2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1993, 1998-1999, 2001-2012 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
5 @setfilename ../../info/internals
6 @node GNU Emacs Internals, Standard Errors, Tips, Top
7 @comment node-name, next, previous, up
8 @appendix GNU Emacs Internals
10 This chapter describes how the runnable Emacs executable is dumped with
11 the preloaded Lisp libraries in it, how storage is allocated, and some
12 internal aspects of GNU Emacs that may be of interest to C programmers.
15 * Building Emacs:: How the dumped Emacs is made.
16 * Pure Storage:: Kludge to make preloaded Lisp functions shareable.
17 * Garbage Collection:: Reclaiming space for Lisp objects no longer used.
18 * Memory Usage:: Info about total size of Lisp objects made so far.
19 * Writing Emacs Primitives:: Writing C code for Emacs.
20 * Object Internals:: Data formats of buffers, windows, processes.
24 @section Building Emacs
25 @cindex building Emacs
28 This section explains the steps involved in building the Emacs
29 executable. You don't have to know this material to build and install
30 Emacs, since the makefiles do all these things automatically. This
31 information is pertinent to Emacs maintenance.
33 Compilation of the C source files in the @file{src} directory
34 produces an executable file called @file{temacs}, also called a
35 @dfn{bare impure Emacs}. It contains the Emacs Lisp interpreter and I/O
36 routines, but not the editing commands.
38 @cindex @file{loadup.el}
39 The command @w{@samp{temacs -l loadup}} uses @file{temacs} to create
40 the real runnable Emacs executable. These arguments direct
41 @file{temacs} to evaluate the Lisp files specified in the file
42 @file{loadup.el}. These files set up the normal Emacs editing
43 environment, resulting in an Emacs that is still impure but no longer
47 It takes some time to load the standard Lisp files. Luckily,
48 you don't have to do this each time you run Emacs; @file{temacs} can
49 dump out an executable program called @file{emacs} that has these files
50 preloaded. @file{emacs} starts more quickly because it does not need to
51 load the files. This is the Emacs executable that is normally
54 @vindex preloaded-file-list
55 @cindex dumped Lisp files
56 To create @file{emacs}, use the command @samp{temacs -batch -l loadup
57 dump}. The purpose of @samp{-batch} here is to prevent @file{temacs}
58 from trying to initialize any of its data on the terminal; this ensures
59 that the tables of terminal information are empty in the dumped Emacs.
60 The argument @samp{dump} tells @file{loadup.el} to dump a new executable
61 named @file{emacs}. The variable @code{preloaded-file-list} stores a
62 list of the Lisp files that were dumped with the @file{emacs} executable.
64 If you port Emacs to a new operating system, and are not able to
65 implement dumping, then Emacs must load @file{loadup.el} each time it
68 @cindex @file{site-load.el}
69 You can specify additional files to preload by writing a library named
70 @file{site-load.el} that loads them. You may need to rebuild Emacs
71 with an added definition
74 #define SITELOAD_PURESIZE_EXTRA @var{n}
78 to make @var{n} added bytes of pure space to hold the additional files;
79 see @file{src/puresize.h}.
80 (Try adding increments of 20000 until it is big enough.) However, the
81 advantage of preloading additional files decreases as machines get
82 faster. On modern machines, it is usually not advisable.
84 After @file{loadup.el} reads @file{site-load.el}, it finds the
85 documentation strings for primitive and preloaded functions (and
86 variables) in the file @file{etc/DOC} where they are stored, by
87 calling @code{Snarf-documentation} (@pxref{Definition of
88 Snarf-documentation,, Accessing Documentation}).
90 @cindex @file{site-init.el}
91 @cindex preloading additional functions and variables
92 You can specify other Lisp expressions to execute just before dumping
93 by putting them in a library named @file{site-init.el}. This file is
94 executed after the documentation strings are found.
96 If you want to preload function or variable definitions, there are
97 three ways you can do this and make their documentation strings
98 accessible when you subsequently run Emacs:
102 Arrange to scan these files when producing the @file{etc/DOC} file,
103 and load them with @file{site-load.el}.
106 Load the files with @file{site-init.el}, then copy the files into the
107 installation directory for Lisp files when you install Emacs.
110 Specify a @code{nil} value for @code{byte-compile-dynamic-docstrings}
111 as a local variable in each of these files, and load them with either
112 @file{site-load.el} or @file{site-init.el}. (This method has the
113 drawback that the documentation strings take up space in Emacs all the
117 It is not advisable to put anything in @file{site-load.el} or
118 @file{site-init.el} that would alter any of the features that users
119 expect in an ordinary unmodified Emacs. If you feel you must override
120 normal features for your site, do it with @file{default.el}, so that
121 users can override your changes if they wish. @xref{Startup Summary}.
123 In a package that can be preloaded, it is sometimes necessary (or
124 useful) to delay certain evaluations until Emacs subsequently starts
125 up. The vast majority of such cases relate to the values of
126 customizable variables. For example, @code{tutorial-directory} is a
127 variable defined in @file{startup.el}, which is preloaded. The default
128 value is set based on @code{data-directory}. The variable needs to
129 access the value of @code{data-directory} when Emacs starts, not when
130 it is dumped, because the Emacs executable has probably been installed
131 in a different location since it was dumped.
133 @defun custom-initialize-delay symbol value
134 This function delays the initialization of @var{symbol} to the next
135 Emacs start. You normally use this function by specifying it as the
136 @code{:initialize} property of a customizable variable. (The argument
137 @var{value} is unused, and is provided only for compatiblity with the
138 form Custom expects.)
