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Appendix A 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.

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A.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 developers.

Building Emacs requires GNU Make version 3.81 or later.

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 would run ‘temacs’ and direct it to load ‘loadup.el’. The loadup library loads additional Lisp libraries, which set up the normal Emacs editing environment. After this step, the Emacs executable is no longer bare.

Because it takes some time to load the standard Lisp files, the ‘temacs’ executable usually isn’t run directly by users. Instead, as one of the last steps of building Emacs, the command ‘temacs -batch -l loadup dump’ is run. The special ‘dump’ argument causes temacs to dump out an executable program, called ‘emacs’, which has all the standard Lisp files preloaded. (The ‘-batch’ argument prevents ‘temacs’ from trying to initialize any of its data on the terminal, so that the tables of terminal information are empty in the dumped Emacs.)

The dumped ‘emacs’ executable (also called a pure Emacs) is the one which is installed. The variable preloaded-file-list stores a list of the Lisp files preloaded into the dumped Emacs. If you port Emacs to a new operating system, and are not able to implement dumping, then Emacs must load ‘loadup.el’ each time it starts.

You can specify additional files to preload by writing a library named ‘site-load.el’ that loads them. You may need to rebuild Emacs with an added definition


to make n added bytes of pure space to hold the additional files; see ‘src/puresize.h’. (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 (@pxref{Definition of Snarf-documentation,, 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:

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. @xref{Startup Summary}. Note that if either ‘site-load.el’ or ‘site-init.el’ changes load-path, the changes will be lost after dumping. @xref{Library Search}. To make a permanent change to load-path, use the ‘--enable-locallisppath’ option of configure.

In a package that can be preloaded, it is sometimes necessary (or useful) to delay certain evaluations until Emacs subsequently starts up. The vast majority of such cases relate to the values of customizable variables. For example, tutorial-directory is a variable defined in ‘startup.el’, which is preloaded. The default value is set based on data-directory. The variable needs to access the value of data-directory when Emacs starts, not when it is dumped, because the Emacs executable has probably been installed in a different location since it was dumped.

Function: custom-initialize-delay symbol value

This function delays the initialization of symbol to the next Emacs start. You normally use this function by specifying it as the :initialize property of a customizable variable. (The argument value is unused, and is provided only for compatibility with the form Custom expects.)

In the unlikely event that you need a more general functionality than custom-initialize-delay provides, you can use before-init-hook (@pxref{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’.

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A.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 section Garbage Collection). 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’ allocates dynamic memory for the part that didn’t fit. The resulting image will work, but garbage collection (see section Garbage Collection) is disabled in this situation, causing a memory leak. Such an overflow normally won’t happen unless you try to preload additional libraries or add features to the standard ones. Emacs will display a warning about the overflow when it starts. If this happens, you should increase the compilation parameter SYSTEM_PURESIZE_EXTRA in the file ‘src/puresize.h’ and rebuild Emacs.

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, but without text properties, 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 preloaded Lisp files.

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 shareable 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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A.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. Different types of Lisp objects, such as symbols, cons cells, small vectors, markers, etc., are segregated in distinct blocks in memory. (Large vectors, long strings, buffers and certain other editing types, which are fairly large, are allocated in individual blocks, one per object; small strings are packed into blocks of 8k bytes, and small vectors are packed into blocks of 4k bytes).

Beyond the basic vector, a lot of objects like window, buffer, and frame are managed as if they were vectors. The corresponding C data structures include the struct vectorlike_header field whose size member contains the subtype enumerated by enum pvec_type and an information about how many Lisp_Object fields this structure contains and what the size of the rest data is. This information is needed to calculate the memory footprint of an object, and used by the vector allocation code while iterating over the vector blocks.

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. 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. Unreachable vectors from vector blocks are coalesced to create largest possible free areas; if a free area spans a complete 4k block, that block is freed. Otherwise, the free area is recorded in a free list array, where each entry corresponds to a free list of areas of the same size. Large vectors, buffers, and other large objects are allocated and freed individually.

