A buffer is a Lisp object containing text to be edited. Buffers are used to hold the contents of files that are being visited; there may also be buffers that are not visiting files. While several buffers may exist at one time, exactly one buffer is designated the current buffer at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows.
Buffers in Emacs editing are objects that have distinct names and hold text that can be edited. Buffers appear to Lisp programs as a special data type. You can think of the contents of a buffer as a string that you can extend; insertions and deletions may occur in any part of the buffer. See section Text.
A Lisp buffer object contains numerous pieces of information. Some of this information is directly accessible to the programmer through variables, while other information is accessible only through special-purpose functions. For example, the visited file name is directly accessible through a variable, while the value of point is accessible only through a primitive function.
Buffer-specific information that is directly accessible is
stored in buffer-local variable bindings, which are
variable values that are effective only in a particular buffer.
This feature allows each buffer to override the values of certain
variables. Most major modes override variables such as
fill-column or comment-column in this
way. For more information about buffer-local variables and
functions related to them, see section Buffer-Local Variables.
For functions and variables related to visiting files in buffers, see section Visiting Files and section Saving Buffers. For functions and variables related to the display of buffers in windows, see section Buffers and Windows.
t if object is a buffer, nil
otherwise.
There are, in general, many buffers in an Emacs session. At any time, one of them is designated as the current buffer. This is the buffer in which most editing takes place, because most of the primitives for examining or changing text in a buffer operate implicitly on the current buffer (see section Text). Normally the buffer that is displayed on the screen in the selected window is the current buffer, but this is not always so: a Lisp program can temporarily designate any buffer as current in order to operate on its contents, without changing what is displayed on the screen.
The way to designate a current buffer in a Lisp program is by
calling set-buffer. The specified buffer remains
current until a new one is designated.
When an editing command returns to the editor command loop, the
command loop designates the buffer displayed in the selected window
as current, to prevent confusion: the buffer that the cursor is in
when Emacs reads a command is the buffer that the command will
apply to. (See section Command
Loop.) Therefore, set-buffer is not the way to
switch visibly to a different buffer so that the user can edit it.
For this, you must use the functions described in section Displaying Buffers in Windows.
However, Lisp functions that change to a different current
buffer should not depend on the command loop to set it back
afterwards. Editing commands written in Emacs Lisp can be called
from other programs as well as from the command loop. It is
convenient for the caller if the subroutine does not change which
buffer is current (unless, of course, that is the subroutine's
purpose). Therefore, you should normally use
set-buffer within a save-current-buffer
or save-excursion (see section Excursions) form that will restore the
current buffer when your function is done. Here is an example, the
code for the command append-to-buffer (with the
documentation string abridged):
(defun append-to-buffer (buffer start end)
"Append to specified buffer the text of the region.
..."
(interactive "BAppend to buffer: \nr")
(let ((oldbuf (current-buffer)))
(save-current-buffer
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end))))
This function binds a local variable to record the current
buffer, and then save-current-buffer arranges to make
it current again. Next, set-buffer makes the specified
buffer current. Finally, insert-buffer-substring
copies the string from the original current buffer to the specified
(and now current) buffer.
If the buffer appended to happens to be displayed in some window, the next redisplay will show how its text has changed. Otherwise, you will not see the change immediately on the screen. The buffer becomes current temporarily during the execution of the command, but this does not cause it to be displayed.
If you make local bindings (with let or function
arguments) for a variable that may also have buffer-local bindings,
make sure that the same buffer is current at the beginning and at
the end of the local binding's scope. Otherwise you might bind it
in one buffer and unbind it in another! There are two ways to do
this. In simple cases, you may see that nothing ever changes the
current buffer within the scope of the binding. Otherwise, use
save-current-buffer or save-excursion to
make sure that the buffer current at the beginning is current again
whenever the variable is unbound.
It is not reliable to change the current buffer back with
set-buffer, because that won't do the job if a quit
happens while the wrong buffer is current. Here is what
not to do:
(let (buffer-read-only
(obuf (current-buffer)))
(set-buffer ...)
