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.

A.1 Building Emacs

The first step in building Emacs is to compile the C sources. This produces a program called ‘temacs’, also called a bare impure Emacs. It contains the Emacs Lisp interpreter and I/O routines, but not the editing commands.

Then, to create a working Emacs editor, issue the ‘temacs -l loadup’ command. This directs ‘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 which is still impure but no longer bare.

It takes a long 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’ which has these files preloaded. ‘emacs’ starts more quickly because it does not need to load the files. This is the program 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.

When the ‘emacs’ executable is started, it automatically loads the user’s ‘.emacs’ file, or the default initialization file ‘default.el’ if the user has none. (@xref{Starting Up}.) With the ‘.emacs’ file, you can produce a version of Emacs that suits you and is not the same as the version other people use. With ‘default.el’, you can customize Emacs for all the users at your site who don’t choose to customize it for themselves. (For further reflection: why is this different from the case of the barber who shaves every man who doesn’t shave himself?)

On some systems, dumping does not work. Then, you must start Emacs with the ‘temacs -l loadup’ command each time you use it. This takes a long time, but since you need to start Emacs once a day at most—and once a week or less frequently if you never log out—the extra time is not too severe a problem.

Before ‘emacs’ is dumped, the documentation strings for primitive and preloaded functions (and variables) need to be found in the file where they are stored. This is done by calling Snarf-documentation (@pxref{Accessing Documentation}). These strings were moved out of ‘emacs’ to make it smaller. @xref{Documentation Basics}.

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

Command: emacs-version

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

(emacs-version)
  ⇒ "GNU Emacs 18.36.1 of Fri Feb 27 1987 on slug
     (berkeley-unix)"

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

Variable: emacs-build-time

The value of this variable is the time at which Emacs was built at the local site.

emacs-build-time
     ⇒ "Fri Feb 27 14:55:57 1987"

Variable: emacs-version

The value of this variable is the version of Emacs being run. It is a string, e.g. "18.36.1".

A.2 Pure Storage

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

Pure storage is allocated only while ‘temacs’ is loading the standard preloaded Lisp libraries. In the file ‘emacs’, it is marked as read-only (on operating systems which 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 will have to increase the compilation parameter PURESIZE in the file ‘config.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 of object in pure storage and returns it. It copies strings by simply making a new string with the same characters in pure storage. It recursively copies the contents of vectors and cons cells. It does not make copies of symbols, or any other objects, but just returns them unchanged. It signals an error if asked to copy markers.

This function is used only while Emacs is being built and dumped; it is called only in the file ‘emacs/lisp/loaddefs.el’.

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). It is set to nil when Emacs is saved out as ‘emacs’. The flag is set and reset in the C sources.

You should not change this flag in a running Emacs.

A.3 Garbage Collection

When a program creates a list or the user defines a new function (such as by loading a library), then 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, buffers and certain other editing types, which are fairly large, are allocated in individual blocks, one per object, while 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 scanning all the objects that have been allocated and marking those that are still accessible to Lisp programs. To begin with, all the symbols, their values and associated function definitions, and any data presently on the stack, are accessible. Any objects which can be reached indirectly through other accessible objects are also accessible.

When this is finished, all inaccessible objects 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 notice. That is what the garbage collector arranges to do.

Unused cons cells are chained together onto a free list for future allocation; likewise for symbols and markers. The accessible strings are compacted so they are contiguous in memory; then the rest of the space formerly occupied by strings is made available to the string creation functions. Vectors, buffers, windows and other large objects are individually allocated and freed using malloc.

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-markers . free-markers)
 used-string-chars 
 used-vector-slots
 (used-floats . free-floats))

(garbage-collect)
     ⇒ ((3435 . 2332) (1688 . 0) (57 . 417) 24510 3839 (4 . 1))

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-markers

The number of markers in use.

free-markers

The number of markers 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.

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 request 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 new 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 100,000. 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 which 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.

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.

A.4 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, then 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 (NULL(args))
    return Qnil;

  args_left = args;
  GCPRO1 (args_left);

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

  UNGCPRO;
  return val;
}

Let’s start with a precise explanation of the arguments to the DEFUN macro. Here are the general names for them:

DEFUN (lname, fname, sname, min, max, interactive, doc)

lname

This is the name of the Lisp symbol to define with this function; 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. For or, no arguments are required.

max

This is the maximum number of arguments that the function accepts. Alternatively, it can be UNEVALLED, indicating a special form that receives unevaluated arguments. A function with the equivalent of an &rest argument would have MANY in this position. Both UNEVALLED and MANY are macros. This argument must be one of these macros or a number at least as large as min. It may not be greater than six.

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 an interactive function taking no arguments.

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 list of argument names that every C function must have, followed by ordinary C declarations for them. Normally, all the arguments must be declared as Lisp_Object. If the function has no upper limit on the number of arguments in Lisp, then in C it receives two arguments: the number of Lisp arguments, and the address of a block containing their values. These 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 know that one pointer to the object is protected; 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 a string is moved, any pointers to it that the garbage collector does not know about will not be properly relocated. Therefore, all pointers to strings must be protected 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. There are also GCPRO3 and GCPRO4.

