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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990-1993, 1998-1999, 2001-2019 Free Software
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@c Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@node GNU Emacs Internals
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@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.
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* Pure Storage::        Kludge to make preloaded Lisp functions shareable.
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* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
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* Stack-allocated Objects::    Temporary conses and strings on C stack.
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* Memory Usage::        Info about total size of Lisp objects made so far.
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* C Dialect::           What C variant Emacs is written in.
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* Writing Emacs Primitives::   Writing C code for Emacs.
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* Writing Dynamic Modules::    Writing loadable modules for Emacs.
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* Object Internals::    Data formats of buffers, windows, processes.
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* C Integer Types::     How C integer types are used inside Emacs.
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@end menu

@node Building Emacs
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@section Building Emacs
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@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
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information is pertinent to Emacs developers.
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  Building Emacs requires GNU Make version 3.81 or later.

  Compilation of the C source files in the @file{src} directory
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produces an executable file called @file{temacs}, also called a
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@dfn{bare impure Emacs}.  It contains the Emacs Lisp interpreter and
I/O routines, but not the editing commands.
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@cindex @file{loadup.el}
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  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}.
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@cindex dumping Emacs
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@cindex @option{--temacs} option, and dumping method
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  Because it takes some time to load the standard Lisp files, the
@file{temacs} executable usually isn't run directly by users.
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Instead, one of the last steps of building Emacs runs the command
@w{@samp{temacs -batch -l loadup --temacs=@var{dump-method}}}.  The
special option @option{--temacs} tells @command{temacs} how to record
all the standard preloaded Lisp functions and variables, so that when
you subsequently run Emacs, it will start much faster.  The
@option{--temacs} option requires an argument @var{dump-method}, which
can be one of the following:

@table @samp
@item pdump
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@cindex dump file
Record the preloaded Lisp data in a @dfn{dump file}.  This
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method produces an additional data file which Emacs will load at
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startup.  The produced dump file is usually called @file{emacs.pdmp},
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and is installed in the Emacs @code{exec-directory} (@pxref{Help
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Functions}).  This method is the most preferred one, as it does not
require Emacs to employ any special techniques of memory allocation,
which might get in the way of various memory-layout techniques used by
modern systems to enhance security and privacy.

@item pbootstrap
@cindex bootstrapping Emacs
Like @samp{pdump}, but used while @dfn{bootstrapping} Emacs, when no
previous Emacs binary and no @file{*.elc} byte-compiled Lisp files are
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available.  The produced dump file is usually named
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@file{bootstrap-emacs.pdmp} in this case.

@item dump
@cindex unexec
This method causes @command{temacs} to dump out an executable program,
called @file{emacs}, which has all the standard Lisp files already
preloaded into it.  (The @samp{-batch} argument prevents
@command{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.)  This method is also known as @dfn{unexec}, because it
produces a program file from a running process, and thus is in some
sense the opposite of executing a program to start a process.
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Although this method was the way that Emacs traditionally saved its
state, it is now deprecated.
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@item bootstrap
Like @samp{dump}, but used when bootstrapping Emacs with the
@code{unexec} method.
@end table
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@cindex preloaded Lisp files
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@vindex preloaded-file-list
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  The dumped @file{emacs} executable (also called a @dfn{pure} Emacs)
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is the one which is installed.  If the portable dumper was used to
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build Emacs, the @file{emacs} executable is actually an exact copy of
@file{temacs}, and the corresponding @file{emacs.pdmp} file is
installed as well.  The variable @code{preloaded-file-list} stores a
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list of the preloaded Lisp files recorded in the dump file or
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in the dumped Emacs executable.  If you port Emacs to a new operating
system, and are not able to implement dumping of any kind, then Emacs
must load @file{loadup.el} each time it starts.
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@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.

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

@noindent
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to make @var{n} added bytes of pure space to hold the additional files;
see @file{src/puresize.h}.
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(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
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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.)
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@end itemize

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@cindex change @code{load-path} at configure time
@cindex @option{--enable-locallisppath} option to @command{configure}
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  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}.
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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}.
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  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
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@var{value} is unused, and is provided only for compatibility with the
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form Custom expects.)
@end defun
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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}).
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@defun dump-emacs-portable to-file &optional track-referrers
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This function dumps the current state of Emacs into a dump
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file @var{to-file}, using the @code{pdump} method.  Normally, the
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dump file is called @file{@var{emacs-name}.dmp}, where
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@var{emacs-name} is the name of the Emacs executable file.  The
optional argument @var{track-referrers}, if non-@code{nil}, causes the
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portable dumper to keep additional information to help track
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down the provenance of object types that are not yet supported by the
@code{pdump} method.

