{.hys} read-only
number register.
@endDefreq

@cindex soft hyphen character
@cindex character, soft hyphen
@glindex hy
@rqindex char
@rqindex tr
@Defreq {shc, [@Var{char}]}
Set the soft hyphen character to @var{char}.  If the argument is
omitted, the soft hyphen character is set to the default character
@code{\(hy} (this is the start-up value of @code{gtroff} also).  The
soft hyphen character is the character 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

@rqindex hpf
@rqindex hw
@pindex troffrc
@pindex troffrc-end
@Defreq {hla, language}
@Defregx {.hla}
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} request 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 current hyphenation language register
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@w{
}@code{v}.
@endDefreq

@cindex double-spacing
@Defreq {ls, [@Var{nnn}]}
@Defregx {.L}
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 current line spacing register
The read-only number register @code{.L} contains the current line
spacing setting.
@endDefreq

@c XXX document \n[nl]
@c XXX document \n[nl] == -1 if vertical position is zero

@Defesc {\\x, ', spacing, '}
@Defregx {.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@w{ }@code{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 vertical line space register
The @code{.a} read-only number register contains the most recent
(nonnegative) extra vertical line space.

@c XXX
@ignore
@Example
... example of inline equation ...
@endExample
@end ignore
@endDefesc

@rqindex sp
@cindex no-space mode
@cindex mode, no-space
@cindex blank lines, disabling
@cindex lines, blank, disabling
@Defreq {ns, }
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.

@cindex top-level diversion
@cindex diversion, top-level
This request is useful for macros which want to avoid that subsequent
macros inadvertently insert some vertical space before the text starts
(for example, to set up the first paragraph after a section header).  It
has no effect if not called within the top-level diversion
(@pxref{Diversions}).

@c XXX xref
@endDefreq

@cindex top-level diversion
@cindex diversion, top-level
@Defreq {rs, }
Disable no-space mode.  It has no effect if not called within the
top-level diversion (@pxref{Diversions}).

@c XXX xref
@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, , , }
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

@Defreq {ta, [@Var{n1} @Var{n2} @dots{} @Var{nn} @t{T} @Var{r1} @Var{r2} @dots{} @Var{rn}]}
@Defregx {.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 2iR
@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}}.  This value
is used even for tty output devices (contrary to @acronym{UNIX}
@code{nroff} which has tab stops preset every 0.8@dmn{i}).

@c XXX xref implementation differences
@end itemize

@cindex current tab settings register
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
@endDefreq

@cindex tab repetition character
@cindex character, tab repetition
@Defreq {tc, [@Var{fill-char}]}
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}).
@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 character (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 character defaults to a period character and
not to space.

@Defesc {\\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

@cindex leader repetition character
@cindex character, leader repetition
@Defreq {lc, [@Var{fill-char}]}
Declare the leader character.  Without an argument, leaders act the
same as tabs (i.e., using whitespace for filling).  @code{gtroff}'s
start-up value is @samp{.}.  The value of this @dfn{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
@cindex delimiting character for fields
@cindex character, field delimiting
@cindex field padding character
@cindex padding character for fields
@cindex character, field padding
@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 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

@rqindex .
@rqindex '
@cindex control character
@cindex character, control
@cindex no-break control character
@cindex character, no-break control
@cindex control character, no-break
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 @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 @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

@esindex \\
@Defreq {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

@cindex escape character
@cindex character, escape
@Defreq {ec, [@Var{c}]}
Set the escape character to @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 (like @TeX{}) to
`intern' macros, i.e., to convert a macro definition into an internal
form which is independent of its representation.  If a macro is
called, it is executed literally.
@endDefreq

@Defesc {\\e, , , }
This escape sequence prints the current escape character (which is the
backslash character @samp{\} by default).
@endDefesc

A @dfn{translation} is a mapping of an input character to an output
character.  The default mappings are given in the font definition files
for the specific output device (@pxref{Font Files}); all mappings (both
with @code{tr} and in the font definition files) occur at output time,
i.e., the input character gets assigned the metric information of the
mapped output character.

@Defreq {tr, @Var{a}@Var{b}@Var{c}@Var{d}@dots{}}
Translate character @var{a} to @var{b}, character @var{c} to @var{d},
etc.  If there is an odd number of arguments, the last one is
translated to the space character.

Some notes:

@itemize @bullet
@item
@esindex \(
@esindex \[
@esindex \'
@esindex \`
@esindex \-
@esindex \_
@esindex \C
@esindex \N
@rqindex char
@cindex special character
@cindex character, special
@cindex numbered character
@cindex character, numbered
Special characters (@code{\(@var{xx}}, @code{\[@var{xxx}]},
@code{\C'@var{xxx}'}, @code{\'}, @code{\`}, @code{\-}, @code{\_}),
characters defined with the @code{char} request, and numbered characters
(@code{\N'@var{xxx}'}) can be translated also.

@item
@esindex \e
The @code{\e} escape can be translated also.

@item
@esindex \%
@esindex \~
Characters can be mapped onto the @code{\%} and @code{\~} escapes (but
@code{\%} and @code{\~} can't be mapped onto another character).

@item
@cindex backspace character
@cindex character, backspace
@cindex leader character
@cindex character, leader
@cindex newline character
@cindex character, newline
@cindex tab character
@cindex character, tab
@esindex \a
@esindex \t
The following characters can't be translated: space (with one exception,
see below), backspace, newline, leader (and @code{\a}), tab (and
@code{\t}).

@item
@rqindex shc
Translations are not considered for finding the soft hyphen character
set with the @code{shc} request.

@item
@esindex \&
The character 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 pair as an argument of @code{tr}.  Additionally, it is not
possible to map the space character to any other character; 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 character has been constructed (this happens at the
moment immediately before the character is appended to an output
character list, either by direct output, in a macro, diversion, or
string), it is no longer affected by @code{tr}.

@c XXX xref

@item
Without an argument, the @code{tr} request is ignored.
@end itemize
@endDefreq

@esindex \!
@cindex @code{\!}, and @code{trnt}
@Defreq {trnt, @Var{a}@Var{b}@Var{c}@Var{d}@dots{}}
@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}.

@pindex troffrc
@pindex troffrc-end
@Defreq {troff, }
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

@pindex tty.tmac
@Defreq {nroff, }
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 XXX move the following to grotty section

@pindex less
@cindex Teletype
@cindex ISO 6249 SGR
@cindex terminal control sequences
@cindex control sequences, for terminals
For tty output devices, underlining is done by emitting sequences of
@samp{_} and @samp{\b} (the backspace character) before the actual
character.  Literally, this is printing an underline character, then
moving back one character position, and printing the actual character
at the same position as the underline character (similar to a
typewriter).  Usually, a modern terminal can't interpret this (and the
original Teletype machines for which this sequence was appropriate are
no longer in use).  You need a pager program like @code{less} which
translates this into ISO 6429 SGR sequences to control terminals.

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

@node Line Layout, Page Layout, 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
@cindex margin, left
@cindex page offset
@cindex offset, page
@dfn{Page offset} -- this is the leftmost position of text on the final
output, defining the @dfn{left margin}.

@item in
@cindex indentation
@cindex line indentation
@dfn{Indentation} -- this is the distance from the left margin where
text is printed.

@item ll
@cindex line length
@cindex length of line
@dfn{Line length} -- this is the distance from the left margin to right
margin.
@end ftable

@c XXX improve example

@Example
.in +.5i
.ll -.5i
A bunch of really boring text which should
be indented from both margins.
Replace me with a better (and more) example!
.in -.5i
.ll +.5i
@endExample

@pindex troffrc
@Defreq {po, [@Var{offset}]}
@Defreqx {po, @t{+}@Var{offset}}
@Defreqx {po, @t{-}@Var{offset}}
@Defregx {.o}
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@w{ }@code{m} (and
not@w{ }@code{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

@Defreq {in, [@Var{indent}]}
@Defreqx {in, @t{+}@Var{indent}}
@Defreqx {in, @t{-}@Var{indent}}
@Defregx {.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@w{ }@code{m}.

