Using Multiple Buffers
**********************

The text you are editing in Emacs resides in an object called a buffer.  Each time you visit a file, a buffer is created to hold the file's text.  Each time you invoke Dired, a buffer is created to hold the directory listing.  If you send a message with C-x m, a buffer named *mail* is used to hold the text of the message.  When you ask for a command's documentation, that appears in a buffer called *Help*.

Each buffer has a unique name, which can be of any length.  When a buffer is displayed in a window, its name is shown in the mode line (see Mode Line).  The distinction between upper and lower case matters in buffer names.  Most buffers are made by visiting files, and their names are derived from the files' names; however, you can also create an empty buffer with any name you want.  A newly started Emacs has a buffer named *scratch*, which is not associated with any file and can be used for evaluating Lisp expressions in Emacs (see Lisp Interaction).

At any time, one and only one buffer is current.  This is also called the selected buffer.  We often say that a command operates on ``the buffer''; this really means that the command operates on the current buffer (most commands do).  When there is only one Emacs window, the buffer displayed in that window is current. When there are multiple windows present, the buffer displayed in the selected window is current.  See Windows.

Each buffer records individually what file it is visiting (if any), whether it is modified, and what major mode and minor modes are in effect (see Major Modes).  Any Emacs variable can be made local to a particular buffer, meaning its value in that buffer can be different from the value in other buffers.  See Locals.

A buffer's size cannot be larger than some maximum, which is defined by the largest buffer position representable by the Emacs integer data type.  This is because Emacs tracks buffer positions using that data type.  For 32-bit machines, the largest buffer size is 256 megabytes.

Creating and Selecting Buffers
==============================

C-x b BUFFER RET
     Select or create a buffer named BUFFER (switch-to-buffer).

C-x 4 b BUFFER RET
     Similar, but select BUFFER in another window (switch-to-buffer-other-window).

C-x 5 b BUFFER RET
     Similar, but select BUFFER in a separate frame (switch-to-buffer-other-frame).

C-x LEFT
     Select the previous buffer in the list of existing buffers.

C-x RIGHT
     Select the next buffer in the list of existing buffers.

C-u M-g M-g
C-u M-g g
     Read a number N and move to line N in the most recently selected buffer other than the current buffer.

To select the buffer named BUFNAME, type C-x b BUFNAME RET.  This runs the command switch-to-buffer with argument BUFNAME.  While entering the buffer name, you can use the usual minibuffer completion and history commands (see Minibuffer).  An empty argument to C-x b specifies the buffer that was current most recently among those not now displayed in any window.

If you specify a buffer that does not exist, C-x b creates a new, empty buffer that is not visiting any file, and selects it for editing.  First, however, Emacs might prompt you for confirmation, in case you entered the wrong buffer name.  Emacs asks for confirmation only if the last key you typed, before submitting the minibuffer input with RET, was TAB (minibuffer-complete).  This catches a common mistake, in which one types RET before realizing that TAB did not complete far enough to yield the desired buffer name (see Completion).  Emacs asks for confirmation by putting the message [Confirm] in the minibuffer; type RET again to confirm and visit the buffer.

The variable confirm-nonexistent-file-or-buffer controls whether Emacs asks for confirmation before visiting a buffer that did not previously exist.  The default value, after-completion, gives the behavior we have just described.  If the value is nil, Emacs never asks for confirmation; for any other non-nil value, Emacs always asks for confirmation.  This variable also affects the find-file command (see Visiting).

One reason to create a new buffer is to use it for making temporary notes.  If you try to save it, Emacs asks for the file name to use. The variable default-major-mode determines the new buffer's major mode; the default value is Fundamental mode.  See Major Modes.

For conveniently switching between a few buffers, use the commands C-x LEFT and C-x RIGHT.  C-x RIGHT (previous-buffer) selects the previous buffer (following the order of most recent selection in the current frame), while C-x LEFT (next-buffer) moves through buffers in the reverse direction.