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Although the portable dumper code can run on many platforms, the dump
files that it produces are not portable---they can be loaded only by
the Emacs executable that dumped them.

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

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@defun dump-emacs to-file from-file
@cindex unexec
This function dumps the current state of Emacs into an executable file
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@var{to-file}, using the @code{unexec} method.  It takes symbols from
@var{from-file} (this is normally the executable file @file{temacs}).
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This function cannot be used in an Emacs that was already dumped.
This function is deprecated, and by default Emacs is built without
@code{unexec} support so this function is not available.
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@end defun

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@defun pdumper-stats
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If the current Emacs session restored its state from a dump
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file, this function returns information about the dump file and the
time it took to restore the Emacs state.  The value is an alist
@w{@code{((dumped-with-pdumper . t) (load-time . @var{time})
(dump-file-name . @var{file}))}},
where @var{file} is the name of the dump file, and @var{time} is the
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time in seconds it took to restore the state from the dump file.
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If the current session was not restored from a dump file, the
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value is nil.
@end defun

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@node Pure Storage
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@section Pure Storage
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@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
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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.
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  Pure storage is allocated only while @command{temacs} is loading the
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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
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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.
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@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;
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it is usually called only in preloaded Lisp files.
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@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
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building Emacs initially (allowing those functions to be shareable and
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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
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@section Garbage Collection
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@cindex memory allocation
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  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
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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
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  Beyond the basic vector, a lot of objects like markers, overlays and
buffers are managed as if they were vectors.  The corresponding C data
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structures include the @code{union vectorlike_header} field whose
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@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.
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@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.
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  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
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(sweep) phase of the garbage collector arranges to reuse them.
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@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
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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.
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@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.)

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@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:
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@example
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((@code{conses} @var{cons-size} @var{used-conses} @var{free-conses})
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 (@code{symbols} @var{symbol-size} @var{used-symbols} @var{free-symbols})
 (@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}))
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@end example

Here is an example:

@example
(garbage-collect)
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      @result{} ((conses 16 49126 8058) (symbols 48 14607 0)
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                 (strings 32 2942 2607)
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                 (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))
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@end example

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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.
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@table @var
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@item cons-size
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Internal size of a cons cell, i.e., @code{sizeof (struct Lisp_Cons)}.
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@item used-conses
The number of cons cells in use.

@item free-conses
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The number of cons cells for which space has been obtained from
the operating system, but that are not currently being used.
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@item symbol-size
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Internal size of a symbol, i.e., @code{sizeof (struct Lisp_Symbol)}.
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@item used-symbols
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The number of symbols in use.

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@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 string-size
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Internal size of a string header, i.e., @code{sizeof (struct Lisp_String)}.
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@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
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Size in bytes of a vector of length 1, including its header.
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@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.
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Slot counts might include some or all overhead from vector headers,
depending on the platform.
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@item free-slots
The number of free slots in all vector blocks.
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@item float-size
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Internal size of a float object, i.e., @code{sizeof (struct Lisp_Float)}.
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(Do not confuse it with the native platform @code{float} or @code{double}.)
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@item used-floats
The number of floats in use.

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

@item interval-size
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Internal size of an interval object, i.e., @code{sizeof (struct interval)}.
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@item used-intervals
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The number of intervals in use.
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@item free-intervals
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The number of intervals for which space has been obtained from
the operating system, but that are not currently being used.
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@item buffer-size
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Internal size of a buffer, i.e., @code{sizeof (struct buffer)}.
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(Do not confuse with the value returned by @code{buffer-size} function.)
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@item used-buffers
The number of buffer objects in use.  This includes killed buffers
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invisible to users, i.e., all buffers in @code{all_buffers} list.
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@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.
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@end table

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

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.
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@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

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  Control over the garbage collector via @code{gc-cons-threshold} and
@code{gc-cons-percentage} is only approximate.  Although Emacs checks
for threshold exhaustion regularly, for efficiency reasons it does not
do so immediately after every change to the heap or to
@code{gc-cons-threshold} or @code{gc-cons-percentage}, so exhausting
the threshold does not immediately trigger garbage collection.  Also,
for efficency in threshold calculations Emacs approximates the heap
size, which counts the bytes used by currently-accessible objects in
the heap.