...
(set-buffer obuf))
Using save-current-buffer, as shown here, handles
quitting, errors, and throw, as well as ordinary
evaluation.
(let (buffer-read-only)
(save-current-buffer
(set-buffer ...)
...))
(current-buffer)
=> #<buffer buffers.texi>
This function returns the buffer identified by buffer-or-name. An error is signaled if buffer-or-name does not identify an existing buffer.
save-current-buffer macro
saves the identity of the current buffer, evaluates the
body forms, and finally restores that buffer as current.
The return value is the value of the last form in body.
The current buffer is restored even in case of an abnormal exit via
throw or error (see section Nonlocal Exits). If the buffer that used to be current has been killed by the
time of exit from save-current-buffer, then it is not
made current again, of course. Instead, whichever buffer was
current just before exit remains current.
with-current-buffer macro
saves the identity of the current buffer, makes buffer
current, evaluates the body forms, and finally restores
the buffer. The return value is the value of the last form in
body. The current buffer is restored even in case of an
abnormal exit via throw or error (see section Nonlocal Exits).
with-temp-buffer macro
evaluates the body forms with a temporary buffer as the
current buffer. It saves the identity of the current buffer,
creates a temporary buffer and makes it current, evaluates the
body forms, and finally restores the previous current
buffer while killing the temporary buffer. The return value is the value of the last form in
body. You can return the contents of the temporary
buffer by using (buffer-string) as the last form.
The current buffer is restored even in case of an abnormal exit
via throw or error (see section Nonlocal Exits).
See also with-temp-file in section Writing to Files.
Each buffer has a unique name, which is a string. Many of the functions that work on buffers accept either a buffer or a buffer name as an argument. Any argument called buffer-or-name is of this sort, and an error is signaled if it is neither a string nor a buffer. Any argument called buffer must be an actual buffer object, not a name.
Buffers that are ephemeral and generally uninteresting to the
user have names starting with a space, so that the
list-buffers and buffer-menu commands
don't mention them. A name starting with space also initially
disables recording undo information; see section Undo.
If buffer-name returns nil, it means
that buffer has been killed. See section Killing Buffers.
(buffer-name)
=> "buffers.texi"
(setq foo (get-buffer "temp"))
=> #<buffer temp>
(kill-buffer foo)
=> nil
(buffer-name foo)
=> nil
foo
=> #<killed buffer>
Ordinarily, rename-buffer signals an error if
newname is already in use. However, if unique
is non-nil, it modifies newname to make a
name that is not in use. Interactively, you can make
unique non-nil with a numeric prefix
argument.
One application of this command is to rename the `*shell*' buffer to some other name, thus making it possible to create a second shell buffer under the name `*shell*'.
nil. If
buffer-or-name is a buffer, it is returned as given.
(That is not very useful, so the argument is usually a name.) For
example:
(setq b (get-buffer "lewis"))
=> #<buffer lewis>
(get-buffer b)
=> #<buffer lewis>
(get-buffer "Frazzle-nots")
=> nil
See also the function get-buffer-create in section
Creating Buffers.
See the related function generate-new-buffer in
section Creating Buffers.
The buffer file name is the name of the file that is
visited in that buffer. When a buffer is not visiting a file, its
buffer file name is nil. Most of the time, the buffer
name is the same as the nondirectory part of the buffer file name,
but the buffer file name and the buffer name are distinct and can
be set independently. See section Visiting Files.
buffer-file-name returns nil. If
buffer is not supplied, it defaults to the current
buffer.
(buffer-file-name (other-buffer))
=> "/usr/user/lewis/manual/files.texi"
nil if it is not visiting a file. It is a permanent
local, unaffected by kill-local-variables.
buffer-file-name
=> "/usr/user/lewis/manual/buffers.texi"
It is risky to change this variable's value without doing
various other things. Normally it is better to use
set-visited-file-name (see below); some of the things
done there, such as changing the buffer name, are not strictly
necessary, but others are essential to avoid confusing Emacs.
nil if no file is visited. It is a permanent local,
unaffected by kill-local-variables. See section Truenames.
nil if no file or a
nonexistent file is visited. It is a permanent local, unaffected by
kill-local-variables. The value is normally a list of the form
(filenum devnum). This pair of
numbers uniquely identifies the file among all files accessible on
the system. See the function file-attributes, in
section Other Information about
Files, for more information about them.
nil. The argument
filename, which must be a string, is expanded (see
section Functions that Expand
Filenames), then compared against the visited file names of all
live buffers.