141 In the unlikely event that you need a more general functionality than
142 @code{custom-initialize-delay} provides, you can use
143 @code{before-init-hook} (@pxref{Startup Summary}).
145 @defun dump-emacs to-file from-file
147 This function dumps the current state of Emacs into an executable file
148 @var{to-file}. It takes symbols from @var{from-file} (this is normally
149 the executable file @file{temacs}).
151 If you want to use this function in an Emacs that was already dumped,
152 you must run Emacs with @samp{-batch}.
156 @section Pure Storage
159 Emacs Lisp uses two kinds of storage for user-created Lisp objects:
160 @dfn{normal storage} and @dfn{pure storage}. Normal storage is where
161 all the new data created during an Emacs session are kept
162 (@pxref{Garbage Collection}). Pure storage is used for certain data
163 in the preloaded standard Lisp files---data that should never change
164 during actual use of Emacs.
166 Pure storage is allocated only while @file{temacs} is loading the
167 standard preloaded Lisp libraries. In the file @file{emacs}, it is
168 marked as read-only (on operating systems that permit this), so that
169 the memory space can be shared by all the Emacs jobs running on the
170 machine at once. Pure storage is not expandable; a fixed amount is
171 allocated when Emacs is compiled, and if that is not sufficient for
172 the preloaded libraries, @file{temacs} allocates dynamic memory for
173 the part that didn't fit. The resulting image will work, but garbage
174 collection (@pxref{Garbage Collection}) is disabled in this situation,
175 causing a memory leak. Such an overflow normally won't happen unless
176 you try to preload additional libraries or add features to the
177 standard ones. Emacs will display a warning about the overflow when
178 it starts. If this happens, you should increase the compilation
179 parameter @code{SYSTEM_PURESIZE_EXTRA} in the file
180 @file{src/puresize.h} and rebuild Emacs.
182 @defun purecopy object
183 This function makes a copy in pure storage of @var{object}, and returns
184 it. It copies a string by simply making a new string with the same
185 characters, but without text properties, in pure storage. It
186 recursively copies the contents of vectors and cons cells. It does
187 not make copies of other objects such as symbols, but just returns
188 them unchanged. It signals an error if asked to copy markers.
190 This function is a no-op except while Emacs is being built and dumped;
191 it is usually called only in preloaded Lisp files.
194 @defvar pure-bytes-used
195 The value of this variable is the number of bytes of pure storage
196 allocated so far. Typically, in a dumped Emacs, this number is very
197 close to the total amount of pure storage available---if it were not,
198 we would preallocate less.
202 This variable determines whether @code{defun} should make a copy of the
203 function definition in pure storage. If it is non-@code{nil}, then the
204 function definition is copied into pure storage.
206 This flag is @code{t} while loading all of the basic functions for
207 building Emacs initially (allowing those functions to be shareable and
208 non-collectible). Dumping Emacs as an executable always writes
209 @code{nil} in this variable, regardless of the value it actually has
210 before and after dumping.
212 You should not change this flag in a running Emacs.
215 @node Garbage Collection
216 @section Garbage Collection
217 @cindex garbage collection
219 @cindex memory allocation
220 When a program creates a list or the user defines a new function (such
221 as by loading a library), that data is placed in normal storage. If
222 normal storage runs low, then Emacs asks the operating system to
223 allocate more memory in blocks of 1k bytes. Each block is used for one
224 type of Lisp object, so symbols, cons cells, markers, etc., are
225 segregated in distinct blocks in memory. (Vectors, long strings,
226 buffers and certain other editing types, which are fairly large, are
227 allocated in individual blocks, one per object, while small strings are
228 packed into blocks of 8k bytes.)
230 It is quite common to use some storage for a while, then release it by
231 (for example) killing a buffer or deleting the last pointer to an
232 object. Emacs provides a @dfn{garbage collector} to reclaim this
233 abandoned storage. (This name is traditional, but ``garbage recycler''
234 might be a more intuitive metaphor for this facility.)
236 The garbage collector operates by finding and marking all Lisp objects
237 that are still accessible to Lisp programs. To begin with, it assumes
238 all the symbols, their values and associated function definitions, and
239 any data presently on the stack, are accessible. Any objects that can
240 be reached indirectly through other accessible objects are also
243 When marking is finished, all objects still unmarked are garbage. No
244 matter what the Lisp program or the user does, it is impossible to refer
245 to them, since there is no longer a way to reach them. Their space
246 might as well be reused, since no one will miss them. The second
247 (``sweep'') phase of the garbage collector arranges to reuse them.
249 @c ??? Maybe add something describing weak hash tables here?
252 The sweep phase puts unused cons cells onto a @dfn{free list}
253 for future allocation; likewise for symbols and markers. It compacts
254 the accessible strings so they occupy fewer 8k blocks; then it frees the
255 other 8k blocks. Vectors, buffers, windows, and other large objects are
256 individually allocated and freed using @code{malloc} and @code{free}.
