27. Buffers

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.

27.1 Buffer Basics

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.

Function: bufferp object

This function returns t if object is a buffer, nil otherwise.

27.2 The Current Buffer

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))))

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))

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 ...)
    ...))

Function: current-buffer

This function returns the current buffer.

(current-buffer) => #<buffer buffers.texi>

Function: set-buffer buffer-or-name

This function makes buffer-or-name the current buffer. This does not display the buffer in any window, so the user cannot necessarily see the buffer. But Lisp programs will now operate on it.

This function returns the buffer identified by buffer-or-name. An error is signaled if buffer-or-name does not identify an existing buffer.

Special Form: save-current-buffer body...

The 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.

Macro: with-current-buffer buffer body...

The 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).

Macro: with-temp-buffer body...

The 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.

27.3 Buffer Names

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.

Function: buffer-name &optional buffer

This function returns the name of buffer as a string. If buffer is not supplied, it defaults to the current buffer.

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>

Command: rename-buffer newname &optional unique

This function renames the current buffer to newname. An error is signaled if newname is not a string, or if there is already a buffer with that name. The function returns newname.

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.)

Function: get-buffer buffer-or-name

This function returns the buffer specified by buffer-or-name. If buffer-or-name is a string and there is no buffer with that name, the value is 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 27.9 Creating Buffers.

Function: generate-new-buffer-name starting-name &rest ignore

This function returns a name that would be unique for a new buffer--but does not create the buffer. It starts with starting-name, and produces a name not currently in use for any buffer by appending a number inside of `<...>'.

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>"

See the related function generate-new-buffer in 27.9 Creating Buffers.

27.4 Buffer File Name

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.

Function: buffer-file-name &optional buffer

This function returns the absolute file name of the file that buffer is visiting. If buffer is not visiting any file, 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"

Variable: buffer-file-name

This buffer-local variable contains the name of the file being visited in the current buffer, or 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"

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.

Variable: buffer-file-truename

This buffer-local variable holds the truename of the file visited in the current buffer, or nil if no file is visited. It is a permanent local, unaffected by kill-all-local-variables. See section 25.6.3 Truenames.

Variable: buffer-file-number

This buffer-local variable holds the file number and directory device number of the file visited in the current buffer, or 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.

Function: get-file-buffer filename

This function returns the buffer visiting file filename. If there is no such buffer, it returns 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>

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.

Command: set-visited-file-name filename &optional no-query along-with-file

If filename is a non-empty string, this function changes the name of the file visited in the current buffer to filename. (If the buffer had no visited file, this gives it one.) The next time the buffer is saved it will go in the newly-specified file. This command marks the buffer as modified, since it does not (as far as Emacs knows) match the contents of filename, even if it matched the former visited file.

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.

Variable: list-buffers-directory

This buffer-local variable specifies a string to display in a buffer listing where the visited file name would go, for buffers that don't have a visited file name. Dired buffers use this variable.

27.5 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 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.

Function: buffer-modified-p &optional buffer

This function returns 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.

Function: set-buffer-modified-p flag

This function marks the current buffer as modified if flag is non-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))

Command: not-modified

This command marks the current buffer as unmodified, and not needing to be saved. With prefix arg, it marks the buffer as modified, so that it will be saved at the next suitable occasion.

Don't use this function in programs, since it prints a message in the echo area; use set-buffer-modified-p (above) instead.

Function: buffer-modified-tick &optional buffer

This function returns buffer's modification-count. This is a counter that increments every time the buffer is modified. If buffer is nil (or omitted), the current buffer is used.

27.6 Comparison of Modification Time

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.

Function: verify-visited-file-modtime buffer

This function compares what buffer has recorded for the modification time of its visited file against the actual modification time of the file as recorded by the operating system. The two should be the same unless some other process has written the file since Emacs visited or saved it.

The function returns t if the last actual modification time and Emacs's recorded modification time are the same, nil otherwise.

