@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1993, 1998-1999, 2001-2019 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node GNU Emacs Internals
@appendix GNU Emacs Internals

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

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

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

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

  Building Emacs requires GNU Make version 3.81 or later.

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

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

@cindex dumping Emacs
@cindex @option{--temacs} option, and dumping method
  Because it takes some time to load the standard Lisp files, the
@file{temacs} executable usually isn't run directly by users.
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
@cindex dump file
Record the preloaded Lisp data in a @dfn{dump file}.  This
method produces an additional data file which Emacs will load at
startup.  The produced dump file is usually called @file{emacs.pdmp},
and is installed in the Emacs @code{exec-directory} (@pxref{Help
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
available.  The produced dump file is usually named
@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.
Although this method was the way that Emacs traditionally saved its
state, it is now deprecated.

@item bootstrap
Like @samp{dump}, but used when bootstrapping Emacs with the
@code{unexec} method.
@end table

@cindex preloaded Lisp files
@vindex preloaded-file-list
  The dumped @file{emacs} executable (also called a @dfn{pure} Emacs)
is the one which is installed.  If the portable dumper was used to
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
list of the preloaded Lisp files recorded in the dump file or
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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@defun dump-emacs-portable to-file &optional track-referrers
This function dumps the current state of Emacs into a dump
file @var{to-file}, using the @code{pdump} method.  Normally, the
dump file is called @file{@var{emacs-name}.dmp}, where
@var{emacs-name} is the name of the Emacs executable file.  The
optional argument @var{track-referrers}, if non-@code{nil}, causes the
portable dumper to keep additional information to help track
down the provenance of object types that are not yet supported by the
@code{pdump} method.

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.

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

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

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.
@end defun

@defun pdumper-stats
If the current Emacs session restored its state from a dump
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
time in seconds it took to restore the state from the dump file.
If the current session was not restored from a dump file, the
value is nil.
@end defun

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

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

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

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

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

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

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

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

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

@node Garbage Collection
@section Garbage Collection

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

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

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

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

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

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

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

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

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

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

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

Here is an example:

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

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

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

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

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

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

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

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

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

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

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

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

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

@item vector-size
Size in bytes of a vector of length 1, including its header.

@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.
Slot counts might include some or all overhead from vector headers,
depending on the platform.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@defvar vector-cells-consed
The total number of vector cells that have been allocated so far
in this Emacs session.
This includes vector-like objects such as markers and overlays, plus
certain objects not visible to users.
@end defvar

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

@end table

  After the call to the @code{DEFUN} macro, you must write the
argument list for the C function, including the types for the
arguments.  If the primitive accepts a fixed maximum number of Lisp
arguments, there must be one C argument for each Lisp argument, and
each argument must be of type @code{Lisp_Object}.  (Various macros and
functions for creating values of type @code{Lisp_Object} are declared
in the file @file{lisp.h}.)  If the primitive 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}).
@cindex type checking internals

@cindex garbage collection protection
@cindex protect C variables from garbage collection
  Within the function @code{For} itself, the local variable
@code{args} refers to objects controlled by Emacs's stack-marking
garbage collector.  Although the garbage collector does not reclaim
objects reachable from C @code{Lisp_Object} stack variables, it may
move 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
indirectly.

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

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

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

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

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

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

@anchor{Defining Lisp variables in C}
@vindex byte-boolean-vars
@cindex defining Lisp variables in C
@cindex @code{DEFVAR_INT}, @code{DEFVAR_LISP}, @code{DEFVAR_BOOL}, @code{DEFSYM}
  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.

  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

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

@cindex @code{staticpro}, protection from GC
  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:

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

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

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

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

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

    @dots{}

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

    @dots{}

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

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

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

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

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

  If you define a function which is side-effect free or pure, give it
a non-@code{nil} @code{side-effect-free} or @code{pure} property,
respectively (@pxref{Standard Properties}).

@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.

