Change default values.

[emacs.git] / lispref / internals.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1998, 1999
@c   Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/internals
@node GNU Emacs Internals, Standard Errors, Tips, Top
@comment  node-name,  next,  previous,  up
@appendix GNU Emacs Internals

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

@menu
* Building Emacs::      How to the dumped Emacs is made.
* Pure Storage::        A kludge to make preloaded Lisp functions sharable.
* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
* Memory Usage::        Info about total size of Lisp objects made so far.
* Writing Emacs Primitives::   Writing C code for Emacs.
* Object Internals::    Data formats of buffers, windows, processes.
@end menu

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

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

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

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

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

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

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

@cindex @file{site-load.el}

  You can specify additional files to preload by writing a library named
@file{site-load.el} that loads them.  You may need to add a definition

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

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

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

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

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

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

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

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

  It is not advisable to put anything in @file{site-load.el} or
@file{site-init.el} that would alter any of the features that users
expect in an ordinary unmodified Emacs.  If you feel you must override
normal features for your site, do it with @file{default.el}, so that
users can override your changes if they wish.  @xref{Startup Summary}.

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

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

@node Pure Storage
@appendixsec Pure Storage
@cindex pure storage

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

  Pure storage is allocated only while @file{temacs} is loading the
standard preloaded Lisp libraries.  In the file @file{emacs}, it is
marked as read-only (on operating systems that permit this), so that
the memory space can be shared by all the Emacs jobs running on the
machine at once.  Pure storage is not expandable; a fixed amount is
allocated when Emacs is compiled, and if that is not sufficient for the
preloaded libraries, @file{temacs} crashes.  If that happens, you must
increase the compilation parameter @code{PURESIZE} in the file
@file{src/puresize.h}.  This normally won't happen unless you try to
preload additional libraries or add features to the standard ones.

@defun purecopy object
This function makes a copy in pure storage of @var{object}, and returns
it.  It copies a string by simply making a new string with the same
characters in pure storage.  It recursively copies the contents of
vectors and cons cells.  It does not make copies of other objects such
as symbols, but just returns them unchanged.  It signals an error if
asked to copy markers.

This function is a no-op except while Emacs is being built and dumped;
it is usually called only in the file @file{emacs/lisp/loaddefs.el}, but
a few packages call it just in case you decide to preload them.
@end defun

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

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

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

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

@node Garbage Collection
@appendixsec Garbage Collection
@cindex garbage collector

@cindex memory allocation
  When a program creates a list or the user defines a new function (such
as by loading a library), that data is placed in normal storage.  If
normal storage runs low, then Emacs asks the operating system to
allocate more memory in blocks of 1k bytes.  Each block is used for one
type of Lisp object, so symbols, cons cells, markers, etc., are
segregated in distinct blocks in memory.  (Vectors, long strings,
buffers and certain other editing types, which are fairly large, are
allocated in individual blocks, one per object, while small strings are
packed into blocks of 8k bytes.)

  It is quite common to use some storage for a while, then release it by
(for example) killing a buffer or deleting the last pointer to an
object.  Emacs provides a @dfn{garbage collector} to reclaim this
abandoned storage.  (This name is traditional, but ``garbage recycler''
might be a more intuitive metaphor for this facility.)

  The garbage collector operates by finding and marking all Lisp objects
that are still accessible to Lisp programs.  To begin with, it assumes
all the symbols, their values and associated function definitions, and
any data presently on the stack, are accessible.  Any objects that can
be reached indirectly through other accessible objects are also
accessible.

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

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

@cindex free list
  The sweep phase puts unused cons cells onto a @dfn{free list}
for future allocation; likewise for symbols and markers.  It compacts
the accessible strings so they occupy fewer 8k blocks; then it frees the
other 8k blocks.  Vectors, buffers, windows, and other large objects are
individually allocated and freed using @code{malloc} and @code{free}.

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

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

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

@code{garbage-collect} returns a list containing the following
information:

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

Here is an example:

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

Here is a table explaining each element:

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

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

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

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

@item used-miscs
The number of miscellaneous objects in use.  These include markers and
overlays, plus certain objects not visible to users.

