emacs/doc/lispref/internals.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1998, 1999, 2001, 2002, 2003,
@c   2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011  Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/internals
@node GNU Emacs Internals, Standard Errors, Tips, Top
@comment  node-name,  next,  previous,  up
@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::        A kludge to make preloaded Lisp functions sharable.
* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
* Memory Usage::        Info about total size of Lisp objects made so far.
* Writing Emacs Primitives::   Writing C code for Emacs.
* Object Internals::    Data formats of buffers, windows, processes.
@end menu

@node Building Emacs
@appendixsec 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 maintenance.

   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{@samp{temacs -l loadup}} uses @file{temacs} to create
the real runnable Emacs executable.  These arguments direct
@file{temacs} to evaluate the Lisp files specified in the file
@file{loadup.el}.  These files set up the normal Emacs editing
environment, resulting in an Emacs that is still impure but no longer
bare.

@cindex dumping Emacs
  It takes a substantial time to load the standard Lisp files.  Luckily,
you don't have to do this each time you run Emacs; @file{temacs} can
dump out an executable program called @file{emacs} that has these files
preloaded.  @file{emacs} starts more quickly because it does not need to
load the files.  This is the Emacs executable that is normally
installed.

@vindex preloaded-file-list
@cindex dumped Lisp files
  To create @file{emacs}, use the command @samp{temacs -batch -l loadup
dump}.  The purpose of @samp{-batch} here is to prevent @file{temacs}
from trying to initialize any of its data on the terminal; this ensures
that the tables of terminal information are empty in the dumped Emacs.
The argument @samp{dump} tells @file{loadup.el} to dump a new executable
named @file{emacs}.  The variable @code{preloaded-file-list} stores a
list of the Lisp files that were dumped with the @file{emacs} executable.

  Some operating systems don't support dumping.  On those systems, you
must start Emacs with the @samp{temacs -l loadup} command each time you
use it.  This takes a substantial time, but since you need to start
Emacs once a day at most---or once a week if you never log out---the
extra time is not too severe a problem.

@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 add a 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.
(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 non-@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

  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}.

  In a package that can be preloaded, it is sometimes useful to
specify a computation to be done when Emacs subsequently starts up.
For this, use @code{eval-at-startup}:

@defmac eval-at-startup body@dots{}
This evaluates the @var{body} forms, either immediately if running in
an Emacs that has already started up, or later when Emacs does start
up.  Since the value of the @var{body} forms is not necessarily
available when the @code{eval-at-startup} form is run, that form
always returns @code{nil}.
@end defmac

@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
@appendixsec 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; see the
following section for information on normal storage.  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 @file{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.  If that happens, you should increase the
compilation parameter @code{PURESIZE} in the file
@file{src/puresize.h} and rebuild Emacs, even though the resulting
image will work: 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.

@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 the file @file{emacs/lisp/loaddefs.el}, but
a few packages call it just in case you decide to preload them.
@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 sharable 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
@appendixsec Garbage Collection
@cindex 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 in blocks of 1k bytes.  Each block is used for one
type of Lisp object, so symbols, cons cells, markers, etc., are
segregated in distinct blocks in memory.  (Vectors, long strings,
buffers and certain other editing types, which are fairly large, are
allocated in individual blocks, one per object, while small strings are
packed into blocks of 8k bytes.)

  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.  (This name is traditional, but ``garbage recycler''
might be a more intuitive metaphor for this facility.)

  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.  Vectors, buffers, windows, and other large objects are
individually allocated and freed using @code{malloc} and @code{free}.

@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 containing the following
information:

@example
@group
((@var{used-conses} . @var{free-conses})
 (@var{used-syms} . @var{free-syms})
@end group
 (@var{used-miscs} . @var{free-miscs})
 @var{used-string-chars}
 @var{used-vector-slots}
 (@var{used-floats} . @var{free-floats})
 (@var{used-intervals} . @var{free-intervals})
 (@var{used-strings} . @var{free-strings}))
@end example

Here is an example:

@example
@group
(garbage-collect)
     @result{} ((106886 . 13184) (9769 . 0)
                (7731 . 4651) 347543 121628
                (31 . 94) (1273 . 168)
                (25474 . 3569))
@end group
@end example

Here is a table explaining each element:

@table @var
@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 used-syms
The number of symbols in use.

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

@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 used-string-chars
The total size of all strings, in characters.

@item used-vector-slots
The total number of elements of existing vectors.

@item used-floats
@c Emacs 19 feature
The number of floats in use.