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 with information on amount of space in use, where each entry has the form ‘(name size used)’ or ‘(name size used free)’. In the entry, name is a symbol describing the kind of objects this entry represents, size is the number of bytes used by each one, used is the number of those objects that were found live in the heap, and optional free is the number of those objects that are not live but that Emacs keeps around for future allocations. So an overall result is:

((conses cons-size used-conses free-conses)
 (symbols symbol-size used-symbols free-symbols)
 (miscs misc-size used-miscs free-miscs)
 (strings string-size used-strings free-strings)
 (string-bytes byte-size used-bytes)
 (vectors vector-size used-vectors)
 (vector-slots slot-size used-slots free-slots)
 (floats float-size used-floats free-floats)
 (intervals interval-size used-intervals free-intervals)
 (buffers buffer-size used-buffers)
 (heap unit-size total-size free-size))

Here is an example:

      ⇒ ((conses 16 49126 8058) (symbols 48 14607 0)
                 (miscs 40 34 56) (strings 32 2942 2607)
                 (string-bytes 1 78607) (vectors 16 7247)
                 (vector-slots 8 341609 29474) (floats 8 71 102)
                 (intervals 56 27 26) (buffers 944 8)
                 (heap 1024 11715 2678))

Below is a table explaining each element. Note that last heap entry is optional and present only if an underlying malloc implementation provides mallinfo function.


Internal size of a cons cell, i.e., sizeof (struct Lisp_Cons).


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.


Internal size of a symbol, i.e., sizeof (struct Lisp_Symbol).


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.


Internal size of a miscellaneous entity, i.e., sizeof (union Lisp_Misc), which is a size of the largest type enumerated in enum Lisp_Misc_Type.


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.


Internal size of a string header, i.e., sizeof (struct Lisp_String).


The number of string headers in use.


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


This is used for convenience and equals to sizeof (char).


The total size of all string data in bytes.


Internal size of a vector header, i.e., sizeof (struct Lisp_Vector).


The number of vector headers allocated from the vector blocks.


Internal size of a vector slot, always equal to sizeof (Lisp_Object).


The number of slots in all used vectors.


The number of free slots in all vector blocks.


Internal size of a float object, i.e., sizeof (struct Lisp_Float). (Do not confuse it with the native platform float or double.)


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.


Internal size of an interval object, i.e., sizeof (struct interval).


The number of intervals in use.


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


Internal size of a buffer, i.e., sizeof (struct buffer). (Do not confuse with the value returned by buffer-size function.)


The number of buffer objects in use. This includes killed buffers invisible to users, i.e., all buffers in all_buffers list.


The unit of heap space measurement, always equal to 1024 bytes.


Total heap size, in unit-size units.


Heap space which is not currently used, in unit-size units.

If there was overflow in pure space (see section Pure Storage), garbage-collect returns nil, because a real garbage collection cannot be done.

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.

Variable: post-gc-hook

This is a normal hook that is run at the end of garbage collection. Garbage collection is inhibited while the hook functions run, so be careful writing them.

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. You can use the result returned by garbage-collect to get an information about size of the particular object type; 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 interpreter is called.

The initial threshold value is GC_DEFAULT_THRESHOLD, defined in ‘alloc.c’. Since it’s defined in word_size units, the value is 400,000 for the default 32-bit configuration and 800,000 for the 64-bit one. 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 1/10th of GC_DEFAULT_THRESHOLD. A value less than this minimum will remain in effect only until the subsequent garbage collection, at which time garbage-collect will set the threshold back to the minimum.

User Option: gc-cons-percentage

The value of this variable specifies the amount of consing before a garbage collection occurs, as a fraction of the current heap size. This criterion and gc-cons-threshold apply in parallel, and garbage collection occurs only when both criteria are satisfied.

As the heap size increases, the time to perform a garbage collection increases. Thus, it can be desirable to do them less frequently in proportion.

The value returned 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.

Variable: memory-full

This variable is t if Emacs is nearly out of memory for Lisp objects, and nil otherwise.

Function: memory-use-counts

This returns a list of numbers that count the number of objects created in this Emacs session. Each of these counters increments for a certain kind of object. See the documentation string for details.

Function: memory-info

This functions returns an amount of total system memory and how much of it is free. On an unsupported system, the value may be nil.

Variable: gcs-done

This variable contains the total number of garbage collections done so far in this Emacs session.

Variable: gc-elapsed

This variable contains the total number of seconds of elapsed time during garbage collection so far in this Emacs session, as a floating-point number.