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

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

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

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

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

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

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

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

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

@defopt garbage-collection-messages
If this variable is non-@code{nil}, Emacs displays a message at the
beginning and end of garbage collection.  The default value is
@code{nil}, meaning there are no such messages.
@end defopt

@defopt gc-cons-threshold
The value of this variable is the number of bytes of storage that must
be allocated for Lisp objects after one garbage collection in order to
trigger another garbage collection.  A cons cell counts as eight bytes,
a string as one byte per character plus a few bytes of overhead, and so
on; space allocated to the contents of buffers does not count.  Note
that the subsequent garbage collection does not happen immediately when
the threshold is exhausted, but only the next time the Lisp evaluator is
called.

The initial threshold value is 400,000.  If you specify a larger
value, garbage collection will happen less often.  This reduces the
amount of time spent garbage collecting, but increases total memory use.
You may want to do this when running a program that creates lots of
Lisp data.

You can make collections more frequent by specifying a smaller value,
down to 10,000.  A value less than 10,000 will remain in effect only
until the subsequent garbage collection, at which time
@code{garbage-collect} will set the threshold back to 10,000.
@end defopt

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

@c Emacs 19 feature
@defun memory-limit
This function returns the address of the last byte Emacs has allocated,
divided by 1024.  We divide the value by 1024 to make sure it fits in a
Lisp integer.

You can use this to get a general idea of how your actions affect the
memory usage.
@end defun

@node Memory Usage
@section Memory Usage

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

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

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

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

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

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

@defvar misc-objects-consed
The total number of miscellaneous objects that have been allocated so
far in this Emacs session.  These include markers and overlays, plus
certain objects not visible to users.
@end defvar

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

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

@node Writing Emacs Primitives
@appendixsec Writing Emacs Primitives
@cindex primitive function internals

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

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

@cindex garbage collection protection
@smallexample
@group
DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
  doc: /* Eval args until one of them yields non-nil, then return that value.
The remaining args are not evalled at all.
If all args return nil, return nil.
@end group
@group
usage: (or CONDITIONS ...)  */)
  (args)
     Lisp_Object args;
@{
  register Lisp_Object val;
  Lisp_Object args_left;
  struct gcpro gcpro1;
@end group

@group
  if (NILP (args))
    return Qnil;

  args_left = args;
  GCPRO1 (args_left);
@end group

@group
  do
    @{
      val = Feval (Fcar (args_left));
      if (!NILP (val))
        break;
      args_left = Fcdr (args_left);
    @}
  while (!NILP (args_left));
@end group

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

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

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

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

@item fname
This is the C function name for this function.  This is
the name that is used in C code for calling the function.  The name is,
by convention, @samp{F} prepended to the Lisp name, with all dashes
(@samp{-}) in the Lisp name changed to underscores.  Thus, to call this
function from C code, call @code{For}.  Remember that the arguments must
be of type @code{Lisp_Object}; various macros and functions for creating
values of type @code{Lisp_Object} are declared in the file
@file{lisp.h}.

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

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

@item max
This is the maximum number of arguments that the function accepts, if
there is a fixed maximum.  Alternatively, it can be @code{UNEVALLED},
indicating a special form that receives unevaluated arguments, or
@code{MANY}, indicating an unlimited number of evaluated arguments (the
equivalent of @code{&rest}).  Both @code{UNEVALLED} and @code{MANY} are
macros.  If @var{max} is a number, it may not be less than @var{min} and
it may not be greater than seven.

@item interactive
This is an interactive specification, a string such as might be used as
the argument of @code{interactive} in a Lisp function.  In the case of
@code{or}, it is 0 (a null pointer), indicating that @code{or} cannot be
called interactively.  A value of @code{""} indicates a function that
should receive no arguments when called interactively.

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

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

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

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

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

  For most data types, it suffices to protect at least one pointer to
the object; as long as the object is not recycled, all pointers to it
remain valid.  This is not so for strings, because the garbage collector
can move them.  When the garbage collector moves a string, it relocates
all the pointers it knows about; any other pointers become invalid.
Therefore, you must protect all pointers to strings across any point
where garbage collection may be possible.

