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 which 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 which have distinct names and hold text that can be edited. Buffers appear to Lisp programs as a special data type. The contents of a buffer may be viewed as an extendable string; 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 only accessible 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.
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 or
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>
nil.
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. See the definition of set-visited-file-name in
`files.el'; 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. See section Truenames.
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.
When the function set-visited-file-name is called interactively, it
prompts for filename in the minibuffer.
See also clear-visited-file-modtime and
verify-visited-file-modtime in section Buffer Modification.
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))
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.
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.
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 (fn), in which
case the proposed buffer modification is not allowed.
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 change the text.
nil.
nil, then read-only buffers and read-only
characters may be modified. The value of buffer-read-only does
not matter when inhibit-read-only is non-nil.
If inhibit-read-only is t, all read-only text
properties have no effect (see section Properties with Special Meanings). If
inhibit-read-only is a list, then read-only text
properties are ignored 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 deletes 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. Several functions, notably other-buffer, use this
ordering. A buffer list displayed for the user also follows this order.
(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")
This list is a copy of a list used inside Emacs; modifying it has no effect on the ordering of buffers.
If buffer-or-name is not supplied (or if it is not a buffer),
then other-buffer returns the first buffer on the buffer list
that is not visible in any window in a visible frame.
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.
If buffer-or-name is nil or omitted, this means to bury
the current buffer. In addition, this switches to some other buffer
(obtained using other-buffer) in the selected window. If the
buffer is displayed in a window other than the selected one, it remains
there.
If you wish 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; 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 according to the variable
default-major-mode. 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 by the value of
default-major-mode. 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 space available for other use.
The buffer object for the buffer which 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.
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
non-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.
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 designate any buffer as current temporarily 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 one to which the command will apply.
(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-excursion that will
restore the current buffer when your function is done
(see section Excursions). 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-excursion
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end))))
This function binds a local variable to the current buffer, and then
save-excursion records the values of point, the mark, and the
original buffer. Next, set-buffer makes another buffer current.
Finally, insert-buffer-substring copies the string from the
original current buffer to the new 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-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-excursion, as shown below, handles quitting, errors
and throw as well as ordinary evaluation.
(let (buffer-read-only)
(save-excursion
(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.