258 @cindex CL note---allocate more storage
260 @b{Common Lisp note:} Unlike other Lisps, GNU Emacs Lisp does not
261 call the garbage collector when the free list is empty. Instead, it
262 simply requests the operating system to allocate more storage, and
263 processing continues until @code{gc-cons-threshold} bytes have been
266 This means that you can make sure that the garbage collector will not
267 run during a certain portion of a Lisp program by calling the garbage
268 collector explicitly just before it (provided that portion of the
269 program does not use so much space as to force a second garbage
273 @deffn Command garbage-collect
274 This command runs a garbage collection, and returns information on
275 the amount of space in use. (Garbage collection can also occur
276 spontaneously if you use more than @code{gc-cons-threshold} bytes of
277 Lisp data since the previous garbage collection.)
279 @code{garbage-collect} returns a list containing the following
284 ((@var{used-conses} . @var{free-conses})
285 (@var{used-syms} . @var{free-syms})
287 (@var{used-miscs} . @var{free-miscs})
288 @var{used-string-chars}
289 @var{used-vector-slots}
290 (@var{used-floats} . @var{free-floats})
291 (@var{used-intervals} . @var{free-intervals})
292 (@var{used-strings} . @var{free-strings}))
300 @result{} ((106886 . 13184) (9769 . 0)
301 (7731 . 4651) 347543 121628
302 (31 . 94) (1273 . 168)
307 Here is a table explaining each element:
311 The number of cons cells in use.
314 The number of cons cells for which space has been obtained from the
315 operating system, but that are not currently being used.
318 The number of symbols in use.
321 The number of symbols for which space has been obtained from the
322 operating system, but that are not currently being used.
325 The number of miscellaneous objects in use. These include markers and
326 overlays, plus certain objects not visible to users.
329 The number of miscellaneous objects for which space has been obtained
330 from the operating system, but that are not currently being used.
332 @item used-string-chars
333 The total size of all strings, in characters.
335 @item used-vector-slots
336 The total number of elements of existing vectors.
340 The number of floats in use.
344 The number of floats for which space has been obtained from the
345 operating system, but that are not currently being used.
348 The number of intervals in use. Intervals are an internal
349 data structure used for representing text properties.
352 The number of intervals for which space has been obtained
353 from the operating system, but that are not currently being used.
356 The number of strings in use.
359 The number of string headers for which the space was obtained from the
360 operating system, but which are currently not in use. (A string
361 object consists of a header and the storage for the string text
362 itself; the latter is only allocated when the string is created.)
365 If there was overflow in pure space (@pxref{Pure Storage}),
366 @code{garbage-collect} returns @code{nil}, because a real garbage
367 collection can not be done in this situation.
370 @defopt garbage-collection-messages
371 If this variable is non-@code{nil}, Emacs displays a message at the
372 beginning and end of garbage collection. The default value is
377 This is a normal hook that is run at the end of garbage collection.
378 Garbage collection is inhibited while the hook functions run, so be
379 careful writing them.
382 @defopt gc-cons-threshold
383 The value of this variable is the number of bytes of storage that must
384 be allocated for Lisp objects after one garbage collection in order to
385 trigger another garbage collection. A cons cell counts as eight bytes,
386 a string as one byte per character plus a few bytes of overhead, and so
387 on; space allocated to the contents of buffers does not count. Note
388 that the subsequent garbage collection does not happen immediately when
389 the threshold is exhausted, but only the next time the Lisp evaluator is
392 The initial threshold value is 800,000. If you specify a larger
393 value, garbage collection will happen less often. This reduces the
394 amount of time spent garbage collecting, but increases total memory use.
395 You may want to do this when running a program that creates lots of
398 You can make collections more frequent by specifying a smaller value,
399 down to 10,000. A value less than 10,000 will remain in effect only
400 until the subsequent garbage collection, at which time
401 @code{garbage-collect} will set the threshold back to 10,000.
404 @defopt gc-cons-percentage
405 The value of this variable specifies the amount of consing before a
406 garbage collection occurs, as a fraction of the current heap size.
407 This criterion and @code{gc-cons-threshold} apply in parallel, and
408 garbage collection occurs only when both criteria are satisfied.
410 As the heap size increases, the time to perform a garbage collection
411 increases. Thus, it can be desirable to do them less frequently in
415 The value returned by @code{garbage-collect} describes the amount of
416 memory used by Lisp data, broken down by data type. By contrast, the
417 function @code{memory-limit} provides information on the total amount of
418 memory Emacs is currently using.
422 This function returns the address of the last byte Emacs has allocated,
423 divided by 1024. We divide the value by 1024 to make sure it fits in a
426 You can use this to get a general idea of how your actions affect the
431 This variable is @code{t} if Emacs is close to out of memory for Lisp
432 objects, and @code{nil} otherwise.
435 @defun memory-use-counts
436 This returns a list of numbers that count the number of objects
437 created in this Emacs session. Each of these counters increments for
438 a certain kind of object. See the documentation string for details.
442 This variable contains the total number of garbage collections
443 done so far in this Emacs session.
447 This variable contains the total number of seconds of elapsed time
448 during garbage collection so far in this Emacs session, as a floating
453 @section Memory Usage
456 These functions and variables give information about the total amount
457 of memory allocation that Emacs has done, broken down by data type.
458 Note the difference between these and the values returned by
459 @code{garbage-collect}; those count objects that currently exist, but
460 these count the number or size of all allocations, including those for
461 objects that have since been freed.
463 @defvar cons-cells-consed
464 The total number of cons cells that have been allocated so far
465 in this Emacs session.
468 @defvar floats-consed
469 The total number of floats that have been allocated so far
470 in this Emacs session.
473 @defvar vector-cells-consed
474 The total number of vector cells that have been allocated so far
475 in this Emacs session.
478 @defvar symbols-consed
479 The total number of symbols that have been allocated so far
480 in this Emacs session.
483 @defvar string-chars-consed
484 The total number of string characters that have been allocated so far
485 in this Emacs session.
488 @defvar misc-objects-consed
489 The total number of miscellaneous objects that have been allocated so
490 far in this Emacs session. These include markers and overlays, plus
491 certain objects not visible to users.