@item free-floats
@c Emacs 19 feature
The number of floats for which space has been obtained from the
operating system, but that are not currently being used.

@item used-intervals
The number of intervals in use.  Intervals are an internal
data structure used for representing text properties.

@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 used-strings
The number of strings in use.

@item free-strings
The number of string headers for which the space was obtained from the
operating system, but which are currently not in use.  (A string
object consists of a header and the storage for the string text
itself; the latter is only allocated when the string is created.)
@end table

If there was overflow in pure space (see the previous section),
@code{garbage-collect} returns @code{nil}, because a real garbage
collection can not be done in this situation.
@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}, meaning there are no such messages.
@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.  A cons cell counts as eight bytes,
a string as one byte per character plus a few bytes of overhead, and so
on; 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 evaluator is
called.

The initial threshold value is 400,000.  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 10,000.  A value less than 10,000 will remain in effect only
until the subsequent garbage collection, at which time
@code{garbage-collect} will set the threshold back to 10,000.
@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.

@c Emacs 19 feature
@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 close to 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

@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 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 Emacs session.
@end defvar

@defvar misc-objects-consed
The total number of miscellaneous objects that have been allocated so
far in this Emacs 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 Writing Emacs Primitives
@appendixsec 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.)

@cindex garbage collection protection
@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)
@{
  register Lisp_Object val = Qnil;
  struct gcpro gcpro1;
@end group

@group
  GCPRO1 (args);
@end group

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

@group
  UNGCPRO;
  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}.  Remember that the arguments 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}.

@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 may not be less than @var{min} and
it may not be greater than eight.

@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.

@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.
@end table

  After the call to the @code{DEFUN} macro, you must write the
argument list that every C function must have, including the types for
the arguments.  For a function with a fixed maximum number of
arguments, declare a C argument for each Lisp argument, and give them
all type @code{Lisp_Object}.  When a Lisp function has no upper limit
on the number of arguments, its implementation in C actually receives
exactly two arguments: the first is the number of Lisp arguments, and
the second is the address of a block containing their values.  They
have types @code{int} and @w{@code{Lisp_Object *}}.

@cindex @code{GCPRO} and @code{UNGCPRO}
@cindex protect C variables from garbage collection
  Within the function @code{For} itself, note the use of the macros
@code{GCPRO1} and @code{UNGCPRO}.  @code{GCPRO1} is used to
``protect'' a variable from garbage collection---to inform the garbage
collector that it must look in that variable and regard its contents
as an accessible object.  GC protection is necessary whenever you call
@code{Feval} or anything that can directly or indirectly call
@code{Feval}.  At such a time, any Lisp object that this function may
refer to again must be protected somehow.

  It suffices to ensure that at least one pointer to each object is
GC-protected; that way, the object cannot be recycled, so all pointers
to it remain valid.  Thus, a particular local variable can do without
protection if it is certain that the object it points to will be
preserved by some other pointer (such as another local variable which
has a @code{GCPRO})@footnote{Formerly, strings were a special
exception; in older Emacs versions, every local variable that might
point to a string needed a @code{GCPRO}.}.  Otherwise, the local
variable needs a @code{GCPRO}.

  The macro @code{GCPRO1} protects just one local variable.  If you
want to protect two variables, use @code{GCPRO2} instead; repeating
@code{GCPRO1} will not work.  Macros @code{GCPRO3}, @code{GCPRO4},
@code{GCPRO5}, and @code{GCPRO6} also exist.  All these macros
implicitly use local variables such as @code{gcpro1}; you must declare
these explicitly, with type @code{struct gcpro}.  Thus, if you use
@code{GCPRO2}, you must declare @code{gcpro1} and @code{gcpro2}.
Alas, we can't explain all the tricky details here.

  @code{UNGCPRO} cancels the protection of the variables that are
protected in the current function.  It is necessary to do this
explicitly.

  Built-in functions that take a variable number of arguments actually
accept two arguments at the C level: the number of Lisp arguments, and
a @code{Lisp_Object *} pointer to a C vector containing those Lisp
arguments.  This C vector may be part of a Lisp vector, but it need
not be.  The responsibility for using @code{GCPRO} to protect the Lisp
arguments from GC if necessary rests with the caller in this case,
since the caller allocated or found the storage for them.

  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}.

  Do not use static variables within functions---place all static
variables at top level in the file.  This is necessary because Emacs on
some operating systems defines the keyword @code{static} as a null
macro.  (This definition is used because those systems put all variables
declared static in a place that becomes read-only after dumping, whether
they have initializers or not.)