The indentation is associated with the current environment.

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

@Defreq {ti, offset}
@Defreqx {ti, @t{+}@Var{offset}}
@Defreqx {ti, @t{-}@Var{offset}}
@Defregx {.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.  The default scaling indicator is@w{ }@code{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

@Defreq {ll, [@Var{length}]}
@Defreqx {ll, @t{+}@Var{length}}
@Defreqx {ll, @t{-}@Var{length}}
@Defregx {.l}
@Defregx {.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@w{ }@code{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.

@cindex current line length register
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 Page Layout, Page Control, Line Layout, gtroff Reference
@section Page Layout
@cindex page layout
@cindex layout, page

@code{gtroff} provides some very primitive operations for controlling
page layout.

@cindex page length
@cindex length of page
@Defreq {pl, [@Var{length}]}
@Defreqx {pl, @t{+}@Var{length}}
@Defreqx {pl, @t{-}@Var{length}}
@Defregx {.p}
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@w{ }@code{v}.

@cindex current page length register
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).

@cindex title line
@cindex three-part title
@cindex page number character
@Defreq {tl, @t{'}@Var{left}@t{'}@Var{center}@t{'}@Var{right}@t{'}}
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

@cindex length of title line
@cindex title line, length
@cindex current title line length register
@Defreq {lt, [@Var{length}]}
@Defreqx {lt, @t{+}@Var{length}}
@Defreqx {lt, @t{-}@Var{length}}
@Defregx {.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@w{ }@code{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

@cindex page number
@cindex number, page
@Defreq {pn, page}
@Defreqx {pn, @t{+}@Var{page}}
@Defreqx {pn, @t{-}@Var{page}}
@Defregx {.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

@cindex current page number register
@Defreg {%}
A read-write register holding the current page number.
@endDefreg

@cindex changing the page number character
@cindex page number character, changing
@vindex %
@Defreq {pc, [@Var{char}]}
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 @code{%}.
@endDefreq

@xref{Traps}.


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

@node Page Control, Fonts, Page Layout, gtroff Reference
@section Page Control
@cindex page control
@cindex control, page

@rqindex pn
@cindex new page
@Defreq {bp, [@Var{page}]}
@Defreqx {bp, @t{+}@Var{page}}
@Defreqx {bp, @t{-}@Var{page}}
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 top-level diversion
@cindex diversion, top-level
@code{bp} has no effect if not called within the top-level diversion
(@pxref{Diversions}).
@endDefreq

@cindex orphan line
@Defreq {ne, [@Var{space}]}
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 unit 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
@endDefreq

@rqindex os
@rqindex ne
@Defreq {sv, [@Var{space}]}
@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 (bottom page boundary), the space is
output immediately (ignoring a partial 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 unit is @samp{v}.
@endDefreq


@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 changing fonts
@cindex fonts, changing

@rqindex sty
@rqindex fam
@kindex styles
@kindex family
@pindex DESC
@Defreq {ft, [@Var{font}]}
@Defescx {\\f, , f, }
@Defescx {\\f, @lparen{}, fn, }
@Defescx {\\f, @lbrack{}, font, @rbrack}
The @code{ft} request and the @code{\f} escape change the current font
to @var{font} (one-character name @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 or with
the @code{family} command in the @file{DESC} file).

@cindex previous font
@cindex font, previous
With no argument or using @samp{P} as an argument, @code{.ft} switches
to the previous font.  Use @code{\fP} or @code{\f[P]} to do this with
the escape.

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

@xref{Font Positions}, for an alternative syntax.
@endDefreq

@rqindex ft
@rqindex ul
@rqindex bd
@esindex \f
@rqindex cs
@rqindex tkf
@rqindex special
@rqindex fspecial
@rqindex fp
@rqindex code
@Defreq {ftr, f [@Var{g}]}
Translate font@w{ }@var{f} to font@w{ }@var{g}.  Whenever a font named
@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{code} requests,
font@w{ }@var{g} is used.  If @var{g} is missing or equal to @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, only @sc{PostScript} fonts 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}).

@Defreq {fam, [@Var{family}]}
@Defregx {.fam}
Switch font family to @var{family}.  If no argument is given, switch
back to the previous font family.  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
@endDefreq

@rqindex cs
@rqindex bd
@rqindex tkf
@rqindex uf
@rqindex fspecial
@Defreq {sty, n style}
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.  When it is a style, the font that is actually used is the font
the name of which 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@w{
}@samp{R} and the current font family is@w{ }@samp{T}, then font
@samp{TR} will be used.  If the current font is not a style, then the
current family is ignored.  When the requests @code{cs}, @code{bd},
@code{tkf}, @code{uf}, or @code{fspecial} are applied to a style, then
they will instead be applied to the member of the current family
corresponding to that style.

@var{n} 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} always checks 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.

@Defreq {fp, pos font [@Var{external-name}]}
@Defregx {.f}
@Defregx {.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 current font position register
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
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.

@rqindex sty
@rqindex fam
@kindex styles
@kindex family
@pindex DESC
@Defreq {ft, nnn}
@Defescx {\\f, , n, }
@Defescx {\\f, @lparen{}, nn, }
@Defescx {\\f, @lbrack{}, nnn, @rbrack}
Change the current font position to @var{nnn} (one-digit position
@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 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').

@c XXX

Please note that currently the distinction between glyphs and
characters in this reference is not clearly carried out.  This will be
improved eventually in the next revision.

@cindex symbol
@cindex special fonts
@kindex fonts
@pindex DESC
@rqindex fspecial
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.

@xref{Font Files}, and @ref{Special Fonts}, for more details.

@Defesc {\\, @lparen{}, nm, }
@Defescx {\\, @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.

If @var{name} is undefined, a warning of type @samp{char} is generated,
and the escape is ignored.  @xref{Debugging}, for information about
warnings.

The list of available symbols is device dependent; see @ref{Glyph Name
Index} for some of them discussed in this reference.

@c XXX list of common symbols
@endDefesc

@Defesc {\\C, ', xxx, '}
Typeset the character named @var{xxx}.  Normally it is more convenient
to use @code{\[@var{xxx}]}, but @code{\C} has the advantage that it is
compatible with newer versions of @code{ditroff} and is available in
compatibility mode.
@endDefesc

@rqindex char
@cindex unicode
@Defesc {\\N, ', n, '}
Typeset the character with code@w{ }@var{n} in the current font (this
is @strong{not} the input character code).  @var{n} can be any
integer.  Most devices only have characters 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 character 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 characters
@cindex characters, unnamed
The code of each character is given in the fourth column in the font
description file after the @code{charset} command.  It is possible to
include unnamed characters in the font description file by using a
name of @samp{---}; the @code{\N} escape sequence is the only way to
use these.
@endDefesc

@c XXX should be `glyph', not `character'

@cindex character properties
@cindex properties of characters
@Defreq {cflags, n c1 c2 @dots{}}
Each character has certain properties associated with it.  These
properties can be modified with the @code{cflags} request.  The first
argument is the the sum of the desired flags and the remaining
arguments are the characters to have those properties.  It is possible
to omit the spaces between the characters.