If you wish to verify the conformance of a module to the Emacs dynamic
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.

@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}.

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
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
normally done in the module initialization function (@pxref{module
initialization function}), after verifying the @acronym{API}
compatibility.

@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
@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}.
@end deftypefn

@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

@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
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.

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

@deftypefn Function bool copy_string_contents (emacs_env *@var{env}, emacs_value @var{arg}, char *@var{buf}, ptrdiff_t *@var{len})
This function stores the UTF-8 encoded text of a Lisp string specified
by @var{arg} in the array of @code{char} pointed by @var{buf}, which
should have enough space to hold at least @code{*@var{len}} bytes,
including the terminating null byte.  The argument @var{len} must not
be a @code{NULL} pointer, and, when the function is called, it should
point to a value that specifies the size of @var{buf} in bytes.

If the buffer size specified by @code{*@var{len}} is large enough to
hold the string's text, the function stores in @code{*@var{len}} the
actual number of bytes copied to @var{buf}, including the terminating
null byte, and returns @code{true}.  If the buffer is too small, the
function raises the @code{args-out-of-range} error condition, stores
the required number of bytes in @code{*@var{len}}, and returns
@code{false}.  @xref{Module Nonlocal}, for how to handle pending error
conditions.

The argument @var{buf} can be a @code{NULL} pointer, in which case the
function stores in @code{*@var{len}} the number of bytes required for
storing the contents of @var{arg}, and returns @code{true}.  This is
how you can determine the size of @var{buf} needed to store a
particular string: first call @code{copy_string_contents} with
@code{NULL} as @var{buf}, then allocate enough memory to hold the
number of bytes stored by the function in @code{*@var{len}}, and call
the function again with non-@code{NULL} @var{buf} to actually perform
the text copying.
@end deftypefn

@deftypefn Function emacs_value vec_get (emacs_env *@var{env}, emacs_value @var{vector}, ptrdiff_t @var{index})
This function returns the element of @var{vector} at @var{index}.  The
@var{index} of the first vector element is zero.  The function raises
the @code{args-out-of-range} error condition if the value of
@var{index} is invalid.  To extract C data from the value the function
returns, use the other extraction functions described here, as
appropriate for the Lisp data type stored in that element of the
vector.
@end deftypefn

@deftypefn Function ptrdiff_t vec_size (emacs_env *@var{env}, emacs_value @var{vector})
This function returns the number of elements in @var{vector}.
@end deftypefn

@deftypefn Function void vec_set (emacs_env *@var{env}, emacs_value @var{vector}, ptrdiff_t @var{index}, emacs_value @var{value})
This function stores @var{value} in the element of @var{vector} whose
index is @var{index}.  It raises the @code{args-out-of-range} error
condition if the value of @var{index} is invalid.
@end deftypefn

The following @acronym{API} functions create @code{emacs_value}
objects from basic C data types.  They all return the created
@code{emacs_value} object.

@deftypefn Function emacs_value make_integer (emacs_env *@var{env}, intmax_t @var{n})
This function takes an integer argument @var{n} and returns the
corresponding @code{emacs_value} object.  It returns either a fixnum
or a bignum depending on whether the value of @var{n} is inside the
limits set by @code{most-negative-fixnum} and
@code{most-positive-fixnum} (@pxref{Integer Basics}).
@end deftypefn

@deftypefn Function emacs_value make_float (emacs_env *@var{env}, double @var{d})
This function takes a @code{double} argument @var{d} and returns the
corresponding Emacs floating-point value.
@end deftypefn

@deftypefn Function emacs_value make_time (emacs_env *@var{env}, struct timespec @var{time})
This function, which is available since Emacs 27, takes a @code{struct
timespec} argument @var{time} and returns the corresponding Emacs
timestamp as a pair @code{(@var{ticks} . @var{hz})}.  @xref{Time of
Day}.  The return value represents exactly the same timestamp as
@var{time}: all input values are representable, and there is never a
loss of precision.  @code{@var{time}.tv_sec} and
@code{@var{time}.tv_nsec} can be arbitrary values.  In particular,
there's no requirement that @var{time} be normalized.  This means that
@code{@var{time}.tv_nsec} can be negative or larger than 999,999,999.
@end deftypefn