  The macro @code{GCPRO1} protects just one local variable.  If you want
to protect two, use @code{GCPRO2} instead; repeating @code{GCPRO1} will
not work.  Macros @code{GCPRO3} and @code{GCPRO4} also exist.

  These macros implicitly use local variables such as @code{gcpro1}; you
must declare these explicitly, with type @code{struct gcpro}.  Thus, if
you use @code{GCPRO2}, you must declare @code{gcpro1} and @code{gcpro2}.
Alas, we can't explain all the tricky details here.

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

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

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

@example
defsubr (&@var{subr-structure-name});
@end example

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

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

@vindex byte-boolean-vars
  The function @code{syms_of_@var{filename}} is also the place to define
any C variables that are to be visible as Lisp variables.
@code{DEFVAR_LISP} makes a C variable of type @code{Lisp_Object} visible
in Lisp.  @code{DEFVAR_INT} makes a C variable of type @code{int}
visible in Lisp with a value that is always an integer.
@code{DEFVAR_BOOL} makes a C variable of type @code{int} visible in Lisp
with a value that is either @code{t} or @code{nil}.  Note that variables
defined with @code{DEFVAR_BOOL} are automatically added to the list
@code{byte-boolean-vars} used by the byte compiler.

  If you define a file-scope C variable of type @code{Lisp_Object},
you must protect it from garbage-collection by calling @code{staticpro}
in @code{syms_of_@var{filename}}, like this:

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

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

@smallexample
@group
DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
  Scoordinates_in_window_p, 2, 2,
  "xSpecify coordinate pair: \nXExpression which evals to window: ",
  "Return non-nil if COORDINATES is in WINDOW.\n\  
COORDINATES is a cons of the form (X . Y), X and Y being distances\n\
...
@end group
@group
If they are on the border between WINDOW and its right sibling,\n\
   `vertical-line' is returned.")
  (coordinates, window)
     register Lisp_Object coordinates, window;
@{
  int x, y;
@end group

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

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

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

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

@group
    case 3:                     /* On right border of window.  */
      return Qvertical_line;
@end group

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

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

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

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

  If you define a function which is side-effect free, update the code in
@file{byte-opt.el} which binds @code{side-effect-free-fns} and
@code{side-effect-and-error-free-fns} to include it.  This will help the
optimizer.

@node Object Internals
@appendixsec Object Internals
@cindex object internals

  GNU Emacs Lisp manipulates many different types of data.  The actual
data are stored in a heap and the only access that programs have to it
is through pointers.  Pointers are thirty-two bits wide in most
implementations.  Depending on the operating system and type of machine
for which you compile Emacs, twenty-eight bits are used to address the
object, and the remaining four bits are used for a GC mark bit and the
tag that identifies the object's type.

  Because Lisp objects are represented as tagged pointers, it is always
possible to determine the Lisp data type of any object.  The C data type
@code{Lisp_Object} can hold any Lisp object of any data type.  Ordinary
variables have type @code{Lisp_Object}, which means they can hold any
type of Lisp value; you can determine the actual data type only at run
time.  The same is true for function arguments; if you want a function
to accept only a certain type of argument, you must check the type
explicitly using a suitable predicate (@pxref{Type Predicates}).
@cindex type checking internals

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

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

  Buffers contain fields not directly accessible by the Lisp programmer.
We describe them here, naming them by the names used in the C code.
Many are accessible indirectly in Lisp programs via Lisp primitives.

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

Here is a list of the @code{struct buffer_text} fields:

@table @code
@item beg
This field contains the actual address of the buffer contents.

@item gpt
This holds the character position of the gap in the buffer.

@item z
This field contains the character position of the end of the buffer
text.

@item gpt_byte
Contains the byte position of the gap.

@item z_byte
Holds the byte position of the end of the buffer text.

@item gap_size
Contains the size of buffer's gap.

@item modiff
This field counts buffer-modification events for this buffer.  It is
incremented for each such event, and never otherwise changed.

@item save_modiff
Contains the previous value of @code{modiff}, as of the last time a
buffer was visited or saved in a file.
    
@item overlay_modiff
Counts modifications to overlays analogous to @code{modiff}.
       
@item beg_unchanged
Holds the number of characters at the start of the text that are known
to be unchanged since the last redisplay that finished.
    