(get-file-buffer "buffers.texi")
=> #<buffer buffers.texi>
In unusual circumstances, there can be more than one buffer visiting the same file name. In such cases, this function returns the first such buffer in the buffer list.
If filename is nil or the empty string,
that stands for "no visited file". In this case,
set-visited-file-name marks the buffer as having no
visited file.
Normally, this function asks the user for confirmation if the
specified file already exists. If no-query is
non-nil, that prevents asking this question.
If along-with-file is non-nil, that
means to assume that the former visited file has been renamed to
filename.
When the function set-visited-file-name is called
interactively, it prompts for filename in the
minibuffer.
Emacs keeps a flag called the modified flag for each
buffer, to record whether you have changed the text of the buffer.
This flag is set to t whenever you alter the contents
of the buffer, and cleared to nil when you save it.
Thus, the flag shows whether there are unsaved changes. The flag
value is normally shown in the mode line (see section Variables Used in the Mode Line), and
controls saving (see section Saving
Buffers) and auto-saving (see section Auto-Saving).
Some Lisp programs set the flag explicitly. For example, the
function set-visited-file-name sets the flag to
t, because the text does not match the newly-visited
file, even if it is unchanged from the file formerly visited.
The functions that modify the contents of buffers are described in section Text.
t if the buffer buffer has been modified
since it was last read in from a file or saved, or nil
otherwise. If buffer is not supplied, the current buffer
is tested.
nil, or as unmodified if the flag is
nil. Another effect of calling this function is to cause
unconditional redisplay of the mode line for the current buffer. In
fact, the function force-mode-line-update works by
doing this:
(set-buffer-modified-p (buffer-modified-p))
Don't use this function in programs, since it prints a message
in the echo area; use set-buffer-modified-p (above)
instead.
nil (or omitted), the current buffer is used.
Suppose that you visit a file and make changes in its buffer, and meanwhile the file itself is changed on disk. At this point, saving the buffer would overwrite the changes in the file. Occasionally this may be what you want, but usually it would lose valuable information. Emacs therefore checks the file's modification time using the functions described below before saving the file.
The function returns t if the last actual
modification time and Emacs's recorded modification time are the
same, nil otherwise.
This function is called in set-visited-file-name
and other exceptional places where the usual test to avoid
overwriting a changed file should not be done.
(high . low). (This is the
same format that file-attributes uses to return time
values; see section Other Information
about Files.)
nil, and otherwise to the last modification time of
the visited file. If time is not nil, it should have the
form (high . low) or
(high low), in either case
containing two integers, each of which holds 16 bits of the
time.
This function is useful if the buffer was not read from the file normally, or if the file itself has been changed for some known benign reason.
Depending on the user's
answer, the function may return normally, in which case the
modification of the buffer proceeds, or it may signal a
file-supersession error with data
(filename), in which case the proposed
buffer modification is not allowed.
This function is called automatically by Emacs on the proper occasions. It exists so you can customize Emacs by redefining it. See the file `userlock.el' for the standard definition.
See also the file locking mechanism in section File Locks.
If a buffer is read-only, then you cannot change its contents, although you may change your view of the contents by scrolling and narrowing.
Read-only buffers are used in two kinds of situations:
buffer-read-only to nil (with
let) or bind inhibit-read-only to
t around the places where they themselves change the
text.
nil.
nil, then read-only buffers and read-only
characters may be modified. Read-only characters in a buffer are
those that have non-nil read-only
properties (either text properties or overlay properties). See
section Properties with Special
Meanings, for more information about text properties. See
section Overlays, for more
information about overlays and their properties. If inhibit-read-only is t, all
read-only character properties have no effect. If
inhibit-read-only is a list, then
read-only character properties have no effect if they
are members of the list (comparison is done with
eq).
buffer-read-only explicitly to the
proper value, t or nil.
buffer-read-only error if the current buffer is
read-only. See section Interactive
Call, for another way to signal an error if the current buffer
is read-only.