494 @defvar intervals-consed
495 The total number of intervals that have been allocated so far
496 in this Emacs session.
499 @defvar strings-consed
500 The total number of strings that have been allocated so far in this
504 @node Writing Emacs Primitives
505 @section Writing Emacs Primitives
506 @cindex primitive function internals
507 @cindex writing Emacs primitives
509 Lisp primitives are Lisp functions implemented in C. The details of
510 interfacing the C function so that Lisp can call it are handled by a few
511 C macros. The only way to really understand how to write new C code is
512 to read the source, but we can explain some things here.
514 An example of a special form is the definition of @code{or}, from
515 @file{eval.c}. (An ordinary function would have the same general
518 @cindex garbage collection protection
521 DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
522 doc: /* Eval args until one of them yields non-nil, then return that
523 value. The remaining args are not evalled at all.
524 If all args return nil, return nil.
527 usage: (or CONDITIONS ...) */)
530 register Lisp_Object val = Qnil;
541 val = Feval (XCAR (args));
555 @cindex @code{DEFUN}, C macro to define Lisp primitives
556 Let's start with a precise explanation of the arguments to the
557 @code{DEFUN} macro. Here is a template for them:
560 DEFUN (@var{lname}, @var{fname}, @var{sname}, @var{min}, @var{max}, @var{interactive}, @var{doc})
565 This is the name of the Lisp symbol to define as the function name; in
566 the example above, it is @code{or}.
569 This is the C function name for this function. This is
570 the name that is used in C code for calling the function. The name is,
571 by convention, @samp{F} prepended to the Lisp name, with all dashes
572 (@samp{-}) in the Lisp name changed to underscores. Thus, to call this
573 function from C code, call @code{For}. Remember that the arguments must
574 be of type @code{Lisp_Object}; various macros and functions for creating
575 values of type @code{Lisp_Object} are declared in the file
579 This is a C variable name to use for a structure that holds the data for
580 the subr object that represents the function in Lisp. This structure
581 conveys the Lisp symbol name to the initialization routine that will
582 create the symbol and store the subr object as its definition. By
583 convention, this name is always @var{fname} with @samp{F} replaced with
587 This is the minimum number of arguments that the function requires. The
588 function @code{or} allows a minimum of zero arguments.
591 This is the maximum number of arguments that the function accepts, if
592 there is a fixed maximum. Alternatively, it can be @code{UNEVALLED},
593 indicating a special form that receives unevaluated arguments, or
594 @code{MANY}, indicating an unlimited number of evaluated arguments (the
595 equivalent of @code{&rest}). Both @code{UNEVALLED} and @code{MANY} are
596 macros. If @var{max} is a number, it may not be less than @var{min} and
597 it may not be greater than eight.
600 This is an interactive specification, a string such as might be used as
601 the argument of @code{interactive} in a Lisp function. In the case of
602 @code{or}, it is 0 (a null pointer), indicating that @code{or} cannot be
603 called interactively. A value of @code{""} indicates a function that
604 should receive no arguments when called interactively. If the value
605 begins with a @samp{(}, the string is evaluated as a Lisp form.
608 This is the documentation string. It uses C comment syntax rather
609 than C string syntax because comment syntax requires nothing special
610 to include multiple lines. The @samp{doc:} identifies the comment
611 that follows as the documentation string. The @samp{/*} and @samp{*/}
612 delimiters that begin and end the comment are not part of the
613 documentation string.
615 If the last line of the documentation string begins with the keyword
616 @samp{usage:}, the rest of the line is treated as the argument list
617 for documentation purposes. This way, you can use different argument
618 names in the documentation string from the ones used in the C code.
619 @samp{usage:} is required if the function has an unlimited number of
622 All the usual rules for documentation strings in Lisp code
623 (@pxref{Documentation Tips}) apply to C code documentation strings
627 After the call to the @code{DEFUN} macro, you must write the
628 argument list that every C function must have, including the types for
629 the arguments. For a function with a fixed maximum number of
630 arguments, declare a C argument for each Lisp argument, and give them
631 all type @code{Lisp_Object}. When a Lisp function has no upper limit
632 on the number of arguments, its implementation in C actually receives
633 exactly two arguments: the first is the number of Lisp arguments, and
634 the second is the address of a block containing their values. They
635 have types @code{int} and @w{@code{Lisp_Object *}}.
637 @cindex @code{GCPRO} and @code{UNGCPRO}
638 @cindex protect C variables from garbage collection
639 Within the function @code{For} itself, note the use of the macros
640 @code{GCPRO1} and @code{UNGCPRO}. @code{GCPRO1} is used to
641 ``protect'' a variable from garbage collection---to inform the garbage
642 collector that it must look in that variable and regard its contents
643 as an accessible object. GC protection is necessary whenever you call
644 @code{Feval} or anything that can directly or indirectly call
645 @code{Feval}. At such a time, any Lisp object that this function may
646 refer to again must be protected somehow.
648 It suffices to ensure that at least one pointer to each object is
649 GC-protected; that way, the object cannot be recycled, so all pointers
650 to it remain valid. Thus, a particular local variable can do without
651 protection if it is certain that the object it points to will be
652 preserved by some other pointer (such as another local variable which
653 has a @code{GCPRO})@footnote{Formerly, strings were a special
654 exception; in older Emacs versions, every local variable that might
655 point to a string needed a @code{GCPRO}.}. Otherwise, the local
656 variable needs a @code{GCPRO}.
