2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1998, 1999, 2002, 2003, 2004,
4 @c 2005, 2006 Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../info/internals
7 @node GNU Emacs Internals, Standard Errors, Tips, Top
8 @comment node-name, next, previous, up
9 @appendix GNU Emacs Internals
11 This chapter describes how the runnable Emacs executable is dumped with
12 the preloaded Lisp libraries in it, how storage is allocated, and some
13 internal aspects of GNU Emacs that may be of interest to C programmers.
16 * Building Emacs:: How the dumped Emacs is made.
17 * Pure Storage:: A kludge to make preloaded Lisp functions sharable.
18 * Garbage Collection:: Reclaiming space for Lisp objects no longer used.
19 * Memory Usage:: Info about total size of Lisp objects made so far.
20 * Writing Emacs Primitives:: Writing C code for Emacs.
21 * Object Internals:: Data formats of buffers, windows, processes.
25 @appendixsec Building Emacs
26 @cindex building Emacs
29 This section explains the steps involved in building the Emacs
30 executable. You don't have to know this material to build and install
31 Emacs, since the makefiles do all these things automatically. This
32 information is pertinent to Emacs maintenance.
34 Compilation of the C source files in the @file{src} directory
35 produces an executable file called @file{temacs}, also called a
36 @dfn{bare impure Emacs}. It contains the Emacs Lisp interpreter and I/O
37 routines, but not the editing commands.
39 @cindex @file{loadup.el}
40 The command @w{@samp{temacs -l loadup}} uses @file{temacs} to create
41 the real runnable Emacs executable. These arguments direct
42 @file{temacs} to evaluate the Lisp files specified in the file
43 @file{loadup.el}. These files set up the normal Emacs editing
44 environment, resulting in an Emacs that is still impure but no longer
47 It takes a substantial 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 To create @file{emacs}, use the command @samp{temacs -batch -l loadup
55 dump}. The purpose of @samp{-batch} here is to prevent @file{temacs}
56 from trying to initialize any of its data on the terminal; this ensures
57 that the tables of terminal information are empty in the dumped Emacs.
58 The argument @samp{dump} tells @file{loadup.el} to dump a new executable
61 Some operating systems don't support dumping. On those systems, you
62 must start Emacs with the @samp{temacs -l loadup} command each time you
63 use it. This takes a substantial time, but since you need to start
64 Emacs once a day at most---or once a week if you never log out---the
65 extra time is not too severe a problem.
67 @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 add a definition
73 #define SITELOAD_PURESIZE_EXTRA @var{n}
77 to make @var{n} added bytes of pure space to hold the additional files.
78 (Try adding increments of 20000 until it is big enough.) However, the
79 advantage of preloading additional files decreases as machines get
80 faster. On modern machines, it is usually not advisable.
82 After @file{loadup.el} reads @file{site-load.el}, it finds the
83 documentation strings for primitive and preloaded functions (and
84 variables) in the file @file{etc/DOC} where they are stored, by
85 calling @code{Snarf-documentation} (@pxref{Definition of
86 Snarf-documentation,, Accessing Documentation}).
88 @cindex @file{site-init.el}
89 You can specify other Lisp expressions to execute just before dumping
90 by putting them in a library named @file{site-init.el}. This file is
91 executed after the documentation strings are found.
93 If you want to preload function or variable definitions, there are
94 three ways you can do this and make their documentation strings
95 accessible when you subsequently run Emacs:
99 Arrange to scan these files when producing the @file{etc/DOC} file,
100 and load them with @file{site-load.el}.
103 Load the files with @file{site-init.el}, then copy the files into the
104 installation directory for Lisp files when you install Emacs.
107 Specify a non-@code{nil} value for
108 @code{byte-compile-dynamic-docstrings} as a local variable in each of these
109 files, and load them with either @file{site-load.el} or
110 @file{site-init.el}. (This method has the drawback that the
111 documentation strings take up space in Emacs all the time.)
114 It is not advisable to put anything in @file{site-load.el} or
115 @file{site-init.el} that would alter any of the features that users
116 expect in an ordinary unmodified Emacs. If you feel you must override
117 normal features for your site, do it with @file{default.el}, so that
118 users can override your changes if they wish. @xref{Startup Summary}.
120 In a package that can be preloaded, it is sometimes useful to
121 specify a computation to be done when Emacs subsequently starts up.
122 For this, use @code{eval-at-startup}:
124 @defmac eval-at-startup body@dots{}
125 This evaluates the @var{body} forms, either immediately if running in
126 an Emacs that has already started up, or later when Emacs does start
127 up. Since the value of the @var{body} forms is not necessarily
128 available when the @code{eval-at-startup} form is run, that form
129 always returns @code{nil}.
132 @defun dump-emacs to-file from-file
134 This function dumps the current state of Emacs into an executable file
135 @var{to-file}. It takes symbols from @var{from-file} (this is normally
136 the executable file @file{temacs}).
138 If you want to use this function in an Emacs that was already dumped,
139 you must run Emacs with @samp{-batch}.
143 @appendixsec Pure Storage
146 Emacs Lisp uses two kinds of storage for user-created Lisp objects:
147 @dfn{normal storage} and @dfn{pure storage}. Normal storage is where
148 all the new data created during an Emacs session are kept; see the
149 following section for information on normal storage. Pure storage is
150 used for certain data in the preloaded standard Lisp files---data that
151 should never change during actual use of Emacs.