The buffer list is a list of all live buffers. Creating
a buffer adds it to this list, and killing a buffer excises it. The
order of the buffers in the list is based primarily on how recently
each buffer has been displayed in the selected window. Buffers move
to the front of the list when they are selected and to the end when
they are buried (see bury-buffer, below). Several
functions, notably other-buffer, use this ordering. A
buffer list displayed for the user also follows this order.
In addition to the fundamental Emacs buffer list, each frame has
its own version of the buffer list, in which the buffers that have
been selected in that frame come first, starting with the buffers
most recently selected in that frame. (This order is
recorded in frame's buffer-list frame
parameter; see section Window Frame
Parameters.) The buffers that were never selected in
frame come afterward, ordered according to the
fundamental Emacs buffer list.
If frame is a frame, this returns frame's
buffer list. If frame is nil, the
fundamental Emacs buffer list is used: all the buffers appear in
order of most recent selection, regardless of which frames they
were selected in.
(buffer-list)
=> (#<buffer buffers.texi>
#<buffer *Minibuf-1*> #<buffer buffer.c>
#<buffer *Help*> #<buffer TAGS>)
;; Note that the name of the minibuffer
;; begins with a space!
(mapcar (function buffer-name) (buffer-list))
=> ("buffers.texi" " *Minibuf-1*"
"buffer.c" "*Help*" "TAGS")
The list that buffer-list returns is constructed
specifically by buffer-list; it is not an internal
Emacs data structure, and modifying it has no effect on the order
of buffers. If you want to change the order of buffers in the
frame-independent buffer list, here is an easy way:
(defun reorder-buffer-list (new-list)
(while new-list
(bury-buffer (car new-list))
(setq new-list (cdr new-list))))
With this method, you can specify any order for the list, but there is no danger of losing a buffer or adding something that is not a valid live buffer.
To change the order or value of a frame's buffer list, set the
frame's buffer-list frame parameter with
modify-frame-parameters (see section Access to Frame Parameters).
If buffer is not supplied (or if it is not a buffer),
then other-buffer returns the first buffer in the
selected frame's buffer list that is not now visible in any window
in a visible frame.
If frame has a non-nil
buffer-predicate parameter, then
other-buffer uses that predicate to decide which
buffers to consider. It calls the predicate once for each buffer,
and if the value is nil, that buffer is ignored. See
section Window Frame
Parameters.
If visible-ok is nil,
other-buffer avoids returning a buffer visible in any
window on any visible frame, except as a last resort. If
visible-ok is non-nil, then it does not
matter whether a buffer is displayed somewhere or not.
If no suitable buffer exists, the buffer `*scratch*' is returned (and created, if necessary).
other-buffer to return. bury-buffer operates on each frame's
buffer-list parameter as well as the frame-independent
Emacs buffer list; therefore, the buffer that you bury will come
last in the value of (buffer-list frame)
and in the value of (buffer-list nil).
If buffer-or-name is nil or omitted,
this means to bury the current buffer. In addition, if the buffer
is displayed in the selected window, this switches to some other
buffer (obtained using other-buffer) in the selected
window. But if the buffer is displayed in some other window, it
remains displayed there.
To replace a buffer in all the windows that display it, use
replace-buffer-in-windows. See section Buffers and Windows.
This section describes the two primitives for creating buffers.
get-buffer-create creates a buffer if it finds no
existing buffer with the specified name;
generate-new-buffer always creates a new buffer and
gives it a unique name.
Other functions you can use to create buffers include
with-output-to-temp-buffer (see section Temporary Displays) and
create-file-buffer (see section Visiting Files). Starting a subprocess
can also create a buffer (see section Processes).
An error is signaled if name is not a string.