@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
@glindex hy
@glindex em
lines can be broken after the character (initially the characters
@samp{-\(hy\(em} have this property)

@item 8
@cindex overlapping characters
@cindex characters, overlapping
@glindex ul
@glindex rn
@glindex ru
the character overlaps horizontally (initially the characters
@samp{\(ul\(rn\(ru} have this property)

@item 16
@glindex br
the character overlaps vertically (initially character @samp{\(br} has
this property)

@item 32
@cindex transparent characters
@cindex character, transparent
@cindex '
@cindex "
@cindex ]
@cindex )
@cindex *
@glindex dg
@glindex rq
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{"')]*\(dg\(rq} have this property).
@end table
@endDefreq

@cindex defining characters
@cindex characters, defining
@cindex creating new characters
@cindex escape character
@cindex character, escape
@rqindex tr
@rqindex cp
@rqindex rc
@rqindex lc
@esindex \l
@esindex \L
@esindex \&
@esindex \e
@rqindex hcode
@Defreq {char, c [@Var{string}]}
Define a new character@w{ }@var{c} to be @var{string} (which can be
empty).  Every time character@w{ }@var{c} 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 character defined by this request can be used just
like a normal character provided by the output device.  In particular,
other characters can be translated to it with the @code{tr} request;
it can be made the leader character by the @code{lc} request; repeated
patterns can be drawn with the character using the @code{\l} and
@code{\L} escape sequences; words containing the character can be
hyphenated correctly, if the @code{hcode} request is used to give the
character a hyphenation code.  There is a special anti-recursion
feature: Use of character within the character's definition is handled
like normal characters not defined with @code{char}.
@endDefreq

@cindex removing character definition
@cindex character, removing definition
@Defreq {rchar, c1 c2 @dots{}}
Remove the definitions of characters @var{c1}, @var{c2},@w{
}@enddots{} This undoes the effect of a @code{char} 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

@c XXX

To be written.

@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 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.  These are no longer necessary in GNU
@code{troff}.  Nevertheless, they are supported.

@cindex underlining
@Defreq {ul, [@Var{lines}]}
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.

@cindex underline font
@cindex font, for underlining
@rqindex uf
This command is associated with the current environment.  The
underline font can be changed with the @code{uf} request.

@c XXX @xref should be changed to grotty

@xref{Troff and Nroff Mode}, for a discussion how underlining is
implemented in for tty output devices, and which problems can arise.

The @code{ul} request does not underline spaces.
@endDefreq

@cindex continuous underlining
@cindex underlining, continuous
@Defreq {cu, [@Var{lines}]}
The @code{cu} request is similar to @code{ul} but underlines spaces as
well (if a tty output device is used).
@endDefreq

@cindex underline font
@cindex font for underlining
@rqindex ul
@rqindex cu
@Defreq {uf, font}
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

@cindex imitating bold face
@cindex bold face, imitating
@Defreq {bd, font [@Var{offset}]}
@Defreqx {bd, font1 font2 [@Var{offset}]}
@Defregx {.b}
Artificially create a bold font by printing each character 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 characters is 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}.

@rqindex fspecial
@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

@cindex constant character space mode
@cindex mode for constant character space
@cindex character, constant space
@rqindex ps
@Defreq {cs, font [@Var{width} [@Var{em-size}]]}
Switch to and from constant character space mode.  If activated, the
width of every character 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 character space mode is deactivated.

Default unit 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.  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 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).

@cindex ligatures enabled register
@Defreq {lg, [@Var{flag}]}
@Defregx {.lg}
The ligature mechanism can be switched on or off with the @code{lg}
request; 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 character 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 characters
have the same width don't use kerning.

@cindex kerning enabled register
@Defreq {kern, [@Var{flag}]}
@Defregx {.kern}
Kerning can be activated with the @code{kern} request.  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
@cindex character, zero width space
@cindex space character, zero width
If the font description file contains pairwise kerning information,
characters from that font are kerned.  Kerning between two characters
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 characters.
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}
Enable track kerning for font@w{ }@var{f}.  If the current font is@w{
}@var{f} the width of every character 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 unit is @samp{z} for @var{s1} and @var{s2}, @samp{p} for
@var{n1} and @var{n2}.
@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.

@cindex italic correction
@cindex correction, italic
@cindex correction between italic and roman character
@cindex roman character, correction after italic character
@cindex italic character, correction before roman character
@Defesc {\\/, , , }
Increase the width of the preceding character so that the spacing
between that character and the following character is correct if the
following character is a roman character.  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 @code{f}
overlaps the top left of the right parenthesis.  Use this escape
sequence whenever an italic character is immediately followed by a
roman character 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

@cindex left italic correction
@cindex correction, left italic
@cindex roman character, correction before italic character
@cindex italic character, correction after roman character
@Defesc {\\\,, , , }
Modify the spacing of the following character so that the spacing
between that character and the preceding character is correct if the
preceding character is a roman character.  Use this escape sequence
whenever a roman character is immediately followed by an italic
character 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 characters.

@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


@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 character.@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}.

@menu
* Changing Type Sizes::
* Fractional Type Sizes::
@end menu

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

@node Changing Type Sizes, Fractional Type Sizes, Sizes, Sizes
@subsection Changing Type Sizes
@cindex changing type sizes
@cindex type sizes, changing

@Defreq {ps, [@Var{size}]}
@Defreqx {ps, @t{+}@Var{size}}
@Defreqx {ps, @t{-}@Var{size}}
@Defescx {\\s, , size, }
@Defregx {.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 unit of @code{size} is @samp{z}.  If @code{size} is zero or
negative, it is set to 1@dmn{u}.

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

@xref{Fractional Type Sizes}, for yet another syntactical form of
using the @code{\s} escape.

Some devices may only have certain permissible sizes, in which case
@code{gtroff} rounds to the nearest permissible size.
@endDefreq

@cindex current type size register
@cindex current vertical spacing register
@Defreq {vs, [@Var{space}]}
@Defreqx {vs, @t{+}@Var{space}}
@Defreqx {vs, @t{-}@Var{space}}
@Defregx {.v}
Change (increase, decrease) the vertical spacing by @var{space}.  The
default unit 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}.

@vindex .V
@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

@c XXX example

@ignore
@Example
... .sz macro example?? ...
@endExample
@end ignore

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

@node Fractional Type Sizes,  , Changing Type Sizes, Sizes
@subsection Fractional Type Sizes
@cindex fractional type sizes
@cindex type sizes, fractional

@cindex @code{s} unit
@cindex unit, @code{s}
@cindex @code{z} unit
@cindex unit, @code{z}
@rqindex ps
@rqindex cs
@rqindex tkf
@esindex \H
@esindex \s
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.

@vindex .s
@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

@cindex last-requested point size register
@cindex point size, last-requested
@vindex .ps
@vindex .s
@Defreg {.psr}
@Defregx {.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 for a DVI
device, 10.95@dmn{pt} are actually used (as specified in the
@file{DESC} file).

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} 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} scaled points;
@var{n} 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).

@cindex string interpolation
@cindex string expansion
@cindex interpolation of strings
@cindex expansion of strings
@Defreq {ds, name [@Var{string}]}
@Defescx {\\*, , n, }
@Defescx {\\*, @lparen{}, nm, }
@Defescx {\\*, @lbrack{}, name, @rbrack{}}
Define and access a string variable @var{name} (one-character name
@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. a string variables can be called within string
variables.

If the string named by the @code{\*} 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}
@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}
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

@esindex \@key{RET}
@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.

@cindex name space of macros and strings
@cindex macros, shared name space with strings
@cindex strings, shared name space with macros
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.

@rqindex chop
@rqindex unformat
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

@cindex appending to strings
@cindex strings, appending
@Defreq {as, name [@Var{string}]}
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
@endDefreq

Rudimentary string manipulation routines are given with the next two
requests.

@cindex substring
@Defreq {substring, str n1 [@Var{n2}]}
Replace the string in register@w{ }@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

@cindex length of a string
@cindex string, length of
@Defreq {length, reg str}
Compute the length of @var{str} and returns it in the number
register@w{ }@var{reg}.  If @var{reg} doesn't exist, it is created.

@Example
.ds xxx abcdefgh
.length yyy xxx
\n[yyy]
    @result{} 8
@endExample
@endDefreq

@cindex rename request
@cindex rename macro
@cindex rename string
@Defreq {rn, xx yy}
Rename the request, macro, or string @var{xx} to @var{yy}.
@endDefreq

@cindex remove request
@cindex remove macro
@cindex remove string
@Defreq {rm, xx}
Remove the request, macro, or string @var{xx}.  @code{gtroff} treats
subsequent invocations as if the object had never been defined.
@endDefreq

@cindex alias
@Defreq {als, new old}
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 by @code{gtroff} automatically.
@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

@rqindex if
@rqindex while
@cindex @code{if}, operators to use with it
@cindex @code{while}, operators to use with it
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
@rqindex nroff
True if the document is being processed in nroff mode (i.e., the
@code{.nroff} command has been issued).