153 Pure storage is allocated only while @file{temacs} is loading the
154 standard preloaded Lisp libraries. In the file @file{emacs}, it is
155 marked as read-only (on operating systems that permit this), so that
156 the memory space can be shared by all the Emacs jobs running on the
157 machine at once. Pure storage is not expandable; a fixed amount is
158 allocated when Emacs is compiled, and if that is not sufficient for
159 the preloaded libraries, @file{temacs} allocates dynamic memory for
160 the part that didn't fit. If that happens, you should increase the
161 compilation parameter @code{PURESIZE} in the file
162 @file{src/puresize.h} and rebuild Emacs, even though the resulting
163 image will work. Such an overflow normally won't happen unless you
164 try to preload additional libraries or add features to the standard
165 ones. Emacs will display a warning about the overflow when it
168 @defun purecopy object
169 This function makes a copy in pure storage of @var{object}, and returns
170 it. It copies a string by simply making a new string with the same
171 characters, but without text properties, in pure storage. It
172 recursively copies the contents of vectors and cons cells. It does
173 not make copies of other objects such as symbols, but just returns
174 them unchanged. It signals an error if asked to copy markers.
176 This function is a no-op except while Emacs is being built and dumped;
177 it is usually called only in the file @file{emacs/lisp/loaddefs.el}, but
178 a few packages call it just in case you decide to preload them.
181 @defvar pure-bytes-used
182 The value of this variable is the number of bytes of pure storage
183 allocated so far. Typically, in a dumped Emacs, this number is very
184 close to the total amount of pure storage available---if it were not,
185 we would preallocate less.
189 This variable determines whether @code{defun} should make a copy of the
190 function definition in pure storage. If it is non-@code{nil}, then the
191 function definition is copied into pure storage.
193 This flag is @code{t} while loading all of the basic functions for
194 building Emacs initially (allowing those functions to be sharable and
195 non-collectible). Dumping Emacs as an executable always writes
196 @code{nil} in this variable, regardless of the value it actually has
197 before and after dumping.
199 You should not change this flag in a running Emacs.
202 @node Garbage Collection
203 @appendixsec Garbage Collection
204 @cindex garbage collector
206 @cindex memory allocation
207 When a program creates a list or the user defines a new function (such
208 as by loading a library), that data is placed in normal storage. If
209 normal storage runs low, then Emacs asks the operating system to
210 allocate more memory in blocks of 1k bytes. Each block is used for one
211 type of Lisp object, so symbols, cons cells, markers, etc., are
212 segregated in distinct blocks in memory. (Vectors, long strings,
213 buffers and certain other editing types, which are fairly large, are
214 allocated in individual blocks, one per object, while small strings are
215 packed into blocks of 8k bytes.)
217 It is quite common to use some storage for a while, then release it by
218 (for example) killing a buffer or deleting the last pointer to an
219 object. Emacs provides a @dfn{garbage collector} to reclaim this
220 abandoned storage. (This name is traditional, but ``garbage recycler''
221 might be a more intuitive metaphor for this facility.)
223 The garbage collector operates by finding and marking all Lisp objects
224 that are still accessible to Lisp programs. To begin with, it assumes
225 all the symbols, their values and associated function definitions, and
226 any data presently on the stack, are accessible. Any objects that can
227 be reached indirectly through other accessible objects are also
230 When marking is finished, all objects still unmarked are garbage. No
231 matter what the Lisp program or the user does, it is impossible to refer
232 to them, since there is no longer a way to reach them. Their space
233 might as well be reused, since no one will miss them. The second
234 (``sweep'') phase of the garbage collector arranges to reuse them.
236 @c ??? Maybe add something describing weak hash tables here?
239 The sweep phase puts unused cons cells onto a @dfn{free list}
240 for future allocation; likewise for symbols and markers. It compacts
241 the accessible strings so they occupy fewer 8k blocks; then it frees the
242 other 8k blocks. Vectors, buffers, windows, and other large objects are
243 individually allocated and freed using @code{malloc} and @code{free}.
245 @cindex CL note---allocate more storage
247 @b{Common Lisp note:} Unlike other Lisps, GNU Emacs Lisp does not
248 call the garbage collector when the free list is empty. Instead, it
249 simply requests the operating system to allocate more storage, and
250 processing continues until @code{gc-cons-threshold} bytes have been
253 This means that you can make sure that the garbage collector will not
254 run during a certain portion of a Lisp program by calling the garbage
255 collector explicitly just before it (provided that portion of the
256 program does not use so much space as to force a second garbage
260 @deffn Command garbage-collect
261 This command runs a garbage collection, and returns information on
262 the amount of space in use. (Garbage collection can also occur
263 spontaneously if you use more than @code{gc-cons-threshold} bytes of
264 Lisp data since the previous garbage collection.)
266 @code{garbage-collect} returns a list containing the following
271 ((@var{used-conses} . @var{free-conses})
272 (@var{used-syms} . @var{free-syms})
274 (@var{used-miscs} . @var{free-miscs})
275 @var{used-string-chars}
276 @var{used-vector-slots}
277 (@var{used-floats} . @var{free-floats})
278 (@var{used-intervals} . @var{free-intervals})
279 (@var{used-strings} . @var{free-strings}))
287 @result{} ((106886 . 13184) (9769 . 0)
288 (7731 . 4651) 347543 121628
289 (31 . 94) (1273 . 168)
294 Here is a table explaining each element:
298 The number of cons cells in use.
301 The number of cons cells for which space has been obtained from the
302 operating system, but that are not currently being used.
305 The number of symbols in use.
308 The number of symbols for which space has been obtained from the
309 operating system, but that are not currently being used.
312 The number of miscellaneous objects in use. These include markers and
313 overlays, plus certain objects not visible to users.
316 The number of miscellaneous objects for which space has been obtained
317 from the operating system, but that are not currently being used.