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A.4 Stack-allocated Objects

The garbage collector described above is used to manage data visible from Lisp programs, as well as most of the data internally used by the Lisp interpreter. Sometimes it may be useful to allocate temporary internal objects using the C stack of the interpreter. This can help performance, as stack allocation is typically faster than using heap memory to allocate and the garbage collector to free. The downside is that using such objects after they are freed results in undefined behavior, so uses should be well thought out and carefully debugged by using the GC_CHECK_MARKED_OBJECTS feature (see ‘src/alloc.c’). In particular, stack-allocated objects should never be made visible to user Lisp code.

Currently, cons cells and strings can be allocated this way. This is implemented by C macros like AUTO_CONS and AUTO_STRING that define a named Lisp_Object with block lifetime. These objects are not freed by the garbage collector; instead, they have automatic storage duration, i.e., they are allocated like local variables and are automatically freed at the end of execution of the C block that defined the object.

For performance reasons, stack-allocated strings are limited to ASCII characters, and many of these strings are immutable, i.e., calling ASET on them produces undefined behavior.

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A.5 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 session.

Variable: misc-objects-consed

The total number of miscellaneous objects that have been allocated so far in this 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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A.6 C Dialect

The C part of Emacs is portable to C99 or later: C11-specific features such as ‘<stdalign.h>’ and ‘_Noreturn’ are not used without a check, typically at configuration time, and the Emacs build procedure provides a substitute implementation if necessary. Some C11 features, such as anonymous structures and unions, are too difficult to emulate, so they are avoided entirely.

At some point in the future the base C dialect will no doubt change to C11.

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A.7 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,
  doc: /* Eval args until one of them yields non-nil, then return
that value.
The remaining args are not evalled at all.
If all args return nil, return nil.
usage: (or CONDITIONS...)  */)
  (Lisp_Object args)
  Lisp_Object val = Qnil;
  while (CONSP (args))
      val = eval_sub (XCAR (args));
      if (!NILP (val))
      args = XCDR (args);
  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.


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 must be more than min but less than 8.


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. If the value begins with a ‘"(’, the string is evaluated as a Lisp form. For example:

DEFUN ("foo", Ffoo, Sfoo, 0, UNEVALLED,
       "(list (read-char-by-name \"Insert character: \")\
              (prefix-numeric-value current-prefix-arg)\
  doc: /* … /*)

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

If the last line of the documentation string begins with the keyword ‘usage:’, the rest of the line is treated as the argument list for documentation purposes. This way, you can use different argument names in the documentation string from the ones used in the C code. ‘usage:’ is required if the function has an unlimited number of arguments.

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

After the call to the DEFUN macro, you must write the argument list for the C function, including the types for the arguments. If the primitive accepts a fixed maximum number of Lisp arguments, there must be one C argument for each Lisp argument, and each argument must be of type Lisp_Object. (Various macros and functions for creating values of type Lisp_Object are declared in the file ‘lisp.h’.) If the primitive has no upper limit on the number of Lisp arguments, it must have exactly two C arguments: the first is the number of Lisp arguments, and the second is the address of a block containing their values. These have types int and Lisp_Object * respectively. Since Lisp_Object can hold any Lisp object of any data type, you can determine the actual data type only at run time; so if you want a primitive to accept only a certain type of argument, you must check the type explicitly using a suitable predicate (@pxref{Type Predicates}).

Within the function For itself, the local variable args refers to objects controlled by Emacs’s stack-marking garbage collector. Although the garbage collector does not reclaim objects reachable from C Lisp_Object stack variables, it may move non-object components of an object, such as string contents; so functions that access non-object components must take care to refetch their addresses after performing Lisp evaluation. Lisp evaluation can occur via calls to eval_sub or Feval, either directly or indirectly.

Note the call to the QUIT macro inside the loop: this macro checks whether the user pressed C-g, and if so, aborts the processing. You should do that in any loop that can potentially require a large number of iterations; in this case, the list of arguments could be very long. This increases Emacs responsiveness and improves user experience.

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 Pure Storage.

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 (&sname);

Here sname 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 want to make a Lisp variables that is defined in C behave like one declared with defcustom, add an appropriate entry to ‘cus-start.el’.