@item t
@rqindex troff
True if the document is being processed in troff mode (i.e., the
@code{.troff} command has been issued).

@item v
Always false.

@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, character 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{|} character 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 c @var{ch}
@rqindex char
True if there is a character @var{ch} available; @var{ch} is either an
@acronym{ASCII} character or a special character (@code{\(@var{ch}} or
@code{\[@var{ch}]}); the condition is also true if @var{ch} 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

@c XXX .nop request

@Defreq {ie, expr anything}
@Defreqx {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 this is a bug in makeinfo: you can't have `@{' as an argument
@c to deffn

@esindex \@{
@esindex \@}
@c @Defesc {\\@@@{, , , }
@c @Defescx {\\@@@}, , , }
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

@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.

@rqindex de
@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

@rqindex while
@cindex @code{break}, in a @code{while} loop
@cindex @code{continue}, in a @code{while} loop
@Defreq {break, }
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, }
Finishes 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.

@Defreq {de, 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}}.

Note that no leading whitespace is allowed in the line containing the
ending token (either @samp{..} or the 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

@c XXX add info about macro definitions in macros.

@c XXX give example for end macro.

@c XXX add info about indirect macro calls:
@c
@c .de xxx
@c from yyy\c
@c ..
@c
@c test \*[xxx] test
@c   => test from yyy test

@c XXX info about common identifier pool for strings, macros, and
@c     diversions.
@endDefreq

@cindex appending, to a macro
@Defreq {am, xx}
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
@endDefreq

@cindex alias
@Defreq {als, new old}
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.

The @code{de}, @code{am}, @code{di}, @code{da}, @code{ds},
and @code{as} requests 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.
@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

@esindex \n
@esindex \$
@esindex \*
@esindex \\
@esindex \@key{RET}
@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

@vindex .$
The arguments to a macro can be examined using a variety of escapes.
The number of arguments is available in the @code{.$} number register.
Any individual argument can be retrieved with one of the following
escapes:

@cindex copy-in mode, and macro arguments
@Defesc {\\$, n, , }
@Defescx {\\$, @lparen{}, nn, }
@Defescx {\\$, @lbrack{}, nnn, @rbrack{}}
The escapes @code{\$@var{n}}, @code{\$(@var{nn}} and
@code{\$[@var{nnn}]} 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}]}
Shifts the arguments 1@w{ }position, or as
many positions as specified by its argument.  After executing this
request, argument@w{ }@var{i} becomes argument @var{i}-@var{n};
arguments 1 to@w{ }@var{n} are no longer available.  Shifting by
negative amounts is currently undefined.
@endDefreq

@Defesc {\\$*, , , }
@Defescx {\\$@@, , , }
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.
@endDefesc

@rqindex als
@cindex @code{als}, use with @code{\$0}
@Defesc {\\$0, , , }
The name used to invoke the current macro.
The @code{als} request can make a macro have more than one name.

@Example
.de vl
.ie \\n(.$=1 .ds Vl Pre-Release Version
.el          .ds Vl Version \\$3, \\$4.
..
@endExample

@noindent
This would be called as

@Example
.vl $Id$
@endExample
@endDefesc

@xref{Request Arguments}.


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

@node Page Motions, Drawing Requests, Writing Macros, gtroff Reference
@section Page Motions
@cindex page motions
@cindex motions, page

@cindex @code{sp}, as vertical page motion
@Defreq {sp, [@Var{len}]}
Motions up and down the page can be done with the @code{sp} request.
However, this causes a break so that the actual effect is to move to the
left margin and then to the specified location.
@endDefreq

@Defreq {mk, [@Var{reg}]}
@Defreqx {rt, reg}
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.  The @code{rt}
request returns @emph{upwards} to the location given in the register
name given as an argument; with no argument it returns to the
location marked with the @code{mk} request.

@c XXX example
@ignore
@Example
... dual column example ...
@endExample
@end ignore
@endDefreq

The following escapes give fine control of movements about the page.

@cindex vertical motion
@cindex motion, vertical
@Defesc {\\v, ', e, '}
The @code{\v'@var{e}'} escape enables arbitrary vertical motion from the
current location on the page.  The argument@w{ }@var{e} specifies the
distance to move; positive is downwards and negative upwards.  The
default unit for this escape @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.
@endDefesc

There are some special case escapes for vertical motion.

@ftable @code
@item \r
move upwards@w{ }1@dmn{v}.

@item \u
move upwards@w{ }.5@dmn{v}.

@item \d
move down@w{ }.5@dmn{v}.
@end ftable

@cindex inserting horizontal space
@cindex horizontal space
@cindex space, horizontal
@Defesc {\\h, ', e, '}
The @code{\h'@var{e}'} escape provides horizontal motions.  The
expression@w{ }@var{e} indicates how far to move: positive is rightwards
and negative leftwards.
@c XXX Is there a default unit for this?
@endDefesc

There are a number of special case escapes for horizontal motion:

@ftable @code
@item \@key{SP}
An unbreakable and unpaddable (i.e.@: not expanded during filling)
space.  (Note: This is a backslash followed by a space.)

@item \~
An unbreakable space that stretches like a normal inter-word space
when a line is adjusted.

@item \|
A 1/6@dmn{th} em space.  Ignored for tty output devices (rounded to
zero).

@item \^
A 1/12@dmn{th} em space.  Ignored for tty output devices (rounded to
zero).

@item \0
A space the size of a digit.

@item \&
@cindex zero width space character
@cindex character, zero width space
@cindex space character, zero width
A zero width space.

@item \)
Like @code{\&} except that it behaves like a character declared with the
@code{cflags} request to be transparent for the purposes of end of
sentence recognition.
@end ftable

The following string sets the @TeX{} logo:

@Example
.ds TeX T\h'-.1667m'\v'.224m'E\v'-.224m'\h'-.125m'X
@endExample

@cindex width escape
@cindex escape, width
@Defesc {\\w, ', text, '}
Used as @code{\w'@var{text}'},
returns 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).

@c XXX example

@ignore
@Example
... strlen example ...
@endExample
@end ignore

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
@vindex st
@vindex sb
The highest and lowest point, respectively, in @var{text}.

@item rst
@itemx rsb
@vindex rst
@vindex rsb
Like the @code{st} and @code{sb} registers, but takes account of the
heights and depths of characters.

@item ct
@vindex ct
Defines the kinds of characters occurring in @var{text}:

@table @asis
@item 0
only short characters, no descenders or tall characters.

@item 1
at least one descender.

@item 2
at least one tall character.

@item 3
at least one each of a descender and a tall character.
@end table

@item ssc
@vindex ssc
The amount of horizontal space (possibly negative) that should be added
to the last character before a subscript.

@item skw
@vindex skw
How far to right of the center of the last character in the @code{\w}
argument, the center of an accent from a roman font should be placed
over that character.
@end table
@endDefesc

@Defesc {\\k, ', x, '}
Stores the current horizontal position in register @var{x}.
Use this, for example, to return to the beginning of a string
for highlighting or other decoration.
@endDefesc

@Defreg {.k}
A read-only number register containing the current horizontal output
position.
@endDefreg

@c XXX documentation


@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.

@cindex drawing horizontal lines
@cindex horizontal line, drawing
@cindex line, horizontal, drawing
@Defesc {\\l, ', l c, '}
Draws a line rightwards from the current
location.  The full syntax for this escape is:

@Example
\l'@var{l}@var{c}'
@endExample

@noindent
where @var{l} is the length of the line to be drawn, starting at the
current location; positive numbers draw to the right, and negative
numbers draw towards the left.  This can also be specified absolutely
(i.e.@: with a leading @samp{|}) which draws back to the beginning
of the line.