319 @item used-string-chars
320 The total size of all strings, in characters.
322 @item used-vector-slots
323 The total number of elements of existing vectors.
327 The number of floats in use.
331 The number of floats for which space has been obtained from the
332 operating system, but that are not currently being used.
335 The number of intervals in use. Intervals are an internal
336 data structure used for representing text properties.
339 The number of intervals for which space has been obtained
340 from the operating system, but that are not currently being used.
343 The number of strings in use.
346 The number of string headers for which the space was obtained from the
347 operating system, but which are currently not in use. (A string
348 object consists of a header and the storage for the string text
349 itself; the latter is only allocated when the string is created.)
353 @defopt garbage-collection-messages
354 If this variable is non-@code{nil}, Emacs displays a message at the
355 beginning and end of garbage collection. The default value is
356 @code{nil}, meaning there are no such messages.
360 This is a normal hook that is run at the end of garbage collection.
361 Garbage collection is inhibited while the hook functions run, so be
362 careful writing them.
365 @defopt gc-cons-threshold
366 The value of this variable is the number of bytes of storage that must
367 be allocated for Lisp objects after one garbage collection in order to
368 trigger another garbage collection. A cons cell counts as eight bytes,
369 a string as one byte per character plus a few bytes of overhead, and so
370 on; space allocated to the contents of buffers does not count. Note
371 that the subsequent garbage collection does not happen immediately when
372 the threshold is exhausted, but only the next time the Lisp evaluator is
375 The initial threshold value is 400,000. If you specify a larger
376 value, garbage collection will happen less often. This reduces the
377 amount of time spent garbage collecting, but increases total memory use.
378 You may want to do this when running a program that creates lots of
381 You can make collections more frequent by specifying a smaller value,
382 down to 10,000. A value less than 10,000 will remain in effect only
383 until the subsequent garbage collection, at which time
384 @code{garbage-collect} will set the threshold back to 10,000.
387 @defopt gc-cons-percentage
388 The value of this variable specifies the amount of consing before a
389 garbage collection occurs, as a fraction of the current heap size.
390 This criterion and @code{gc-cons-threshold} apply in parallel, and
391 garbage collection occurs only when both criteria are satisfied.
393 As the heap size increases, the time to perform a garbage collection
394 increases. Thus, it can be desirable to do them less frequently in
398 The value returned by @code{garbage-collect} describes the amount of
399 memory used by Lisp data, broken down by data type. By contrast, the
400 function @code{memory-limit} provides information on the total amount of
401 memory Emacs is currently using.
405 This function returns the address of the last byte Emacs has allocated,
406 divided by 1024. We divide the value by 1024 to make sure it fits in a
409 You can use this to get a general idea of how your actions affect the
414 This variable is @code{t} if Emacs is close to out of memory for Lisp
415 objects, and @code{nil} otherwise.
418 @defun memory-use-counts
419 This returns a list of numbers that count the number of objects
420 created in this Emacs session. Each of these counters increments for
421 a certain kind of object. See the documentation string for details.
425 This variable contains the total number of garbage collections
426 done so far in this Emacs session.
430 This variable contains the total number of seconds of elapsed time
431 during garbage collection so far in this Emacs session, as a floating
436 @section Memory Usage
438 These functions and variables give information about the total amount
439 of memory allocation that Emacs has done, broken down by data type.
440 Note the difference between these and the values returned by
441 @code{(garbage-collect)}; those count objects that currently exist, but
442 these count the number or size of all allocations, including those for
443 objects that have since been freed.
445 @defvar cons-cells-consed
446 The total number of cons cells that have been allocated so far
447 in this Emacs session.
450 @defvar floats-consed
451 The total number of floats that have been allocated so far
452 in this Emacs session.
455 @defvar vector-cells-consed
456 The total number of vector cells that have been allocated so far
457 in this Emacs session.
460 @defvar symbols-consed
461 The total number of symbols that have been allocated so far
462 in this Emacs session.
465 @defvar string-chars-consed
466 The total number of string characters that have been allocated so far
467 in this Emacs session.
470 @defvar misc-objects-consed
471 The total number of miscellaneous objects that have been allocated so
472 far in this Emacs session. These include markers and overlays, plus
473 certain objects not visible to users.
476 @defvar intervals-consed
477 The total number of intervals that have been allocated so far
478 in this Emacs session.
481 @defvar strings-consed
482 The total number of strings that have been allocated so far in this
486 @node Writing Emacs Primitives
487 @appendixsec Writing Emacs Primitives
488 @cindex primitive function internals
490 Lisp primitives are Lisp functions implemented in C. The details of
491 interfacing the C function so that Lisp can call it are handled by a few
492 C macros. The only way to really understand how to write new C code is
493 to read the source, but we can explain some things here.
495 An example of a special form is the definition of @code{or}, from
496 @file{eval.c}. (An ordinary function would have the same general
499 @cindex garbage collection protection
502 DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
503 doc: /* Eval args until one of them yields non-nil, then return that value.
504 The remaining args are not evalled at all.
505 If all args return nil, return nil.
508 usage: (or CONDITIONS ...) */)
512 register Lisp_Object val = Qnil;
523 val = Feval (XCAR (args));
537 Let's start with a precise explanation of the arguments to the
538 @code{DEFUN} macro. Here is a template for them:
541 DEFUN (@var{lname}, @var{fname}, @var{sname}, @var{min}, @var{max}, @var{interactive}, @var{doc})
546 This is the name of the Lisp symbol to define as the function name; in
547 the example above, it is @code{or}.
