emacs/lispref/internals.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993 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 preload Lisp libraries into Emacs.
* Pure Storage::        A kludge to make preloaded Lisp functions sharable.
* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
* Writing Emacs Primitives::   Writing C code for Emacs.
* Object Internals::    Data formats of buffers, windows, processes.
@end menu

@node Building Emacs, Pure Storage, GNU Emacs Internals, GNU Emacs Internals
@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 increase the
value of @code{PURESIZE}, in @file{src/puresize.h}, to make room for 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.

@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}.  However, if
they might alter the behavior that users expect from an ordinary
unmodified Emacs, it is better to put them in @file{default.el}, so that
users can override them if they wish.  @xref{Start-up Summary}.

  Before @file{loadup.el} dumps the new executable, it finds the
documentation strings for primitive and preloaded functions (and
variables) in the file where they are stored, by calling
@code{Snarf-documentation} (@pxref{Accessing Documentation}).  These
strings were moved out of the @file{emacs} executable to make it
smaller.  @xref{Documentation Basics}.

@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 use this function in an Emacs that was already dumped, you must
set @code{command-line-processed} to @code{nil} first for good results.
@xref{Command Line Arguments}.
@end defun

@deffn Command emacs-version
  This function returns a string describing the version of Emacs that is
running.  It is useful to include this string in bug reports.

@example
@group
(emacs-version)
  @result{} "GNU Emacs 19.29.1 (i386-debian-linux) \
 of Tue Jun  6 1995 on balloon"
@end group
@end example

Called interactively, the function prints the same information in the
echo area.
@end deffn

@defvar emacs-build-time
The value of this variable is the time at which Emacs was built at the
local site.

@example
@group
emacs-build-time
     @result{} "Tue Jun  6 14:55:57 1995"
@end group
@end example
@end defvar

@defvar emacs-version
The value of this variable is the version of Emacs being run.  It is a
string such as @code{"19.29.1"}.
@end defvar

  The following two variables did not exist before Emacs version 19.23,
which reduces their usefulness at present, but we hope they will be
convenient in the future.

@defvar emacs-major-version
The major version number of Emacs, as an integer.  For Emacs version
19.29, the value is 19.
@end defvar

@defvar emacs-minor-version
The minor version number of Emacs, as an integer.  For Emacs version
19.29, the value is 29.
@end defvar

@node Pure Storage, Garbage Collection, Building Emacs, GNU Emacs Internals
@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 is 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 of @var{object} in pure storage and returns
it.  It copies strings 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, Writing Emacs Primitives, Pure Storage, GNU Emacs Internals
@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.

@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-markers} . @var{free-markers})
 @var{used-string-chars} 
 @var{used-vector-slots}
 (@var{used-floats} . @var{free-floats}))

@group
(garbage-collect)
     @result{} ((3435 . 2332) (1688 . 0)
           (57 . 417) 24510 3839 (4 . 1))
@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-markers
The number of markers in use.

@item free-markers
The number of markers 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.
@end table
@end deffn

@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 300,000.  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

@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 Writing Emacs Primitives, Object Internals, Garbage Collection, GNU Emacs Internals
@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,
  "Eval args until one of them yields non-nil, then return that value.\n\
The remaining args are not evalled at all.\n\
@end group
@group
If all args return nil, return nil.")
  (args)
     Lisp_Object args;
@{
  register Lisp_Object val;
  Lisp_Object args_left;
  struct gcpro gcpro1;
@end group

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

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

@group
  do
    @{
      val = Feval (Fcar (args_left));
      if (!NULL (val))
        break;
      args_left = Fcdr (args_left);
    @}
  while (!NULL (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 is written just like a
documentation string for a function defined in Lisp, except you must
write @samp{\n\} at the end of each line.  In particular, the first line
should be a single sentence.
@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
they 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.

  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}.

  Here is another example function, with more complicated arguments.
This comes from the code for the X Window System, 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 POSITIONS is in WINDOW.\n\  
  \(POSITIONS is a list, (SCREEN-X SCREEN-Y)\)\n\
@end group
@group
  Returned value is list of positions expressed\n\
  relative to window upper left corner.")
  (coordinate, window)
     register Lisp_Object coordinate, window;
@{
  register Lisp_Object xcoord, ycoord;
@end group

@group
  if (!CONSP (coordinate)) wrong_type_argument (Qlistp, coordinate);
  CHECK_WINDOW (window, 2);
  xcoord = Fcar (coordinate);
  ycoord = Fcar (Fcdr (coordinate));
  CHECK_NUMBER (xcoord, 0);
  CHECK_NUMBER (ycoord, 1);
@end group
@group
  if ((XINT (xcoord) < XINT (XWINDOW (window)->left))
      || (XINT (xcoord) >= (XINT (XWINDOW (window)->left)
                            + XINT (XWINDOW (window)->width))))
    return Qnil;
  XFASTINT (xcoord) -= XFASTINT (XWINDOW (window)->left);
@end group
@group
  if (XINT (ycoord) == (screen_height - 1))
    return Qnil;
@end group
@group
  if ((XINT (ycoord) < XINT (XWINDOW (window)->top))
      || (XINT (ycoord) >= (XINT (XWINDOW (window)->top)
                            + XINT (XWINDOW (window)->height)) - 1))
    return Qnil;
@end group
@group
  XFASTINT (ycoord) -= XFASTINT (XWINDOW (window)->top);
  return (Fcons (xcoord, Fcons (ycoord, Qnil)));
@}
@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.

@node Object Internals,  , Writing Emacs Primitives, GNU Emacs 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-four to twenty-six bits are used to
address the object, and the remaining six to eight bits are used for a
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, Window Internals, Object Internals, Object 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.

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

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

@item modtime
This field contains the modification time of the visited file.  It is
set when the file is written or read.  Every time the buffer is written
to the file, this field is compared to the modification time of the
file.  @xref{Buffer Modification}.

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

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

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

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

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

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

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

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

@item markers
This field contains the chain of all markers that currently point into
the buffer.  Deletion of text in the buffer, and motion of the buffer's
gap, must check each of these markers and perhaps update it.
@xref{Markers}.

@item backed_up
This field is a flag that tells whether a backup file has been made
for the visited file of this buffer.

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

@item local_var_alist
This field contains the association list describing the variables local
in this buffer, and their values, with the exception of local variables
that have special slots in the buffer object.  (Those slots are omitted
from this table.)  @xref{Buffer-Local Variables}.

@item base_buffer
This field holds the buffer's base buffer (if it is an indirect buffer),
or @code{nil}.

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

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

@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.
@end table

@node Window Internals, Process Internals, Buffer Internals, Object 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 buffer
The buffer that the window is displaying.  This may change often during
the life of the window.

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

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

@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 last_modified
The @code{modified} 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 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.

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

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

@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.

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

@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}.
@end table

@node Process Internals,  , Window Internals, Object 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.
@end table