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.

Building Emacs
==============

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 src directory produces an executable file called temacs, also called a bare impure Emacs.  It contains the Emacs Lisp interpreter and I/O routines, but not the editing commands.

The command temacs -l loadup uses temacs to create the real runnable Emacs executable.  These arguments direct temacs to evaluate the Lisp files specified in the 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; temacs can dump out an executable program called emacs that has these files preloaded.  emacs starts more quickly because it does not need to load the files.  This is the Emacs executable that is normally installed.

To create emacs, use the command temacs -batch -l loadup dump.  The purpose of -batch here is to prevent 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 dump tells loadup.el to dump a new executable named emacs.

Some operating systems don't support dumping.  On those systems, you must start Emacs with the 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.

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

     #define SITELOAD_PURESIZE_EXTRA N

to make 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 loadup.el reads site-load.el, it finds the documentation strings for primitive and preloaded functions (and variables) in the file etc/DOC where they are stored, by calling Snarf-documentation (see Accessing Documentation).

You can specify other Lisp expressions to execute just before dumping by putting them in a library named 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:

  * Arrange to scan these files when producing the etc/DOC file, and load them with site-load.el.

  * Load the files with site-init.el, then copy the files into the installation directory for Lisp files when you install Emacs.

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

It is not advisable to put anything in site-load.el or 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 default.el, so that users can override your changes if they wish.  See Startup Summary.

 -- Function: dump-emacs to-file from-file
     This function dumps the current state of Emacs into an executable file TO-FILE.  It takes symbols from FROM-FILE (this is normally the executable file temacs).

     If you want to use this function in an Emacs that was already dumped, you must run Emacs with -batch.

Pure Storage
============

Emacs Lisp uses two kinds of storage for user-created Lisp objects: normal storage and 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 temacs is loading the standard preloaded Lisp libraries.  In the 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, temacs crashes.  If that happens, you must increase the compilation parameter PURESIZE in the file src/puresize.h.  This normally won't happen unless you try to preload additional libraries or add features to the standard ones.

 -- Function: purecopy object
     This function makes a copy in pure storage of 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 emacs/lisp/loaddefs.el, but a few packages call it just in case you decide to preload them.

 -- Variable: 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.

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

     This flag is 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 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.

Garbage Collection
==================

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 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.

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

The sweep phase puts unused cons cells onto a 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 malloc and free.

     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 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).

 -- 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 gc-cons-threshold bytes of Lisp data since the previous garbage collection.)

     garbage-collect returns a list containing the following information:

          ((USED-CONSES . FREE-CONSES)
           (USED-SYMS . FREE-SYMS)

           (USED-MISCS . FREE-MISCS)
           USED-STRING-CHARS
           USED-VECTOR-SLOTS
           (USED-FLOATS . FREE-FLOATS)
           (USED-INTERVALS . FREE-INTERVALS))

     Here is an example:

          (garbage-collect)
               => ((106886 . 13184) (9769 . 0)
                          (7731 . 4651) 347543 121628
                          (31 . 94) (1273 . 168))

     Here is a table explaining each element:

     used-conses
          The number of cons cells in use.

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

     used-syms
          The number of symbols in use.

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

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

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

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

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

     used-floats
          The number of floats in use.

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

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

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

 -- User Option: garbage-collection-messages
     If this variable is non-nil, Emacs displays a message at the beginning and end of garbage collection.  The default value is nil, meaning there are no such messages.

 -- User Option: 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 garbage-collect will set the threshold back to 10,000.

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

 -- Function: 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.

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 (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.

 -- Variable: cons-cells-consed
     The total number of cons cells that have been allocated so far in this Emacs session.

 -- Variable: floats-consed
     The total number of floats that have been allocated so far in this Emacs session.

 -- Variable: vector-cells-consed
     The total number of vector cells that have been allocated so far in this Emacs session.

 -- Variable: symbols-consed
     The total number of symbols that have been allocated so far in this Emacs session.

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

 -- Variable: 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.

 -- Variable: intervals-consed
     The total number of intervals that have been allocated so far in this Emacs session.

Writing Emacs Primitives
========================

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

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

     DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
       "Eval args until one of them yields non-nil; return that value.\n\
     The remaining args are not evalled at all.\n\

     If all args return nil, return nil.")
       (args)
          Lisp_Object args;
     {
       register Lisp_Object val;
       Lisp_Object args_left;
       struct gcpro gcpro1;

       if (NILP (args))
         return Qnil;

       args_left = args;
       GCPRO1 (args_left);

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

       UNGCPRO;
       return val;
     }

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

     DEFUN (LNAME, FNAME, SNAME, MIN, MAX, INTERACTIVE, DOC)

lname
     This is the name of the Lisp symbol to define as the function name; in the example above, it is or.

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, F prepended to the Lisp name, with all dashes (-) in the Lisp name changed to underscores.  Thus, to call this function from C code, call For.  Remember that the arguments must be of type Lisp_Object; various macros and functions for creating values of type Lisp_Object are declared in the file lisp.h.