In addition to using these macros, you must declare the local variables such as gcpro1 which they implicitly use. If you protect two variables, with GCPRO2, you must declare gcpro1 and gcpro2, as it uses them both. Alas, we can’t explain all the tricky details here.

Defining the C function is not enough; you must also create the Lisp symbol for the primitive and store a suitable subr object in its function cell. This is done by adding code to an initialization routine. The code looks like this:

defsubr (&subr-structure-name);

subr-structure-name is the name you used as the third argument to DEFUN.

If you are adding a 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 that function call to it. 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.

This function syms_of_filename is also the place to define any C variables which are to be visible as Lisp variables. DEFVAR_LISP is used to make a C variable of type Lisp_Object visible in Lisp. DEFVAR_INT is used to make a C variable of type int visible in Lisp with a value that is an integer.

Here is another function, with more complicated arguments. This comes from the code for the X Window System, and it demonstrates the use of macros and functions to manipulate Lisp objects.

DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
  Scoordinates_in_window_p, 2, 2,
  "xSpecify coordinate pair: \nXExpression which evals to window: ",
  "Return non-nil if POSITIONS is in WINDOW.\n\  
  \(POSITIONS is a list, (SCREEN-X SCREEN-Y)\)\n\

  Returned value is list of positions expressed\n\
  relative to window upper left corner.")
  (coordinate, window)
     register Lisp_Object coordinate, window;
{
  register Lisp_Object xcoord, ycoord;

  if (!CONSP (coordinate)) wrong_type_argument (Qlistp, coordinate);
  CHECK_WINDOW (window, 2);
  xcoord = Fcar (coordinate);
  ycoord = Fcar (Fcdr (coordinate));
  CHECK_NUMBER (xcoord, 0);
  CHECK_NUMBER (ycoord, 1);

  if ((XINT (xcoord) < XINT (XWINDOW (window)->left))
      || (XINT (xcoord) >= (XINT (XWINDOW (window)->left)
                            + XINT (XWINDOW (window)->width))))
    {
      return Qnil;
    }
  XFASTINT (xcoord) -= XFASTINT (XWINDOW (window)->left);

  if (XINT (ycoord) == (screen_height - 1))
    return Qnil;

  if ((XINT (ycoord) < XINT (XWINDOW (window)->top))
      || (XINT (ycoord) >= (XINT (XWINDOW (window)->top)
                            + XINT (XWINDOW (window)->height)) - 1))
    {
      return Qnil;
    }

  XFASTINT (ycoord) -= XFASTINT (XWINDOW (window)->top);
  return (Fcons (xcoord, Fcons (ycoord, Qnil)));
}

Note that you cannot directly call functions defined in Lisp as, for example, the primitive function Fcons is called above. You must create the appropriate Lisp form, protect everything from garbage collection, and Feval the form, as was done in For above.

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

A.5 Object Internals

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

Because all access to data is through tagged pointers, it is always possible to determine the type of any object. This allows variables to be untyped, and the values assigned to them to be changed without regard to type. Function arguments also can be of any type; if you want a function to accept only a certain type of argument, you must check the type explicitly using a suitable predicate (@pxref{Type Predicates}).

A.5.1 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.

name

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

save_modified

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

modtime

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

auto_save_modified

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

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.

undodata

This field points to the buffer’s undo stack. @xref{Undo}.

syntax_table_v

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

downcase_table

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

upcase_table

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

case_canon_table

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

case_eqv_table

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

display_table

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

markers

This field contains the chain of all markers that point into the buffer. At each deletion or motion of the buffer gap, all of these markers must be checked and perhaps updated. @xref{Markers}.

backed_up

This field is a flag which tells whether a backup file has been made for the visited file of this buffer.

mark

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

local_var_alist

This field contains the association list containing all of the variables local in this buffer, and their values. The function buffer-local-variables returns a copy of this list. @xref{Buffer-Local Variables}.

mode_line_format

This field contains a Lisp object which controls how to display the mode line for this buffer. @xref{Mode Line Format}.

A.5.2 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.

height

The height of the window, measured in lines.

width

The width of the window, measured in columns.

buffer

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

dedicated

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

start

The position in the buffer which 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.

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.)

next

This is the window that is the next in the chain of siblings.

prev

This is the window that is the previous in the chain of siblings.

force_start

This is a flag which, if non-nil, says that the window has been scrolled explicitly by the Lisp program. At the next redisplay, if point is off the screen, instead of scrolling the window to show the text around point, point will be moved to a location that is on the screen.

hscroll

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

use_time

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

display_table

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

A.5.3 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.

flags

A symbol indicating the state of the process. Possible values include run, stop, closed, etc.

reason

An integer, the Unix signal number that the process received that caused the process to terminate or stop. If the process has exited, then this is the exit code it specified.

mark

A marker indicating the position of end of last output from this process inserted into the buffer. This is usually the end of the buffer.

kill_without_query

A flag, non-nil meaning this process should not cause confirmation to be needed if Emacs is killed.

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