Fix typos and minor wording issues in ELisp manual

[emacs.git] / doc / lispref / internals.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1993, 1998-1999, 2001-2018 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node GNU Emacs Internals
@appendix GNU Emacs Internals

This chapter describes how the runnable Emacs executable is dumped with
the preloaded Lisp libraries in it, how storage is allocated, and some
internal aspects of GNU Emacs that may be of interest to C programmers.

@menu
* Building Emacs::      How the dumped Emacs is made.
* Pure Storage::        Kludge to make preloaded Lisp functions shareable.
* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
* Stack-allocated Objects::    Temporary conses and strings on C stack.
* Memory Usage::        Info about total size of Lisp objects made so far.
* C Dialect::           What C variant Emacs is written in.
* Writing Emacs Primitives::   Writing C code for Emacs.
* Object Internals::    Data formats of buffers, windows, processes.
* C Integer Types::     How C integer types are used inside Emacs.
@end menu

@node Building Emacs
@section Building Emacs
@cindex building Emacs
@pindex temacs

  This section explains the steps involved in building the Emacs
executable.  You don't have to know this material to build and install
Emacs, since the makefiles do all these things automatically.  This
information is pertinent to Emacs developers.

  Building Emacs requires GNU Make version 3.81 or later.

  Compilation of the C source files in the @file{src} directory
produces an executable file called @file{temacs}, also called a
@dfn{bare impure Emacs}.  It contains the Emacs Lisp interpreter and
I/O routines, but not the editing commands.

@cindex @file{loadup.el}
  The command @w{@command{temacs -l loadup}} would run @file{temacs}
and direct it to load @file{loadup.el}.  The @code{loadup} library
loads additional Lisp libraries, which set up the normal Emacs editing
environment.  After this step, the Emacs executable is no longer
@dfn{bare}.

@cindex dumping Emacs
  Because it takes some time to load the standard Lisp files, the
@file{temacs} executable usually isn't run directly by users.
Instead, as one of the last steps of building Emacs, the command
@samp{temacs -batch -l loadup dump} is run.  The special @samp{dump}
argument causes @command{temacs} to dump out an executable program,
called @file{emacs}, which has all the standard Lisp files preloaded.
(The @samp{-batch} argument prevents @file{temacs} from trying to
initialize any of its data on the terminal, so that the tables of
terminal information are empty in the dumped Emacs.)

@cindex preloaded Lisp files
@vindex preloaded-file-list
  The dumped @file{emacs} executable (also called a @dfn{pure} Emacs)
is the one which is installed.  The variable
@code{preloaded-file-list} stores a list of the Lisp files preloaded
into the dumped Emacs.  If you port Emacs to a new operating system,
and are not able to implement dumping, then Emacs must load
@file{loadup.el} each time it starts.

@cindex build details
@cindex deterministic build
@cindex @option{--disable-build-details} option to @command{configure}
  By default the dumped @file{emacs} executable records details such
as the build time and host name.  Use the
@option{--disable-build-details} option of @command{configure} to
suppress these details, so that building and installing Emacs twice
from the same sources is more likely to result in identical copies of
Emacs.

@cindex @file{site-load.el}
  You can specify additional files to preload by writing a library named
@file{site-load.el} that loads them.  You may need to rebuild Emacs
with an added definition

@example
#define SITELOAD_PURESIZE_EXTRA @var{n}
@end example

@noindent
to make @var{n} added bytes of pure space to hold the additional files;
see @file{src/puresize.h}.
(Try adding increments of 20000 until it is big enough.)  However, the
advantage of preloading additional files decreases as machines get
faster.  On modern machines, it is usually not advisable.

  After @file{loadup.el} reads @file{site-load.el}, it finds the
documentation strings for primitive and preloaded functions (and
variables) in the file @file{etc/DOC} where they are stored, by
calling @code{Snarf-documentation} (@pxref{Definition of
Snarf-documentation,, Accessing Documentation}).

@cindex @file{site-init.el}
@cindex preloading additional functions and variables
  You can specify other Lisp expressions to execute just before dumping
by putting them in a library named @file{site-init.el}.  This file is
executed after the documentation strings are found.

  If you want to preload function or variable definitions, there are
three ways you can do this and make their documentation strings
accessible when you subsequently run Emacs:

@itemize @bullet
@item
Arrange to scan these files when producing the @file{etc/DOC} file,
and load them with @file{site-load.el}.

@item
Load the files with @file{site-init.el}, then copy the files into the
installation directory for Lisp files when you install Emacs.

@item
Specify a @code{nil} value for @code{byte-compile-dynamic-docstrings}
as a local variable in each of these files, and load them with either
@file{site-load.el} or @file{site-init.el}.  (This method has the
drawback that the documentation strings take up space in Emacs all the
time.)
@end itemize

@cindex change @code{load-path} at configure time
@cindex @option{--enable-locallisppath} option to @command{configure}
  It is not advisable to put anything in @file{site-load.el} or
@file{site-init.el} that would alter any of the features that users
expect in an ordinary unmodified Emacs.  If you feel you must override
normal features for your site, do it with @file{default.el}, so that
users can override your changes if they wish.  @xref{Startup Summary}.
Note that if either @file{site-load.el} or @file{site-init.el} changes
@code{load-path}, the changes will be lost after dumping.
@xref{Library Search}.  To make a permanent change to
@code{load-path}, use the @option{--enable-locallisppath} option
of @command{configure}.

  In a package that can be preloaded, it is sometimes necessary (or
useful) to delay certain evaluations until Emacs subsequently starts
up.  The vast majority of such cases relate to the values of
customizable variables.  For example, @code{tutorial-directory} is a
variable defined in @file{startup.el}, which is preloaded.  The default
value is set based on @code{data-directory}.  The variable needs to
access the value of @code{data-directory} when Emacs starts, not when
it is dumped, because the Emacs executable has probably been installed
in a different location since it was dumped.

@defun custom-initialize-delay symbol value
This function delays the initialization of @var{symbol} to the next
Emacs start.  You normally use this function by specifying it as the
@code{:initialize} property of a customizable variable.  (The argument
@var{value} is unused, and is provided only for compatibility with the
form Custom expects.)
@end defun

In the unlikely event that you need a more general functionality than
@code{custom-initialize-delay} provides, you can use
@code{before-init-hook} (@pxref{Startup Summary}).

@defun dump-emacs to-file from-file
@cindex unexec
This function dumps the current state of Emacs into an executable file
@var{to-file}.  It takes symbols from @var{from-file} (this is normally
the executable file @file{temacs}).

If you want to use this function in an Emacs that was already dumped,
you must run Emacs with @samp{-batch}.
@end defun

@node Pure Storage
@section Pure Storage
@cindex pure storage

  Emacs Lisp uses two kinds of storage for user-created Lisp objects:
@dfn{normal storage} and @dfn{pure storage}.  Normal storage is where
all the new data created during an Emacs session are kept
(@pxref{Garbage Collection}).  Pure storage is used for certain data
in the preloaded standard Lisp files---data that should never change
during actual use of Emacs.