@cindex underscore character
@cindex character, underscore
@cindex line drawing character
@cindex character for line drawing
The optional second parameter @var{c} is a character to draw the line
with.  If this second argument is not specified, @code{gtroff} uses
the underscore character.

@cindex zero width space character
@cindex character, zero width space
@cindex space character, zero width
@esindex \&
To separate the two arguments (to prevent @code{gtroff} from
interpreting a drawing character as a scaling indicator) use @code{\&}.

Here a small useful example:

@Example
.de box
\(br\\$*\(br\l'|0\(rn'\l'|0\(ul'
..
@endExample

@opindex |
@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.  Then the line
drawing escapes both draw from the current location to the beginning of
the @emph{input} line.
@endDefesc

@cindex drawing vertical lines
@cindex vertical line drawing
@cindex line, vertical, drawing
@cindex line drawing character
@cindex character for line drawing
@cindex box rule character
@cindex character, box rule
@Defesc {\\L, ', l c, '}
Draws vertical lines.  Its parameters are
similar to the @code{\l} escape.  The
movement is downwards for positive values,
and upwards for negative values.  The
default character is the box rule character.  As with the vertical
motion escapes, text processing blindly continues where the line
ends.

@c XXX example

@ignore
@Example
...box macro...
@endExample
@end ignore
@endDefesc

@Defesc {\\D, ', command arg @dots{}, '}
The @code{\D} escape provides a variety of drawing functions.
While the previous escapes work on a character device, these
escapes do not.

@table @code
@item \D'l @var{dx} @var{dy}'
Draw a line from the current location to the relative point specified by
(@var{dx},@var{dy}).

@c XXX example

@ignore
@Example
...revised box macro...
@endExample
@end ignore

@item \D'c @var{d}'
@cindex circle drawing
@cindex drawing a circle
Draw a circle with a diameter of @var{d} with the leftmost point at the
current position.

@item \D'C @var{d}'
Draw a solid circle with the same parameters as an outlined circle.

@item \D'e @var{dx} @var{dy}'
@cindex drawing an ellipse
@cindex ellipse drawing
Draw an ellipse with a horizontal diameter of @var{dx} and a vertical
diameter of @var{dy} with the leftmost point at the current position.

@item \D'E @var{dx} @var{dy}'
Draw a solid ellipse with the same parameters as an outlined ellipse.

@item \D'a @var{dx1} @var{dy1} @var{dx2} @var{dy2}'
@cindex arc drawing
@cindex drawing an arc
Draw an arc clockwise from the current location through the two
specified locations (@var{dx1},@var{dy1}) and (@var{dx2},@var{dy2}).

@item \D'~ @var{dx1} @var{dy1} @var{dx2} @var{dy2} ...'
@cindex drawing a spline
@cindex spline drawing
Draw a spline from the current location to (@var{dx1},@var{dy1}) and
then to (@var{dx2},@var{dy2}), and so on.

@item \D'f @var{n}'
@cindex gray shading
@cindex shading
@cindex shades for filling objects
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@w{
}1000 is used.

@item \D'p @var{dx1} @var{dy1} @var{dx2} @var{dy2} ...'
@cindex drawing a polygon
@cindex polygon drawing
Draw a polygon from the current location to (@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.

@c XXX example

@ignore
@Example
... box example (yes, again)...
@endExample
@end ignore

@item \D'P @var{dx1} @var{dy1} @var{dx2} @var{dy2} ...'
Draw a solid polygon with the same parameters as an outlined polygon.

@c XXX example

@ignore
@Example
... shaded box example ...
@endExample
@end ignore

@item \D't @var{n}'
@cindex line thickness
@cindex thickness of lines
Set the current line thickness to @var{n} 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 @code{ditroff}).
@end table
@endDefesc

@cindex pile, character
@cindex character pile
@Defesc {\\b, ', string, '}
@dfn{Piles} a sequence of characters
vertically, and centers it vertically on the current line.  Use it
to build large brackets and braces.

@Example
\b'\(lt\(bv\(lk\(bv\(lb'
@endExample
@endDefesc

@xref{Drawing Functions}.


@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, after a certain number of input
lines or at the end of input.

@menu
* Page Location Traps::
* Diversion Traps::
* Input 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 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

@cindex vertical position trap enable register
@Defreq {vpt, flag}
@Defregx {.vpt}
Enables 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 macro}
Sets 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.

@var{macro} is the name of the macro to execute when the
trap is sprung.

@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
@endDefreq

@cindex distance to next trap
@cindex trap, distance
@Defreg {.t}
A read-only number register holding the distance to the next trap.
@endDefreg

@cindex changing trap location
@cindex trap, changing location
@Defreq {ch, dist macro}
Changes 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.

@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

@rqindex ne
@cindex @code{ne}, and the @code{.trunc} register
@Defreg {.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

@vindex .t
@cindex @code{.t}, and diversions
@Defreq {dt, dist macro}
Sets a trap @emph{within} a diversion.
@var{dist} is the first argument is the location of the trap
(identical to the @code{.wh} request)
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, End-of-input Traps, Diversion Traps, Traps
@subsection Input Line Traps
@cindex input line traps
@cindex traps, input line

@Defreq {it, n macro}
Sets an input line trap.
@var{n} is the number of lines of input which may be read before
@dfn{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
@endDefreq

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

@node End-of-input Traps,  , Input Line Traps, Traps
@subsection End-of-input Traps
@cindex end-of-input traps
@cindex traps, end-of-input

@Defreq {em, macro}
Sets a trap at the end of input.  The @var{macro}
specified 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.

@c XXX describe top-level diversion
@c XXX index entry for top-level diversion

@Defreq {di, macro}
@Defreqx {da, macro}
Begins 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.

@c XXX example

@ignore
@Example
... end-note example ...
@endExample
@end ignore
@endDefreq

@vindex nl
@vindex .h
@cindex nested diversions
@cindex diversion, nested
@Defreg {.z}
@Defregx {.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

@c XXX more info

@Defreg {.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.
@endDefreg

@Defreg {dn}
@Defregx {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
@group
.\" Center text both horizontally & vertically
.de (c
.br
.nf
.di @@c
..
@end group
@group
.de )c
.br
.di
.nr @@s (((\\n(.tu-\\n(dnu)/2u)-1v)
.sp \\n(@@su
.ce 1000
.nf
.@c
.br
.ce 0
.sp \\n(@@su
.br
.fi
.rr @@s
..
@end group
@end example
@endDefreg

@cindex transparent output
@cindex output, transparent
@Defesc {\\!, , , }
@Defescx {\\?, , @Var{anything}, \\?}
Prevents 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.

@c XXX anything is read in copy mode. (what about \! ??)

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 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
@endDefesc

@cindex unformatting diversions
@cindex diversion, unformatting
@Defreq {asciify, div}
@dfn{Unformats} the diversion specified by @var{div}
in such a way that @acronym{ASCII} and space characters 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 @code{n} to@w{ }1.

@Example
.tr @@.
.di x
@@nr n 1
.br
.di
.tr @@@@
.asciify x
.x
@endExample

@xref{Copy-in Mode}.
@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, character height and slant, space
and sentence space size)

@item
page parameters (line length, title length, vertical spacing,
line spacing, indentation, line numbering, 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
@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@w{ }@samp{2}.

@cindex switch environments
@cindex current environment number/name register
@Defreq {ev, env}
@Defregx {.ev}
Switches 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}.

@c XXX example

@ignore
@Example
... page break macro, revised ...
@endExample
@end ignore

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

@cindex copy environment
@Defreq {evc, env}
Copies the environment @var{env} into the current environment.
@endDefreq


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

@node Suppressing output, I/O, Environments, gtroff Reference
@section Suppressing output
@cindex suppressing output

@Defesc {\\O, , num, }
Disables or enables output depending on the value of @var{num}:

@table @samp
@item \O0
Disable any ditroff glyphs from being emitted to the device driver.

@item \O1
Enable output of glyphs.
@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.

The following two forms of @code{\O} are specific to @code{grohtml}.