@deftypefn Function emacs_value make_string (emacs_env *@var{env}, const char *@var{str}, ptrdiff_t @var{strlen})
This function creates an Emacs string from C text string pointed by
@var{str} whose length in bytes, not including the terminating null
byte, is @var{strlen}.  The original string in @var{str} can be either
an @acronym{ASCII} string or a UTF-8 encoded non-@acronym{ASCII}
string; it can include embedded null bytes, and doesn't have to end in
a terminating null byte at @code{@var{str}[@var{strlen}]}.  The
function raises the @code{overflow-error} error condition if
@var{strlen} is negative or exceeds the maximum length of an Emacs
string.
@end deftypefn

If you define the preprocessor macro @code{EMACS_MODULE_GMP} before
including the header @file{emacs-module.h}, you can also convert
between Emacs integers and GMP @code{mpz_t} values.  @xref{GMP
Basics,,,gmp}.  If @code{EMACS_MODULE_GMP} is defined,
@file{emacs-module.h} wraps @code{mpz_t} in the following structure:

@deftp struct emacs_mpz value
struct emacs_mpz @{ mpz_t value; @};
@end deftp

@noindent
Then you can use the following additional functions:

@deftypefn Function bool extract_big_integer (emacs_env *@var{env}, emacs_value @var{arg}, struct emacs_mpz *@var{result})
This function, which is available since Emacs 27, extracts the
integral value of @var{arg} into @var{result}.  @var{result} must not
be @code{NULL}.  @code{@var{result}->value} must be an initialized
@code{mpz_t} object.  @xref{Initializing Integers,,,gmp}.  If
@var{arg} is an integer, Emacs will store its value into
@code{@var{result}->value}.  After you have finished using
@code{@var{result}->value}, you should free it using @code{mpz_clear}
or similar.
@end deftypefn

@deftypefn Function emacs_value make_big_integer (emacs_env *@var{env}, const struct emacs_mpz *@var{value})
This function, which is available since Emacs 27, takes an
arbitrary-sized integer argument and returns a corresponding
@code{emacs_value} object.  @var{value} must not be @code{NULL}.
@code{@var{value}->value} must be an initialized @code{mpz_t} object.
@xref{Initializing Integers,,,gmp}.  Emacs will return a corresponding
integral object.  After you have finished using
@code{@var{value}->value}, you should free it using @code{mpz_clear}
or similar.
@end deftypefn

The following example uses GMP to calculate the next probable prime
after a given integer:

@example
#include <assert.h>
#include <gmp.h>

#define EMACS_MODULE_GMP
#include <emacs-module.h>

static emacs_value
next_prime (emacs_env *env, ptrdiff_t nargs, emacs_value *args,
            void *data)
@{
  assert (nargs == 1);
  emacs_mpz p;
  mpz_init (p.value);
  env->extract_big_integer (env, args[0], &p);
  mpz_nextprime (p.value, p.value);
  emacs_value result = env->make_big_integer (env, &p);
  mpz_clear (p.value);
  return result;
@}
@end example

The @acronym{API} does not provide functions to manipulate Lisp data
structures, for example, create lists with @code{cons} and @code{list}
(@pxref{Building Lists}), extract list members with @code{car} and
@code{cdr} (@pxref{List Elements}), create vectors with @code{vector}
(@pxref{Vector Functions}), etc.  For these, use @code{intern} and
@code{funcall}, described in the next subsection, to call the
corresponding Lisp functions.