658 The macro @code{GCPRO1} protects just one local variable. If you
659 want to protect two variables, use @code{GCPRO2} instead; repeating
660 @code{GCPRO1} will not work. Macros @code{GCPRO3}, @code{GCPRO4},
661 @code{GCPRO5}, and @code{GCPRO6} also exist. All these macros
662 implicitly use local variables such as @code{gcpro1}; you must declare
663 these explicitly, with type @code{struct gcpro}. Thus, if you use
664 @code{GCPRO2}, you must declare @code{gcpro1} and @code{gcpro2}.
665 Alas, we can't explain all the tricky details here.
667 @code{UNGCPRO} cancels the protection of the variables that are
668 protected in the current function. It is necessary to do this
671 Built-in functions that take a variable number of arguments actually
672 accept two arguments at the C level: the number of Lisp arguments, and
673 a @code{Lisp_Object *} pointer to a C vector containing those Lisp
674 arguments. This C vector may be part of a Lisp vector, but it need
675 not be. The responsibility for using @code{GCPRO} to protect the Lisp
676 arguments from GC if necessary rests with the caller in this case,
677 since the caller allocated or found the storage for them.
679 You must not use C initializers for static or global variables unless
680 the variables are never written once Emacs is dumped. These variables
681 with initializers are allocated in an area of memory that becomes
682 read-only (on certain operating systems) as a result of dumping Emacs.
685 Do not use static variables within functions---place all static
686 variables at top level in the file. This is necessary because Emacs on
687 some operating systems defines the keyword @code{static} as a null
688 macro. (This definition is used because those systems put all variables
689 declared static in a place that becomes read-only after dumping, whether
690 they have initializers or not.)
692 @cindex @code{defsubr}, Lisp symbol for a primitive
693 Defining the C function is not enough to make a Lisp primitive
694 available; you must also create the Lisp symbol for the primitive and
695 store a suitable subr object in its function cell. The code looks like
699 defsubr (&@var{subr-structure-name});
703 Here @var{subr-structure-name} is the name you used as the third
704 argument to @code{DEFUN}.
706 If you add a new primitive to a file that already has Lisp primitives
707 defined in it, find the function (near the end of the file) named
708 @code{syms_of_@var{something}}, and add the call to @code{defsubr}
709 there. If the file doesn't have this function, or if you create a new
710 file, add to it a @code{syms_of_@var{filename}} (e.g.,
711 @code{syms_of_myfile}). Then find the spot in @file{emacs.c} where all
712 of these functions are called, and add a call to
713 @code{syms_of_@var{filename}} there.
715 @anchor{Defining Lisp variables in C}
716 @vindex byte-boolean-vars
717 @cindex defining Lisp variables in C
718 @cindex @code{DEFVAR_INT}, @code{DEFVAR_LISP}, @code{DEFVAR_BOOL}
719 The function @code{syms_of_@var{filename}} is also the place to define
720 any C variables that are to be visible as Lisp variables.
721 @code{DEFVAR_LISP} makes a C variable of type @code{Lisp_Object} visible
722 in Lisp. @code{DEFVAR_INT} makes a C variable of type @code{int}
723 visible in Lisp with a value that is always an integer.
724 @code{DEFVAR_BOOL} makes a C variable of type @code{int} visible in Lisp
725 with a value that is either @code{t} or @code{nil}. Note that variables
726 defined with @code{DEFVAR_BOOL} are automatically added to the list
727 @code{byte-boolean-vars} used by the byte compiler.
729 @cindex @code{staticpro}, protection from GC
730 If you define a file-scope C variable of type @code{Lisp_Object},
731 you must protect it from garbage-collection by calling @code{staticpro}
732 in @code{syms_of_@var{filename}}, like this:
735 staticpro (&@var{variable});
738 Here is another example function, with more complicated arguments.
739 This comes from the code in @file{window.c}, and it demonstrates the use
740 of macros and functions to manipulate Lisp objects.
744 DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
745 Scoordinates_in_window_p, 2, 2,
746 "xSpecify coordinate pair: \nXExpression which evals to window: ",
747 "Return non-nil if COORDINATES is in WINDOW.\n\
748 COORDINATES is a cons of the form (X . Y), X and Y being distances\n\
752 If they are on the border between WINDOW and its right sibling,\n\
753 `vertical-line' is returned.")
754 (coordinates, window)
755 register Lisp_Object coordinates, window;
761 CHECK_LIVE_WINDOW (window, 0);
762 CHECK_CONS (coordinates, 1);
763 x = XINT (Fcar (coordinates));
764 y = XINT (Fcdr (coordinates));
768 switch (coordinates_in_window (XWINDOW (window), &x, &y))
770 case 0: /* NOT in window at all. */
775 case 1: /* In text part of window. */
776 return Fcons (make_number (x), make_number (y));
780 case 2: /* In mode line of window. */
785 case 3: /* On right border of window. */
786 return Qvertical_line;
797 Note that C code cannot call functions by name unless they are defined
798 in C. The way to call a function written in Lisp is to use
799 @code{Ffuncall}, which embodies the Lisp function @code{funcall}. Since
800 the Lisp function @code{funcall} accepts an unlimited number of
801 arguments, in C it takes two: the number of Lisp-level arguments, and a
802 one-dimensional array containing their values. The first Lisp-level
803 argument is the Lisp function to call, and the rest are the arguments to
804 pass to it. Since @code{Ffuncall} can call the evaluator, you must
805 protect pointers from garbage collection around the call to
808 The C functions @code{call0}, @code{call1}, @code{call2}, and so on,
809 provide handy ways to call a Lisp function conveniently with a fixed
810 number of arguments. They work by calling @code{Ffuncall}.