@item end_unchanged
Holds the number of characters at the end of the text that are known to
be unchanged since the last redisplay that finished.
    
@item unchanged_modified
Contains the value of @code{modiff} at the time of the last redisplay
that finished.  If this value matches @code{modiff},
@code{beg_unchanged} and @code{end_unchanged} contain no useful
information.
    
@item overlay_unchanged_modified
Contains the value of @code{overlay_modiff} at the time of the last
redisplay that finished.  If this value matches @code{overlay_modiff},
@code{beg_unchanged} and @code{end_unchanged} contain no useful
information.
       
@item markers
The markers that refer to this buffer.  This is actually a single
marker, and successive elements in its marker @code{chain} are the other
markers referring to this buffer text.

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

The fields of @code{struct buffer} are:

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

@item own_text
This is a @code{struct buffer_text} structure.  In an ordinary buffer,
it holds the buffer contents.  In indirect buffers, this field is not
used.

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

@item pt
Contains the character position of point in a buffer.

@item pt_byte
Contains the byte position of point in a buffer.

@item begv
This field contains the character position of the beginning of the 
accessible range of text in the buffer.

@item begv_byte
This field contains the byte position of the beginning of the 
accessible range of text in the buffer.

@item zv
This field contains the character position of the end of the 
accessible range of text in the buffer.

@item zv_byte
This field contains the byte position of the end of the 
accessible range of text in the buffer.

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

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

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

@item auto_save_modified
This field contains the time when the buffer was last auto-saved.

@item auto_save_failure_time
The time at which we detected a failure to auto-save, or -1 if we didn't
have a failure.

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

@item clip_changed
This flag is set when narrowing changes in a buffer.

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

@item undo_list
This field points to the buffer's undo list.  @xref{Undo}.

@item name
The buffer name is a string that names the buffer.  It is guaranteed to
be unique.  @xref{Buffer Names}.

@item filename
The name of the file visited in this buffer, or @code{nil}.
 
@item directory
The directory for expanding relative file names.

@item save_length
Length of the file this buffer is visiting, when last read or saved.
This and other fields concerned with saving are not kept in the
@code{buffer_text} structure because indirect buffers are never saved.

@item auto_save_file_name
File name used for auto-saving this buffer.  This is not in the 
@code{buffer_text} because it's not used in indirect buffers at all.

@item read_only
Non-@code{nil} means this buffer is read-only.

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

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

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

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

@item mode_line_format
Mode line element that controls the format of the mode line.  If this
is @code{nil}, no mode line will be displayed.

@item header_line_format
This field is analoguous to @code{mode_line_format} for the mode 
line displayed at the top of windows.

@item keymap
This field holds the buffer's local keymap.  @xref{Keymaps}.

@item abbrev_table
This buffer's local abbrevs.

@item syntax_table
This field contains the syntax table for the buffer.  @xref{Syntax Tables}.

@item category_table
This field contains the category table for the buffer.

@item case_fold_search
The value of @code{case-fold-search} in this buffer.

@item tab_width
The value of @code{tab-width} in this buffer.

@item fill_column
The value of @code{fill-column} in this buffer.

@item left_margin
The value of @code{left-margin} in this buffer.

@item auto_fill_function
The value of @code{auto-fill-function} in this buffer.

@item downcase_table
This field contains the conversion table for converting text to lower case.
@xref{Case Tables}.

@item upcase_table
This field contains the conversion table for converting text to upper case.
@xref{Case Tables}.

@item case_canon_table
This field contains the conversion table for canonicalizing text for
case-folding search.  @xref{Case Tables}.

@item case_eqv_table
This field contains the equivalence table for case-folding search.
@xref{Case Tables}.

@item truncate_lines
The value of @code{truncate-lines} in this buffer.

@item ctl_arrow
The value of @code{ctl-arrow} in this buffer.

@item selective_display
The value of @code{selective-display} in this buffer.

@item selective_display_ellipsis
The value of @code{selective-display-ellipsis} in this buffer.

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

@item overwrite_mode
The value of @code{overwrite_mode} in this buffer.

@item abbrev_mode
The value of @code{abbrev-mode} in this buffer.