@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{subr-structure-name});
@end example

@noindent
Here @var{subr-structure-name} 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 @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,
  "xSpecify coordinate pair: \nXExpression which evals to window: ",
  "Return non-nil if COORDINATES is in WINDOW.\n\
COORDINATES is a cons of the form (X . Y), X and Y being distances\n\
...
@end group
@group
If they are on the border between WINDOW and its right sibling,\n\
   `vertical-line' is returned.")
  (coordinates, window)
     register Lisp_Object coordinates, window;
@{
  int x, y;
@end group

@group
  CHECK_LIVE_WINDOW (window, 0);
  CHECK_CONS (coordinates, 1);
  x = XINT (Fcar (coordinates));
  y = XINT (Fcdr (coordinates));
@end group

@group
  switch (coordinates_in_window (XWINDOW (window), &x, &y))
    @{
    case 0:                     /* NOT in window at all. */
      return Qnil;
@end group

@group
    case 1:                     /* In text part of window. */
      return Fcons (make_number (x), make_number (y));
@end group

@group
    case 2:                     /* In mode line of window. */
      return Qmode_line;
@end group

@group
    case 3:                     /* On right border of window.  */
      return Qvertical_line;
@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.  Since @code{Ffuncall} can call the evaluator, you must
protect pointers from garbage collection around the call to
@code{Ffuncall}.

  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} which 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
@appendixsec Object Internals
@cindex object internals

  GNU Emacs Lisp manipulates many different types of data.  The actual
data are stored in a heap and the only access that programs have to it
is through pointers.  Each pointer is 32 bits wide on 32-bit machines,
and 64 bits wide on 64-bit machines; three of these bits are used for
the tag that identifies the object's type, and the remainder are used
to address the object.

  Because Lisp objects are represented as tagged pointers, it is always
possible to determine the Lisp data type of any object.  The C data type
@code{Lisp_Object} can hold any Lisp object of any data type.  Ordinary
variables have type @code{Lisp_Object}, which means they can hold any
type of Lisp value; you can determine the actual data type only at run
time.  The same is true for function arguments; if you want a function
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

@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
@appendixsubsec Buffer Internals
@cindex internals, of buffer
@cindex buffer internals

  Two structures 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 compelete 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 next
Points to the next buffer, in the chain of all buffers (including
killed buffers).  This chain is used only for garbage collection, in
order to collect killed buffers properly.  Note that vectors, and most
kinds of objects allocated as vectors, are all on one chain, but
buffers are on a separate chain of their own.

@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 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.

@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 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}.  @xref{Standard
Buffer-Local Variables}.

@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 selective_display
@itemx selective_display_ellipses
@itemx overwrite_mode
@itemx abbrev_mode
@itemx display_table
@itemx mark_active
@itemx enable_multibyte_characters
@itemx buffer_file_coding_system
@itemx auto_save_file_format
@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}.
@xref{Standard Buffer-Local Variables}.

@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
@appendixsubsec Window Internals
@cindex internals, of window
@cindex window internals

  Windows have the following accessible fields:

@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.

@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.

@item next
@itemx prev
The next sibling and previous sibling of this window.  @code{next} is
@code{nil} if the window is the rightmost or bottommost in its group;
@code{prev} is @code{nil} if it is the leftmost or topmost 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_width
@itemx right_margin_width
The widths of the left and right margins in this window.  A value of
@code{nil} means to use the buffer's value of @code{left-margin-width}
or @code{right-margin-width}.

@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 preempted 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
The type of 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 on.

@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.

@item region_showing
If the region (or part of it) is highlighted in this window, this field
holds the mark position that made one end of that region.  Otherwise,
this field is @code{nil}.

@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
A glyph matrix describing the current display of this window.

@item desired_matrix
A glyph matrix describing the desired display of this window.
@end table

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

  The fields of a process are:

@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 instead of a buffer,
or @code{nil}.

@item sentinel
A function called whenever the process receives a signal, or @code{nil}.

@item buffer
The associated buffer of the process.

@item pid
An integer, the operating system's process @acronym{ID}.

@item childp
A flag, non-@code{nil} if this is really a child process.
It is @code{nil} for a network or serial connection.

@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_low
@itemx raw_status_high
These two fields record 16 bits each of the process status 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 @acronym{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 subtty
The file descriptor for the terminal that the subprocess is using.  (On
some systems, there is no need to record this, so the value is
@code{nil}.)

@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 encoding_carryover
Size of carryover in 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