Normally, @code{emacs_value} objects have a rather short lifetime: it
ends when the @code{emacs_env} pointer used for their creation goes
out of scope.  Occasionally, you may need to create @dfn{global
references}: @code{emacs_value} objects that live as long as you
wish.  Use the following two functions to manage such objects.

@deftypefn Function emacs_value make_global_ref (emacs_env *@var{env}, emacs_value @var{value})
This function returns a global reference for @var{value}.
@end deftypefn

@deftypefn Function void free_global_ref (emacs_env *@var{env}, emacs_value @var{global_value})
This function frees the @var{global_value} previously created by
@code{make_global_ref}.  The @var{global_value} is no longer valid
after the call.  Your module code should pair each call to
@code{make_global_ref} with the corresponding @code{free_global_ref}.
@end deftypefn

@cindex user pointer, using in module functions
An alternative to keeping around C data structures that need to be
passed to module functions later is to create @dfn{user pointer}
objects.  A user pointer, or @code{user-ptr}, object is a Lisp object
that encapsulates a C pointer and can have an associated finalizer
function, which is called when the object is garbage-collected
(@pxref{Garbage Collection}).  The module @acronym{API} provides
functions to create and access @code{user-ptr} objects.  These
functions raise the @code{wrong-type-argument} error condition if they
are called on @code{emacs_value} that doesn't represent a
@code{user-ptr} object.

@deftypefn Function emacs_value make_user_ptr (emacs_env *@var{env}, emacs_finalizer @var{fin}, void *@var{ptr})
This function creates and returns a @code{user-ptr} object which wraps
the C pointer @var{ptr}.  The finalizer function @var{fin} can be a
@code{NULL} pointer (meaning no finalizer), or it can be a function of
the following signature:

@example
typedef void (*emacs_finalizer) (void *@var{ptr});
@end example

@noindent
If @var{fin} is not a @code{NULL} pointer, it will be called with the
@var{ptr} as the argument when the @code{user-ptr} object is
garbage-collected.  Don't run any expensive code in a finalizer,
because GC must finish quickly to keep Emacs responsive.
@end deftypefn

@deftypefn Function void *get_user_ptr (emacs_env *@var{env}, emacs_value val)
This function extracts the C pointer from the Lisp object represented
by @var{val}.
@end deftypefn

@deftypefn Function void set_user_ptr (emacs_env *@var{env}, emacs_value @var{value}, void *@var{ptr})
This function sets the C pointer embedded in the @code{user-ptr}
object represented by @var{value} to @var{ptr}.
@end deftypefn

@deftypefn Function emacs_finalizer get_user_finalizer (emacs_env *@var{env}, emacs_value val)
This function returns the finalizer of the @code{user-ptr} object
represented by @var{val}, or @code{NULL} if it doesn't have a finalizer.
@end deftypefn

@deftypefn Function void set_user_finalizer (emacs_env *@var{env}, emacs_value @var{val}, emacs_finalizer @var{fin})
This function changes the finalizer of the @code{user-ptr} object
represented by @var{val} to be @var{fin}.  If @var{fin} is a
@code{NULL} pointer, the @code{user-ptr} object will have no finalizer.
@end deftypefn

@node Module Misc
@subsection Miscellaneous Convenience Functions for Modules

  This subsection describes a few convenience functions provided by
the module @acronym{API}.  Like the functions described in previous
subsections, all of them are actually function pointers, and need to
be called via the @code{emacs_env} pointer.  Description of functions
that were introduced after Emacs 25 calls out the first version where
they became available.

@deftypefn Function bool eq (emacs_env *@var{env}, emacs_value @var{val1}, emacs_value @var{val2})
This function returns @code{true} if the Lisp objects represented by
@var{val1} and @var{val2} are identical, @code{false} otherwise.  This
is the same as the Lisp function @code{eq} (@pxref{Equality
Predicates}), but avoids the need to intern the objects represented by
the arguments.