Function: clear-visited-file-modtime

This function clears out the record of the last modification time of the file being visited by the current buffer. As a result, the next attempt to save this buffer will not complain of a discrepancy in file modification times.

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.

Function: visited-file-modtime

This function returns the buffer's recorded last file modification time, as a list of the form (high . low). (This is the same format that file-attributes uses to return time values; see 25.6.4 Other Information about Files.)

Function: set-visited-file-modtime &optional time

This function updates the buffer's record of the last modification time of the visited file, to the value specified by time if time is not 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.

Function: ask-user-about-supersession-threat filename

This function is used to ask a user how to proceed after an attempt to modify an obsolete buffer visiting file filename. An obsolete buffer is an unmodified buffer for which the associated file on disk is newer than the last save-time of the buffer. This means some other program has probably altered the file.

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.

27.7 Read-Only Buffers

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:

• A buffer visiting a write-protected file is normally read-only.

  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.

• Modes such as Dired and Rmail make buffers read-only when altering the contents with the usual editing commands would probably be a mistake.

  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.

Variable: buffer-read-only

This buffer-local variable specifies whether the buffer is read-only. The buffer is read-only if this variable is non-nil.

Variable: inhibit-read-only

If this variable is non-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).

Command: toggle-read-only

This command changes whether the current buffer is read-only. It is intended for interactive use; do not use it in programs. At any given point in a program, you should know whether you want the read-only flag on or off; so you can set buffer-read-only explicitly to the proper value, t or nil.

Function: barf-if-buffer-read-only

This function signals a 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.

27.8 The Buffer List

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.

Function: buffer-list &optional frame

This function returns the buffer list, including all buffers, even those whose names begin with a space. The elements are actual buffers, not their names.

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 29.3.1 Access to Frame Parameters).

Function: other-buffer &optional buffer visible-ok frame

This function returns the first buffer in the buffer list other than buffer. Usually this is the buffer selected most recently (in frame frame or else the currently selected frame, see section 29.9 Input Focus), aside from buffer. Buffers whose names start with a space are not considered at all.

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).

Command: bury-buffer &optional buffer-or-name

This function puts buffer-or-name at the end of the buffer list, without changing the order of any of the other buffers on the list. This buffer therefore becomes the least desirable candidate for 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.

27.9 Creating Buffers

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).

Function: get-buffer-create name

This function returns a buffer named name. It returns an existing buffer with that name, if one exists; otherwise, it creates a new buffer. The buffer does not become the current buffer--this function does not change which buffer is current.

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 23.1.3 How Emacs Chooses a Major Mode.

Function: generate-new-buffer name

This function returns a newly created, empty buffer, but does not make it current. If there is no buffer named name, then that is the name of the new buffer. If that name is in use, this function adds suffixes of the form `<n>' to name, where n is an integer. It tries successive integers starting with 2 until it finds an available name.

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 23.1.3 How Emacs Chooses a Major Mode.

See the related function generate-new-buffer-name in 27.3 Buffer Names.

27.10 Killing Buffers

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)))

Command: kill-buffer buffer-or-name

This function kills the buffer buffer-or-name, freeing all its memory for other uses or to be returned to the operating system. It returns 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

Variable: kill-buffer-query-functions

After confirming unsaved changes, 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.

Variable: kill-buffer-hook

This is a normal hook run by 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.

Variable: buffer-offer-save

This variable, if non-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.

27.11 Indirect Buffers

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.

Command: make-indirect-buffer base-buffer name

This creates an indirect buffer named name whose base buffer is base-buffer. The argument base-buffer may be a buffer or a string. If base-buffer is an indirect buffer, its base buffer is used as the base for the new buffer.

Function: buffer-base-buffer buffer

This function returns the base buffer of buffer. If buffer is not indirect, the value is nil. Otherwise, the value is another buffer, which is never an indirect buffer.

27.12 The Buffer Gap

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.

Function: gap-position

This function returns the current gap position in the current buffer.

Function: gap-size

This function returns the current gap size of the current buffer.