550 This is the C function name for this function. This is
551 the name that is used in C code for calling the function. The name is,
552 by convention, @samp{F} prepended to the Lisp name, with all dashes
553 (@samp{-}) in the Lisp name changed to underscores. Thus, to call this
554 function from C code, call @code{For}. Remember that the arguments must
555 be of type @code{Lisp_Object}; various macros and functions for creating
556 values of type @code{Lisp_Object} are declared in the file
560 This is a C variable name to use for a structure that holds the data for
561 the subr object that represents the function in Lisp. This structure
562 conveys the Lisp symbol name to the initialization routine that will
563 create the symbol and store the subr object as its definition. By
564 convention, this name is always @var{fname} with @samp{F} replaced with
568 This is the minimum number of arguments that the function requires. The
569 function @code{or} allows a minimum of zero arguments.
572 This is the maximum number of arguments that the function accepts, if
573 there is a fixed maximum. Alternatively, it can be @code{UNEVALLED},
574 indicating a special form that receives unevaluated arguments, or
575 @code{MANY}, indicating an unlimited number of evaluated arguments (the
576 equivalent of @code{&rest}). Both @code{UNEVALLED} and @code{MANY} are
577 macros. If @var{max} is a number, it may not be less than @var{min} and
578 it may not be greater than eight.
581 This is an interactive specification, a string such as might be used as
582 the argument of @code{interactive} in a Lisp function. In the case of
583 @code{or}, it is 0 (a null pointer), indicating that @code{or} cannot be
584 called interactively. A value of @code{""} indicates a function that
585 should receive no arguments when called interactively.
588 This is the documentation string. It uses C comment syntax rather
589 than C string syntax because comment syntax requires nothing special
590 to include multiple lines. The @samp{doc:} identifies the comment
591 that follows as the documentation string. The @samp{/*} and @samp{*/}
592 delimiters that begin and end the comment are not part of the
593 documentation string.
595 If the last line of the documentation string begins with the keyword
596 @samp{usage:}, the rest of the line is treated as the argument list
597 for documentation purposes. This way, you can use different argument
598 names in the documentation string from the ones used in the C code.
599 @samp{usage:} is required if the function has an unlimited number of
602 All the usual rules for documentation strings in Lisp code
603 (@pxref{Documentation Tips}) apply to C code documentation strings
607 After the call to the @code{DEFUN} macro, you must write the argument
608 name list that every C function must have, followed by ordinary C
609 declarations for the arguments. For a function with a fixed maximum
610 number of arguments, declare a C argument for each Lisp argument, and
611 give them all type @code{Lisp_Object}. When a Lisp function has no
612 upper limit on the number of arguments, its implementation in C actually
613 receives exactly two arguments: the first is the number of Lisp
614 arguments, and the second is the address of a block containing their
615 values. They have types @code{int} and @w{@code{Lisp_Object *}}.
617 Within the function @code{For} itself, note the use of the macros
618 @code{GCPRO1} and @code{UNGCPRO}. @code{GCPRO1} is used to ``protect''
619 a variable from garbage collection---to inform the garbage collector that
620 it must look in that variable and regard its contents as an accessible
621 object. This is necessary whenever you call @code{Feval} or anything
622 that can directly or indirectly call @code{Feval}. At such a time, any
623 Lisp object that you intend to refer to again must be protected somehow.
624 @code{UNGCPRO} cancels the protection of the variables that are
625 protected in the current function. It is necessary to do this explicitly.
627 It suffices to ensure that at least one pointer to each object is
628 GC-protected; as long as the object is not recycled, all pointers to
629 it remain valid. So if you are sure that a local variable points to
630 an object that will be preserved by some other pointer, that local
631 variable does not need a @code{GCPRO}. (Formerly, strings were an
632 exception to this rule; in older Emacs versions, every pointer to a
633 string needed to be marked by GC.)
635 The macro @code{GCPRO1} protects just one local variable. If you
636 want to protect two, use @code{GCPRO2} instead; repeating
637 @code{GCPRO1} will not work. Macros, @code{GCPRO3}, @code{GCPRO4},
638 @code{GCPRO5}, and @code{GCPRO6} also exist. These macros implicitly
639 use local variables such as @code{gcpro1}; you must declare these
640 explicitly, with type @code{struct gcpro}. Thus, if you use
641 @code{GCPRO2}, you must declare @code{gcpro1} and @code{gcpro2}.
642 Alas, we can't explain all the tricky details here.
644 Built-in functions that take a variable number of arguments actually
645 accept two arguments at the C level: the number of Lisp arguments, and
646 a @code{Lisp_Object *} pointer to a C vector containing those Lisp
647 arguments. This C vector may be part of a Lisp vector, but it need
648 not be. The responsibility for using @code{GCPRO} to protect the Lisp
649 arguments from GC if necessary rests with the caller in this case,
650 since the caller allocated or found the storage for them.
652 You must not use C initializers for static or global variables unless
653 the variables are never written once Emacs is dumped. These variables
654 with initializers are allocated in an area of memory that becomes
655 read-only (on certain operating systems) as a result of dumping Emacs.
658 Do not use static variables within functions---place all static
659 variables at top level in the file. This is necessary because Emacs on
660 some operating systems defines the keyword @code{static} as a null
661 macro. (This definition is used because those systems put all variables
662 declared static in a place that becomes read-only after dumping, whether
663 they have initializers or not.)
665 Defining the C function is not enough to make a Lisp primitive
666 available; you must also create the Lisp symbol for the primitive and
667 store a suitable subr object in its function cell. The code looks like
671 defsubr (&@var{subr-structure-name});
675 Here @var{subr-structure-name} is the name you used as the third
676 argument to @code{DEFUN}.
