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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 | How to preload Lisp libraries into Emacs. | |
A.2 Pure Storage | A kludge to make preloaded Lisp functions sharable. | |
A.3 Garbage Collection | Reclaiming space for Lisp objects no longer used. | |
A.5 Object Internals | Data formats of buffers, windows, processes. | |
A.4 Writing Emacs Primitives | Writing C code for Emacs. |
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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}.
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}.
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.
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"
The value of this variable is the version of Emacs being run. It is a
string, e.g. "18.36.1"
.
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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.
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’.
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.
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.
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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).
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:
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 markers in use.
The number of markers 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 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.
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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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)
This is the name of the Lisp symbol to define with this
function; 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. For
or
, no arguments are required.
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.
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.
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.
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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 | Components of a buffer structure. | |
A.5.2 Window Internals | Components of a window structure. | |
A.5.3 Process Internals | Components of a process structure. |
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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}.
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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.
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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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