  Pure storage is allocated only while @command{temacs} is loading the
standard preloaded Lisp libraries.  In the file @file{emacs}, it is
marked as read-only (on operating systems that permit this), so that
the memory space can be shared by all the Emacs jobs running on the
machine at once.  Pure storage is not expandable; a fixed amount is
allocated when Emacs is compiled, and if that is not sufficient for
the preloaded libraries, @file{temacs} allocates dynamic memory for
the part that didn't fit.  The resulting image will work, but garbage
collection (@pxref{Garbage Collection}) is disabled in this situation,
causing a memory leak.  Such an overflow normally won't happen unless
you try to preload additional libraries or add features to the
standard ones.  Emacs will display a warning about the overflow when
it starts.  If this happens, you should increase the compilation
parameter @code{SYSTEM_PURESIZE_EXTRA} in the file
@file{src/puresize.h} and rebuild Emacs.

@defun purecopy object
This function makes a copy in pure storage of @var{object}, and returns
it.  It copies a string by simply making a new string with the same
characters, but without text properties, in pure storage.  It
recursively copies the contents of vectors and cons cells.  It does
not make copies of other objects such as symbols, but just returns
them unchanged.  It signals an error if asked to copy markers.

This function is a no-op except while Emacs is being built and dumped;
it is usually called only in preloaded Lisp files.
@end defun

@defvar pure-bytes-used
The value of this variable is the number of bytes of pure storage
allocated so far.  Typically, in a dumped Emacs, this number is very
close to the total amount of pure storage available---if it were not,
we would preallocate less.
@end defvar

@defvar purify-flag
This variable determines whether @code{defun} should make a copy of the
function definition in pure storage.  If it is non-@code{nil}, then the
function definition is copied into pure storage.

This flag is @code{t} while loading all of the basic functions for
building Emacs initially (allowing those functions to be shareable and
non-collectible).  Dumping Emacs as an executable always writes
@code{nil} in this variable, regardless of the value it actually has
before and after dumping.

You should not change this flag in a running Emacs.
@end defvar

@node Garbage Collection
@section Garbage Collection

@cindex memory allocation
  When a program creates a list or the user defines a new function
(such as by loading a library), that data is placed in normal storage.
If normal storage runs low, then Emacs asks the operating system to
allocate more memory.  Different types of Lisp objects, such as
symbols, cons cells, small vectors, markers, etc., are segregated in
distinct blocks in memory.  (Large vectors, long strings, buffers and
certain other editing types, which are fairly large, are allocated in
individual blocks, one per object; small strings are packed into blocks
of 8k bytes, and small vectors are packed into blocks of 4k bytes).

@cindex vector-like objects, storage
@cindex storage of vector-like Lisp objects
  Beyond the basic vector, a lot of objects like window, buffer, and
frame are managed as if they were vectors.  The corresponding C data
structures include the @code{union vectorlike_header} field whose
@code{size} member contains the subtype enumerated by @code{enum pvec_type}
and an information about how many @code{Lisp_Object} fields this structure
contains and what the size of the rest data is.  This information is
needed to calculate the memory footprint of an object, and used
by the vector allocation code while iterating over the vector blocks.

@cindex garbage collection
  It is quite common to use some storage for a while, then release it
by (for example) killing a buffer or deleting the last pointer to an
object.  Emacs provides a @dfn{garbage collector} to reclaim this
abandoned storage.  The garbage collector operates by finding and
marking all Lisp objects that are still accessible to Lisp programs.
To begin with, it assumes all the symbols, their values and associated
function definitions, and any data presently on the stack, are
accessible.  Any objects that can be reached indirectly through other
accessible objects are also accessible.

  When marking is finished, all objects still unmarked are garbage.  No
matter what the Lisp program or the user does, it is impossible to refer
to them, since there is no longer a way to reach them.  Their space
might as well be reused, since no one will miss them.  The second
(sweep) phase of the garbage collector arranges to reuse them.

@c ??? Maybe add something describing weak hash tables here?

@cindex free list
  The sweep phase puts unused cons cells onto a @dfn{free list}
for future allocation; likewise for symbols and markers.  It compacts
the accessible strings so they occupy fewer 8k blocks; then it frees the
other 8k blocks.  Unreachable vectors from vector blocks are coalesced
to create largest possible free areas; if a free area spans a complete
4k block, that block is freed.  Otherwise, the free area is recorded
in a free list array, where each entry corresponds to a free list
of areas of the same size.  Large vectors, buffers, and other large
objects are allocated and freed individually.

@cindex CL note---allocate more storage
@quotation
@b{Common Lisp note:} Unlike other Lisps, GNU Emacs Lisp does not
call the garbage collector when the free list is empty.  Instead, it
simply requests the operating system to allocate more storage, and
processing continues until @code{gc-cons-threshold} bytes have been
used.

This means that you can make sure that the garbage collector will not
run during a certain portion of a Lisp program by calling the garbage
collector explicitly just before it (provided that portion of the
program does not use so much space as to force a second garbage
collection).
@end quotation

@deffn Command garbage-collect
This command runs a garbage collection, and returns information on
the amount of space in use.  (Garbage collection can also occur
spontaneously if you use more than @code{gc-cons-threshold} bytes of
Lisp data since the previous garbage collection.)

@code{garbage-collect} returns a list with information on amount of space in
use, where each entry has the form @samp{(@var{name} @var{size} @var{used})}
or @samp{(@var{name} @var{size} @var{used} @var{free})}.  In the entry,
@var{name} is a symbol describing the kind of objects this entry represents,
@var{size} is the number of bytes used by each one, @var{used} is the number
of those objects that were found live in the heap, and optional @var{free} is
the number of those objects that are not live but that Emacs keeps around for
future allocations.  So an overall result is:

@example
((@code{conses} @var{cons-size} @var{used-conses} @var{free-conses})
 (@code{symbols} @var{symbol-size} @var{used-symbols} @var{free-symbols})
 (@code{miscs} @var{misc-size} @var{used-miscs} @var{free-miscs})
 (@code{strings} @var{string-size} @var{used-strings} @var{free-strings})
 (@code{string-bytes} @var{byte-size} @var{used-bytes})
 (@code{vectors} @var{vector-size} @var{used-vectors})
 (@code{vector-slots} @var{slot-size} @var{used-slots} @var{free-slots})
 (@code{floats} @var{float-size} @var{used-floats} @var{free-floats})
 (@code{intervals} @var{interval-size} @var{used-intervals} @var{free-intervals})
 (@code{buffers} @var{buffer-size} @var{used-buffers})
 (@code{heap} @var{unit-size} @var{total-size} @var{free-size}))
@end example

Here is an example:

@example
(garbage-collect)
      @result{} ((conses 16 49126 8058) (symbols 48 14607 0)
                 (miscs 40 34 56) (strings 32 2942 2607)
                 (string-bytes 1 78607) (vectors 16 7247)
                 (vector-slots 8 341609 29474) (floats 8 71 102)
                 (intervals 56 27 26) (buffers 944 8)
                 (heap 1024 11715 2678))
@end example

Below is a table explaining each element.  Note that last @code{heap} entry
is optional and present only if an underlying @code{malloc} implementation
provides @code{mallinfo} function.

@table @var
@item cons-size
Internal size of a cons cell, i.e., @code{sizeof (struct Lisp_Cons)}.

@item used-conses
The number of cons cells in use.

@item free-conses
The number of cons cells for which space has been obtained from
the operating system, but that are not currently being used.

@item symbol-size
Internal size of a symbol, i.e., @code{sizeof (struct Lisp_Symbol)}.

@item used-symbols
The number of symbols in use.