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 FNAME with F replaced with S.

min
     This is the minimum number of arguments that the function requires.  The function or allows a minimum of zero arguments.

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

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

doc
     This is the documentation string.  It is written just like a documentation string for a function defined in Lisp, except you must write \n\ at the end of each line.  In particular, the first line should be a single sentence.

After the call to the 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 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 int and Lisp_Object *.

Within the function For itself, note the use of the macros GCPRO1 and UNGCPRO.  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 Feval or anything that can directly or indirectly call Feval.  At such a time, any Lisp object that you intend to refer to again must be protected somehow. 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 GCPRO1 protects just one local variable.  If you want to protect two, use GCPRO2 instead; repeating GCPRO1 will not work.  Macros GCPRO3 and GCPRO4 also exist.

These macros implicitly use local variables such as gcpro1; you must declare these explicitly, with type struct gcpro.  Thus, if you use GCPRO2, you must declare gcpro1 and 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 stored in 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. See 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 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:

     defsubr (&SUBR-STRUCTURE-NAME);

Here SUBR-STRUCTURE-NAME is the name you used as the third argument to 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 syms_of_SOMETHING, and add the call to defsubr there.  If the file doesn't have this function, or if you create a new file, add to it a syms_of_FILENAME (e.g., syms_of_myfile).  Then find the spot in emacs.c where all of these functions are called, and add a call to syms_of_FILENAME there.

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

If you define a file-scope C variable of type Lisp_Object, you must protect it from garbage-collection by calling staticpro in syms_of_FILENAME, like this:

     staticpro (&VARIABLE);

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

     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\
     ...

     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;

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

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

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

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

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

         default:
           abort ();
         }
     }

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 Ffuncall, which embodies the Lisp function funcall.  Since the Lisp function 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 Ffuncall can call the evaluator, you must protect pointers from garbage collection around the call to Ffuncall.

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

eval.c is a very good file to look through for examples; 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 byte-opt.el which binds side-effect-free-fns and side-effect-and-error-free-fns to include it.  This will help the optimizer.

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 Lisp_Object can hold any Lisp object of any data type.  Ordinary variables have type 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 (see Type Predicates).

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 buffer_text structure contains fields describing the text of a buffer; the buffer structure holds other fields.  In the case of indirect buffers, two or more buffer structures reference the same buffer_text structure.

Here is a list of the struct buffer_text fields:

beg
     This field contains the Actual address of the buffer contents.

got
     This holds the character position of the gap in the buffer.

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

gpt_byte
     Contains the byte position of the gap.

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

gap_size
     Contains the size of buffer's gap.

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

save_modiff
     Contains the previous value of modiff, as of the last time a buffer was visited or saved in a file.

overlay_modiff
     Counts modifications to overlays analogous to modiff.

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.

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.

unchanged_modified
     Contains the value of modiff at the time of the last redisplay that finished.  If this value matches modiff, beg_unchanged and end_unchanged contain no useful information.

overlay_unchanged_modified
     Contains the value of overlay_modiff at the time of the last redisplay that finished.  If this value matches overlay_modiff, beg_unchanged and end_unchanged contain no useful information.

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

intervals
     Contains the interval tree which records the text properties of this buffer.

The fields of struct buffer are:

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.

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

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

pt
     Contains the character position of point in a buffer.

pt_byte
     Contains the byte position of point in a buffer.

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

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

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

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

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

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 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.)

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.  See Buffer Modification.

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

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

last_window_start
     This field contains the window-start position in the buffer as of the last time the buffer was displayed in a window.

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

prevent_redisplay_optimizations_p
     A flag indicating the redisplay optiomizations should not be used to display this buffer.

undo_list
     This field points to the buffer's undo list.  See Undo.

name
     The buffer name is a string that names the buffer.  It is guaranteed to be unique.  See Buffer Names.

filename
     The name of the file visited in this buffer, or nil.

directory
     The directory for expanding relative file names.

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 buffer_text structure because indirect buffers are never saved.

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

read_only
     Non-nil means this buffer is read-only.

mark
     This field contains the mark for the buffer.  The mark is a marker, hence it is also included on the list markers.  See The Mark.

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.)  See Buffer-Local Variables.

major_mode
     Symbol naming the major mode of this buffer, e.g., lisp-mode.

mode_name
     Pretty name of major mode, e.g., "Lisp".

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

header_line_format
     This field is analoguous to mode_line_format for the mode line displayed at the top of windows.

keymap
     This field holds the buffer's local keymap.  See Keymaps.

abbrev_table
     This buffer's local abbrevs.

syntax_table
     This field contains the syntax table for the buffer.  See Syntax Tables.

category_table
     This field contains the category table for the buffer.

case_fold_search
     The value of case-fold-search in this buffer.

tab_width
     The value of tab-width in this buffer.

fill_column
     The value of fill-column in this buffer.

left_margin
     The value of left-margin in this buffer.

auto_fill_function
     The value of auto-fill-function in this buffer.

downcase_table
     This field contains the conversion table for converting text to lower case. See Case Tables.

upcase_table
     This field contains the conversion table for converting text to upper case. See Case Tables.