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, 0,
  doc: /* Return non-nil if COORDINATES are in WINDOW.
  or `right-margin' is returned.  */)
  (register Lisp_Object coordinates, Lisp_Object window)
  struct window *w;
  struct frame *f;
  int x, y;
  Lisp_Object lx, ly;
  w = XWINDOW (window);
  f = XFRAME (w->frame);
  CHECK_CONS (coordinates);
  lx = Fcar (coordinates);
  ly = Fcdr (coordinates);
  switch (coordinates_in_window (w, x, y))
    case ON_NOTHING:            /* NOT in window at all.  */
      return Qnil;

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

    case ON_SCROLL_BAR:         /* On scroll-bar of window.  */
      /* Historically we are supposed to return nil in this case.  */
      return Qnil;
      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.

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’ that binds side-effect-free-fns and side-effect-and-error-free-fns so that the compiler optimizer knows about it.

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A.8 Object Internals

Emacs Lisp provides a rich set of the data types. Some of them, like cons cells, integers and strings, are common to nearly all Lisp dialects. Some others, like markers and buffers, are quite special and needed to provide the basic support to write editor commands in Lisp. To implement such a variety of object types and provide an efficient way to pass objects between the subsystems of an interpreter, there is a set of C data structures and a special type to represent the pointers to all of them, which is known as tagged pointer.

In C, the tagged pointer is an object of type Lisp_Object. Any initialized variable of such a type always holds the value of one of the following basic data types: integer, symbol, string, cons cell, float, vectorlike or miscellaneous object. Each of these data types has the corresponding tag value. All tags are enumerated by enum Lisp_Type and placed into a 3-bit bitfield of the Lisp_Object. The rest of the bits is the value itself. Integers are immediate, i.e., directly represented by those value bits, and all other objects are represented by the C pointers to a corresponding object allocated from the heap. Width of the Lisp_Object is platform- and configuration-dependent: usually it’s equal to the width of an underlying platform pointer (i.e., 32-bit on a 32-bit machine and 64-bit on a 64-bit one), but also there is a special configuration where Lisp_Object is 64-bit but all pointers are 32-bit. The latter trick was designed to overcome the limited range of values for Lisp integers on a 32-bit system by using 64-bit long long type for Lisp_Object.

The following C data structures are defined in ‘lisp.h’ to represent the basic data types beyond integers:

struct Lisp_Cons

Cons cell, an object used to construct lists.

struct Lisp_String

String, the basic object to represent a sequence of characters.

struct Lisp_Vector

Array, a fixed-size set of Lisp objects which may be accessed by an index.

struct Lisp_Symbol

Symbol, the unique-named entity commonly used as an identifier.

struct Lisp_Float

Floating-point value.

union Lisp_Misc

Miscellaneous kinds of objects which don’t fit into any of the above.

These types are the first-class citizens of an internal type system. Since the tag space is limited, all other types are the subtypes of either Lisp_Vectorlike or Lisp_Misc. Vector subtypes are enumerated by enum pvec_type, and nearly all complex objects like windows, buffers, frames, and processes fall into this category. The rest of special types, including markers and overlays, are enumerated by enum Lisp_Misc_Type and form the set of subtypes of Lisp_Misc.

Below there is a description of a few subtypes of Lisp_Vectorlike. Buffer object represents the text to display and edit. Window is the part of display structure which shows the buffer or used as a container to recursively place other windows on the same frame. (Do not confuse Emacs Lisp window object with the window as an entity managed by the user interface system like X; in Emacs terminology, the latter is called frame.) Finally, process object is used to manage the subprocesses.

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A.8.1 Buffer Internals

Two structures (see ‘buffer.h’) 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 are some of the fields in struct buffer_text:


The address of the buffer contents.


The character and byte positions of the buffer gap. @xref{Buffer Gap}.


The character and byte positions of the end of the buffer text.


The size of buffer’s gap. @xref{Buffer Gap}.


These fields count the number of buffer-modification events performed in this buffer. modiff is incremented after each buffer-modification event, and is never otherwise changed; save_modiff contains the value of modiff the last time the buffer was visited or saved; chars_modiff counts only modifications to the characters in the buffer, ignoring all other kinds of changes; and overlay_modiff counts only modifications to the overlays.


The number of characters at the start and end of the text that are known to be unchanged since the last complete redisplay.


The values of modiff and overlay_modiff, respectively, after the last complete redisplay. If their current values match modiff or overlay_modiff, that means 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.


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

Some of the fields of struct buffer are:


A header of type struct vectorlike_header is common to all vectorlike objects.


A struct buffer_text structure that ordinarily holds the buffer contents. In indirect buffers, this field is not used.