There are no @acronym{API} functions for other equality predicates, so
you will need to use @code{intern} and @code{funcall}, described
below, to perform more complex equality tests.
@end deftypefn

@deftypefn Function bool is_not_nil (emacs_env *@var{env}, emacs_value @var{val})
This function tests whether the Lisp object represented by @var{val}
is non-@code{nil}; it returns @code{true} or @code{false} accordingly.

Note that you could implement an equivalent test by using
@code{intern} to get an @code{emacs_value} representing @code{nil},
then use @code{eq}, described above, to test for equality.  But using
this function is more convenient.
@end deftypefn

@deftypefn Function emacs_value type_of (emacs_env *@var{env}, emacs_value @code{object})
This function returns the type of @var{object} as a value that
represents a symbol: @code{string} for a string, @code{integer} for an
integer, @code{process} for a process, etc.  @xref{Type Predicates}.
You can use @code{intern} and @code{eq} to compare against known type
symbols, if your code needs to depend on the object type.
@end deftypefn

@anchor{intern}
@deftypefn Function emacs_value intern (emacs_env *@var{env}, const char *name)
This function returns an interned Emacs symbol whose name is
@var{name}, which should be an @acronym{ASCII} null-terminated string.
It creates a new symbol if one does not already exist.

Together with @code{funcall}, described below, this function provides
a means for invoking any Lisp-callable Emacs function, provided that
its name is a pure @acronym{ASCII} string.  For example, here's how to
intern a symbol whose name @code{name_str} is non-@acronym{ASCII}, by
calling the more powerful Emacs @code{intern} function
(@pxref{Creating Symbols}):

@example
emacs_value fintern = env->intern (env, "intern");
emacs_value sym_name =
  env->make_string (env, name_str, strlen (name_str));
emacs_value intern_args[] = @{ sym_name, env->intern (env, "nil") @};
emacs_value symbol = env->funcall (env, fintern, 2, intern_args);
@end example

@end deftypefn

@deftypefn Function emacs_value funcall (emacs_env *@var{env}, emacs_value @var{func}, ptrdiff_t @var{nargs}, emacs_value *@var{args})
This function calls the specified @var{func} passing it @var{nargs}
arguments from the array pointed to by @var{args}.  The argument
@var{func} can be a function symbol (e.g., returned by @code{intern}
described above), a module function returned by @code{make_function}
(@pxref{Module Functions}), a subroutine written in C, etc.  If
@var{nargs} is zero, @var{args} can be a @code{NULL} pointer.

The function returns the value that @var{func} returned.
@end deftypefn

If your module includes potentially long-running code, it is a good
idea to check from time to time in that code whether the user wants to
quit, e.g., by typing @kbd{C-g} (@pxref{Quitting}).  The following
function, which is available since Emacs 26.1, is provided for that
purpose.

@anchor{should_quit}
@deftypefn Function bool should_quit (emacs_env *@var{env})
This function returns @code{true} if the user wants to quit.  In that
case, we recommend that your module function aborts any on-going
processing and returns as soon as possible.  In most cases, use
@code{process_input} instead.
@end deftypefn

To process input events in addition to checking whether the user wants
to quit, use the following function, which is available since Emacs
27.1.

@anchor{process_input}
@deftypefn Function enum emacs_process_input_result process_input (emacs_env *@var{env})
This function processes pending input events.  It returns
@code{emacs_process_input_quit} if the user wants to quit or an error
occurred while processing signals.  In that case, we recommend that
your module function aborts any on-going processing and returns as
soon as possible.  If the module code may continue running,
@code{process_input} returns @code{emacs_process_input_continue}.  The
return value is @code{emacs_process_input_continue} if and only if
there is no pending nonlocal exit in @code{env}.  If the module
continues after calling @code{process_input}, global state such as
variable values and buffer content may have been modified in arbitrary
ways.
@end deftypefn

@node Module Nonlocal
@subsection Nonlocal Exits in Modules
@cindex nonlocal exits, in modules