case_canon_table
     This field contains the conversion table for canonicalizing text for case-folding search.  See Case Tables.

case_eqv_table
     This field contains the equivalence table for case-folding search. See Case Tables.

truncate_lines
     The value of truncate-lines in this buffer.

ctl_arrow
     The value of ctl-arrow in this buffer.

selective_display
     The value of selective-display in this buffer.

selective_display_ellipsis
     The value of selective-display-ellipsis in this buffer.

minor_modes
     An alist of the minor modes of this buffer.

overwrite_mode
     The value of overwrite_mode in this buffer.

abbrev_mode
     The value of abbrev-mode in this buffer.

display_table
     This field contains the buffer's display table, or nil if it doesn't have one.  See Display Tables.

save_modified
     This field contains the time when the buffer was last saved, as an integer. See Buffer Modification.

mark_active
     This field is non-nil if the buffer's mark is active.

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.

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.

overlay_center
     This field holds the current overlay center position.  See Overlays.

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

buffer_file_coding_system
     The value of buffer-file-coding-system in this buffer.

file_format
     The value of buffer-file-format in this buffer.

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.

begv_marker
     In an indirect buffer, or a buffer that is the base of an indirect buffer, this holds a marker that records begv for this buffer when the buffer is not current.

zv_marker
     In an indirect buffer, or a buffer that is the base of an indirect buffer, this holds a marker that records zv for this buffer when the buffer is not current.

file_truename
     The truename of the visited file, or nil.

invisibility_spec
     The value of buffer-invisibility-spec in this buffer.

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

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

left_margin_width
     The value of left-margin-width in this buffer.

right_margin_width
     The value of right-margin-width in this buffer.

indicate_empty_lines
     Non-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.

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

scroll_up_aggressively
     The value of scroll-up-aggressively in this buffer.

scroll_down_aggressively
     The value of scroll-down-aggressively in this buffer.

Window Internals
----------------

Windows have the following accessible fields:

frame
     The frame that this window is on.

mini_p
     Non-nil if this window is a minibuffer window.

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.

hchild
     In a window subdivided horizontally by child windows, the leftmost child. Otherwise, nil.

vchild
     In a window subdivided vertically by child windows, the topmost child. Otherwise, nil.

next
     The next sibling of this window.  It is nil in a window that is the rightmost or bottommost of a group of siblings.

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

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

top
     This is the top edge of the window, measured in lines.  (The top line on the screen is line 0.)

height
     The height of the window, measured in lines.

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).

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

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

pointm
     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.

force_start
     If this flag is non-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.

optional_new_start
     Set to a non-nil value when we have explicitly changed the value of start, but don't want the next redisplay to be obliged to use the new value.

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

start_at_line_beg
     Non-nil means current value of start was the beginning of a line when it was chosen.

too_small_ok
     Non-nil means don't delete this window for becoming ``too small''.

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

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

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

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

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

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

last_had_star
     A non-nil value means the window's buffer was ``modified'' when the window was last updated.

vertical_scroll_bar
     This window's vertical scroll bar.

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

right_margin_width
     Likewise for the right margin.

     ignore

last_mark_x

last_mark_y
     ???Not used.

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

window_end_bytepos
     The byte position corresponding to window_end_pos.

window_end_vpos
     The window-relative vertical position of the line containing window_end_pos.

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

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

     ignore

orig_height

orig_top
     ??? Are temporary storage areas.

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

last_cursor
     The value of cursor as of the last redisplay that finished.

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

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

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

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

last_cursor_off_p
     This field contains the value of cursor_off_p as of the time of the last redisplay.

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

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

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

dedicated
     Non-nil if this window is dedicated to its buffer.

display_table
     The window's display table, or nil if none is specified for it.

update_mode_line
     Non-nil means this window's mode line needs to be updated.

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

base_line_pos
     The position in the buffer for which the line number is known, or nil meaning none is known.

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 nil.

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

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

desired_matrix
     A glyph matrix describing the desired display of this window.

Process Internals
-----------------

The fields of a process are:

name
     A string, the name of the process.

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

filter
     A function used to accept output from the process instead of a buffer, or nil.

sentinel
     A function called whenever the process receives a signal, or nil.

buffer
     The associated buffer of the process.

pid
     An integer, the Unix process ID.

childp
     A flag, non-nil if this is really a child process. It is nil for a network connection.

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.

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

raw_status_low
raw_status_high
     These two fields record 16 bits each of the process status returned by the wait system call.

status
     The process status, as process-status should return it.

tick
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.

pty_flag
     Non-nil if communication with the subprocess uses a PTY; nil if it uses a pipe.

infd
     The file descriptor for input from the process.

outfd
     The file descriptor for output to the process.

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 nil.)

tty_name
     The name of the terminal that the subprocess is using, or nil if it is using pipes.

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

decoding_buf
     A working buffer for decoding.

decoding_carryover
     Size of carryover in decoding.

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

encoding_buf
     A working buffer for enecoding.

encoding_carryover
     Size of carryover in encoding.

inherit_coding_system_flag
     Flag to set coding-system of the process buffer from the coding system used to decode process output.