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  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
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This function returns an estimate of the total amount of bytes of
virtual memory that Emacs is currently using, divided by 1024.
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You can use this to get a general idea of how your actions affect the
memory usage.
@end defun

@defvar memory-full
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This variable is @code{t} if Emacs is nearly out of memory for Lisp
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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

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@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}.
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@end defun
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@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
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during garbage collection so far in this Emacs session, as a
floating-point number.
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@end defvar

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

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@cindex stack allocated Lisp objects
@cindex Lisp objects, stack-allocated
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  The garbage collector described above is used to manage data visible
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from Lisp programs, as well as most of the data internally used by the
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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
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is implemented by C macros like @code{AUTO_CONS} and
@code{AUTO_STRING} that define a named @code{Lisp_Object} with block
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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
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of execution of the C block that defined the object.
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  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.
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@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
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@code{garbage-collect}; those count objects that currently exist, but
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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.
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This includes vector-like objects such as markers and overlays, plus
certain objects not visible to users.
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@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
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in this session.
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@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

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

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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,
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typically at configuration time, and the Emacs build procedure
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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.
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At some point in the future the base C dialect will no doubt change to C11.
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@node Writing Emacs Primitives
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@section Writing Emacs Primitives
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@cindex primitive function internals
@cindex writing Emacs primitives

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  Lisp primitives are Lisp functions implemented in C@.  The details of
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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,
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       doc: /* Eval args until one of them yields non-nil,
then return that value.
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The remaining args are not evalled at all.
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If all args return nil, return nil.
@end group
@group
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usage: (or CONDITIONS...)  */)
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  (Lisp_Object args)
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@{
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  Lisp_Object val = Qnil;
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@end group

@group
  while (CONSP (args))
    @{
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      val = eval_sub (XCAR (args));
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      if (!NILP (val))
        break;
      args = XCDR (args);
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      maybe_quit ();
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    @}
@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
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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}.
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@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
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macros.  If @var{max} is a number, it must be more than @var{min} but
less than 8.
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@cindex interactive specification in primitives
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@item interactive
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This is an interactive specification, a string such as might be used
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as the argument of @code{interactive} in a Lisp function
(@pxref{Using Interactive}).  In the case
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of @code{or}, it is @code{0} (a null pointer), indicating that @code{or}
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cannot be called interactively.  A value of @code{""} indicates a
function that should receive no arguments when called interactively.
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If the value begins with a @samp{"(}, the string is evaluated as a
Lisp form.  For example:

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@example
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@group
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DEFUN ("foo", Ffoo, Sfoo, 0, 3,
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       "(list (read-char-by-name \"Insert character: \")\
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              (prefix-numeric-value current-prefix-arg)\
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              t)",
       doc: /* @dots{} */)
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@end group
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@end example
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@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.

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Some primitives have multiple definitions, one per platform (e.g.,
@code{x-create-frame}).  In such cases, rather than writing the
same documentation string in each definition, only one definition has
the actual documentation.  The others have placeholders beginning with
@samp{SKIP}, which are ignored by the function that parses the
@file{DOC} file.

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All the usual rules for documentation strings in Lisp code
(@pxref{Documentation Tips}) apply to C code documentation strings
too.
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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
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       doc: /* @dots{} */
       attributes: @var{attr1} @var{attr2} @dots{})
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@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

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@end table

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  After the call to the @code{DEFUN} macro, you must write the
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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
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in the file @file{lisp.h}.)  If the primitive is a special form, it
must accept a Lisp list containing its unevaluated Lisp arguments as a
single argument of type @code{Lisp_Object}.  If the primitive has no
upper limit on the number of evaluated 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{ptrdiff_t} 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}).
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@cindex type checking internals
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@cindex garbage collection protection
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@cindex protect C variables from garbage collection
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  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
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move some of the components of an object, such as the contents of a
string or the text of a buffer.  Therefore, functions that access
these components must take care to refetch their addresses after
performing Lisp evaluation.  This means that instead of keeping C
pointers to string contents or buffer text, the code should keep the
buffer or string position, and recompute the C pointer from the
position after performing Lisp evaluation.  Lisp evaluation can occur
via calls to @code{eval_sub} or @code{Feval}, either directly or
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indirectly.
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@cindex @code{maybe_quit}, use in Lisp primitives
  Note the call to @code{maybe_quit} inside the loop: this function
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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.