@item free-symbols
The number of symbols for which space has been obtained from
the operating system, but that are not currently being used.

@item misc-size
Internal size of a miscellaneous entity, i.e.,
@code{sizeof (union Lisp_Misc)}, which is a size of the
largest type enumerated in @code{enum Lisp_Misc_Type}.

@item used-miscs
The number of miscellaneous objects in use.  These include markers
and overlays, plus certain objects not visible to users.

@item free-miscs
The number of miscellaneous objects for which space has been obtained
from the operating system, but that are not currently being used.

@item string-size
Internal size of a string header, i.e., @code{sizeof (struct Lisp_String)}.

@item used-strings
The number of string headers in use.

@item free-strings
The number of string headers for which space has been obtained
from the operating system, but that are not currently being used.

@item byte-size
This is used for convenience and equals to @code{sizeof (char)}.

@item used-bytes
The total size of all string data in bytes.

@item vector-size
Internal size of a vector header, i.e., @code{sizeof (struct Lisp_Vector)}.

@item used-vectors
The number of vector headers allocated from the vector blocks.

@item slot-size
Internal size of a vector slot, always equal to @code{sizeof (Lisp_Object)}.

@item used-slots
The number of slots in all used vectors.

@item free-slots
The number of free slots in all vector blocks.

@item float-size
Internal size of a float object, i.e., @code{sizeof (struct Lisp_Float)}.
(Do not confuse it with the native platform @code{float} or @code{double}.)

@item used-floats
The number of floats in use.

@item free-floats
The number of floats for which space has been obtained from
the operating system, but that are not currently being used.

@item interval-size
Internal size of an interval object, i.e., @code{sizeof (struct interval)}.

@item used-intervals
The number of intervals in use.

@item free-intervals
The number of intervals for which space has been obtained from
the operating system, but that are not currently being used.

@item buffer-size
Internal size of a buffer, i.e., @code{sizeof (struct buffer)}.
(Do not confuse with the value returned by @code{buffer-size} function.)

@item used-buffers
The number of buffer objects in use.  This includes killed buffers
invisible to users, i.e., all buffers in @code{all_buffers} list.

@item unit-size
The unit of heap space measurement, always equal to 1024 bytes.

@item total-size
Total heap size, in @var{unit-size} units.

@item free-size
Heap space which is not currently used, in @var{unit-size} units.
@end table

If there was overflow in pure space (@pxref{Pure Storage}),
@code{garbage-collect} returns @code{nil}, because a real garbage
collection cannot be done.
@end deffn

@defopt garbage-collection-messages
If this variable is non-@code{nil}, Emacs displays a message at the
beginning and end of garbage collection.  The default value is
@code{nil}.
@end defopt

@defvar post-gc-hook
This is a normal hook that is run at the end of garbage collection.
Garbage collection is inhibited while the hook functions run, so be
careful writing them.
@end defvar

@defopt gc-cons-threshold
The value of this variable is the number of bytes of storage that must
be allocated for Lisp objects after one garbage collection in order to
trigger another garbage collection.  You can use the result returned by
@code{garbage-collect} to get an information about size of the particular
object type; space allocated to the contents of buffers does not count.
Note that the subsequent garbage collection does not happen immediately
when the threshold is exhausted, but only the next time the Lisp interpreter
is called.

The initial threshold value is @code{GC_DEFAULT_THRESHOLD}, defined in
@file{alloc.c}.  Since it's defined in @code{word_size} units, the value
is 400,000 for the default 32-bit configuration and 800,000 for the 64-bit
one.  If you specify a larger value, garbage collection will happen less
often.  This reduces the amount of time spent garbage collecting, but
increases total memory use.  You may want to do this when running a program
that creates lots of Lisp data.

You can make collections more frequent by specifying a smaller value, down
to 1/10th of @code{GC_DEFAULT_THRESHOLD}.  A value less than this minimum
will remain in effect only until the subsequent garbage collection, at which
time @code{garbage-collect} will set the threshold back to the minimum.
@end defopt

@defopt gc-cons-percentage
The value of this variable specifies the amount of consing before a
garbage collection occurs, as a fraction of the current heap size.
This criterion and @code{gc-cons-threshold} apply in parallel, and
garbage collection occurs only when both criteria are satisfied.

As the heap size increases, the time to perform a garbage collection
increases.  Thus, it can be desirable to do them less frequently in
proportion.
@end defopt

  The value returned by @code{garbage-collect} describes the amount of
memory used by Lisp data, broken down by data type.  By contrast, the
function @code{memory-limit} provides information on the total amount of
memory Emacs is currently using.

@defun memory-limit
This function returns the address of the last byte Emacs has allocated,
divided by 1024.  We divide the value by 1024 to make sure it fits in a
Lisp integer.

You can use this to get a general idea of how your actions affect the
memory usage.
@end defun

@defvar memory-full
This variable is @code{t} if Emacs is nearly out of memory for Lisp
objects, and @code{nil} otherwise.
@end defvar

@defun memory-use-counts
This returns a list of numbers that count the number of objects
created in this Emacs session.  Each of these counters increments for
a certain kind of object.  See the documentation string for details.
@end defun

@defun memory-info
This functions returns an amount of total system memory and how much
of it is free.  On an unsupported system, the value may be @code{nil}.
@end defun

@defvar gcs-done
This variable contains the total number of garbage collections
done so far in this Emacs session.
@end defvar

@defvar gc-elapsed
This variable contains the total number of seconds of elapsed time
during garbage collection so far in this Emacs session, as a
floating-point number.
@end defvar

@node Stack-allocated Objects
@section Stack-allocated Objects

@cindex stack allocated Lisp objects
@cindex Lisp objects, stack-allocated
  The garbage collector described above is used to manage data visible
from Lisp programs, as well as most of the data internally used by the
Lisp interpreter.  Sometimes it may be useful to allocate temporary
internal objects using the C stack of the interpreter.  This can help
performance, as stack allocation is typically faster than using heap
memory to allocate and the garbage collector to free.  The downside is
that using such objects after they are freed results in undefined
behavior, so uses should be well thought out and carefully debugged by
using the @code{GC_CHECK_MARKED_OBJECTS} feature (see
@file{src/alloc.c}).  In particular, stack-allocated objects should
never be made visible to user Lisp code.

  Currently, cons cells and strings can be allocated this way.  This
is implemented by C macros like @code{AUTO_CONS} and
@code{AUTO_STRING} that define a named @code{Lisp_Object} with block
lifetime.  These objects are not freed by the garbage collector;
instead, they have automatic storage duration, i.e., they are
allocated like local variables and are automatically freed at the end
of execution of the C block that defined the object.

  For performance reasons, stack-allocated strings are limited to
@acronym{ASCII} characters, and many of these strings are immutable,
i.e., calling @code{ASET} on them produces undefined behavior.

@node Memory Usage
@section Memory Usage
@cindex memory usage

  These functions and variables give information about the total amount
of memory allocation that Emacs has done, broken down by data type.
Note the difference between these and the values returned by
@code{garbage-collect}; those count objects that currently exist, but
these count the number or size of all allocations, including those for
objects that have since been freed.