A pointer to the buffer_text structure 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.


A pointer to the next buffer, in the chain of all buffers, including killed buffers. This chain is used only for allocation and garbage collection, in order to collect killed buffers properly.


The character and byte positions of point in a buffer.


The character and byte positions of the beginning of the accessible range of text in the buffer.


The character and byte positions 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.)


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


The time when the buffer was last auto-saved.


The window-start position in the buffer as of the last time the buffer was displayed in a window.


This flag indicates that narrowing has changed in the buffer. @xref{Narrowing}.


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


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


These fields hold, respectively, a list of overlays that end at or before the current overlay center, and a list of overlays that end after the current overlay center. @xref{Managing Overlays}. overlays_before is sorted in order of decreasing end position, and overlays_after is sorted in order of increasing beginning position.


A Lisp string that names the buffer. It is guaranteed to be unique. @xref{Buffer Names}.


The 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.


The directory for expanding relative file names. This is the value of the buffer-local variable default-directory (@pxref{File Name Expansion}).


The name of the file visited in this buffer, or nil. This is the value of the buffer-local variable buffer-file-name (@pxref{Buffer File Name}).


These fields store the values of Lisp variables that are automatically buffer-local (@pxref{Buffer-Local Variables}), whose corresponding variable names have the additional prefix buffer- and have underscores replaced with dashes. For instance, undo_list stores the value of buffer-undo-list.


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


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


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


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


These fields store the buffer’s local keymap (@pxref{Keymaps}), abbrev table (@pxref{Abbrev Tables}), syntax table (@pxref{Syntax Tables}), category table (@pxref{Categories}), and display table (@pxref{Display Tables}).


These fields store the conversion tables for converting text to lower case, upper case, and for canonicalizing text for case-fold search. @xref{Case Tables}.


An alist of the minor modes of this buffer.


These fields are only used in an indirect buffer, or in a buffer that is the base of an indirect buffer. Each holds a marker that records pt, begv, and zv respectively, for this buffer when the buffer is not current.


These fields store the values of Lisp variables that are automatically buffer-local (@pxref{Buffer-Local Variables}), whose corresponding variable names have underscores replaced with dashes. For instance, mode_line_format stores the value of mode-line-format.


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

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A.8.2 Window Internals

The fields of a window (for a complete list, see the definition of struct window in ‘window.h’) include:


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.


These fields contain the window’s leftmost child and its topmost child respectively. hchild is used if the window is subdivided horizontally by child windows, and vchild if it is subdivided vertically. In a live window, only one of hchild, vchild, and buffer (q.v.) is non-nil.


The next sibling and previous sibling of this window. next is nil if the window is the right-most or bottom-most in its group; prev is nil if it is the left-most or top-most in its group.


The left-hand edge of the window, measured in columns, relative to the leftmost column in the frame (column 0).


The top edge of the window, measured in lines, relative to the topmost line in the frame (line 0).


The width and height of the window, measured in columns and lines respectively. The 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.


A marker pointing to the position in the buffer that is the first character 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.


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 widths of the left and right margins in this window. A value of nil means no margin.


The widths of the left and right fringes in this window. A value of nil or t means use the values of the frame.


A non-nil value means the fringes outside the display margins; othersize they are between the margin and the text.


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 pre-empted, since in that case the display that window_end_pos was computed for did not get onto the screen.


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, height, and width of the 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 off. This is used for blinking the cursor.


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 it is a buffer, don’t display the line number as long as the window shows that buffer.


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


Glyph matrices describing the current and desired display of this window.

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A.8.3 Process Internals

The fields of a process (for a complete list, see the definition of struct Lisp_Process in ‘process.h’) include:


A string, the name of the process.


A list containing the command arguments that were used to start this process. For a network or serial process, it is nil if the process is running or t if the process is stopped.


A function used to accept output from the process.


A function called whenever the state of the process changes.


The associated buffer of the process.


An integer, the operating system’s process ID. Pseudo-processes such as network or serial connections use a value of 0.


A flag, t if this is really a child process. For a network or serial connection, it is a plist based on the arguments to make-network-process or make-serial-process.


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


The raw process status, as 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 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 encoding.


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


Symbol indicating the type of process: real, network, serial.

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A.9 C Integer Types

Here are some guidelines for use of integer types in the Emacs C source code. These guidelines sometimes give competing advice; common sense is advised.

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