To select a buffer in a window other than the current one, type C-x 4 b (switch-to-buffer-other-window).  This prompts for a buffer name using the minibuffer, displays that buffer in another window, and selects that window.  By default, if displaying the buffer causes two vertically adjacent windows to be displayed, the heights of those windows are evened out; to countermand that and preserve the window configuration, set the variable even-window-heights to nil.

Similarly, C-x 5 b (switch-to-buffer-other-frame) prompts for a buffer name, displays that buffer in another frame, and selects that frame.

In addition, C-x C-f, and any other command for visiting a file, can also be used to switch to an existing file-visiting buffer. See Visiting.

You can control how certain buffers are handled by these commands by customizing the variables special-display-buffer-names, special-display-regexps, same-window-buffer-names, and same-window-regexps.  See Force Same Window, and Special Buffer Frames, for more about these variables.  In addition, if the value of display-buffer-reuse-frames is non-nil, and the buffer you want to switch to is already displayed in some frame, Emacs will just raise that frame.

C-u M-g M-g, that is goto-line with a plain prefix argument, reads a number N using the minibuffer, selects the most recently selected buffer other than the current buffer in another window, and then moves point to the beginning of line number N in that buffer.  This is mainly useful in a buffer that refers to line numbers in another buffer: if point is on or just after a number, goto-line uses that number as the default for N.  Note that prefix arguments other than just C-u behave differently. C-u 4 M-g M-g goes to line 4 in the current buffer, without reading a number from the minibuffer.  (Remember that M-g M-g without prefix argument reads a number N and then moves to line number N in the current buffer.  See Moving Point.)

Emacs uses buffer names that start with a space for internal purposes. It treats these buffers specially in minor ways---for example, by default they do not record undo information.  It is best to avoid using such buffer names yourself.

Listing Existing Buffers
========================

C-x C-b
     List the existing buffers (list-buffers).

To display a list of existing buffers, type C-x C-b.  Each line in the list shows one buffer's name, major mode and visited file. The buffers are listed in the order that they were current; the buffers that were current most recently come first.

. in the first field of a line indicates that the buffer is current.  % indicates a read-only buffer.  * indicates that the buffer is ``modified.''  If several buffers are modified, it may be time to save some with C-x s (see Save Commands). Here is an example of a buffer list:

     CRM Buffer                Size  Mode              File
     . * .emacs                3294  Emacs-Lisp        ~/.emacs
      %  *Help*                 101  Help
         search.c             86055  C                 ~/cvs/emacs/src/search.c
      %  src                  20959  Dired by name     ~/cvs/emacs/src/
       * *mail*                  42  Mail
      %  HELLO                 1607  Fundamental       ~/cvs/emacs/etc/HELLO
      %  NEWS                481184  Outline           ~/cvs/emacs/etc/NEWS
         *scratch*              191  Lisp Interaction
       * *Messages*            1554  Fundamental

The buffer *Help* was made by a help request (see Help); it is not visiting any file.  The buffer src was made by Dired on the directory ~/cvs/emacs/src/.  You can list only buffers that are visiting files by giving the command a prefix argument, as in C-u C-x C-b.

list-buffers omits buffers whose names begin with a space, unless they visit files: such buffers are used internally by Emacs.

Miscellaneous Buffer Operations
===============================

C-x C-q
     Toggle read-only status of buffer (toggle-read-only).

M-x rename-buffer RET NAME RET
     Change the name of the current buffer.

M-x rename-uniquely
     Rename the current buffer by adding <NUMBER> to the end.

M-x view-buffer RET BUFFER RET
     Scroll through buffer BUFFER.

A buffer can be read-only, which means that commands to change its contents are not allowed.  The mode line indicates read-only buffers with %% or %* near the left margin.  Read-only buffers are usually made by subsystems such as Dired and Rmail that have special commands to operate on the text; also by visiting a file whose access control says you cannot write it.