@item display_table
This field contains the buffer's display table, or @code{nil} if it doesn't
have one.  @xref{Display Tables}.

@item save_modified
This field contains the time when the buffer was last saved, as an integer.
@xref{Buffer Modification}.

@item mark_active
This field is non-@code{nil} if the buffer's mark is active.

@item overlays_before
This field holds a list of the overlays in this buffer that end at or
before the current overlay center position.  They are sorted in order of
decreasing end position.

@item overlays_after
This field holds a list of the overlays in this buffer that end after
the current overlay center position.  They are sorted in order of
increasing beginning position.

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

@item enable_multibyte_characters
This field holds the buffer's local value of
@code{enable-multibyte-characters}---either @code{t} or @code{nil}.

@item buffer_file_coding_system
The value of @code{buffer-file-coding-system} in this buffer.

@item file_format
The value of @code{buffer-file-format} in this buffer.

@item pt_marker
In an indirect buffer, or a buffer that is the base of an indirect
buffer, this holds a marker that records point for this buffer when the
buffer is not current.

@item begv_marker
In an indirect buffer, or a buffer that is the base of an indirect
buffer, this holds a marker that records @code{begv} for this buffer
when the buffer is not current.
 
@item zv_marker
In an indirect buffer, or a buffer that is the base of an indirect
buffer, this holds a marker that records @code{zv} for this buffer when
the buffer is not current.

@item file_truename
The truename of the visited file, or @code{nil}.

@item invisibility_spec
The value of @code{buffer-invisibility-spec} in this buffer.

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

@item display_count
This field is incremented each time the buffer is displayed in a window.

@item left_margin_width
The value of @code{left-margin-width} in this buffer.

@item right_margin_width
The value of @code{right-margin-width} in this buffer.

@item indicate_empty_lines
Non-@code{nil} means indicate empty lines (lines with no text) with a
small bitmap in the fringe, when using a window system that can do it.

@item display_time
This holds a time stamp that is updated each time this buffer is
displayed in a window.

@item scroll_up_aggressively
The value of @code{scroll-up-aggressively} in this buffer.
 
@item scroll_down_aggressively
The value of @code{scroll-down-aggressively} in this buffer.
@end table

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

  Windows have the following accessible fields:

@table @code
@item frame
The frame that this window is on.

@item mini_p
Non-@code{nil} if this window is a minibuffer window.

@item parent
Internally, Emacs arranges windows in a tree; each group of siblings has
a parent window whose area includes all the siblings.  This field points
to a window's parent.

Parent windows do not display buffers, and play little role in display
except to shape their child windows.  Emacs Lisp programs usually have
no access to the parent windows; they operate on the windows at the
leaves of the tree, which actually display buffers.

The following four fields also describe the window tree structure.

@item hchild
In a window subdivided horizontally by child windows, the leftmost child.
Otherwise, @code{nil}.

@item vchild
In a window subdivided vertically by child windows, the topmost child.
Otherwise, @code{nil}.

@item next
The next sibling of this window.  It is @code{nil} in a window that is
the rightmost or bottommost of a group of siblings.

@item prev
The previous sibling of this window.  It is @code{nil} in a window that
is the leftmost or topmost of a group of siblings.

@item left
This is the left-hand edge of the window, measured in columns.  (The
leftmost column on the screen is @w{column 0}.)

@item top
This is the top edge of the window, measured in lines.  (The top line on
the screen is @w{line 0}.)

@item height
The height of the window, measured in lines.

@item width
The width of the window, measured in columns.  This width includes the
scroll bar and fringes, and/or the separator line on the right of the
window (if any).

@item buffer
The buffer that the window is displaying.  This may change often during
the life of the window.

@item start
The position in the buffer that is the first character to be displayed
in the window.

@item pointm
@cindex window point internals
This is the value of point in the current buffer when this window is
selected; when it is not selected, it retains its previous value.

@item force_start
If this flag is non-@code{nil}, it says that the window has been
scrolled explicitly by the Lisp program.  This affects what the next
redisplay does if point is off the screen: instead of scrolling the
window to show the text around point, it moves point to a location that
is on the screen.

@item frozen_window_start_p
This field is set temporarily to 1 to indicate to redisplay that 
@code{start} of this window should not be changed, even if point
gets invisible.