678 If you add a new primitive to a file that already has Lisp primitives
679 defined in it, find the function (near the end of the file) named
680 @code{syms_of_@var{something}}, and add the call to @code{defsubr}
681 there. If the file doesn't have this function, or if you create a new
682 file, add to it a @code{syms_of_@var{filename}} (e.g.,
683 @code{syms_of_myfile}). Then find the spot in @file{emacs.c} where all
684 of these functions are called, and add a call to
685 @code{syms_of_@var{filename}} there.
687 @anchor{Defining Lisp variables in C}
688 @vindex byte-boolean-vars
689 The function @code{syms_of_@var{filename}} is also the place to define
690 any C variables that are to be visible as Lisp variables.
691 @code{DEFVAR_LISP} makes a C variable of type @code{Lisp_Object} visible
692 in Lisp. @code{DEFVAR_INT} makes a C variable of type @code{int}
693 visible in Lisp with a value that is always an integer.
694 @code{DEFVAR_BOOL} makes a C variable of type @code{int} visible in Lisp
695 with a value that is either @code{t} or @code{nil}. Note that variables
696 defined with @code{DEFVAR_BOOL} are automatically added to the list
697 @code{byte-boolean-vars} used by the byte compiler.
699 If you define a file-scope C variable of type @code{Lisp_Object},
700 you must protect it from garbage-collection by calling @code{staticpro}
701 in @code{syms_of_@var{filename}}, like this:
704 staticpro (&@var{variable});
707 Here is another example function, with more complicated arguments.
708 This comes from the code in @file{window.c}, and it demonstrates the use
709 of macros and functions to manipulate Lisp objects.
713 DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
714 Scoordinates_in_window_p, 2, 2,
715 "xSpecify coordinate pair: \nXExpression which evals to window: ",
716 "Return non-nil if COORDINATES is in WINDOW.\n\
717 COORDINATES is a cons of the form (X . Y), X and Y being distances\n\
721 If they are on the border between WINDOW and its right sibling,\n\
722 `vertical-line' is returned.")
723 (coordinates, window)
724 register Lisp_Object coordinates, window;
730 CHECK_LIVE_WINDOW (window, 0);
731 CHECK_CONS (coordinates, 1);
732 x = XINT (Fcar (coordinates));
733 y = XINT (Fcdr (coordinates));
737 switch (coordinates_in_window (XWINDOW (window), &x, &y))
739 case 0: /* NOT in window at all. */
744 case 1: /* In text part of window. */
745 return Fcons (make_number (x), make_number (y));
749 case 2: /* In mode line of window. */
754 case 3: /* On right border of window. */
755 return Qvertical_line;
766 Note that C code cannot call functions by name unless they are defined
767 in C. The way to call a function written in Lisp is to use
768 @code{Ffuncall}, which embodies the Lisp function @code{funcall}. Since
769 the Lisp function @code{funcall} accepts an unlimited number of
770 arguments, in C it takes two: the number of Lisp-level arguments, and a
771 one-dimensional array containing their values. The first Lisp-level
772 argument is the Lisp function to call, and the rest are the arguments to
773 pass to it. Since @code{Ffuncall} can call the evaluator, you must
774 protect pointers from garbage collection around the call to
777 The C functions @code{call0}, @code{call1}, @code{call2}, and so on,
778 provide handy ways to call a Lisp function conveniently with a fixed
779 number of arguments. They work by calling @code{Ffuncall}.
781 @file{eval.c} is a very good file to look through for examples;
782 @file{lisp.h} contains the definitions for some important macros and
785 If you define a function which is side-effect free, update the code
786 in @file{byte-opt.el} which binds @code{side-effect-free-fns} and
787 @code{side-effect-and-error-free-fns} so that the compiler optimizer
790 @node Object Internals
791 @appendixsec Object Internals
792 @cindex object internals
794 GNU Emacs Lisp manipulates many different types of data. The actual
795 data are stored in a heap and the only access that programs have to it
796 is through pointers. Pointers are thirty-two bits wide in most
797 implementations. Depending on the operating system and type of machine
798 for which you compile Emacs, twenty-nine bits are used to address the
799 object, and the remaining three bits are used for the tag that
800 identifies the object's type.
802 Because Lisp objects are represented as tagged pointers, it is always
803 possible to determine the Lisp data type of any object. The C data type
804 @code{Lisp_Object} can hold any Lisp object of any data type. Ordinary
805 variables have type @code{Lisp_Object}, which means they can hold any
806 type of Lisp value; you can determine the actual data type only at run
807 time. The same is true for function arguments; if you want a function
808 to accept only a certain type of argument, you must check the type
809 explicitly using a suitable predicate (@pxref{Type Predicates}).
810 @cindex type checking internals
813 * Buffer Internals:: Components of a buffer structure.
814 * Window Internals:: Components of a window structure.
815 * Process Internals:: Components of a process structure.
818 @node Buffer Internals
819 @appendixsubsec Buffer Internals
820 @cindex internals, of buffer
821 @cindex buffer internals
823 Buffers contain fields not directly accessible by the Lisp programmer.
824 We describe them here, naming them by the names used in the C code.
825 Many are accessible indirectly in Lisp programs via Lisp primitives.
827 Two structures are used to represent buffers in C. The
828 @code{buffer_text} structure contains fields describing the text of a
829 buffer; the @code{buffer} structure holds other fields. In the case
830 of indirect buffers, two or more @code{buffer} structures reference
831 the same @code{buffer_text} structure.
833 Here is a list of the @code{struct buffer_text} fields:
837 This field contains the actual address of the buffer contents.
840 This holds the character position of the gap in the buffer.
844 This field contains the character position of the end of the buffer
848 Contains the byte position of the gap.
851 Holds the byte position of the end of the buffer text.
854 Contains the size of buffer's gap. @xref{Buffer Gap}.
857 This field counts buffer-modification events for this buffer. It is
858 incremented for each such event, and never otherwise changed.
