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E. 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.

E.1 Building Emacs  How to the dumped Emacs is made.
E.2 Pure Storage  A kludge to make preloaded Lisp functions sharable.
E.3 Garbage Collection  Reclaiming space for Lisp objects no longer used.
E.4 Memory Usage  Info about total size of Lisp objects made so far.
E.5 Writing Emacs Primitives  Writing C code for Emacs.
E.6 Object Internals  Data formats of buffers, windows, processes.

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E.1 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


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 section 24.2 Access to Documentation Strings).

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:

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 section 40.1.1 Summary: Sequence of Actions at Startup.

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'.

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E.2 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.

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E.3 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.

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-floats . free-floats)
 (used-intervals . free-intervals)
 (used-strings . free-strings))

Here is an example:

     => ((106886 . 13184) (9769 . 0)
                (7731 . 4651) 347543 121628
                (31 . 94) (1273 . 168)
                (25474 . 3569))

Here is a table explaining each element:

The number of cons cells in use.

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

The number of symbols in use.

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

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

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

The total size of all strings, in characters.

The total number of elements of existing vectors.

The number of floats in use.

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

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

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

The number of strings in use.

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.)

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.

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E.4 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.

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

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E.5 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.")
     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);

      val = Feval (Fcar (args_left));
      if (!NILP (val))
      args_left = Fcdr (args_left);
  while (!NILP (args_left));

  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)

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

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'.

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'.

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

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.

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.

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 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. See section E.2 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;

      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.

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E.6 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 section 2.6 Type Predicates).

E.6.1 Buffer Internals  Components of a buffer structure.
E.6.2 Window Internals  Components of a window structure.
E.6.3 Process Internals  Components of a process structure.

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E.6.1 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:

This field contains the actual address of the buffer contents.

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

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

Contains the byte position of the gap.

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

Contains the size of buffer's gap.

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

Contains the previous value of modiff, as of the last time a buffer was visited or saved in a file.
Counts modifications to overlays analogous to modiff.
Holds the number of characters at the start of the text that are known to be unchanged since the last redisplay that finished.
Holds the number of characters at the end of the text that are known to be unchanged since the last redisplay that finished.
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.
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.
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.

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

The fields of struct buffer are:

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.

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

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.

Contains the character position of point in a buffer.

Contains the byte position of point in a buffer.

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

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

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

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

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

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.)

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 section 27.5 Buffer Modification.

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

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

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

This flag is set when narrowing changes in a buffer.

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

This field points to the buffer's undo list. See section 32.9 Undo.

The buffer name is a string that names the buffer. It is guaranteed to be unique. See section 27.3 Buffer Names.

The name of the file visited in this buffer, or nil.
The directory for expanding relative file names.

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.

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.

Non-nil means this buffer is read-only.

This field contains the mark for the buffer. The mark is a marker, hence it is also included on the list markers. See section 31.7 The Mark.

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 section 11.10 Buffer-Local Variables.

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

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

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

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

This field holds the buffer's local keymap. See section 22. Keymaps.

This buffer's local abbrevs.

This field contains the syntax table for the buffer. See section 35. Syntax Tables.

This field contains the category table for the buffer.

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

The value of tab-width in this buffer.

The value of fill-column in this buffer.

The value of left-margin in this buffer.

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

This field contains the conversion table for converting text to lower case. See section 4.9 The Case Table.

This field contains the conversion table for converting text to upper case. See section 4.9 The Case Table.

This field contains the conversion table for canonicalizing text for case-folding search. See section 4.9 The Case Table.

This field contains the equivalence table for case-folding search. See section 4.9 The Case Table.

The value of truncate-lines in this buffer.

The value of ctl-arrow in this buffer.

The value of selective-display in this buffer.

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

An alist of the minor modes of this buffer.

The value of overwrite_mode in this buffer.

The value of abbrev-mode in this buffer.

This field contains the buffer's display table, or nil if it doesn't have one. See section 38.17 Display Tables.

This field contains the time when the buffer was last saved, as an integer. See section 27.5 Buffer Modification.

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

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.

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.

This field holds the current overlay center position. See section 38.9 Overlays.

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

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

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

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.

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.
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.

The truename of the visited file, or nil.

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

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

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

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

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

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.

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

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

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E.6.2 Window Internals

Windows have the following accessible fields:

The frame that this window is on.

Non-nil if this window is a minibuffer window.

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.

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

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

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

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

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

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

The height of the window, measured in lines.

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).

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

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

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.

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.

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.

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

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

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

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

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

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

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

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

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

This window's vertical scroll bar.

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.

Likewise for the right margin.

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.

The byte position corresponding to window_end_pos.

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

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.

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

A structure describing where the cursor is in this window.

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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.

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

A glyph matrix describing the current display of this window.

A glyph matrix describing the desired display of this window.

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E.6.3 Process Internals

The fields of a process are:

A string, the name of the process.

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

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

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

The associated buffer of the process.

An integer, the Unix process ID.

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

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.

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

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

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

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.

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

The file descriptor for input from the process.

The file descriptor for output to the process.

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.)

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

Coding-system for decoding the input from this process.

A working buffer for decoding.

Size of carryover in decoding.

Coding-system for encoding the output to this process.

A working buffer for enecoding.

Size of carryover in encoding.

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

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