@defvar cons-cells-consed
The total number of cons cells that have been allocated so far
in this Emacs session.
@end defvar

@defvar floats-consed
The total number of floats that have been allocated so far
in this Emacs session.
@end defvar

@defvar vector-cells-consed
The total number of vector cells that have been allocated so far
in this Emacs session.
@end defvar

@defvar symbols-consed
The total number of symbols that have been allocated so far
in this Emacs session.
@end defvar

@defvar string-chars-consed
The total number of string characters that have been allocated so far
in this session.
@end defvar

@defvar misc-objects-consed
The total number of miscellaneous objects that have been allocated so
far in this session.  These include markers and overlays, plus
certain objects not visible to users.
@end defvar

@defvar intervals-consed
The total number of intervals that have been allocated so far
in this Emacs session.
@end defvar

@defvar strings-consed
The total number of strings that have been allocated so far in this
Emacs session.
@end defvar

@node C Dialect
@section C Dialect
@cindex C programming language

The C part of Emacs is portable to C99 or later: C11-specific features such
as @samp{<stdalign.h>} and @samp{_Noreturn} are not used without a check,
typically at configuration time, and the Emacs build procedure
provides a substitute implementation if necessary.  Some C11 features,
such as anonymous structures and unions, are too difficult to emulate,
so they are avoided entirely.

At some point in the future the base C dialect will no doubt change to C11.

@node Writing Emacs Primitives
@section Writing Emacs Primitives
@cindex primitive function internals
@cindex writing Emacs primitives

  Lisp primitives are Lisp functions implemented in C@.  The details of
interfacing the C function so that Lisp can call it are handled by a few
C macros.  The only way to really understand how to write new C code is
to read the source, but we can explain some things here.

  An example of a special form is the definition of @code{or}, from
@file{eval.c}.  (An ordinary function would have the same general
appearance.)

@smallexample
@group
DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
  doc: /* Eval args until one of them yields non-nil, then return
that value.
The remaining args are not evalled at all.
If all args return nil, return nil.
@end group
@group
usage: (or CONDITIONS...)  */)
  (Lisp_Object args)
@{
  Lisp_Object val = Qnil;
@end group

@group
  while (CONSP (args))
    @{
      val = eval_sub (XCAR (args));
      if (!NILP (val))
        break;
      args = XCDR (args);
      maybe_quit ();
    @}
@end group

@group
  return val;
@}
@end group
@end smallexample

@cindex @code{DEFUN}, C macro to define Lisp primitives
  Let's start with a precise explanation of the arguments to the
@code{DEFUN} macro.  Here is a template for them:

@example
DEFUN (@var{lname}, @var{fname}, @var{sname}, @var{min}, @var{max}, @var{interactive}, @var{doc})
@end example

@table @var
@item lname
This is the name of the Lisp symbol to define as the function name; in
the example above, it is @code{or}.

@item fname
This is the C function name for this function.  This is the name that
is used in C code for calling the function.  The name is, by
convention, @samp{F} prepended to the Lisp name, with all dashes
(@samp{-}) in the Lisp name changed to underscores.  Thus, to call
this function from C code, call @code{For}.

@item sname
This is a C variable name to use for a structure that holds the data for
the subr object that represents the function in Lisp.  This structure
conveys the Lisp symbol name to the initialization routine that will
create the symbol and store the subr object as its definition.  By
convention, this name is always @var{fname} with @samp{F} replaced with
@samp{S}.

@item min
This is the minimum number of arguments that the function requires.  The
function @code{or} allows a minimum of zero arguments.

@item max
This is the maximum number of arguments that the function accepts, if
there is a fixed maximum.  Alternatively, it can be @code{UNEVALLED},
indicating a special form that receives unevaluated arguments, or
@code{MANY}, indicating an unlimited number of evaluated arguments (the
equivalent of @code{&rest}).  Both @code{UNEVALLED} and @code{MANY} are
macros.  If @var{max} is a number, it must be more than @var{min} but
less than 8.

@cindex interactive specification in primitives
@item interactive
This is an interactive specification, a string such as might be used
as the argument of @code{interactive} in a Lisp function.  In the case
of @code{or}, it is 0 (a null pointer), indicating that @code{or}
cannot be called interactively.  A value of @code{""} indicates a
function that should receive no arguments when called interactively.
If the value begins with a @samp{"(}, the string is evaluated as a
Lisp form.  For example:

@example
@group
DEFUN ("foo", Ffoo, Sfoo, 0, UNEVALLED, 0
       "(list (read-char-by-name \"Insert character: \")\
              (prefix-numeric-value current-prefix-arg)\
              t))",
  doc: /* @dots{} */)
@end group
@end example

@item doc
This is the documentation string.  It uses C comment syntax rather
than C string syntax because comment syntax requires nothing special
to include multiple lines.  The @samp{doc:} identifies the comment
that follows as the documentation string.  The @samp{/*} and @samp{*/}
delimiters that begin and end the comment are not part of the
documentation string.

If the last line of the documentation string begins with the keyword
@samp{usage:}, the rest of the line is treated as the argument list
for documentation purposes.  This way, you can use different argument
names in the documentation string from the ones used in the C code.
@samp{usage:} is required if the function has an unlimited number of
arguments.

All the usual rules for documentation strings in Lisp code
(@pxref{Documentation Tips}) apply to C code documentation strings
too.

The documentation string can be followed by a list of C function
attributes for the C function that implements the primitive, like
this:

@example
@group
DEFUN ("bar", Fbar, Sbar, 0, UNEVALLED, 0
  doc: /* @dots{} */
  attributes: @var{attr1} @var{attr2} @dots{})
@end group
@end example

@noindent
You can specify more than a single attribute, one after the other.
Currently, only the following attributes are recognized:

@table @code
@item noreturn
Declares the C function as one that never returns.  This corresponds
to the C11 keyword @code{_Noreturn} and to @w{@code{__attribute__
((__noreturn__))}} attribute of GCC (@pxref{Function Attributes,,,
gcc, Using the GNU Compiler Collection}).

@item const
Declares that the function does not examine any values except its
arguments, and has no effects except the return value.  This
corresponds to @w{@code{__attribute__ ((__const__))}} attribute of
GCC.

@item noinline
This corresponds to @w{@code{__attribute__ ((__noinline__))}}
attribute of GCC, which prevents the function from being considered
for inlining.  This might be needed, e.g., to countermand effects of
link-time optimizations on stack-based variables.
@end table

@end table

  After the call to the @code{DEFUN} macro, you must write the
argument list for the C function, including the types for the
arguments.  If the primitive accepts a fixed maximum number of Lisp
arguments, there must be one C argument for each Lisp argument, and
each argument must be of type @code{Lisp_Object}.  (Various macros and
functions for creating values of type @code{Lisp_Object} are declared
in the file @file{lisp.h}.)  If the primitive has no upper limit on
the number of Lisp arguments, it must have exactly two C arguments:
the first is the number of Lisp arguments, and the second is the
address of a block containing their values.  These have types
@code{int} and @w{@code{Lisp_Object *}} respectively.  Since
@code{Lisp_Object} can hold any Lisp object of any data type, you
can determine the actual data type only at run time; so if you want
a primitive to accept only a certain type of argument, you must check
the type explicitly using a suitable predicate (@pxref{Type Predicates}).
@cindex type checking internals

@cindex garbage collection protection
@cindex protect C variables from garbage collection
  Within the function @code{For} itself, the local variable
@code{args} refers to objects controlled by Emacs's stack-marking
garbage collector.  Although the garbage collector does not reclaim
objects reachable from C @code{Lisp_Object} stack variables, it may
move non-object components of an object, such as string contents; so
functions that access non-object components must take care to refetch
their addresses after performing Lisp evaluation.  Lisp evaluation can
occur via calls to @code{eval_sub} or @code{Feval}, either directly or
indirectly.