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

@noindent
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Here @var{sname} is the name you used as the third argument to @code{DEFUN}.
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  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
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@cindex @code{DEFVAR_INT}, @code{DEFVAR_LISP}, @code{DEFVAR_BOOL}, @code{DEFSYM}
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  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.

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  These macros all expect three arguments:

@table @code
@item lname
The name of the variable to be used by Lisp programs.
@item vname
The name of the variable in the C sources.
@item doc
The documentation for the variable, as a C
comment.  @xref{Documentation Basics} for more details.
@end table

  By convention, when defining variables of a ``native'' type
(@code{int} and @code{bool}), the name of the C variable is the name
of the Lisp variable with @code{-} replaced by @code{_}.  When the
variable has type @code{Lisp_Object}, the convention is to also prefix
the C variable name with @code{V}.  i.e.

@smallexample
DEFVAR_INT ("my-int-variable", my_int_variable,
           doc: /* An integer variable.  */);

DEFVAR_LISP ("my-lisp-variable", Vmy_lisp_variable,
           doc: /* A Lisp variable.  */);
@end smallexample

  There are situations in Lisp where you need to refer to the symbol
itself rather than the value of that symbol.  One such case is when
temporarily overriding the value of a variable, which in Lisp is done
with @code{let}.  In C sources, this is done by defining a
corresponding, constant symbol, and using @code{specbind}.  By
convention, @code{Qmy_lisp_variable} corresponds to
@code{Vmy_lisp_variable}; to define it, use the @code{DEFSYM} macro.
i.e.

@smallexample
DEFSYM (Qmy_lisp_variable, "my-lisp-variable");
@end smallexample

  To perform the actual binding:

@smallexample
specbind (Qmy_lisp_variable, Qt);
@end smallexample

  In Lisp symbols sometimes need to be quoted, to achieve the same
effect in C you again use the corresponding constant symbol
@code{Qmy_lisp_variable}.  For example, when creating a buffer-local
variable (@pxref{Buffer-Local Variables}) in Lisp you would write:

@smallexample
(make-variable-buffer-local 'my-lisp-variable)
@end smallexample

In C the corresponding code uses @code{Fmake_variable_buffer_local} in
combination with @code{DEFSYM}, i.e.

@smallexample
DEFSYM (Qmy_lisp_variable, "my-lisp-variable");
Fmake_variable_buffer_local (Qmy_lisp_variable);
@end smallexample

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@cindex defining customization variables in C
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  If you want to make a Lisp variable that is defined in C behave
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like one declared with @code{defcustom}, add an appropriate entry to
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@file{cus-start.el}.  @xref{Variable Definitions}, for a description of
the format to use.
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@cindex @code{staticpro}, protection from GC
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  If you directly 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:
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@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,
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       Scoordinates_in_window_p, 2, 2, 0,
       doc: /* Return non-nil if COORDINATES are in WINDOW.
  @dots{}
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@end group
@group
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  or `right-margin' is returned.  */)
  (register Lisp_Object coordinates, Lisp_Object window)
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@{
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  struct window *w;
  struct frame *f;
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  int x, y;
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  Lisp_Object lx, ly;
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@end group

@group
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  w = decode_live_window (window);
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  f = XFRAME (w->frame);
  CHECK_CONS (coordinates);
  lx = Fcar (coordinates);
  ly = Fcdr (coordinates);
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  CHECK_NUMBER (lx);
  CHECK_NUMBER (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);
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@end group

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

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

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

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

  Note that C code cannot call functions by name unless they are defined
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in C@.  The way to call a function written in Lisp is to use
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@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
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pass to it.
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  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.