If you wish to make changes in a read-only buffer, use the command C-x C-q (toggle-read-only).  It makes a read-only buffer writable, and makes a writable buffer read-only.  This works by setting the variable buffer-read-only, which has a local value in each buffer and makes the buffer read-only if its value is non-nil.  If you have files under version control, you may find it convenient to bind C-x C-q to vc-toggle-read-only instead.  This will guard you against an operation that will confuse most modern version-conmtrol systems. See Version Control.

M-x rename-buffer changes the name of the current buffer.  You specify the new name as a minibuffer argument; there is no default. If you specify a name that is in use for some other buffer, an error happens and no renaming is done.

M-x rename-uniquely renames the current buffer to a similar name with a numeric suffix added to make it both different and unique. This command does not need an argument.  It is useful for creating multiple shell buffers: if you rename the *shell* buffer, then do M-x shell again, it makes a new shell buffer named *shell*; meanwhile, the old shell buffer continues to exist under its new name.  This method is also good for mail buffers, compilation buffers, and most Emacs features that create special buffers with particular names.  (With some of these features, such as M-x compile, M-x grep an M-x info, you need to switch to some other buffer before using the command, in order for it to make a different buffer.)

M-x view-buffer is much like M-x view-file (see Misc File Ops) except that it examines an already existing Emacs buffer. View mode provides commands for scrolling through the buffer conveniently but not for changing it.  When you exit View mode with q, that switches back to the buffer (and the position) which was previously displayed in the window.  Alternatively, if you exit View mode with e, the buffer and the value of point that resulted from your perusal remain in effect.

The commands M-x append-to-buffer and M-x insert-buffer can be used to copy text from one buffer to another.  See Accumulating Text.

Killing Buffers
===============

If you continue an Emacs session for a while, you may accumulate a large number of buffers.  You may then find it convenient to kill the buffers you no longer need.  On most operating systems, killing a buffer releases its space back to the operating system so that other programs can use it.  Here are some commands for killing buffers:

C-x k BUFNAME RET
     Kill buffer BUFNAME (kill-buffer).

M-x kill-some-buffers
     Offer to kill each buffer, one by one.

M-x kill-matching-buffers
     Offer to kill all buffers matching a regular expression.

C-x k (kill-buffer) kills one buffer, whose name you specify in the minibuffer.  The default, used if you type just RET in the minibuffer, is to kill the current buffer.  If you kill the current buffer, another buffer becomes current: one that was current in the recent past but is not displayed in any window now.  If you ask to kill a file-visiting buffer that is modified, then you must confirm with yes before the buffer is killed.

The command M-x kill-some-buffers asks about each buffer, one by one.  An answer of y means to kill the buffer, just like kill-buffer.  This command ignores buffers whose names begin with a space, which are used internally by Emacs.

The command M-x kill-matching-buffers prompts for a regular expression and kills all buffers whose names match that expression. See Regexps.  Like kill-some-buffers, it asks for confirmation before each kill.  This command normally ignores buffers whose names begin with a space, which are used internally by Emacs. To kill internal buffers as well, call kill-matching-buffers with a prefix argument.

The buffer menu feature is also convenient for killing various buffers.  See Several Buffers.

If you want to do something special every time a buffer is killed, you can add hook functions to the hook kill-buffer-hook (see Hooks).

If you run one Emacs session for a period of days, as many people do, it can fill up with buffers that you used several days ago.  The command M-x clean-buffer-list is a convenient way to purge them; it kills all the unmodified buffers that you have not used for a long time.  An ordinary buffer is killed if it has not been displayed for three days; however, you can specify certain buffers that should never be killed automatically, and others that should be killed if they have been unused for a mere hour.

You can also have this buffer purging done for you, every day at midnight, by enabling Midnight mode.  Midnight mode operates each day at midnight; at that time, it runs clean-buffer-list, or whichever functions you have placed in the normal hook midnight-hook (see Hooks).  To enable Midnight mode, use the Customization buffer to set the variable midnight-mode to t.  See Easy Customization.

Operating on Several Buffers
============================

M-x buffer-menu
     Begin editing a buffer listing all Emacs buffers.