812 @file{eval.c} is a very good file to look through for examples;
813 @file{lisp.h} contains the definitions for some important macros and
816 If you define a function which is side-effect free, update the code
817 in @file{byte-opt.el} which binds @code{side-effect-free-fns} and
818 @code{side-effect-and-error-free-fns} so that the compiler optimizer
821 @node Object Internals
822 @section Object Internals
823 @cindex object internals
825 GNU Emacs Lisp manipulates many different types of data. The actual
826 data are stored in a heap and the only access that programs have to it
827 is through pointers. Each pointer is 32 bits wide on 32-bit machines,
828 and 64 bits wide on 64-bit machines; three of these bits are used for
829 the tag that identifies the object's type, and the remainder are used
830 to address the object.
832 Because Lisp objects are represented as tagged pointers, it is always
833 possible to determine the Lisp data type of any object. The C data type
834 @code{Lisp_Object} can hold any Lisp object of any data type. Ordinary
835 variables have type @code{Lisp_Object}, which means they can hold any
836 type of Lisp value; you can determine the actual data type only at run
837 time. The same is true for function arguments; if you want a function
838 to accept only a certain type of argument, you must check the type
839 explicitly using a suitable predicate (@pxref{Type Predicates}).
840 @cindex type checking internals
843 * Buffer Internals:: Components of a buffer structure.
844 * Window Internals:: Components of a window structure.
845 * Process Internals:: Components of a process structure.
848 @node Buffer Internals
849 @subsection Buffer Internals
850 @cindex internals, of buffer
851 @cindex buffer internals
853 Two structures are used to represent buffers in C. The
854 @code{buffer_text} structure contains fields describing the text of a
855 buffer; the @code{buffer} structure holds other fields. In the case
856 of indirect buffers, two or more @code{buffer} structures reference
857 the same @code{buffer_text} structure.
859 Here are some of the fields in @code{struct buffer_text}:
863 The address of the buffer contents.
867 The character and byte positions of the buffer gap. @xref{Buffer
872 The character and byte positions of the end of the buffer text.
875 The size of buffer's gap. @xref{Buffer Gap}.
880 @itemx overlay_modiff
881 These fields count the number of buffer-modification events performed
882 in this buffer. @code{modiff} is incremented after each
883 buffer-modification event, and is never otherwise changed;
884 @code{save_modiff} contains the value of @code{modiff} the last time
885 the buffer was visited or saved; @code{chars_modiff} counts only
886 modifications to the characters in the buffer, ignoring all other
887 kinds of changes; and @code{overlay_modiff} counts only modifications
892 The number of characters at the start and end of the text that are
893 known to be unchanged since the last complete redisplay.
895 @item unchanged_modified
896 @itemx overlay_unchanged_modified
897 The values of @code{modiff} and @code{overlay_modiff}, respectively,
898 after the last complete redisplay. If their current values match
899 @code{modiff} or @code{overlay_modiff}, that means
900 @code{beg_unchanged} and @code{end_unchanged} contain no useful
904 The markers that refer to this buffer. This is actually a single
905 marker, and successive elements in its marker @code{chain} are the other
906 markers referring to this buffer text.
909 The interval tree which records the text properties of this buffer.
912 Some of the fields of @code{struct buffer} are:
916 Points to the next buffer, in the chain of all buffers (including
917 killed buffers). This chain is used only for garbage collection, in
918 order to collect killed buffers properly. Note that vectors, and most
919 kinds of objects allocated as vectors, are all on one chain, but
920 buffers are on a separate chain of their own.
923 A @code{struct buffer_text} structure that ordinarily holds the buffer
924 contents. In indirect buffers, this field is not used.
927 A pointer to the @code{buffer_text} structure for this buffer. In an
928 ordinary buffer, this is the @code{own_text} field above. In an
929 indirect buffer, this is the @code{own_text} field of the base buffer.
933 The character and byte positions of point in a buffer.
937 The character and byte positions of the beginning of the accessible
938 range of text in the buffer.
942 The character and byte positions of the end of the accessible range of
946 In an indirect buffer, this points to the base buffer. In an ordinary
950 This field contains flags indicating that certain variables are local
951 in this buffer. Such variables are declared in the C code using
952 @code{DEFVAR_PER_BUFFER}, and their buffer-local bindings are stored
953 in fields in the buffer structure itself. (Some of these fields are
954 described in this table.)
957 The modification time of the visited file. It is set when the file is
958 written or read. Before writing the buffer into a file, this field is
959 compared to the modification time of the file to see if the file has
960 changed on disk. @xref{Buffer Modification}.
962 @item auto_save_modified
963 The time when the buffer was last auto-saved.
965 @item last_window_start
966 The @code{window-start} position in the buffer as of the last time the
967 buffer was displayed in a window.
970 This flag indicates that narrowing has changed in the buffer.
973 @item prevent_redisplay_optimizations_p
974 This flag indicates that redisplay optimizations should not be used to
978 This field holds the current overlay center position. @xref{Managing
981 @item overlays_before
982 @itemx overlays_after
983 These fields hold, respectively, a list of overlays that end at or
984 before the current overlay center, and a list of overlays that end
985 after the current overlay center. @xref{Managing Overlays}.
986 @code{overlays_before} is sorted in order of decreasing end position,
987 and @code{overlays_after} is sorted in order of increasing beginning
991 A Lisp string that names the buffer. It is guaranteed to be unique.
995 The length of the file this buffer is visiting, when last read or
996 saved. This and other fields concerned with saving are not kept in
997 the @code{buffer_text} structure because indirect buffers are never
1001 The directory for expanding relative file names. This is the value of
1002 the buffer-local variable @code{default-directory} (@pxref{File Name Expansion}).