861 Contains the previous value of @code{modiff}, as of the last time a
862 buffer was visited or saved in a file.
865 Counts modifications to overlays analogous to @code{modiff}.
868 Holds the number of characters at the start of the text that are known
869 to be unchanged since the last redisplay that finished.
872 Holds the number of characters at the end of the text that are known to
873 be unchanged since the last redisplay that finished.
875 @item unchanged_modified
876 Contains the value of @code{modiff} at the time of the last redisplay
877 that finished. If this value matches @code{modiff},
878 @code{beg_unchanged} and @code{end_unchanged} contain no useful
881 @item overlay_unchanged_modified
882 Contains the value of @code{overlay_modiff} at the time of the last
883 redisplay that finished. If this value matches @code{overlay_modiff},
884 @code{beg_unchanged} and @code{end_unchanged} contain no useful
888 The markers that refer to this buffer. This is actually a single
889 marker, and successive elements in its marker @code{chain} are the other
890 markers referring to this buffer text.
893 Contains the interval tree which records the text properties of this
897 The fields of @code{struct buffer} are:
901 Points to the next buffer, in the chain of all buffers including killed
902 buffers. This chain is used only for garbage collection, in order to
903 collect killed buffers properly. Note that vectors, and most kinds of
904 objects allocated as vectors, are all on one chain, but buffers are on a
905 separate chain of their own.
908 This is a @code{struct buffer_text} structure. In an ordinary buffer,
909 it holds the buffer contents. In indirect buffers, this field is not
913 This points to the @code{buffer_text} structure that is used for this
914 buffer. In an ordinary buffer, this is the @code{own_text} field above.
915 In an indirect buffer, this is the @code{own_text} field of the base
919 Contains the character position of point in a buffer.
922 Contains the byte position of point in a buffer.
925 This field contains the character position of the beginning of the
926 accessible range of text in the buffer.
929 This field contains the byte position of the beginning of the
930 accessible range of text in the buffer.
933 This field contains the character position of the end of the
934 accessible range of text in the buffer.
937 This field contains the byte position of the end of the
938 accessible range of text in the buffer.
941 In an indirect buffer, this points to the base buffer. In an ordinary
944 @item local_var_flags
945 This field contains flags indicating that certain variables are local in
946 this buffer. Such variables are declared in the C code using
947 @code{DEFVAR_PER_BUFFER}, and their buffer-local bindings are stored in
948 fields in the buffer structure itself. (Some of these fields are
949 described in this table.)
952 This field contains the modification time of the visited file. It is
953 set when the file is written or read. Before writing the buffer into a
954 file, this field is compared to the modification time of the file to see
955 if the file has changed on disk. @xref{Buffer Modification}.
957 @item auto_save_modified
958 This field contains the time when the buffer was last auto-saved.
960 @item auto_save_failure_time
961 The time at which we detected a failure to auto-save, or -1 if we didn't
964 @item last_window_start
965 This field contains the @code{window-start} position in the buffer as of
966 the last time the buffer was displayed in a window.
969 This flag is set when narrowing changes in a buffer.
971 @item prevent_redisplay_optimizations_p
972 this flag indicates that redisplay optimizations should not be used
973 to display this buffer.
976 This field points to the buffer's undo list. @xref{Undo}.
979 The buffer name is a string that names the buffer. It is guaranteed to
980 be unique. @xref{Buffer Names}.
983 The name of the file visited in this buffer, or @code{nil}.
986 The directory for expanding relative file names.
989 Length of the file this buffer is visiting, when last read or saved.
990 This and other fields concerned with saving are not kept in the
991 @code{buffer_text} structure because indirect buffers are never saved.
993 @item auto_save_file_name
994 File name used for auto-saving this buffer. This is not in the
995 @code{buffer_text} because it's not used in indirect buffers at all.
998 Non-@code{nil} means this buffer is read-only.
1001 This field contains the mark for the buffer. The mark is a marker,
1002 hence it is also included on the list @code{markers}. @xref{The Mark}.
1004 @item local_var_alist
1005 This field contains the association list describing the buffer-local
1006 variable bindings of this buffer, not including the built-in
1007 buffer-local bindings that have special slots in the buffer object.
1008 (Those slots are omitted from this table.) @xref{Buffer-Local
1012 Symbol naming the major mode of this buffer, e.g., @code{lisp-mode}.
1015 Pretty name of major mode, e.g., @code{"Lisp"}.
1017 @item mode_line_format
1018 Mode line element that controls the format of the mode line. If this
1019 is @code{nil}, no mode line will be displayed.
1021 @item header_line_format
1022 This field is analoguous to @code{mode_line_format} for the mode
1023 line displayed at the top of windows.
1026 This field holds the buffer's local keymap. @xref{Keymaps}.
1029 This buffer's local abbrevs.
1032 This field contains the syntax table for the buffer. @xref{Syntax Tables}.
1034 @item category_table
1035 This field contains the category table for the buffer.
1037 @item case_fold_search
1038 The value of @code{case-fold-search} in this buffer.
1041 The value of @code{tab-width} in this buffer.
1044 The value of @code{fill-column} in this buffer.
1047 The value of @code{left-margin} in this buffer.
1049 @item auto_fill_function
1050 The value of @code{auto-fill-function} in this buffer.
1052 @item downcase_table
1053 This field contains the conversion table for converting text to lower case.
1057 This field contains the conversion table for converting text to upper case.
1060 @item case_canon_table
1061 This field contains the conversion table for canonicalizing text for
1062 case-folding search. @xref{Case Tables}.