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  If you define a function which is side-effect free or pure, give it
a non-@code{nil} @code{side-effect-free} or @code{pure} property,
respectively (@pxref{Standard Properties}).
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@node Writing Dynamic Modules
@section Writing Dynamically-Loaded Modules
@cindex writing emacs modules
@cindex dynamic modules, writing

@cindex module @acronym{API}
  This section describes the Emacs module @acronym{API} and how to use
it as part of writing extension modules for Emacs.  The module
@acronym{API} is defined in the C programming language, therefore the
description and the examples in this section assume the module is
written in C@.  For other programming languages, you will need to use
the appropriate bindings, interfaces and facilities for calling C code.
Emacs C code requires a C99 or later compiler (@pxref{C Dialect}), and
so the code examples in this section also follow that standard.

Writing a module and integrating it into Emacs comprises the following
tasks:

@itemize @bullet
@item
Writing initialization code for the module.

@item
Writing one or more module functions.

@item
Communicating values and objects between Emacs and your module
functions.

@item
Handling of error conditions and nonlocal exits.
@end itemize

@noindent
The following subsections describe these tasks and the @acronym{API}
itself in more detail.

Once your module is written, compile it to produce a shared library,
according to the conventions of the underlying platform.  Then place
the shared library in a directory mentioned in @code{load-path}
(@pxref{Library Search}), where Emacs will find it.

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If you wish to verify the conformance of a module to the Emacs dynamic
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module @acronym{API}, invoke Emacs with the @kbd{--module-assertions}
option.  @xref{Initial Options,,,emacs, The GNU Emacs Manual}.

@menu
* Module Initialization::
* Module Functions::
* Module Values::
* Module Misc::
* Module Nonlocal::
@end menu

@node Module Initialization
@subsection Module Initialization Code
@cindex module initialization

  Begin your module by including the header file @file{emacs-module.h}
and defining the GPL compatibility symbol:

@example
#include <emacs-module.h>

int plugin_is_GPL_compatible;
@end example

The @file{emacs-module.h} file is installed into your system's include
tree as part of the Emacs installation.  Alternatively, you can find
it in the Emacs source tree.

@anchor{module initialization function}
Next, write an initialization function for the module.

@deftypefn Function int emacs_module_init (struct emacs_runtime *@var{runtime})
Emacs calls this function when it loads a module.  If a module does
not export a function named @code{emacs_module_init}, trying to load
the module will signal an error.  The initialization function should
return zero if the initialization succeeds, non-zero otherwise.  In
the latter case, Emacs will signal an error, and the loading of the
module will fail.  If the user presses @kbd{C-g} during the
initialization, Emacs ignores the return value of the initialization
function and quits (@pxref{Quitting}).  (If needed, you can catch user
quitting inside the initialization function, @pxref{should_quit}.)

The argument @var{runtime} is a pointer to a C @code{struct} that
includes 2 public fields: @code{size}, which provides the size of the
structure in bytes; and @code{get_environment}, which provides a
pointer to a function that allows the module initialization function
access to the Emacs environment object and its interfaces.

The initialization function should perform whatever initialization is
required for the module.  In addition, it can perform the following
tasks:

@table @asis
@cindex compatibility, between modules and Emacs
@item Compatibility verification
A module can verify that the Emacs executable which loads the module
is compatible with the module, by comparing the @code{size} member of
the @var{runtime} structure with the value compiled into the module:

@example
int
emacs_module_init (struct emacs_runtime *ert)
@{
  if (ert->size < sizeof (*ert))
    return 1;
@}
@end example

@noindent
If the size of the runtime object passed to the module is smaller than
what it expects, it means the module was compiled for an Emacs version
newer (later) than the one which attempts to load it, i.e.@: the
module might be incompatible with the Emacs binary.

In addition, a module can verify the compatibility of the module
@acronym{API} with what the module expects.  The following sample code
assumes it is part of the @code{emacs_module_init} function shown
above:

@example
  emacs_env *env = ert->get_environment (ert);
  if (env->size < sizeof (*env))
    return 2;
@end example

@noindent
@cindex module runtime environment
This calls the @code{get_environment} function using the pointer
provided in the @code{runtime} structure to retrieve a pointer to the
@acronym{API}'s @dfn{environment}, a C @code{struct} which also has a
@code{size} field holding the size of the structure in bytes.