1005 The name of the file visited in this buffer, or @code{nil}. This is
1006 the value of the buffer-local variable @code{buffer-file-name}
1007 (@pxref{Buffer File Name}).
1011 @itemx auto_save_file_name
1014 @itemx file_truename
1015 @itemx invisibility_spec
1016 @itemx display_count
1018 These fields store the values of Lisp variables that are automatically
1019 buffer-local (@pxref{Buffer-Local Variables}), whose corresponding
1020 variable names have the additional prefix @code{buffer-} and have
1021 underscores replaced with dashes. For instance, @code{undo_list}
1022 stores the value of @code{buffer-undo-list}.
1025 The mark for the buffer. The mark is a marker, hence it is also
1026 included on the list @code{markers}. @xref{The Mark}.
1028 @item local_var_alist
1029 The association list describing the buffer-local variable bindings of
1030 this buffer, not including the built-in buffer-local bindings that
1031 have special slots in the buffer object. (Those slots are omitted
1032 from this table.) @xref{Buffer-Local Variables}.
1035 Symbol naming the major mode of this buffer, e.g., @code{lisp-mode}.
1038 Pretty name of the major mode, e.g., @code{"Lisp"}.
1043 @itemx category_table
1044 @itemx display_table
1045 These fields store the buffer's local keymap (@pxref{Keymaps}), abbrev
1046 table (@pxref{Abbrev Tables}), syntax table (@pxref{Syntax Tables}),
1047 category table (@pxref{Categories}), and display table (@pxref{Display
1050 @item downcase_table
1052 @itemx case_canon_table
1053 These fields store the conversion tables for converting text to lower
1054 case, upper case, and for canonicalizing text for case-fold search.
1058 An alist of the minor modes of this buffer.
1063 These fields are only used in an indirect buffer, or in a buffer that
1064 is the base of an indirect buffer. Each holds a marker that records
1065 @code{pt}, @code{begv}, and @code{zv} respectively, for this buffer
1066 when the buffer is not current.
1068 @item mode_line_format
1069 @itemx header_line_format
1070 @itemx case_fold_search
1074 @itemx auto_fill_function
1075 @itemx truncate_lines
1078 @itemx selective_display
1079 @itemx selective_display_ellipses
1080 @itemx overwrite_mode
1082 @itemx display_table
1084 @itemx enable_multibyte_characters
1085 @itemx buffer_file_coding_system
1086 @itemx auto_save_file_format
1087 @itemx cache_long_line_scans
1088 @itemx point_before_scroll
1089 @itemx left_fringe_width
1090 @itemx right_fringe_width
1091 @itemx fringes_outside_margins
1092 @itemx scroll_bar_width
1093 @itemx indicate_empty_lines
1094 @itemx indicate_buffer_boundaries
1095 @itemx fringe_indicator_alist
1096 @itemx fringe_cursor_alist
1097 @itemx scroll_up_aggressively
1098 @itemx scroll_down_aggressively
1100 @itemx cursor_in_non_selected_windows
1101 These fields store the values of Lisp variables that are automatically
1102 buffer-local (@pxref{Buffer-Local Variables}), whose corresponding
1103 variable names have underscores replaced with dashes. For instance,
1104 @code{mode_line_format} stores the value of @code{mode-line-format}.
1106 @item last_selected_window
1107 This is the last window that was selected with this buffer in it, or @code{nil}
1108 if that window no longer displays this buffer.
1111 @node Window Internals
1112 @subsection Window Internals
1113 @cindex internals, of window
1114 @cindex window internals
1116 Windows have the following accessible fields:
1120 The frame that this window is on.
1123 Non-@code{nil} if this window is a minibuffer window.
1126 Internally, Emacs arranges windows in a tree; each group of siblings has
1127 a parent window whose area includes all the siblings. This field points
1128 to a window's parent.
1130 Parent windows do not display buffers, and play little role in display
1131 except to shape their child windows. Emacs Lisp programs usually have
1132 no access to the parent windows; they operate on the windows at the
1133 leaves of the tree, which actually display buffers.
1137 These fields contain the window's leftmost child and its topmost child
1138 respectively. @code{hchild} is used if the window is subdivided
1139 horizontally by child windows, and @code{vchild} if it is subdivided
1144 The next sibling and previous sibling of this window. @code{next} is
1145 @code{nil} if the window is the right-most or bottom-most in its group;
1146 @code{prev} is @code{nil} if it is the left-most or top-most in its
1150 The left-hand edge of the window, measured in columns, relative to the
1151 leftmost column in the frame (column 0).
1154 The top edge of the window, measured in lines, relative to the topmost
1155 line in the frame (line 0).
1159 The width and height of the window, measured in columns and lines
1160 respectively. The width includes the scroll bar and fringes, and/or
1161 the separator line on the right of the window (if any).
1164 The buffer that the window is displaying.
1167 A marker pointing to the position in the buffer that is the first
1168 character displayed in the window.
1171 @cindex window point internals
1172 This is the value of point in the current buffer when this window is
1173 selected; when it is not selected, it retains its previous value.
1176 If this flag is non-@code{nil}, it says that the window has been
1177 scrolled explicitly by the Lisp program. This affects what the next
1178 redisplay does if point is off the screen: instead of scrolling the
1179 window to show the text around point, it moves point to a location that
1182 @item frozen_window_start_p
1183 This field is set temporarily to 1 to indicate to redisplay that
1184 @code{start} of this window should not be changed, even if point
1187 @item start_at_line_beg
1188 Non-@code{nil} means current value of @code{start} was the beginning of a line
1192 This is the last time that the window was selected. The function
1193 @code{get-lru-window} uses this field.