1064 @item case_eqv_table
1065 This field contains the equivalence table for case-folding search.
1068 @item truncate_lines
1069 The value of @code{truncate-lines} in this buffer.
1072 The value of @code{ctl-arrow} in this buffer.
1074 @item selective_display
1075 The value of @code{selective-display} in this buffer.
1077 @item selective_display_ellipsis
1078 The value of @code{selective-display-ellipsis} in this buffer.
1081 An alist of the minor modes of this buffer.
1083 @item overwrite_mode
1084 The value of @code{overwrite_mode} in this buffer.
1087 The value of @code{abbrev-mode} in this buffer.
1090 This field contains the buffer's display table, or @code{nil} if it doesn't
1091 have one. @xref{Display Tables}.
1094 This field contains the time when the buffer was last saved, as an integer.
1095 @xref{Buffer Modification}.
1098 This field is non-@code{nil} if the buffer's mark is active.
1100 @item overlays_before
1101 This field holds a list of the overlays in this buffer that end at or
1102 before the current overlay center position. They are sorted in order of
1103 decreasing end position.
1105 @item overlays_after
1106 This field holds a list of the overlays in this buffer that end after
1107 the current overlay center position. They are sorted in order of
1108 increasing beginning position.
1110 @item overlay_center
1111 This field holds the current overlay center position. @xref{Overlays}.
1113 @item enable_multibyte_characters
1114 This field holds the buffer's local value of
1115 @code{enable-multibyte-characters}---either @code{t} or @code{nil}.
1117 @item buffer_file_coding_system
1118 The value of @code{buffer-file-coding-system} in this buffer.
1121 The value of @code{buffer-file-format} in this buffer.
1123 @item auto_save_file_format
1124 The value of @code{buffer-auto-save-file-format} in this buffer.
1127 In an indirect buffer, or a buffer that is the base of an indirect
1128 buffer, this holds a marker that records point for this buffer when the
1129 buffer is not current.
1132 In an indirect buffer, or a buffer that is the base of an indirect
1133 buffer, this holds a marker that records @code{begv} for this buffer
1134 when the buffer is not current.
1137 In an indirect buffer, or a buffer that is the base of an indirect
1138 buffer, this holds a marker that records @code{zv} for this buffer when
1139 the buffer is not current.
1142 The truename of the visited file, or @code{nil}.
1144 @item invisibility_spec
1145 The value of @code{buffer-invisibility-spec} in this buffer.
1147 @item last_selected_window
1148 This is the last window that was selected with this buffer in it, or @code{nil}
1149 if that window no longer displays this buffer.
1152 This field is incremented each time the buffer is displayed in a window.
1154 @item left_margin_width
1155 The value of @code{left-margin-width} in this buffer.
1157 @item right_margin_width
1158 The value of @code{right-margin-width} in this buffer.
1160 @item indicate_empty_lines
1161 Non-@code{nil} means indicate empty lines (lines with no text) with a
1162 small bitmap in the fringe, when using a window system that can do it.
1165 This holds a time stamp that is updated each time this buffer is
1166 displayed in a window.
1168 @item scroll_up_aggressively
1169 The value of @code{scroll-up-aggressively} in this buffer.
1171 @item scroll_down_aggressively
1172 The value of @code{scroll-down-aggressively} in this buffer.
1175 @node Window Internals
1176 @appendixsubsec Window Internals
1177 @cindex internals, of window
1178 @cindex window internals
1180 Windows have the following accessible fields:
1184 The frame that this window is on.
1187 Non-@code{nil} if this window is a minibuffer window.
1190 Internally, Emacs arranges windows in a tree; each group of siblings has
1191 a parent window whose area includes all the siblings. This field points
1192 to a window's parent.
1194 Parent windows do not display buffers, and play little role in display
1195 except to shape their child windows. Emacs Lisp programs usually have
1196 no access to the parent windows; they operate on the windows at the
1197 leaves of the tree, which actually display buffers.
1199 The following four fields also describe the window tree structure.
1202 In a window subdivided horizontally by child windows, the leftmost child.
1203 Otherwise, @code{nil}.
1206 In a window subdivided vertically by child windows, the topmost child.
1207 Otherwise, @code{nil}.
1210 The next sibling of this window. It is @code{nil} in a window that is
1211 the rightmost or bottommost of a group of siblings.
1214 The previous sibling of this window. It is @code{nil} in a window that
1215 is the leftmost or topmost of a group of siblings.
1218 This is the left-hand edge of the window, measured in columns. (The
1219 leftmost column on the screen is @w{column 0}.)
1222 This is the top edge of the window, measured in lines. (The top line on
1223 the screen is @w{line 0}.)
1226 The height of the window, measured in lines.
1229 The width of the window, measured in columns. This width includes the
1230 scroll bar and fringes, and/or the separator line on the right of the
1234 The buffer that the window is displaying. This may change often during
1235 the life of the window.
1238 The position in the buffer that is the first character to be displayed
1242 @cindex window point internals
1243 This is the value of point in the current buffer when this window is
1244 selected; when it is not selected, it retains its previous value.
1247 If this flag is non-@code{nil}, it says that the window has been
1248 scrolled explicitly by the Lisp program. This affects what the next
1249 redisplay does if point is off the screen: instead of scrolling the
1250 window to show the text around point, it moves point to a location that
1253 @item frozen_window_start_p
1254 This field is set temporarily to 1 to indicate to redisplay that
1255 @code{start} of this window should not be changed, even if point
1258 @item start_at_line_beg
1259 Non-@code{nil} means current value of @code{start} was the beginning of a line
1263 Non-@code{nil} means don't delete this window for becoming ``too small''.
1265 @item height_fixed_p
1266 This field is temporarily set to 1 to fix the height of the selected
1267 window when the echo area is resized.