@table @samp
@item \O2
Disable any ditroff glyphs from being emitted to the device driver.  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
Enable output of glyphs (the default).  Also write out to @code{stderr}
the page number and four registers encompassing the glyphs previously
written since the last call to @code{\O}.
@end table
@endDefesc


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

@node I/O, Postprocessor Access, Suppressing output, 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:

@cindex including a file
@cindex file inclusion
@Defreq {so, file}
Reads 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.
@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

@cindex transparent output
@cindex output, transparent
@Defreq {cf, file}
@Defreqx {trf, file}
Transparently outputs the contents of @var{file}.  Each line is output
as 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.  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

The request @w{@code{.cf @var{filename}}}, when used in a diversion,
embeds an object in the diversion which, when reread, causes the
contents of @var{filename} to be transparently copied through to the
output.

In @acronym{UNIX} @code{troff}, the contents of @var{filename}
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.  This request causes a line break.

@rqindex trf
With @code{trf}, unlike @code{cf}, the file cannot contain characters
such as NUL that are not valid @code{gtroff} input characters
(@pxref{Identifiers}).  This request causes a line break.
@endDefreq

@Defreq {nx, }
Forces @code{gtroff} to continue processing of
the file specified as an argument.
@endDefreq

@Defreq {rd, }
The @code{rd} request reads from standard input, and includes what is
read as though it were part of the input file.  Text is read until a
blank line is encountered.
@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:

@Example
.ce
\*(td
.sp 2
.nf
.rd
.sp
.rd
.fi
Body of letter.
.bp
.nx repeat.let
@endExample

@rqindex ex
@noindent
When this is run, the following file 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} requests
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}
Pipes 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.
@endDefreq

@Defreq {sy, cmds}
@Defregx {systat}
In @dfn{unsafe} mode, executes the shell command(s) specified by
@var{cmds}.  The output is not saved anyplace, so it is up to the user
to do so.

@c XXX add info about safer and unsafe mode

For example, the following example 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
@samp{H}, @samp{M} and @samp{S}, and then reads those commands in with
the @code{so} request.

@cindex @code{system()} return value register
The @code{systat} read-write number register contains the return value
of the @code{system()} function executed by the last @code{sy} request.
@endDefreq

@Defreq {open, stream file}
@Defreqx {opena, stream file}
Opens the specified @var{file} for writing and
associates the specified @var{stream} with it.

The @code{opena} is like @code{open}, but if the file exists, append to
it instead of truncating it.
@endDefreq

@cindex copy-in mode, and @code{write} requests
@cindex mode, copy-in, and @code{write} requests
@Defreq {write, stream data}
Writes 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.
@endDefreq

@Defreq {close, stream}
Closes the specified @var{stream};
the stream is no longer an acceptable argument to the
@code{write} request.

@c XXX example

@ignore
@Example
... example of open write &c...
@endExample
@end ignore
@endDefreq

@Defesc {\\V, ', xxx, '}
Interpolates the contents of the specified
environment variable, as returned by the function @code{getenv}.
Specify the argument to @code{\V} as an identifier, i.e.@:
@samp{\V@var{x}}, @samp{\V(@var{xx}} or @samp{\V[@var{xxx}]}.  @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}}.
@endDefesc

@Defesc {\\Y, ', xxx, '}
The @code{\Y} escape is called with an identifier (i.e.@:
@code{\Y@var{x}}, @code{\Y(@var{xx}} or @code{\Y[@var{xxx}]}).  This is
approximately equivalent to @samp{\X'\*[@var{xxx}]'}.  However, the
contents of the string or macro @var{xxx} are not interpreted; also it
is permitted for @var{xxx} 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.
@endDefesc

@xref{Output Devices}.


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

@node Miscellaneous, Gtroff Internals, Postprocessor Access, gtroff Reference
@section Miscellaneous
@cindex miscellaneous

This section documents parts of @code{gtroff} which cannot (yet) be
categorized elsewhere in this manual.

@cindex line numbers
@cindex numbers, line
@Defreq {nm, start inc space indent}
Prints line numbers in the left margin.
@var{start} is the line number of the @emph{next}
output line; this defaults to@w{ }1.  @var{inc} indicates on
which lines numbers are printed, i.e.@: 5 means put line numbers on
every 5@w{ }lines; this defaults to@w{ }1.  @var{space} is the
space to be left between the number and the text; this defaults to@w{
}1.  The fourth argument is the indentation of the line numbers.
Without arguments, line numbers are turned off.
@endDefreq

@c XXX xref ln register

@Defreq {nn, [@Var{skip}]}
Temporarily turns off line numbering.  The
argument is the number of lines not to be numbered; this defaults
to@w{ }1.

@c XXX (does this disable incrementing or display?)

@c XXX example

@ignore
@Example
... line numbering example ...
@endExample
@end ignore
@endDefreq

@cindex margin characters
@cindex characters for margins
@Defreq {mc, char dist}
Prints margin characters to the right of the text.
The first argument is the character to be
printed, and the second argument is the distance away from the main body
text.  With no arguments the margin characters are turned off.  If this
occurs before a break, no margin character is printed.

@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.

@c XXX example

@ignore
@Example
... margin char example ...
@endExample
@end ignore
@endDefreq

@pindex soelim
@cindex multi-file documents
@cindex documents, multi-file
@Defreq {lf, line filename}
A debugging aid for
documents which are split into many files, then put together
with @code{soelim} and other preprocessors.  The second argument is the
name of the file and the first argument is the input line number in
that file.  This way @code{gtroff} can produce error messages which are
intelligible to the user.

@c XXX example

@ignore
@Example
... example of soelim'ed doc ...
@endExample
@end ignore
@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}.  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.

For example, the input string @samp{fi\[:u]} is converted in 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}; 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

@esindex \v
@rqindex unformat
@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{\v}.  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.


@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 {tm, string}
Sends the @var{string} to the standard error stream;
this is very useful for printing debugging output among other things.
@endDefreq

@cindex aborting
@Defreq {ab, [@Var{string}]}
Similar to the @code{tm} request, except that
it causes @code{gtroff} to stop processing.  With no argument it
prints @samp{User Abort}.
@endDefreq

@cindex @code{ex}, use in debugging
@cindex exiting
@Defreq {ex, }
The @code{ex} request also causes @code{gtroff} to stop processing
if encountered at the topmost level; 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

@c XXX .tm1, .tmc requests

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.

@cindex dumping symbol table
@cindex symbol table, dumping
@Defreq {pm, }
The @code{pm} request prints out the entire symbol table on @code{stderr}.
@endDefreq

@cindex dumping number registers
@cindex number registers, dumping
@Defreq {pnr, }
Prints the names and contents of all
currently defined number registers on @code{stderr}.
@endDefreq

@cindex dumping traps
@cindex traps, dumping
@Defreq {ptr, }
Prints 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

@cindex flush output
@cindex output, flush
@cindex interactive use of @code{gtroff}
@cindex @code{gtroff}, interactive use
@Defreq {fl, }
Instructs @code{gtroff} to flush its output
immediately.  The intent is for interactive use.
@code{gtroff}; there is little other use for it.  This
request causes a line break.
@endDefreq

@cindex backtrace of input stack
@cindex input stack, backtrace
@Defreq {backtrace, }
The @code{backtrace} request prints a backtrace of the input stack
to the standard error stream.
@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}.

@cindex level of warnings
@cindex warnings, level
@Defreq {warn, [@Var{flags}]}
@Defregx {.warn}
Controls 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
characters.  If an argument is not 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 characters.  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
@rqindex ie
@rqindex el
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
@rqindex di
@rqindex da
@cindex @code{di}, debugging
@cindex @code{da}, debugging
Use of @code{di} or @code{da} without an argument when there is no
current diversion.

@item mac
@itemx 512
@rqindex de
@c XXX more index entries
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
@rqindex nr
@c XXX more index entries
Use of undefined number registers.  When an undefined number register is
used, that register is automatically defined to have a value of@w{ }0.
A definition is automatically made with 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
Use of a tab character where a number was expected.