Finally, you can write a module that will work with older versions of
Emacs, by comparing the size of the environment passed by Emacs with
known sizes, like this:

@example
  emacs_env *env = ert->get_environment (ert);
  if (env->size >= sizeof (struct emacs_env_26))
    emacs_version = 26;  /* Emacs 26 or later.  */
  else if (env->size >= sizeof (struct emacs_env_25))
    emacs_version = 25;
  else
    return 2; /* Unknown or unsupported version.  */
@end example

@noindent
This works because later Emacs versions always @emph{add} members to
the environment, never @emph{remove} any members, so the size can only
grow with new Emacs releases.  Given the version of Emacs, the module
can use only the parts of the module @acronym{API} that existed in
that version, since those parts are identical in later versions.

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@file{emacs-module.h} defines a preprocessor macro
@code{EMACS_MAJOR_VERSION}.  It expands to an integer literal which is
the latest major version of Emacs supported by the header.
@xref{Version Info}.  Note that the value of
@code{EMACS_MAJOR_VERSION} is a compile-time constant and does not
represent the version of Emacs that is currently running and has
loaded your module.  If you want your module to be compatible with
various versions of @file{emacs-module.h} as well as various versions
of Emacs, you can use conditional compilation based on
@code{EMACS_MAJOR_VERSION}.

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We recommend that modules always perform the compatibility
verification, unless they do their job entirely in the initialization
function, and don't access any Lisp objects or use any Emacs functions
accessible through the environment structure.

@item Binding module functions to Lisp symbols
This gives the module functions names so that Lisp code could call it
by that name.  We describe how to do this in @ref{Module Functions}
below.
@end table
@end deftypefn

@node Module Functions
@subsection Writing Module Functions
@cindex writing module functions
@cindex module functions

  The main reason for writing an Emacs module is to make additional
functions available to Lisp programs that load the module.  This
subsection describes how to write such @dfn{module functions}.

A module function has the following general form and signature:

@deftypefn Function emacs_value module_func (emacs_env *@var{env}, ptrdiff_t @var{nargs}, emacs_value *@var{args}, void *@var{data})
The @var{env} argument provides a pointer to the @acronym{API}
environment, needed to access Emacs objects and functions.  The
@var{nargs} argument is the required number of arguments, which can be
zero (see @code{make_function} below for more flexible specification
of the argument number), and @var{args} is a pointer to the array of
the function arguments.  The argument @var{data} points to additional
data required by the function, which was arranged when
@code{make_function} (see below) was called to create an Emacs
function from @code{module_func}.

Module functions use the type @code{emacs_value} to communicate Lisp
objects between Emacs and the module (@pxref{Module Values}).  The
@acronym{API}, described below and in the following subsections,
provides facilities for conversion between basic C data types and the
corresponding @code{emacs_value} objects.

A module function always returns a value.  If the function returns
normally, the Lisp code which called it will see the Lisp object
corresponding to the @code{emacs_value} value the function returned.
However, if the user typed @kbd{C-g}, or if the module function or its
callees signaled an error or exited nonlocally (@pxref{Module
Nonlocal}), Emacs will ignore the returned value and quit or throw as
it does when Lisp code encounters the same situations.
@end deftypefn

After writing your C code for a module function, you should make a
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Lisp function object from it using the @code{make_function} function,
whose pointer is provided in the environment (recall that the pointer
to the environment is returned by @code{get_environment}).  This is
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normally done in the module initialization function (@pxref{module
initialization function}), after verifying the @acronym{API}
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compatibility.
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@deftypefn Function emacs_value make_function (emacs_env *@var{env}, ptrdiff_t @var{min_arity}, ptrdiff_t @var{max_arity}, subr @var{func}, const char *@var{docstring}, void *@var{data})
@vindex emacs_variadic_function
This returns an Emacs function created from the C function @var{func},
whose signature is as described for @code{module_func} above (assumed
here to be @code{typedef}'ed as @code{subr}).  The arguments
@var{min_arity} and @var{max_arity} specify the minimum and maximum
number of arguments that @var{func} can accept.  The @var{max_arity}
argument can have the special value @code{emacs_variadic_function},
which makes the function accept an unlimited number of arguments, like
the @code{&rest} keyword in Lisp (@pxref{Argument List}).