  Emacs Lisp supports nonlocal exits, whereby program control is
transfered from one point in a program to another remote point.
@xref{Nonlocal Exits}.  Thus, Lisp functions called by your module
might exit nonlocally by calling @code{signal} or @code{throw}, and
your module functions must handle such nonlocal exits properly.  Such
handling is needed because C programs will not automatically release
resources and perform other cleanups in these cases; your module code
must itself do it.  The module @acronym{API} provides facilities for
that, described in this subsection.  They are generally available
since Emacs 25; those of them that became available in later releases
explicitly call out the first Emacs version where they became part of
the @acronym{API}.

When some Lisp code called by a module function signals an error or
throws, the nonlocal exit is trapped, and the pending exit and its
associated data are stored in the environment.  Whenever a nonlocal
exit is pending in the environment, any module @acronym{API} function
called with a pointer to that environment will return immediately
without any processing (the functions @code{non_local_exit_check},
@code{non_local_exit_get}, and @code{non_local_exit_clear} are
exceptions from this rule).  If your module function then does nothing
and returns to Emacs, a pending nonlocal exit will cause Emacs to act
on it: signal an error or throw to the corresponding @code{catch}.

So the simplest ``handling'' of nonlocal exits in module functions is
to do nothing special and let the rest of your code to run as if
nothing happened.  However, this can cause two classes of problems:

@itemize @minus
@item
Your module function might use uninitialized or undefined values,
since @acronym{API} functions return immediately without producing the
expected results.

@item
Your module might leak resources, because it might not have the
opportunity to release them.
@end itemize

Therefore, we recommend that your module functions check for nonlocal
exit conditions and recover from them, using the functions described
below.

@deftypefn Function enum emacs_funcall_exit non_local_exit_check (emacs_env *@var{env})
This function returns the kind of nonlocal exit condition stored in
@var{env}.  The possible values are:

@vindex emacs_funcall_exit@r{, enumeration}
@vtable @code
@item emacs_funcall_exit_return
The last @acronym{API} function exited normally.
@item emacs_funcall_exit_signal
The last @acronym{API} function signaled an error.
@item emacs_funcall_exit_throw
The last @acronym{API} function exited via @code{throw}.
@end vtable
@end deftypefn

@deftypefn Function enum emacs_funcall_exit non_local_exit_get (emacs_env *@var{env}, emacs_value *@var{symbol}, emacs_value *@var{data})
This function returns the kind of nonlocal exit condition stored in
@var{env}, like @code{non_local_exit_check} does, but it also returns
the full information about the nonlocal exit, if any.  If the return
value is @code{emacs_funcall_exit_signal}, the function stores the
error symbol in @code{*@var{symbol}} and the error data in
@code{*@var{data}} (@pxref{Signaling Errors}).  If the return value is
@code{emacs_funcall_exit_throw}, the function stores the @code{catch}
tag symbol in @code{*@var{symbol}} and the @code{throw} value in
@code{*@var{data}}.  The function doesn't store anything in memory
pointed by these arguments when the return value is
@code{emacs_funcall_exit_return}.
@end deftypefn

You should check nonlocal exit conditions where it matters: before you
allocated some resource or after you allocated a resource that might
need freeing, or where a failure means further processing is
impossible or infeasible.

Once your module function detected that a nonlocal exit is pending, it
can either return to Emacs (after performing the necessary local
cleanup), or it can attempt to recover from the nonlocal exit.  The
following @acronym{API} functions will help with these tasks.