@item start_at_line_beg
Non-@code{nil} means current value of @code{start} was the beginning of a line
when it was chosen.

@item too_small_ok
Non-@code{nil} means don't delete this window for becoming ``too small''.

@item height_fixed_p
This field is temporarily set to 1 to fix the height of the selected
window when the echo area is resized.

@item use_time
This is the last time that the window was selected.  The function
@code{get-lru-window} uses this field.

@item sequence_number
A unique number assigned to this window when it was created.

@item last_modified
The @code{modiff} field of the window's buffer, as of the last time
a redisplay completed in this window.

@item last_overlay_modified
The @code{overlay_modiff} field of the window's buffer, as of the last
time a redisplay completed in this window.

@item last_point
The buffer's value of point, as of the last time a redisplay completed
in this window.

@item last_had_star
A non-@code{nil} value means the window's buffer was ``modified'' when the
window was last updated.

@item vertical_scroll_bar
This window's vertical scroll bar.

@item left_margin_width
The width of the left margin in this window, or @code{nil} not to
specify it (in which case the buffer's value of @code{left-margin-width}
is used.

@item right_margin_width
Likewise for the right margin.

@ignore
@item last_mark_x
@item last_mark_y
???Not used.
@end ignore

@item window_end_pos
This is computed as @code{z} minus the buffer position of the last glyph
in the current matrix of the window.  The value is only valid if
@code{window_end_valid} is not @code{nil}.

@item window_end_bytepos
The byte position corresponding to @code{window_end_pos}.

@item window_end_vpos
The window-relative vertical position of the line containing
@code{window_end_pos}.

@item window_end_valid
This field is set to a non-@code{nil} value if @code{window_end_pos} is truly
valid.  This is @code{nil} if nontrivial redisplay is preempted since in that
case the display that @code{window_end_pos} was computed for did not get
onto the screen.

@item redisplay_end_trigger
If redisplay in this window goes beyond this buffer position, it runs
run the @code{redisplay-end-trigger-hook}.

@ignore
@item orig_height
@item orig_top
??? Are temporary storage areas.
@end ignore

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

@item last_cursor
The value of @code{cursor} as of the last redisplay that finished.

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

@item phys_cursor_type
The type of cursor that was last displayed on this window.

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

@item cursor_off_p
Non-zero means the cursor in this window is logically on.

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

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

@item hscroll
This is the number of columns that the display in the window is scrolled
horizontally to the left.  Normally, this is 0.

@item vscroll
Vertical scroll amount, in pixels.  Normally, this is 0.

@item dedicated
Non-@code{nil} if this window is dedicated to its buffer.

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

@item update_mode_line
Non-@code{nil} means this window's mode line needs to be updated.

@item base_line_number
The line number of a certain position in the buffer, or @code{nil}.
This is used for displaying the line number of point in the mode line.

@item base_line_pos
The position in the buffer for which the line number is known, or
@code{nil} meaning none is known.

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

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

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

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

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

  The fields of a process are:

@table @code
@item name
A string, the name of the process.

@item command
A list containing the command arguments that were used to start this
process.

@item filter
A function used to accept output from the process instead of a buffer,
or @code{nil}.

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

@item buffer
The associated buffer of the process.

@item pid
An integer, the Unix process @sc{id}.

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

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

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

@item raw_status_low
@itemx raw_status_high
These two fields record 16 bits each of the process status returned by
the @code{wait} system call.

@item status
The process status, as @code{process-status} should return it.

@item tick
@itemx update_tick
If these two fields are not equal, a change in the status of the process
needs to be reported, either by running the sentinel or by inserting a
message in the process buffer.

@item pty_flag
Non-@code{nil} if communication with the subprocess uses a @sc{pty};
@code{nil} if it uses a pipe.

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

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

@item subtty
The file descriptor for the terminal that the subprocess is using.  (On
some systems, there is no need to record this, so the value is
@code{nil}.)

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

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

@item decoding_buf
A working buffer for decoding.

@item decoding_carryover
Size of carryover in decoding.

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

@item encoding_buf
A working buffer for enecoding.

@item encoding_carryover
Size of carryover in encoding.

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