The argument @var{data} is a way to arrange for arbitrary additional
data to be passed to @var{func} when it is called.  Whatever pointer
is passed to @code{make_function} will be passed unaltered to
@var{func}.

The argument @var{docstring} specifies the documentation string for
the function.  It should be either an @acronym{ASCII} string, or a
UTF-8 encoded non-@acronym{ASCII} string, or a @code{NULL} pointer; in
the latter case the function will have no documentation.  The
documentation string can end with a line that specifies the advertised
calling convention, see @ref{Function Documentation}.

Since every module function must accept the pointer to the environment
as its first argument, the call to @code{make_function} could be made
from any module function, but you will normally want to do that from
the module initialization function, so that all the module functions
are known to Emacs once the module is loaded.
@end deftypefn

Finally, you should bind the Lisp function to a symbol, so that Lisp
code could call your function by name.  For that, use the module
@acronym{API} function @code{intern} (@pxref{intern}) whose pointer is
also provided in the environment that module functions can access.

Combining the above steps, code that arranges for a C function
@code{module_func} to be callable as @code{module-func} from Lisp will
look like this, as part of the module initialization function:

@example
 emacs_env *env = ert->get_environment (ert);
 emacs_value func = env->make_function (env, min_arity, max_arity,
                                        module_func, docstring, data);
 emacs_value symbol = env->intern (env, "module-func");
 emacs_value args[] = @{symbol, func@};
 env->funcall (env, env->intern (env, "defalias"), 2, args);
@end example

@noindent
This makes the symbol @code{module-func} known to Emacs by calling
@code{env->intern}, then invokes @code{defalias} from Emacs to bind
the function to that symbol.  Note that it is possible to use
@code{fset} instead of @code{defalias}; the differences are described
in @ref{Defining Functions, defalias}.

Using the module @acronym{API}, it is possible to define more complex
function and data types: interactive functions, inline functions,
macros, etc.  However, the resulting C code will be cumbersome and
hard to read.  Therefore, we recommend that you limit the module code
which creates functions and data structures to the absolute minimum,
and leave the rest for a Lisp package that will accompany your module,
because doing these additional tasks in Lisp is much easier, and will
produce a much more readable code.  For example, given a module
function @code{module-func} defined as above, one way of making an
interactive command @code{module-cmd} based on it is with the
following simple Lisp wrapper:

@lisp
(defun module-cmd (&rest args)
  "Documentation string for the command."
  (interactive @var{spec})
  (apply 'module-func args))
@end lisp

The Lisp package which goes with your module could then load the
module using the @code{module-load} primitive (@pxref{Dynamic
Modules}) when the package is loaded into Emacs.

@node Module Values
@subsection Conversion Between Lisp and Module Values
@cindex module values, conversion

@cindex @code{emacs_value} data type
  With very few exceptions, most modules need to exchange data with
Lisp programs that call them: accept arguments to module functions and
return values from module functions.  For this purpose, the module
@acronym{API} provides the @code{emacs_value} type, which represents
Emacs Lisp objects communicated via the @acronym{API}; it is the
functional equivalent of the @code{Lisp_Object} type used in Emacs C
primitives (@pxref{Writing Emacs Primitives}).  This section describes
the parts of the module @acronym{API} that allow to create
@code{emacs_value} objects corresponding to basic Lisp data types, and
how to access from C data in @code{emacs_value} objects that
correspond to Lisp objects.

All of the functions described below are actually @emph{function
pointers} provided via the pointer to the environment which every
module function accepts.  Therefore, module code should call these
functions through the environment pointer, like this:

@example
emacs_env *env;  /* the environment pointer */
env->some_function (arguments@dots{});
@end example

@noindent
The @code{emacs_env} pointer will usually come from the first argument
to the module function, or from the call to @code{get_environment} if
you need the environment in the module initialization function.

Most of the functions described below became available in Emacs 25,
the first Emacs release that supported dynamic modules.  For the few
functions that became available in later Emacs releases, we mention
the first Emacs version that supported them.

