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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, only 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.
"behind Emacs's back".
27.1 Buffer Basics What is a buffer? 27.2 The Current Buffer Designating a buffer as current so that primitives will access its contents. 27.3 Buffer Names Accessing and changing buffer names. 27.4 Buffer File Name The buffer file name indicates which file is visited. 27.5 Buffer Modification A buffer is modified if it needs to be saved. 27.6 Comparison of Modification Time Determining whether the visited file was changed
27.7 Read-Only Buffers Modifying text is not allowed in a read-only buffer. 27.8 The Buffer List How to look at all the existing buffers. 27.9 Creating Buffers Functions that create buffers. 27.10 Killing Buffers Buffers exist until explicitly killed. 27.11 Indirect Buffers An indirect buffer shares text with some other buffer. 27.12 The Buffer Gap The gap in the buffer.
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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. Although several buffers normally exist, only 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 32. 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 11.10 Buffer-Local Variables.
For functions and variables related to visiting files in buffers, see 25.1 Visiting Files and 25.2 Saving Buffers. For functions and variables related to the display of buffers in windows, see 28.6 Buffers and Windows.
t if object is a buffer,
nil otherwise.
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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 32. 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 21. Command Loop.) Therefore, set-buffer is not the way to
switch visibly to a different buffer so that the user can edit it. For
that, you must use the functions described in 28.7 Displaying Buffers in Windows.
Note: 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 30.3 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))))
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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.
Do not rely on using set-buffer to change the current buffer
back, 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))
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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 ...)
...))
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(current-buffer)
=> #<buffer buffers.texi>
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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 10.5 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 10.5 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 10.5 Nonlocal Exits).
See also with-temp-file in 25.4 Writing to Files.
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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
32.9 Undo.
If buffer-name returns nil, it means that buffer
has been killed. See section 27.10 Killing Buffers.
(buffer-name)
=> "buffers.texi"
(setq foo (get-buffer "temp"))
=> #<buffer temp>
(kill-buffer foo)
=> nil
(buffer-name foo)
=> nil
foo
=> #<killed buffer>
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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.
(This is how the command rename-uniquely is implemented.)
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
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See also the function get-buffer-create in 27.9 Creating Buffers.
If the optional second argument ignore is non-nil, it
should be a string; it makes a difference if it is a name in the
sequence of names to be tried. That name will be considered acceptable,
if it is tried, even if a buffer with that name exists. Thus, if
buffers named `foo', `foo<2>', `foo<3>' and `foo<4>'
exist,
(generate-new-buffer-name "foo")
=> "foo<5>"
(generate-new-buffer-name "foo" "foo<3>")
=> "foo<3>"
(generate-new-buffer-name "foo" "foo<6>")
=> "foo<5>"
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See the related function generate-new-buffer in 27.9 Creating Buffers.
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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 25.1 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"
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nil if it is not visiting a file. It
is a permanent local variable, unaffected by
kill-all-local-variables.
buffer-file-name
=> "/usr/user/lewis/manual/buffers.texi"
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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-all-local-variables. See section 25.6.3 Truenames.
nil if no
file or a nonexistent file is visited. It is a permanent local,
unaffected by kill-all-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 25.6.4 Other Information about Files, for more information
about them.
nil. The argument
filename, which must be a string, is expanded (see section 25.8.4 Functions that Expand Filenames), then compared against the visited file names of all live
buffers.
(get-file-buffer "buffers.texi")
=> #<buffer buffers.texi>
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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.
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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 23.3.2 Variables Used in the Mode Line), and controls saving (see section 25.2 Saving Buffers) and auto-saving (see section 26.2 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 32. 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.
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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
25.6.4 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 25.5 File Locks.
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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:
Here, the purpose is to inform the user that editing the buffer with the aim of saving it in the file may be futile or undesirable. The user who wants to change the buffer text despite this can do so after clearing the read-only flag with C-x C-q.
The special commands of these modes bind 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 32.19.4 Properties with Special Meanings, for more
information about text properties. See section 38.9 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 21.3 Interactive Call, for another way to
signal an error if the current buffer is read-only.
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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 29.3.3 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")
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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))))
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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 29.3.1 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 29.3.3 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 28.6 Buffers and Windows.
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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 38.8 Temporary Displays) and
create-file-buffer (see section 25.1 Visiting Files). Starting a
subprocess can also create a buffer (see section 37. Processes).
An error is signaled if name is not a string.
(get-buffer-create "foo")
=> #<buffer foo>
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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 23.1.3 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>>
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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 23.1.3 How Emacs Chooses a Major Mode.
See the related function generate-new-buffer-name in 27.3 Buffer Names.
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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; if you kill two distinct buffers, they remain
distinct according to eq although both are dead.
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 27.2 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 37.5 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 27.5 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
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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 of this feature is that these functions will
ask for confirmation from the user. 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 23.6 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 11.10 Buffer-Local Variables.
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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.
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 saves 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.
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Emacs buffers are implemented using an invisible gap to make insertion and deletion faster. Insertion works by filling in part of the gap, and deletion adds to the gap. Of course, this means that the gap must first be moved to the locus of the insertion or deletion. Emacs moves the gap only when you try to insert or delete. This is why your first editing command in one part of a large buffer, after previously editing in another far-away part, sometimes involves a noticeable delay.
This mechanism works invisibly, and Lisp code should never be affected by the gap's current location, but these functions are available for getting information about the gap status.
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