@deftypefn Function void non_local_exit_clear (emacs_env *@var{env})
This function clears the pending nonlocal exit conditions and data
from @var{env}.  After calling it, the module @acronym{API} functions
will work normally.  Use this function if your module function can
recover from nonlocal exits of the Lisp functions it calls and
continue, and also before calling any of the following two functions
(or any other @acronym{API} functions, if you want them to perform
their intended processing when a nonlocal exit is pending).
@end deftypefn

@deftypefn Function void non_local_exit_throw (emacs_env *@var{env}, emacs_value @var{tag}, emacs_value @var{value})
This function throws to the Lisp @code{catch} symbol represented by
@var{tag}, passing it @var{value} as the value to return.  Your module
function should in general return soon after calling this function.
One use of this function is when you want to re-throw a non-local exit
from one of the called @acronym{API} or Lisp functions.
@end deftypefn

@deftypefn Function void non_local_exit_signal (emacs_env *@var{env}, emacs_value @var{error}, emacs_value @var{data})
This function signals the error represented by @var{error} with the
specified error data @var{data}.  The module function should return
soon after calling this function.  This function could be useful,
e.g., for signaling errors from module functions to Emacs.
@end deftypefn


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

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

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

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

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

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

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

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

@item struct Lisp_Float
Floating-point value.
@end table

  These types are the first-class citizens of an internal type system.
Since the tag space is limited, all other types are the subtypes of
@code{Lisp_Vectorlike}.  Vector subtypes are enumerated
by @code{enum pvec_type}, and nearly all complex objects like windows, buffers,
frames, and processes fall into this category.

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

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

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

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

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

@table @code
@item beg
The address of the buffer contents.  The buffer contents is a linear C
array of @code{char}, with the gap somewhere in its midst.

@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 (such as text properties); and @code{overlay_modiff}
counts only modifications to the buffer's overlays.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@item name
A Lisp string that names the buffer.  It is guaranteed to be unique.
@xref{Buffer Names}.  This and the following fields have their names
in the C struct definition end in a @code{_} to indicate that they
should not be accessed directly, but via the @code{BVAR} macro, like
this:

@example
  Lisp_Object buf_name = BVAR (buffer, name);
@end example

@item save_length
The length of the file this buffer is visiting, when last read or
saved.  It can have 2 special values: @minus{}1 means auto-saving was
turned off in this buffer, and @minus{}2 means don't turn off
auto-saving if buffer text shrinks a lot.  This and other fields
concerned with saving are not kept in the @code{buffer_text} structure
because indirect buffers are never saved.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@table @code
@item frame
The frame that this window is on, as a Lisp object.

@item mini
Non-zero if this window is a minibuffer window, a window showing the
minibuffer or the echo area.

@item pseudo_window_p
@cindex pseudo window
Non-zero if this window is a @dfn{pseudo window}.  A pseudo window is
either a window used to display the menu bar or the tool bar (when
Emacs uses toolkits that don't display their own menu bar and tool
bar) or a window showing a tooltip on a tooltip frame.  Pseudo windows
are in general not accessible from Lisp code.

@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 the window's parent in that tree, as a Lisp object.  For the
root window of the tree and a minibuffer window this is always
@code{nil}.

Parent windows do not display buffers, and play little role in display
except to shape their child windows.  Emacs Lisp programs cannot
directly manipulate parent windows; they operate on the windows at the
leaves of the tree, which actually display buffers.

@item contents
For a leaf window and windows showing a tooltip, this is the buffer,
as a Lisp object, that the window is displaying.  For an internal
(``parent'') window, this is its first child window.  For a pseudo
window showing a menu or tool bar this is @code{nil}.  It is also
@code{nil} for a window that has been deleted.

@item next
@itemx prev
The next and previous sibling of this window as Lisp objects.
@code{next} is @code{nil} if the window is the right-most or
bottom-most in its group; @code{prev} is @code{nil} if it is the
left-most or top-most in its group.  Whether the sibling is left/right
or up/down is determined by the @code{horizontal} field of the
sibling's parent: if it's non-zero, the siblings are arranged
horizontally.

As a special case, @code{next} of a frame's root window points to the
frame's minibuffer window, provided this is not a minibuffer-only or
minibuffer-less frame.  On such frames @code{prev} of the minibuffer
window points to that frame's root window.  In any other case, the
root window's @code{next} and the minibuffer window's (if present)
@code{prev} fields are @code{nil}.