@item  right-brace
@itemx 4096
@esindex \@}
@cindex @code{\@}}, debugging
Use of @code{\@}} where a number was expected.

@item  missing
@itemx 8192
Requests that are missing non-optional arguments.

@item  input
@itemx 16384
Illegal input characters.

@item  escape
@itemx 32768
Unrecognized escape sequences.  When an unrecognized escape sequence is
encountered, the escape character is ignored.

@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 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, Summary, Debugging, gtroff Reference
@section Implementation Differences
@cindex implementation differences
@cindex differences in implementation
@cindex incompatibilities with Unix @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

@esindex \*
@esindex \n
@cindex @code{\*}, incompatibilities with Unix @code{troff}
@cindex @code{\n}, incompatibilities with Unix @code{troff}
@rqindex cp
@vindex .C
@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).
Compatibility mode can be turned on with the @option{-C} command line
option, and turned on or off with the @code{cp} request.  The number
register @code{.C} is@w{ }1 if compatibility mode is on, 0@w{
}otherwise.

@esindex \A
@esindex \|
@esindex \^
@esindex \&
@esindex \@{
@esindex \@}
@esindex \@key{SP}
@esindex \'
@esindex \`
@esindex \-
@esindex \_
@esindex \!
@esindex \%
@esindex \c
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 point sizes, fractional
@rqindex ps
@cindex @code{ps}, incompatibilities with Unix @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.

@rqindex bd
@rqindex cs
@rqindex tkf
@rqindex tr
@rqindex fp
@cindex @code{bd}, incompatibilities with Unix @code{troff}
@cindex @code{cs}, incompatibilities with Unix @code{troff}
@cindex @code{tkf}, incompatibilities with Unix @code{troff}
@cindex @code{tr}, incompatibilities with Unix @code{troff}
@cindex @code{fp}, incompatibilities with Unix @code{troff}
@cindex input and output characters, compatibility with Unix
@cindex output characters, compatibility with Unix
@cindex characters, input and output, compatibility with Unix
In GNU @code{troff} there is a fundamental difference between
unformatted, input characters, and formatted, output characters.
Everything that affects how an output character is output is stored
with the character; once an output character 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 output characters are constructed from input characters at the
moment immediately before the character 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 output characters in
any combination.  An output character 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

@esindex \e
@esindex \!
@esindex \?
@cindex @code{\e}, incompatibilities with Unix @code{troff}
@cindex @code{\!}, incompatibilities with Unix @code{troff}
@cindex @code{\?}, incompatibilities with Unix @code{troff}
@cindex transparent output, incompatibilities with Unix @code{troff}
@cindex output, transparent, incompatibilities with Unix @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}.  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.

@c XXX .tl compatibility mode -> input stack level
@c XXX .if compatibility mode -> input stack level

@xref{Diversions}, for more information.


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

@node Summary,  , Implementation Differences, gtroff Reference
@section Summary
@cindex summary

@c XXX documentation



@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}
@cindex @code{geqn}

@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}
@cindex @code{gtbl}

@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}
@cindex @code{gpic}

@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}
@cindex @code{ggrn}

@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}

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}
@cindex @code{grefer}

@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}
@cindex @code{gsoelim}

@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}

@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 =====================================================================

@node grops, grodvi, grotty, Output Devices
@section @code{grops}
@cindex @code{grops}

@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}

@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}

@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}

@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}

@c XXX

@menu
* Invoking grohtml::
@end menu

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

@node Invoking grohtml,  , grohtml, grohtml
@subsection Invoking @code{grohtml}
@cindex invoking @code{grohtml}
@cindex @code{grohtml}, invoking

@c XXX


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

@node gxditview,  , grohtml, Output Devices
@section @code{gxditview}
@cindex @code{gxditview}

@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

@c XXX

@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 format output of GNU @code{troff}.  The
output format used by GNU @code{troff} is very similar -- but
not identical -- to that used by
@acronym{UNIX} device-independent @code{troff} (@code{ditroff}).

@menu
* Output Format::
* Device Control::
* Drawing Functions::
* Line Continuation::
@end menu

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

@node Output Format, Device Control, gtroff Output, gtroff Output
@subsection Output Format
@cindex output format
@cindex format of output

@cindex 8-bit input
@cindex input, 8-bit
The output format is text based, as opposed to a binary format (like
@TeX{} DVI).  The output format is @w{8-bit} clean, thus single
characters can have the eighth bit set, as can the names of fonts and
special characters.

The output format consists of single command characters with attached
parameters which are separated from subsequent text by whitespace or a
newline.

The names of characters and fonts can be of arbitrary length; drivers
should not assume that they are only two characters long (as
@code{ditroff} does).

When a character is to be printed, that character is always in the
current font.  Unlike @code{ditroff}, it is not necessary for drivers to
search special fonts to find a character.

@table @code
@item H@var{n}
@c XXX

@item V@var{n}
@c XXX

@item h@var{n}
@c XXX

@item v@var{n}
@c XXX

@item c@var{n}
@c XXX

@item C@var{n}
@c XXX

@item @var{nn}@var{c}
@c XXX

@item t@var{xxx}
@var{xxx} is any sequence of characters terminated by a space or a
newline; the first character should be printed at the current position,
the the current horizontal position should be increased by the width of
the first character, and so on for each character.  The width of the
character is that given in the font file, appropriately scaled for the
current point size, and rounded so that it is a multiple of the
horizontal resolution.  Special characters cannot be printed using this
command.

@kindex tcommand
@pindex DESC@r{, and @code{tcommand}}
This command is only allowed if the @samp{tcommand} line is present in
the @file{DESC} file.

@item u@var{n} @var{xxx}
This is same as the @samp{t} command except that after printing each
character, the current horizontal position is increased by the sum of
the width of that character and@w{ }@var{n}.

This command is only allowed if the @samp{tcommand} line is present in
the @file{DESC} file.

@item n@var{a}@var{b}
@c XXX

@item p@var{n}
@c XXX

@item s@var{n}
@kindex sizescale
@pindex DESC@r{, and @code{sizescale}}
The argument to the @samp{s} command is in scaled points (units of
points/@var{n}, where @var{n} is the argument to the @samp{sizescale}
command in the @file{DESC} file).

@item f@var{n}
@item x @dots{} \n
Device control.
@c XXX more info

@item D@var{c} @var{x}@dots{}\n
@c XXX
@end table

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

@node Device Control, Drawing Functions, Output Format, gtroff Output
@subsection Device Control
@cindex device control
@cindex control of devices

The @samp{x} command is normally followed by a letter or word indicating
the function to perform, followed by white space separated arguments.

The first argument can be abbreviated to the first letter.

@table @code
@item x init
@c XXX

@item x T
@c XXX

@item x res @var{n} @var{h} @var{v}
@c XXX

@item x H
@c XXX more info
The argument to the @w{@samp{x Height}} command is also in scaled
points.
@end table

The first three output commands are guaranteed to be:

@Example
x T device
x res n h v
x init
@endExample

@noindent
For example, the input

@Example
crunchy \fH\s+2frog\s0\fP!?
@endExample

@noindent
produces

@c XXX example

@ignore
@Example
... sample output here ...
@endExample
@end ignore

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

@node Drawing Functions, Line Continuation, Device Control, gtroff Output
@subsection Drawing Functions
@cindex drawing functions
@cindex functions for drawing

@pindex gpic
The @samp{D} drawing command has been extended.  These extensions are
used by GNU @code{pic} only if the @option{-x} option is given.

@xref{Drawing Requests}.

@table @code
@c XXX ...
@item Df @var{n}
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@w{
}1000 is used.  Whatever color a solid object has, it should
completely obscure everything beneath it.  A value greater than@w{ }1000
or less than@w{ }0 can also be used: this means fill with the shade of
gray that is currently being used for lines and text.  Normally this
is black, but some drivers may provide a way of changing this.

@item DC @var{d}
Draw a solid circle with a diameter of@w{ }@var{d} with the leftmost
point at the current position.