1270 This is the last time that the window was selected. The function
1271 @code{get-lru-window} uses this field.
1273 @item sequence_number
1274 A unique number assigned to this window when it was created.
1277 The @code{modiff} field of the window's buffer, as of the last time
1278 a redisplay completed in this window.
1280 @item last_overlay_modified
1281 The @code{overlay_modiff} field of the window's buffer, as of the last
1282 time a redisplay completed in this window.
1285 The buffer's value of point, as of the last time a redisplay completed
1289 A non-@code{nil} value means the window's buffer was ``modified'' when the
1290 window was last updated.
1292 @item vertical_scroll_bar
1293 This window's vertical scroll bar.
1295 @item left_margin_width
1296 The width of the left margin in this window, or @code{nil} not to
1297 specify it (in which case the buffer's value of @code{left-margin-width}
1300 @item right_margin_width
1301 Likewise for the right margin.
1309 @item window_end_pos
1310 This is computed as @code{z} minus the buffer position of the last glyph
1311 in the current matrix of the window. The value is only valid if
1312 @code{window_end_valid} is not @code{nil}.
1314 @item window_end_bytepos
1315 The byte position corresponding to @code{window_end_pos}.
1317 @item window_end_vpos
1318 The window-relative vertical position of the line containing
1319 @code{window_end_pos}.
1321 @item window_end_valid
1322 This field is set to a non-@code{nil} value if @code{window_end_pos} is truly
1323 valid. This is @code{nil} if nontrivial redisplay is preempted since in that
1324 case the display that @code{window_end_pos} was computed for did not get
1327 @item redisplay_end_trigger
1328 If redisplay in this window goes beyond this buffer position, it runs
1329 the @code{redisplay-end-trigger-hook}.
1334 ??? Are temporary storage areas.
1338 A structure describing where the cursor is in this window.
1341 The value of @code{cursor} as of the last redisplay that finished.
1344 A structure describing where the cursor of this window physically is.
1346 @item phys_cursor_type
1347 The type of cursor that was last displayed on this window.
1349 @item phys_cursor_on_p
1350 This field is non-zero if the cursor is physically on.
1353 Non-zero means the cursor in this window is logically on.
1355 @item last_cursor_off_p
1356 This field contains the value of @code{cursor_off_p} as of the time of
1359 @item must_be_updated_p
1360 This is set to 1 during redisplay when this window must be updated.
1363 This is the number of columns that the display in the window is scrolled
1364 horizontally to the left. Normally, this is 0.
1367 Vertical scroll amount, in pixels. Normally, this is 0.
1370 Non-@code{nil} if this window is dedicated to its buffer.
1373 The window's display table, or @code{nil} if none is specified for it.
1375 @item update_mode_line
1376 Non-@code{nil} means this window's mode line needs to be updated.
1378 @item base_line_number
1379 The line number of a certain position in the buffer, or @code{nil}.
1380 This is used for displaying the line number of point in the mode line.
1383 The position in the buffer for which the line number is known, or
1384 @code{nil} meaning none is known.
1386 @item region_showing
1387 If the region (or part of it) is highlighted in this window, this field
1388 holds the mark position that made one end of that region. Otherwise,
1389 this field is @code{nil}.
1391 @item column_number_displayed
1392 The column number currently displayed in this window's mode line, or @code{nil}
1393 if column numbers are not being displayed.
1395 @item current_matrix
1396 A glyph matrix describing the current display of this window.
1398 @item desired_matrix
1399 A glyph matrix describing the desired display of this window.
1402 @node Process Internals
1403 @appendixsubsec Process Internals
1404 @cindex internals, of process
1405 @cindex process internals
1407 The fields of a process are:
1411 A string, the name of the process.
1414 A list containing the command arguments that were used to start this
1418 A function used to accept output from the process instead of a buffer,
1422 A function called whenever the process receives a signal, or @code{nil}.
1425 The associated buffer of the process.
1428 An integer, the operating system's process @acronym{ID}.
1431 A flag, non-@code{nil} if this is really a child process.
1432 It is @code{nil} for a network connection.
1435 A marker indicating the position of the end of the last output from this
1436 process inserted into the buffer. This is often but not always the end
1439 @item kill_without_query
1440 If this is non-@code{nil}, killing Emacs while this process is still
1441 running does not ask for confirmation about killing the process.
1443 @item raw_status_low
1444 @itemx raw_status_high
1445 These two fields record 16 bits each of the process status returned by
1446 the @code{wait} system call.
1449 The process status, as @code{process-status} should return it.
1453 If these two fields are not equal, a change in the status of the process
1454 needs to be reported, either by running the sentinel or by inserting a
1455 message in the process buffer.
1458 Non-@code{nil} if communication with the subprocess uses a @acronym{PTY};
1459 @code{nil} if it uses a pipe.
1462 The file descriptor for input from the process.
1465 The file descriptor for output to the process.
1468 The file descriptor for the terminal that the subprocess is using. (On
1469 some systems, there is no need to record this, so the value is
1473 The name of the terminal that the subprocess is using,
1474 or @code{nil} if it is using pipes.
1476 @item decode_coding_system
1477 Coding-system for decoding the input from this process.
1480 A working buffer for decoding.
1482 @item decoding_carryover
1483 Size of carryover in decoding.
1485 @item encode_coding_system
1486 Coding-system for encoding the output to this process.
1489 A working buffer for encoding.
1491 @item encoding_carryover
1492 Size of carryover in encoding.
1494 @item inherit_coding_system_flag
1495 Flag to set @code{coding-system} of the process buffer from the
1496 coding system used to decode process output.
1500 arch-tag: 4b2c33bc-d7e4-43f5-bc20-27c0db52a53e