The following @acronym{API} functions extract values of various C data
types from @code{emacs_value} objects.  They all raise the
@code{wrong-type-argument} error condition (@pxref{Type Predicates})
if the argument @code{emacs_value} object is not of the type expected
by the function.  @xref{Module Nonlocal}, for details of how signaling
errors works in Emacs modules, and how to catch error conditions
inside the module before they are reported to Emacs.  The
@acronym{API} function @code{type_of} (@pxref{Module Misc, type_of})
can be used to obtain the type of a @code{emacs_value} object.

@deftypefn Function intmax_t extract_integer (emacs_env *@var{env}, emacs_value @var{arg})
This function returns the value of a Lisp integer specified by
@var{arg}.  The C data type of the return value, @code{intmax_t}, is
the widest integral data type supported by the C compiler, typically
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@w{@code{long long}}.  If the value of @var{arg} doesn't fit into an
@code{intmax_t}, the function signals an error using the error symbol
@code{overflow-error}.
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@end deftypefn

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@deftypefn Function bool extract_big_integer (emacs_env *@var{env}, emacs_value @var{arg}, int *@var{sign}, ptrdiff_t *@var{count}, emacs_limb_t *@var{magnitude})
This function, which is available since Emacs 27, extracts the
integral value of @var{arg}.  The value of @var{arg} must be an
integer (fixnum or bignum).  If @var{sign} is not @code{NULL}, it
stores the sign of @var{arg} (-1, 0, or +1) into @code{*sign}.  The
magnitude is stored into @var{magnitude} as follows.  If @var{count}
and @var{magnitude} are bot non-@code{NULL}, then @var{magnitude} must
point to an array of at least @code{*count} @code{unsigned long}
elements.  If @var{magnitude} is large enough to hold the magnitude of
@var{arg}, then this function writes the magnitude into the
@var{magnitude} array in little-endian form, stores the number of
array elements written into @code{*count}, and returns @code{true}.
If @var{magnitude} is not large enough, it stores the required array
size into @code{*count}, signals an error, and returns @code{false}.
If @var{count} is not @code{NULL} and @var{magnitude} is @code{NULL},
then the function stores the required array size into @code{*count}
and returns @code{true}.

Emacs guarantees that the maximum required value of @code{*count}
never exceeds @code{min (PTRDIFF_MAX, SIZE_MAX) / sizeof
(emacs_limb_t)}.  This implies that you can use e.g. @code{malloc
((size_t) (*count * sizeof (emacs_limb_t)))} to allocate the
@code{magnitude} array without integer overflow.
@end deftypefn

@deftp {Type alias} emacs_limb_t
This type is an alias to an otherwise unspecified unsigned integral
type.  It is used as element type for the magnitude arrays for the big
integer conversion functions.
@end deftp

@defvr Macro EMACS_LIMB_MAX
This macro expands to an integer literal specifying the maximum
possible value for an @code{emacs_limb_t} object.
@end defvr

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@deftypefn Function double extract_float (emacs_env *@var{env}, emacs_value @var{arg})
This function returns the value of a Lisp float specified by
@var{arg}, as a C @code{double} value.
@end deftypefn

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@deftypefn Function struct timespec extract_time (emacs_env *@var{env}, emacs_value @var{time})
This function, which is available since Emacs 27, interprets
@var{time} as an Emacs Lisp time value and returns the corresponding
@code{struct timespec}.  @xref{Time of Day}.  @code{struct timespec}
represents a timestamp with nanosecond precision.  It has the
following members:

@table @code
@item time_t tv_sec
Whole number of seconds.
@item long tv_nsec
Fractional seconds as number of nanoseconds, always less than one
billion.
@end table

@noindent
@xref{Elapsed Time,,,libc}.

If @var{time} has higher precision than nanoseconds, then this
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function truncates it to nanosecond precision towards negative
infinity.  This function signals an error if @var{time} (truncated to
nanoseconds) cannot be represented by @code{struct timespec}.  For
example, if @code{time_t} is a 32-bit integral type, then a @var{time}
value of ten billion seconds would signal an error, but a @var{time}
value of 600 picoseconds would get truncated to zero.
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If you need to deal with time values that are not representable by
@code{struct timespec}, or if you want higher precision, call the Lisp
function @code{encode-time} and work with its return value.
@xref{Time Conversion}.
@end deftypefn

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