@cindex @code{maybe_quit}, use in Lisp primitives
  Note the call to @code{maybe_quit} inside the loop: this function
checks whether the user pressed @kbd{C-g}, and if so, aborts the
processing.  You should do that in any loop that can potentially
require a large number of iterations; in this case, the list of
arguments could be very long.  This increases Emacs responsiveness and
improves user experience.

  You must not use C initializers for static or global variables unless
the variables are never written once Emacs is dumped.  These variables
with initializers are allocated in an area of memory that becomes
read-only (on certain operating systems) as a result of dumping Emacs.
@xref{Pure Storage}.

@cindex @code{defsubr}, Lisp symbol for a primitive
  Defining the C function is not enough to make a Lisp primitive
available; you must also create the Lisp symbol for the primitive and
store a suitable subr object in its function cell.  The code looks like
this:

@example
defsubr (&@var{sname});
@end example

@noindent
Here @var{sname} is the name you used as the third argument to @code{DEFUN}.

  If you add a new primitive to a file that already has Lisp primitives
defined in it, find the function (near the end of the file) named
@code{syms_of_@var{something}}, and add the call to @code{defsubr}
there.  If the file doesn't have this function, or if you create a new
file, add to it a @code{syms_of_@var{filename}} (e.g.,
@code{syms_of_myfile}).  Then find the spot in @file{emacs.c} where all
of these functions are called, and add a call to
@code{syms_of_@var{filename}} there.

@anchor{Defining Lisp variables in C}
@vindex byte-boolean-vars
@cindex defining Lisp variables in C
@cindex @code{DEFVAR_INT}, @code{DEFVAR_LISP}, @code{DEFVAR_BOOL}
  The function @code{syms_of_@var{filename}} is also the place to define
any C variables that are to be visible as Lisp variables.
@code{DEFVAR_LISP} makes a C variable of type @code{Lisp_Object} visible
in Lisp.  @code{DEFVAR_INT} makes a C variable of type @code{int}
visible in Lisp with a value that is always an integer.
@code{DEFVAR_BOOL} makes a C variable of type @code{int} visible in Lisp
with a value that is either @code{t} or @code{nil}.  Note that variables
defined with @code{DEFVAR_BOOL} are automatically added to the list
@code{byte-boolean-vars} used by the byte compiler.

@cindex defining customization variables in C
  If you want to make a Lisp variable that is defined in C behave
like one declared with @code{defcustom}, add an appropriate entry to
@file{cus-start.el}.

@cindex @code{staticpro}, protection from GC
  If you define a file-scope C variable of type @code{Lisp_Object},
you must protect it from garbage-collection by calling @code{staticpro}
in @code{syms_of_@var{filename}}, like this:

@example
staticpro (&@var{variable});
@end example

  Here is another example function, with more complicated arguments.
This comes from the code in @file{window.c}, and it demonstrates the use
of macros and functions to manipulate Lisp objects.

@smallexample
@group
DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
  Scoordinates_in_window_p, 2, 2, 0,
  doc: /* Return non-nil if COORDINATES are in WINDOW.
  ...
@end group
@group
  or `right-margin' is returned.  */)
  (register Lisp_Object coordinates, Lisp_Object window)
@{
  struct window *w;
  struct frame *f;
  int x, y;
  Lisp_Object lx, ly;
@end group

@group
  CHECK_LIVE_WINDOW (window);
  w = XWINDOW (window);
  f = XFRAME (w->frame);
  CHECK_CONS (coordinates);
  lx = Fcar (coordinates);
  ly = Fcdr (coordinates);
  CHECK_NUMBER_OR_FLOAT (lx);
  CHECK_NUMBER_OR_FLOAT (ly);
  x = FRAME_PIXEL_X_FROM_CANON_X (f, lx) + FRAME_INTERNAL_BORDER_WIDTH(f);
  y = FRAME_PIXEL_Y_FROM_CANON_Y (f, ly) + FRAME_INTERNAL_BORDER_WIDTH(f);
@end group

@group
  switch (coordinates_in_window (w, x, y))
    @{
    case ON_NOTHING:            /* NOT in window at all.  */
      return Qnil;
@end group

    ...

@group
    case ON_MODE_LINE:          /* In mode line of window.  */
      return Qmode_line;
@end group

    ...

@group
    case ON_SCROLL_BAR:         /* On scroll-bar of window.  */
      /* Historically we are supposed to return nil in this case.  */
      return Qnil;
@end group

@group
    default:
      abort ();
    @}
@}
@end group
@end smallexample

  Note that C code cannot call functions by name unless they are defined
in C@.  The way to call a function written in Lisp is to use
@code{Ffuncall}, which embodies the Lisp function @code{funcall}.  Since
the Lisp function @code{funcall} accepts an unlimited number of
arguments, in C it takes two: the number of Lisp-level arguments, and a
one-dimensional array containing their values.  The first Lisp-level
argument is the Lisp function to call, and the rest are the arguments to
pass to it.

  The C functions @code{call0}, @code{call1}, @code{call2}, and so on,
provide handy ways to call a Lisp function conveniently with a fixed
number of arguments.  They work by calling @code{Ffuncall}.

  @file{eval.c} is a very good file to look through for examples;
@file{lisp.h} contains the definitions for some important macros and
functions.

  If you define a function which is side-effect free, update the code
in @file{byte-opt.el} that binds @code{side-effect-free-fns} and
@code{side-effect-and-error-free-fns} so that the compiler optimizer
knows about it.

@node Object Internals
@section Object Internals
@cindex object internals

  Emacs Lisp provides a rich set of the data types.  Some of them, like cons
cells, integers and strings, are common to nearly all Lisp dialects.  Some
others, like markers and buffers, are quite special and needed to provide
the basic support to write editor commands in Lisp.  To implement such
a variety of object types and provide an efficient way to pass objects between
the subsystems of an interpreter, there is a set of C data structures and
a special type to represent the pointers to all of them, which is known as
@dfn{tagged pointer}.

  In C, the tagged pointer is an object of type @code{Lisp_Object}.  Any
initialized variable of such a type always holds the value of one of the
following basic data types: integer, symbol, string, cons cell, float,
vectorlike or miscellaneous object.  Each of these data types has the
corresponding tag value.  All tags are enumerated by @code{enum Lisp_Type}
and placed into a 3-bit bitfield of the @code{Lisp_Object}.  The rest of the
bits is the value itself.  Integers are immediate, i.e., directly
represented by those @dfn{value bits}, and all other objects are represented
by the C pointers to a corresponding object allocated from the heap.  Width
of the @code{Lisp_Object} is platform- and configuration-dependent: usually
it's equal to the width of an underlying platform pointer (i.e., 32-bit on
a 32-bit machine and 64-bit on a 64-bit one), but also there is a special
configuration where @code{Lisp_Object} is 64-bit but all pointers are 32-bit.
The latter trick was designed to overcome the limited range of values for
Lisp integers on a 32-bit system by using 64-bit @code{long long} type for
@code{Lisp_Object}.

  The following C data structures are defined in @file{lisp.h} to represent
the basic data types beyond integers:

@table @code
@item struct Lisp_Cons
Cons cell, an object used to construct lists.