(get-buffer-create "foo")
=> #<buffer foo>
The major mode for the new buffer is set to Fundamental mode.
The variable default-major-mode is handled at a higher
level. See section How Emacs Chooses a
Major Mode.
An error is signaled if name is not a string.
(generate-new-buffer "bar")
=> #<buffer bar>
(generate-new-buffer "bar")
=> #<buffer bar<2>>
(generate-new-buffer "bar")
=> #<buffer bar<3>>
The major mode for the new buffer is set to Fundamental mode.
The variable default-major-mode is handled at a higher
level. See section How Emacs Chooses a
Major Mode.
See the related function generate-new-buffer-name
in section Buffer Names.
Killing a buffer makes its name unknown to Emacs and makes its text space available for other use.
The buffer object for the buffer that has been killed remains in
existence as long as anything refers to it, but it is specially
marked so that you cannot make it current or display it. Killed
buffers retain their identity, however; two distinct buffers, when
killed, remain distinct according to eq.
If you kill a buffer that is current or displayed in a window, Emacs automatically selects or displays some other buffer instead. This means that killing a buffer can in general change the current buffer. Therefore, when you kill a buffer, you should also take the precautions associated with changing the current buffer (unless you happen to know that the buffer being killed isn't current). See section The Current Buffer.
If you kill a buffer that is the base buffer of one or more indirect buffers, the indirect buffers are automatically killed as well.
The buffer-name of a killed buffer is
nil. You can use this feature to test whether a buffer
has been killed:
(defun buffer-killed-p (buffer) "Return t if BUFFER is killed." (not (buffer-name buffer)))
nil. Any processes that have this buffer as the
process-buffer are sent the SIGHUP
signal, which normally causes them to terminate. (The basic meaning
of SIGHUP is that a dialup line has been
disconnected.) See section Deleting
Processes.
If the buffer is visiting a file and contains unsaved changes,
kill-buffer asks the user to confirm before the buffer
is killed. It does this even if not called interactively. To
prevent the request for confirmation, clear the modified flag
before calling kill-buffer. See section Buffer Modification.
Killing a buffer that is already dead has no effect.
(kill-buffer "foo.unchanged")
=> nil
(kill-buffer "foo.changed")
---------- Buffer: Minibuffer ----------
Buffer foo.changed modified; kill anyway? (yes or no) yes
---------- Buffer: Minibuffer ----------
=> nil
kill-buffer calls the functions in the list
kill-buffer-query-functions, in order of appearance,
with no arguments. The buffer being killed is the current buffer
when they are called. The idea is that these functions ask for
confirmation from the user for various nonstandard reasons. If any
of them returns nil, kill-buffer spares
the buffer's life.
kill-buffer after asking all the questions it is
going to ask, just before actually killing the buffer. The buffer
to be killed is current when the hook functions run. See section Hooks.
nil in a particular buffer, tells
save-buffers-kill-emacs and
save-some-buffers to offer to save that buffer, just
as they offer to save file-visiting buffers. The variable
buffer-offer-save automatically becomes buffer-local
when set for any reason. See section Buffer-Local Variables.
An indirect buffer shares the text of some other buffer, which is called the base buffer of the indirect buffer. In some ways it is the analogue, for buffers, of a symbolic link among files. The base buffer may not itself be an indirect buffer.
The text of the indirect buffer is always identical to the text of its base buffer; changes made by editing either one are visible immediately in the other. This includes the text properties as well as the characters themselves.
But in all other respects, the indirect buffer and its base buffer are completely separate. They have different names, different values of point, different narrowing, different markers and overlays (though inserting or deleting text in either buffer relocates the markers and overlays for both), different major modes, and different buffer-local variables.
An indirect buffer cannot visit a file, but its base buffer can. If you try to save the indirect buffer, that actually works by saving the base buffer.
Killing an indirect buffer has no effect on its base buffer. Killing the base buffer effectively kills the indirect buffer in that it cannot ever again be the current buffer.
nil. Otherwise, the value is
another buffer, which is never an indirect buffer.