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

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

@item pixel_left
@itemx pixel_top
The left-hand and top edges of this window, measured in pixels,
relative to the top-left corner (0, 0) of the window's native frame.

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

@item pixel_width;
@itemx pixel_height;
The total width and height of the window measured in pixels.

@item start
A marker pointing to the position in the buffer that is the first
character (in the logical order, @pxref{Bidirectional Display})
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 old_pointm
The value of @code{pointm} at the last redisplay time.

@item force_start
If this flag is non-@code{nil}, it says that the window has been
scrolled explicitly by the Lisp program, and the value of the the
window's @code{start} was set for redisplay to honor.  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 optional_new_start
This is similar to @code{force_start}, but the next redisplay will
only obey it if point stays visible.

@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-zero value means the window's buffer was modified when the
window was last updated.

@item vertical_scroll_bar_type
@itemx horizontal_scroll_bar_type
The types of this window's vertical and horizontal scroll bars.

@item scroll_bar_width
@itemx scroll_bar_height
The width of this window's vertical scroll bar and the height of this
window's horizontal scroll bar, in pixels.

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

@item left_fringe_width
@itemx right_fringe_width
The pixel widths of the left and right fringes in this window.  A
value of @minus{}1 means use the values of the frame.

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

@item window_end_pos
This is computed as @code{z} minus the buffer position of the last glyph
in the current matrix of the window.  The value is only valid if
@code{window_end_valid} is non-zero.

@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-zero value if @code{window_end_pos} and
@code{window_end_vpos} are truly valid.  This is zero if nontrivial
redisplay is pre-empted, since in that case the display that
@code{window_end_pos} was computed for did not get onto the screen.

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

@item last_cursor_vpos
The window-relative vertical position of the line showing the cursor
as of the last redisplay that finished.

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

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

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

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

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

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

@item hscroll
This is the number of columns that the display in the window is
scrolled horizontally to the left.  Normally, this is 0.  When only
the current line is hscrolled, this describes how much the current
line is scrolled.

@item min_hscroll
Minimum value of @code{hscroll}, set by the user via
@code{set-window-hscroll} (@pxref{Horizontal Scrolling}).  When only
the current line is hscrolled, this describes the horizontal scrolling
of lines other than the current one.

@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 combination_limit
This window's combination limit, meaningful only for a parent window.
If this is @code{t}, then it is not allowed to delete this window and
recombine its child windows with other siblings of this window.

@item window_parameters
The alist of this window's parameters.

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

@item update_mode_line
Non-zero means this window's mode line needs to be updated.

@item mode_line_height
@itemx header_line_height
The height in pixels of the mode line and the header line, or
@minus{}1 if not known.

@item base_line_number
The line number of a certain position in the buffer, or zero.
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
zero meaning none is known.  If it is @minus{}1, don't display
the line number as long as the window shows that buffer.

@item column_number_displayed
The column number currently displayed in this window's mode line, or
@minus{}1 if column numbers are not being displayed.

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

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

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

@table @code
@item name
A Lisp 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 Lisp function used to accept output from the process.

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

@item buffer
The associated buffer of the process.

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

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

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

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

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

@item status
The process status, as @code{process-status} should return it.  This
is a Lisp symbol, a cons cell, or a list.

@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-zero if communication with the subprocess uses a pty; zero if it
uses a pipe.

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

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

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

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

@item decoding_buf
A working buffer for decoding.

@item decoding_carryover
Size of carryover in decoding.

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

@item encoding_buf
A working buffer for encoding.

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

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

@end table

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

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

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

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

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

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

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

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

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

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

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

@item
Prefer @code{intmax_t} for representing values that might be any
signed integer value.
A @code{printf}-family function can print such a value
via a format like @code{"%"PRIdMAX}.

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

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

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