@item struct Lisp_String
String, the basic object to represent a sequence of characters.

@item struct Lisp_Vector
Array, a fixed-size set of Lisp objects which may be accessed by an index.

@item struct Lisp_Symbol
Symbol, the unique-named entity commonly used as an identifier.

@item struct Lisp_Float
Floating-point value.

@item union Lisp_Misc
Miscellaneous kinds of objects which don't fit into any of the above.
@end table

  These types are the first-class citizens of an internal type system.
Since the tag space is limited, all other types are the subtypes of either
@code{Lisp_Vectorlike} or @code{Lisp_Misc}.  Vector subtypes are enumerated
by @code{enum pvec_type}, and nearly all complex objects like windows, buffers,
frames, and processes fall into this category.  The rest of special types,
including markers and overlays, are enumerated by @code{enum Lisp_Misc_Type}
and form the set of subtypes of @code{Lisp_Misc}.

  Below there is a description of a few subtypes of @code{Lisp_Vectorlike}.
Buffer object represents the text to display and edit.  Window is the part
of display structure which shows the buffer or used as a container to
recursively place other windows on the same frame.  (Do not confuse Emacs Lisp
window object with the window as an entity managed by the user interface
system like X; in Emacs terminology, the latter is called frame.)  Finally,
process object is used to manage the subprocesses.

@menu
* Buffer Internals::    Components of a buffer structure.
* Window Internals::    Components of a window structure.
* Process Internals::   Components of a process structure.
@end menu

@node Buffer Internals
@subsection Buffer Internals
@cindex internals, of buffer
@cindex buffer internals

  Two structures (see @file{buffer.h}) are used to represent buffers
in C@.  The @code{buffer_text} structure contains fields describing the
text of a buffer; the @code{buffer} structure holds other fields.  In
the case of indirect buffers, two or more @code{buffer} structures
reference the same @code{buffer_text} structure.

Here are some of the fields in @code{struct buffer_text}:

@table @code
@item beg
The address of the buffer contents.

@item gpt
@itemx gpt_byte
The character and byte positions of the buffer gap.  @xref{Buffer
Gap}.

@item z
@itemx z_byte
The character and byte positions of the end of the buffer text.

@item gap_size
The size of buffer's gap.  @xref{Buffer Gap}.

@item modiff
@itemx save_modiff
@itemx chars_modiff
@itemx overlay_modiff
These fields count the number of buffer-modification events performed
in this buffer.  @code{modiff} is incremented after each
buffer-modification event, and is never otherwise changed;
@code{save_modiff} contains the value of @code{modiff} the last time
the buffer was visited or saved; @code{chars_modiff} counts only
modifications to the characters in the buffer, ignoring all other
kinds of changes; and @code{overlay_modiff} counts only modifications
to the overlays.

@item beg_unchanged
@itemx end_unchanged
The number of characters at the start and end of the text that are
known to be unchanged since the last complete redisplay.

@item unchanged_modified
@itemx overlay_unchanged_modified
The values of @code{modiff} and @code{overlay_modiff}, respectively,
after the last complete redisplay.  If their current values match
@code{modiff} or @code{overlay_modiff}, that means
@code{beg_unchanged} and @code{end_unchanged} contain no useful
information.

@item markers
The markers that refer to this buffer.  This is actually a single
marker, and successive elements in its marker @code{chain} are the other
markers referring to this buffer text.

@item intervals
The interval tree which records the text properties of this buffer.
@end table

Some of the fields of @code{struct buffer} are:

@table @code
@item header
A header of type @code{union vectorlike_header} is common to all
vectorlike objects.

@item own_text
A @code{struct buffer_text} structure that ordinarily holds the buffer
contents.  In indirect buffers, this field is not used.

@item text
A pointer to the @code{buffer_text} structure for this buffer.  In an
ordinary buffer, this is the @code{own_text} field above.  In an
indirect buffer, this is the @code{own_text} field of the base buffer.

@item next
A pointer to the next buffer, in the chain of all buffers, including
killed buffers.  This chain is used only for allocation and garbage
collection, in order to collect killed buffers properly.

@item pt
@itemx pt_byte
The character and byte positions of point in a buffer.

@item begv
@itemx begv_byte
The character and byte positions of the beginning of the accessible
range of text in the buffer.

@item zv
@itemx zv_byte
The character and byte positions of the end of the accessible range of
text in the buffer.

@item base_buffer
In an indirect buffer, this points to the base buffer.  In an ordinary
buffer, it is null.

@item local_flags
This field contains flags indicating that certain variables are local
in this buffer.  Such variables are declared in the C code using
@code{DEFVAR_PER_BUFFER}, and their buffer-local bindings are stored
in fields in the buffer structure itself.  (Some of these fields are
described in this table.)

@item modtime
The modification time of the visited file.  It is set when the file is
written or read.  Before writing the buffer into a file, this field is
compared to the modification time of the file to see if the file has
changed on disk.  @xref{Buffer Modification}.

@item auto_save_modified
The time when the buffer was last auto-saved.

@item last_window_start
The @code{window-start} position in the buffer as of the last time the
buffer was displayed in a window.

@item clip_changed
This flag indicates that narrowing has changed in the buffer.
@xref{Narrowing}.

@item prevent_redisplay_optimizations_p
This flag indicates that redisplay optimizations should not be used to
display this buffer.

@item overlay_center
This field holds the current overlay center position.  @xref{Managing
Overlays}.

@item overlays_before
@itemx overlays_after
These fields hold, respectively, a list of overlays that end at or
before the current overlay center, and a list of overlays that end
after the current overlay center.  @xref{Managing Overlays}.
@code{overlays_before} is sorted in order of decreasing end position,
and @code{overlays_after} is sorted in order of increasing beginning
position.

@c FIXME? the following are now all Lisp_Object BUFFER_INTERNAL_FIELD (foo).

@item name
A Lisp string that names the buffer.  It is guaranteed to be unique.
@xref{Buffer Names}.

@item save_length
The length of the file this buffer is visiting, when last read or
saved.  This and other fields concerned with saving are not kept in
the @code{buffer_text} structure because indirect buffers are never
saved.

@item directory
The directory for expanding relative file names.  This is the value of
the buffer-local variable @code{default-directory} (@pxref{File Name Expansion}).

@item filename
The name of the file visited in this buffer, or @code{nil}.  This is
the value of the buffer-local variable @code{buffer-file-name}
(@pxref{Buffer File Name}).

@item undo_list
@itemx backed_up
@itemx auto_save_file_name
@itemx auto_save_file_format
@itemx read_only
@itemx file_format
@itemx file_truename
@itemx invisibility_spec
@itemx display_count
@itemx display_time
These fields store the values of Lisp variables that are automatically
buffer-local (@pxref{Buffer-Local Variables}), whose corresponding
variable names have the additional prefix @code{buffer-} and have
underscores replaced with dashes.  For instance, @code{undo_list}
stores the value of @code{buffer-undo-list}.

@item mark
The mark for the buffer.  The mark is a marker, hence it is also
included on the list @code{markers}.  @xref{The Mark}.

@item local_var_alist
The association list describing the buffer-local variable bindings of
this buffer, not including the built-in buffer-local bindings that
have special slots in the buffer object.  (Those slots are omitted
from this table.)  @xref{Buffer-Local Variables}.