1195 @item sequence_number
1196 A unique number assigned to this window when it was created.
1199 The @code{modiff} field of the window's buffer, as of the last time
1200 a redisplay completed in this window.
1202 @item last_overlay_modified
1203 The @code{overlay_modiff} field of the window's buffer, as of the last
1204 time a redisplay completed in this window.
1207 The buffer's value of point, as of the last time a redisplay completed
1211 A non-@code{nil} value means the window's buffer was ``modified'' when the
1212 window was last updated.
1214 @item vertical_scroll_bar
1215 This window's vertical scroll bar.
1217 @item left_margin_width
1218 @itemx right_margin_width
1219 The widths of the left and right margins in this window. A value of
1220 @code{nil} means to use the buffer's value of @code{left-margin-width}
1221 or @code{right-margin-width}.
1223 @item window_end_pos
1224 This is computed as @code{z} minus the buffer position of the last glyph
1225 in the current matrix of the window. The value is only valid if
1226 @code{window_end_valid} is not @code{nil}.
1228 @item window_end_bytepos
1229 The byte position corresponding to @code{window_end_pos}.
1231 @item window_end_vpos
1232 The window-relative vertical position of the line containing
1233 @code{window_end_pos}.
1235 @item window_end_valid
1236 This field is set to a non-@code{nil} value if @code{window_end_pos} is truly
1237 valid. This is @code{nil} if nontrivial redisplay is preempted since in that
1238 case the display that @code{window_end_pos} was computed for did not get
1242 A structure describing where the cursor is in this window.
1245 The value of @code{cursor} as of the last redisplay that finished.
1248 A structure describing where the cursor of this window physically is.
1250 @item phys_cursor_type
1251 The type of cursor that was last displayed on this window.
1253 @item phys_cursor_on_p
1254 This field is non-zero if the cursor is physically on.
1257 Non-zero means the cursor in this window is logically on.
1259 @item last_cursor_off_p
1260 This field contains the value of @code{cursor_off_p} as of the time of
1263 @item must_be_updated_p
1264 This is set to 1 during redisplay when this window must be updated.
1267 This is the number of columns that the display in the window is scrolled
1268 horizontally to the left. Normally, this is 0.
1271 Vertical scroll amount, in pixels. Normally, this is 0.
1274 Non-@code{nil} if this window is dedicated to its buffer.
1277 The window's display table, or @code{nil} if none is specified for it.
1279 @item update_mode_line
1280 Non-@code{nil} means this window's mode line needs to be updated.
1282 @item base_line_number
1283 The line number of a certain position in the buffer, or @code{nil}.
1284 This is used for displaying the line number of point in the mode line.
1287 The position in the buffer for which the line number is known, or
1288 @code{nil} meaning none is known.
1290 @item region_showing
1291 If the region (or part of it) is highlighted in this window, this field
1292 holds the mark position that made one end of that region. Otherwise,
1293 this field is @code{nil}.
1295 @item column_number_displayed
1296 The column number currently displayed in this window's mode line, or @code{nil}
1297 if column numbers are not being displayed.
1299 @item current_matrix
1300 A glyph matrix describing the current display of this window.
1302 @item desired_matrix
1303 A glyph matrix describing the desired display of this window.
1306 @node Process Internals
1307 @subsection Process Internals
1308 @cindex internals, of process
1309 @cindex process internals
1311 The fields of a process are:
1315 A string, the name of the process.
1318 A list containing the command arguments that were used to start this
1319 process. For a network or serial process, it is @code{nil} if the
1320 process is running or @code{t} if the process is stopped.
1323 A function used to accept output from the process instead of a buffer,
1327 A function called whenever the process receives a signal, or @code{nil}.
1330 The associated buffer of the process.
1333 An integer, the operating system's process @acronym{ID}.
1336 A flag, non-@code{nil} if this is really a child process.
1337 It is @code{nil} for a network or serial connection.
1340 A marker indicating the position of the end of the last output from this
1341 process inserted into the buffer. This is often but not always the end
1344 @item kill_without_query
1345 If this is non-zero, killing Emacs while this process is still running
1346 does not ask for confirmation about killing the process.
1348 @item raw_status_low
1349 @itemx raw_status_high
1350 These two fields record 16 bits each of the process status returned by
1351 the @code{wait} system call.
1354 The process status, as @code{process-status} should return it.
1358 If these two fields are not equal, a change in the status of the process
1359 needs to be reported, either by running the sentinel or by inserting a
1360 message in the process buffer.
1363 Non-@code{nil} if communication with the subprocess uses a @acronym{PTY};
1364 @code{nil} if it uses a pipe.
1367 The file descriptor for input from the process.
1370 The file descriptor for output to the process.
1373 The file descriptor for the terminal that the subprocess is using. (On
1374 some systems, there is no need to record this, so the value is
1378 The name of the terminal that the subprocess is using,
1379 or @code{nil} if it is using pipes.
1381 @item decode_coding_system
1382 Coding-system for decoding the input from this process.
1385 A working buffer for decoding.
1387 @item decoding_carryover
1388 Size of carryover in decoding.
1390 @item encode_coding_system
1391 Coding-system for encoding the output to this process.
1394 A working buffer for encoding.
1396 @item encoding_carryover
1397 Size of carryover in encoding.
1399 @item inherit_coding_system_flag
1400 Flag to set @code{coding-system} of the process buffer from the
1401 coding system used to decode process output.
1404 Symbol indicating the type of process: @code{real}, @code{network},