@item DE @var{dx} @var{dy}
Draw a solid ellipse with a horizontal diameter of@w{ }@var{dx} and a
vertical diameter of@w{ }@var{dy} with the leftmost point at the current
position.

@item Dp @var{dx1} @var{dy1} @var{dx2} @var{dy2} @dots{} @var{dxn} @var{dyn}
Draw a polygon with automatic closure.  The first vertex is at the
current position, the second vertex at an offset (@var{dx1},@var{dy1})
from the current position, the second vertex at an offset
(@var{dx2},@var{dy2}) from the first vertex, and so on up to the
@var{n}@dmn{th} vertex.  At the moment, GNU @code{pic} only uses this
command to generate triangles and rectangles.

@item DP @var{dx1} @var{dy1} @var{dx2} @var{dy2} @dots{} @var{dxn} @var{dyn}
Like @code{Dp} but draw a solid rather than outlined polygon.

@item Dt @var{n}
@cindex line thickness
@cindex thickness of lines
Set the current line thickness to @var{n}@w{ }machine units.
Traditionally, @acronym{UNIX} @code{troff} drivers use a line thickness
proportional to the current point size; drivers should continue to do
this if no @code{Dt} command has been given, or if a @code{Dt} command
has been given with a negative value of@w{ }@var{n}.  A zero value of@w{
}@var{n} selects the smallest available line thickness.
@end table

@esindex \D
A difficulty arises in how the current position should be changed after
the execution of these commands.  This is not of great importance since
the code generated by GNU @code{pic} does not depend on this.  Given a
drawing command of the form

@Example
\D'@var{c} @var{x1} @var{y1} @var{x2} @var{y2} @dots{} @var{xn} @var{yn}'
@endExample

@esindex \w
@vindex st
@vindex sb
@noindent
where @var{c} is not one of @samp{c}, @samp{e}, @samp{l}, @samp{a} or
@samp{~}, @acronym{UNIX} @code{troff} treats each x@w{ }value
as a horizontal quantity, and each y@w{ }value as a vertical
quantity; it assumes that the width of the drawn object is the sum of
all x@w{ }values, and that the height is the sum of all y@w{ }values.
(The assumption about the height can be seen by examining the @code{st}
and @code{sb} registers after using such a @code{D}@w{ }command in a
@code{\w} escape sequence.)  This rule also holds for all the original
drawing commands with the exception of @code{De}.  For the sake of
compatibility GNU @code{troff} also follows this rule, even though it
produces an ugly result in the case of the @code{Df}, @code{Dt}, and, to
a lesser extent, @code{DE}@w{ }commands.  Thus after executing a
@code{D}@w{ }command of the form

@Example
D@var{c} @var{x1} @var{y1} @var{x2} @var{y2} @dots{} @var{xn} @var{yn}
@endExample

@noindent
the current position should be increased horizontally by the sum of all
x@w{ }values and vertically by the sum of all y@w{ }values.

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

@node Line Continuation,  , Drawing Functions, gtroff Output
@subsection Line Continuation
@cindex line continuation in output commands
@cindex output commands, line continuation

There is a continuation convention which permits the argument to the
@w{@samp{x X}} command to contain newlines: When outputting the argument
to the @w{@samp{x X}} command, GNU @code{troff} follows each newline
in the argument with a @samp{+} character (as usual, it terminates
the entire argument with a newline); thus if the line after the line
containing the @w{@samp{x X}} command starts with @samp{+}, then the
newline ending the line containing the @w{@samp{x X}} command should be
treated as part of the argument to the @w{@samp{x X}} command, the
@samp{+} should be ignored, and the part of the line following the
@samp{+} should be treated like the part of the line following the
@w{@samp{x X}} command.


@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.  Unlike the @code{ditroff} font format,
there is no associated binary format; all files are text files.  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:

@table @code
@item res @var{n}
@kindex res
There are @var{n} machine units per inch.

@item hor @var{n}
@kindex hor
The horizontal resolution is @var{n} machine units.

@item vert @var{n}
@kindex vert
The vertical resolution is @var{n} 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 tcommand
@kindex tcommand
This means that the postprocessor can handle the @samp{t} and @samp{u}
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 a@w{
}0.  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@var{ }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 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.

@c XXX add other commands resp. xrefs to output devices
@c XXX add obsolete commands

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

@node Font File Format,  , DESC File Format, Font Files
@subsection Font File Format
@cindex font file format
@cindex format of font files

A font 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 characters 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
Characters @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
@kindex special
The font is special; this means that when a character 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.  The
@code{charset} line is followed by a sequence of lines.  Each line gives
information for one character.  A line comprises a number of fields
separated by blanks or tabs.  The format is

@c XXX fix it for new HTML additions

@Example
@var{name} @var{metrics} @var{type} @var{code} @var{comment}
@endExample

@cindex 8-bit input
@cindex input, 8-bit
@esindex \N
@kindex ---
@noindent
@var{name} identifies the character: If @var{name} is a single
character@w{ }@var{c} then it corresponds to the @code{gtroff} input
character @var{c}; if it is of the form @samp{\@var{c}} where @var{c} is
a single character, then it corresponds to the @code{gtroff} input
character@w{ }\@var{c}; otherwise it corresponds to the groff input
character @samp{\[@var{name}]}.  (If it is exactly two characters
@var{xx} it can be entered as @samp{\(@var{xx}}.)  @code{gtroff}
supports 8-bit characters; however some utilities have difficulties with
eight-bit characters.  For this reason, there is a convention that the
name @samp{char@var{n}} is equivalent to the single 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 @w{ISO
Latin-1} character set.  The name @samp{---} is special and indicates
that the character is unnamed; such characters can only be used by means
of the @code{\N} escape sequence in @code{gtroff}.

@c XXX input encodings vs. output encodings

The @var{type} field gives the character type:

@table @code
@item 1
the character has an descender, for example, `p';

@item 2
the character has an ascender, for example, `b';

@item 3
the character has both an ascender and a descender, for example, `('.
@end table

The @var{code} field gives the code which the postprocessor uses to
print the character.  The character can also be input to @code{gtroff}
using this code by means of the @code{\N} escape sequence.  The 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.

Anything on the line after the @var{code} field is ignored.

The @var{metrics} field has the form:

@Example
@var{width}[,@var{height}[,@var{depth}[,@var{italic_correction}
  [,@var{left_italic_correction}[,@var{subscript_correction}]]]]]
@endExample

@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 character.  The
@var{height} subfield gives the height of the character (upwards is
positive); if a character 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 character, that is, the distance below
the lowest point below the baseline to which the character extends
(downwards is positive); if a character does not extend below above 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 character when it is immediately to be
followed by a character from a roman font.  The
@var{left_italic_correction} subfield gives the amount of space that
should be added before the character when it is immediately to be
preceded by a character from a roman font.  The
@var{subscript_correction} gives the amount of space that should be
added after a character 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 character
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 character @var{c1} appears next to character
@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, Request Index, File formats, Top
@chapter Installation
@cindex installation

@c XXX



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

@node Request Index, Escape Index, Installation, Top
@chapter 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
@chapter Escape Index

@printindex es



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

@node Operator Index, Register Index, Escape Index, Top
@chapter Operator Index

@printindex op



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

@node Register Index, Macro Index, Operator Index, Top
@chapter Register Index

@printindex vr



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

@node Macro Index, String Index, Register Index, Top
@chapter Macro Index

@printindex ma



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

@node String Index, Glyph Name Index, Macro Index, Top
@chapter String Index

@printindex st



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

@node Glyph Name Index, Font File Keyword Index, String Index, Top
@chapter 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]}.

@printindex gl



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

@node Font File Keyword Index, Program and File Index, Glyph Name Index, Top
@chapter Font File Keyword Index

@printindex ky



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

@node Program and File Index, Concept Index, Font File Keyword Index, Top
@chapter Program  and File Index

@printindex pg



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

@node Concept Index,  , Program and File Index, Top
@chapter Concept Index

@printindex cp



@summarycontents
@contents
@bye