@item major_mode
Symbol naming the major mode of this buffer, e.g., @code{lisp-mode}.

@item mode_name
Pretty name of the major mode, e.g., @code{"Lisp"}.

@item keymap
@itemx abbrev_table
@itemx syntax_table
@itemx category_table
@itemx display_table
These fields store the buffer's local keymap (@pxref{Keymaps}), abbrev
table (@pxref{Abbrev Tables}), syntax table (@pxref{Syntax Tables}),
category table (@pxref{Categories}), and display table (@pxref{Display
Tables}).

@item downcase_table
@itemx upcase_table
@itemx case_canon_table
These fields store the conversion tables for converting text to lower
case, upper case, and for canonicalizing text for case-fold search.
@xref{Case Tables}.

@item minor_modes
An alist of the minor modes of this buffer.

@item pt_marker
@itemx begv_marker
@itemx zv_marker
These fields are only used in an indirect buffer, or in a buffer that
is the base of an indirect buffer.  Each holds a marker that records
@code{pt}, @code{begv}, and @code{zv} respectively, for this buffer
when the buffer is not current.

@item mode_line_format
@itemx header_line_format
@itemx case_fold_search
@itemx tab_width
@itemx fill_column
@itemx left_margin
@itemx auto_fill_function
@itemx truncate_lines
@itemx word_wrap
@itemx ctl_arrow
@itemx bidi_display_reordering
@itemx bidi_paragraph_direction
@itemx selective_display
@itemx selective_display_ellipses
@itemx overwrite_mode
@itemx abbrev_mode
@itemx mark_active
@itemx enable_multibyte_characters
@itemx buffer_file_coding_system
@itemx cache_long_line_scans
@itemx point_before_scroll
@itemx left_fringe_width
@itemx right_fringe_width
@itemx fringes_outside_margins
@itemx scroll_bar_width
@itemx indicate_empty_lines
@itemx indicate_buffer_boundaries
@itemx fringe_indicator_alist
@itemx fringe_cursor_alist
@itemx scroll_up_aggressively
@itemx scroll_down_aggressively
@itemx cursor_type
@itemx cursor_in_non_selected_windows
These fields store the values of Lisp variables that are automatically
buffer-local (@pxref{Buffer-Local Variables}), whose corresponding
variable names have underscores replaced with dashes.  For instance,
@code{mode_line_format} stores the value of @code{mode-line-format}.

@item last_selected_window
This is the last window that was selected with this buffer in it, or @code{nil}
if that window no longer displays this buffer.
@end table

@node Window Internals
@subsection Window Internals
@cindex internals, of window
@cindex window internals

  The fields of a window (for a complete list, see the definition of
@code{struct window} in @file{window.h}) include:

@table @code
@item frame
The frame that this window is on.

@item mini_p
Non-@code{nil} if this window is a minibuffer window.

@item parent
Internally, Emacs arranges windows in a tree; each group of siblings has
a parent window whose area includes all the siblings.  This field points
to a window's parent.

Parent windows do not display buffers, and play little role in display
except to shape their child windows.  Emacs Lisp programs usually have
no access to the parent windows; they operate on the windows at the
leaves of the tree, which actually display buffers.

@c FIXME: These two slots and the 'buffer' slot below were replaced
@c with a single slot 'contents' on 2013-03-28.  --xfq
@item hchild
@itemx vchild
These fields contain the window's leftmost child and its topmost child
respectively.  @code{hchild} is used if the window is subdivided
horizontally by child windows, and @code{vchild} if it is subdivided
vertically.  In a live window, only one of @code{hchild}, @code{vchild},
and @code{buffer} (q.v.@:) is non-@code{nil}.

@item next
@itemx prev
The next sibling and previous sibling of this window.  @code{next} is
@code{nil} if the window is the right-most or bottom-most in its group;
@code{prev} is @code{nil} if it is the left-most or top-most in its
group.

@item left_col
The left-hand edge of the window, measured in columns, relative to the
leftmost column in the frame (column 0).

@item top_line
The top edge of the window, measured in lines, relative to the topmost
line in the frame (line 0).

@item total_cols
@itemx total_lines
The width and height of the window, measured in columns and lines
respectively.  The width includes the scroll bar and fringes, and/or
the separator line on the right of the window (if any).

@item buffer
The buffer that the window is displaying.

@item start
A marker pointing to the position in the buffer that is the first
character displayed in the window.

@item pointm
@cindex window point internals
This is the value of point in the current buffer when this window is
selected; when it is not selected, it retains its previous value.

@item force_start
If this flag is non-@code{nil}, it says that the window has been
scrolled explicitly by the Lisp program.  This affects what the next
redisplay does if point is off the screen: instead of scrolling the
window to show the text around point, it moves point to a location that
is on the screen.

@item frozen_window_start_p
This field is set temporarily to 1 to indicate to redisplay that
@code{start} of this window should not be changed, even if point
gets invisible.

@item start_at_line_beg
Non-@code{nil} means current value of @code{start} was the beginning of a line
when it was chosen.

@item use_time
This is the last time that the window was selected.  The function
@code{get-lru-window} uses this field.

@item sequence_number
A unique number assigned to this window when it was created.

@item last_modified
The @code{modiff} field of the window's buffer, as of the last time
a redisplay completed in this window.

@item last_overlay_modified
The @code{overlay_modiff} field of the window's buffer, as of the last
time a redisplay completed in this window.

@item last_point
The buffer's value of point, as of the last time a redisplay completed
in this window.

@item last_had_star
A non-@code{nil} value means the window's buffer was modified when the
window was last updated.

@item vertical_scroll_bar
This window's vertical scroll bar.

@item left_margin_cols
@itemx right_margin_cols
The widths of the left and right margins in this window.  A value of
@code{nil} means no margin.

@item left_fringe_width
@itemx right_fringe_width
The widths of the left and right fringes in this window.  A value of
@code{nil} or @code{t} means use the values of the frame.

@item fringes_outside_margins
A non-@code{nil} value means the fringes outside the display margins;
othersize they are between the margin and the text.

@item window_end_pos
This is computed as @code{z} minus the buffer position of the last glyph
in the current matrix of the window.  The value is only valid if
@code{window_end_valid} is not @code{nil}.

@item window_end_bytepos
The byte position corresponding to @code{window_end_pos}.

@item window_end_vpos
The window-relative vertical position of the line containing
@code{window_end_pos}.

@item window_end_valid
This field is set to a non-@code{nil} value if @code{window_end_pos} is truly
valid.  This is @code{nil} if nontrivial redisplay is pre-empted, since in that
case the display that @code{window_end_pos} was computed for did not get
onto the screen.

@item cursor
A structure describing where the cursor is in this window.

@item last_cursor
The value of @code{cursor} as of the last redisplay that finished.

@item phys_cursor
A structure describing where the cursor of this window physically is.

@item phys_cursor_type
@c FIXME What is this?
@c itemx phys_cursor_ascent
@itemx phys_cursor_height
@itemx phys_cursor_width
The type, height, and width of the cursor that was last displayed on
this window.

@item phys_cursor_on_p
This field is non-zero if the cursor is physically on.

@item cursor_off_p
Non-zero means the cursor in this window is logically off.  This is
used for blinking the cursor.

@item last_cursor_off_p
This field contains the value of @code{cursor_off_p} as of the time of
the last redisplay.

@item must_be_updated_p
This is set to 1 during redisplay when this window must be updated.

@item hscroll
This is the number of columns that the display in the window is scrolled
horizontally to the left.  Normally, this is 0.

@item vscroll
Vertical scroll amount, in pixels.  Normally, this is 0.

@item dedicated
Non-@code{nil} if this window is dedicated to its buffer.

@item display_table
The window's display table, or @code{nil} if none is specified for it.

@item update_mode_line
Non-@code{nil} means this window's mode line needs to be updated.

@item base_line_number
The line number of a certain position in the buffer, or @code{nil}.
This is used for displaying the line number of point in the mode line.

@item base_line_pos
The position in the buffer for which the line number is known, or
@code{nil} meaning none is known.  If it is a buffer, don't display
the line number as long as the window shows that buffer.

@item column_number_displayed
The column number currently displayed in this window's mode line, or @code{nil}
if column numbers are not being displayed.

@item current_matrix
@itemx desired_matrix
Glyph matrices describing the current and desired display of this window.
@end table

@node Process Internals
@subsection Process Internals
@cindex internals, of process
@cindex process internals

  The fields of a process (for a complete list, see the definition of
@code{struct Lisp_Process} in @file{process.h}) include:

@table @code
@item name
A string, the name of the process.

@item command
A list containing the command arguments that were used to start this
process.  For a network or serial process, it is @code{nil} if the
process is running or @code{t} if the process is stopped.

@item filter
A function used to accept output from the process.

@item sentinel
A function called whenever the state of the process changes.

@item buffer
The associated buffer of the process.

@item pid
An integer, the operating system's process @acronym{ID}.
Pseudo-processes such as network or serial connections use a value of 0.

@item childp
A flag, @code{t} if this is really a child process.  For a network or
serial connection, it is a plist based on the arguments to
@code{make-network-process} or @code{make-serial-process}.

@item mark
A marker indicating the position of the end of the last output from this
process inserted into the buffer.  This is often but not always the end
of the buffer.

@item kill_without_query
If this is non-zero, killing Emacs while this process is still running
does not ask for confirmation about killing the process.

@item raw_status
The raw process status, as returned by the @code{wait} system call.

@item status
The process status, as @code{process-status} should return it.

@item tick
@itemx update_tick
If these two fields are not equal, a change in the status of the process
needs to be reported, either by running the sentinel or by inserting a
message in the process buffer.

@item pty_flag
Non-@code{nil} if communication with the subprocess uses a pty;
@code{nil} if it uses a pipe.

@item infd
The file descriptor for input from the process.

@item outfd
The file descriptor for output to the process.

@item tty_name
The name of the terminal that the subprocess is using,
or @code{nil} if it is using pipes.

@item decode_coding_system
Coding-system for decoding the input from this process.

@item decoding_buf
A working buffer for decoding.

@item decoding_carryover
Size of carryover in decoding.

@item encode_coding_system
Coding-system for encoding the output to this process.

@item encoding_buf
A working buffer for encoding.

@item inherit_coding_system_flag
Flag to set @code{coding-system} of the process buffer from the
coding system used to decode process output.

@item type
Symbol indicating the type of process: @code{real}, @code{network},
@code{serial}.

@end table

@node C Integer Types
@section C Integer Types
@cindex integer types (C programming language)

Here are some guidelines for use of integer types in the Emacs C
source code.  These guidelines sometimes give competing advice; common
sense is advised.

@itemize @bullet
@item
Avoid arbitrary limits.  For example, avoid @code{int len = strlen
(s);} unless the length of @code{s} is required for other reasons to
fit in @code{int} range.

@item
Do not assume that signed integer arithmetic wraps around on overflow.
This is no longer true of Emacs porting targets: signed integer
overflow has undefined behavior in practice, and can dump core or
even cause earlier or later code to behave illogically.  Unsigned
overflow does wrap around reliably, modulo a power of two.

@item
Prefer signed types to unsigned, as code gets confusing when signed
and unsigned types are combined.  Many other guidelines assume that
types are signed; in the rarer cases where unsigned types are needed,
similar advice may apply to the unsigned counterparts (e.g.,
@code{size_t} instead of @code{ptrdiff_t}, or @code{uintptr_t} instead
of @code{intptr_t}).

@item
Prefer @code{int} for Emacs character codes, in the range 0 ..@: 0x3FFFFF@.
More generally, prefer @code{int} for integers known to be in
@code{int} range, e.g., screen column counts.

@item
Prefer @code{ptrdiff_t} for sizes, i.e., for integers bounded by the
maximum size of any individual C object or by the maximum number of
elements in any C array.  This is part of Emacs's general preference
for signed types.  Using @code{ptrdiff_t} limits objects to
@code{PTRDIFF_MAX} bytes, but larger objects would cause trouble
anyway since they would break pointer subtraction, so this does not
impose an arbitrary limit.

@item
Avoid @code{ssize_t} except when communicating to low-level APIs that
have @code{ssize_t}-related limitations.  Although it's equivalent to
@code{ptrdiff_t} on typical platforms, @code{ssize_t} is occasionally
narrower, so using it for size-related calculations could overflow.
Also, @code{ptrdiff_t} is more ubiquitous and better-standardized, has
standard @code{printf} formats, and is the basis for Emacs's internal
size-overflow checking.  When using @code{ssize_t}, please note that
POSIX requires support only for values in the range @minus{}1 ..@:
@code{SSIZE_MAX}.

@item
Prefer @code{intptr_t} for internal representations of pointers, or
for integers bounded only by the number of objects that can exist at
any given time or by the total number of bytes that can be allocated.
Currently Emacs sometimes uses other types when @code{intptr_t} would
be better; fixing this is lower priority, as the code works as-is on
Emacs's current porting targets.

@item
Prefer the Emacs-defined type @code{EMACS_INT} for representing values
converted to or from Emacs Lisp fixnums, as fixnum arithmetic is based
on @code{EMACS_INT}.

@item
When representing a system value (such as a file size or a count of
seconds since the Epoch), prefer the corresponding system type (e.g.,
@code{off_t}, @code{time_t}).  Do not assume that a system type is
signed, unless this assumption is known to be safe.  For example,
although @code{off_t} is always signed, @code{time_t} need not be.

@item
Prefer the Emacs-defined type @code{printmax_t} for representing
values that might be any signed integer that can be printed,
using a @code{printf}-family function.

@item
Prefer @code{intmax_t} for representing values that might be any
signed integer value.

@item
Prefer @code{bool}, @code{false} and @code{true} for booleans.
Using @code{bool} can make programs easier to read and a bit faster than
using @code{int}.  Although it is also OK to use @code{int}, @code{0}
and @code{1}, this older style is gradually being phased out.  When
using @code{bool}, respect the limitations of the replacement
implementation of @code{bool}, as documented in the source file
@file{lib/stdbool.in.h}.  In particular, boolean bitfields should be of type
@code{bool_bf}, not @code{bool}, so that they work correctly even when
compiling Objective C with standard GCC.

@item
In bitfields, prefer @code{unsigned int} or @code{signed int} to
@code{int}, as @code{int} is less portable: it might be signed, and
might not be.  Single-bit bit fields should be @code{unsigned int} or
@code{bool_bf} so that their values are 0 or 1.
@end itemize

@c FIXME Mention src/globals.h somewhere in this file?