M-x buffer-menu-other-window.
     Similar, but do it in another window.

The buffer menu opened by C-x C-b (see List Buffers) does not merely list buffers.  It also allows you to perform various operations on buffers, through an interface similar to Dired (see Dired).  You can save buffers, kill them (here called deleting them, for consistency with Dired), or display them.

To use the buffer menu, type C-x C-b and switch to the window displaying the *Buffer List* buffer.  You can also type M-x buffer-menu to open the buffer menu in the selected window. Alternatively, the command M-x buffer-menu-other-window opens the buffer menu in another window, and selects that window.

The buffer menu is a read-only buffer, and can be changed only through the special commands described in this section.  The usual Emacs cursor motion commands can be used in this buffer.  The following commands apply to the buffer described on the current line:

d
     Request to delete (kill) the buffer, then move down.  The request shows as a D on the line, before the buffer name.  Requested deletions take place when you type the x command.

C-d
     Like d but move up afterwards instead of down.

s
     Request to save the buffer.  The request shows as an S on the line.  Requested saves take place when you type the x command. You may request both saving and deletion for the same buffer.

x
     Perform previously requested deletions and saves.

u
     Remove any request made for the current line, and move down.

DEL
     Move to previous line and remove any request made for that line.

The d, C-d, s and u commands to add or remove flags also move down (or up) one line.  They accept a numeric argument as a repeat count.

These commands operate immediately on the buffer listed on the current line:

~
     Mark the buffer ``unmodified.''  The command ~ does this immediately when you type it.

%
     Toggle the buffer's read-only flag.  The command % does this immediately when you type it.

t
     Visit the buffer as a tags table.  See Select Tags Table.

There are also commands to select another buffer or buffers:

q
     Quit the buffer menu---immediately display the most recent formerly visible buffer in its place.

RET
f
     Immediately select this line's buffer in place of the *Buffer List* buffer.

o
     Immediately select this line's buffer in another window as if by C-x 4 b, leaving *Buffer List* visible.

C-o
     Immediately display this line's buffer in another window, but don't select the window.

1
     Immediately select this line's buffer in a full-screen window.

2
     Immediately set up two windows, with this line's buffer selected in one, and the previously current buffer (aside from the buffer *Buffer List*) displayed in the other.

b
     Bury the buffer listed on this line.

m
     Mark this line's buffer to be displayed in another window if you exit with the v command.  The request shows as a > at the beginning of the line.  (A single buffer may not have both a delete request and a display request.)

v
     Immediately select this line's buffer, and also display in other windows any buffers previously marked with the m command.  If you have not marked any buffers, this command is equivalent to 1.

There is also a command that affects the entire buffer list:

T
     Delete, or reinsert, lines for non-file buffers.  This command toggles the inclusion of such buffers in the buffer list.

What buffer-menu actually does is create and switch to a suitable buffer, and turn on Buffer Menu mode in it.  Everything else described above is implemented by the special commands provided in Buffer Menu mode.  One consequence of this is that you can switch from the *Buffer List* buffer to another Emacs buffer, and edit there.  You can reselect the *Buffer List* buffer later, to perform the operations already requested, or you can kill it, or pay no further attention to it.

Normally, the buffer *Buffer List* is not updated automatically when buffers are created and killed; its contents are just text.  If you have created, deleted or renamed buffers, the way to update *Buffer List* to show what you have done is to type g (revert-buffer).  You can make this happen regularly every auto-revert-interval seconds if you enable Auto Revert mode in this buffer, as long as it is not marked modified.  Global Auto Revert mode applies to the *Buffer List* buffer only if global-auto-revert-non-file-buffers is non-nil. See global-auto-revert-non-file-buffers, for details.

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 between files.

M-x make-indirect-buffer RET BASE-BUFFER RET INDIRECT-NAME RET
     Create an indirect buffer named INDIRECT-NAME whose base buffer is BASE-BUFFER.

M-x clone-indirect-buffer RET
     Create an indirect buffer that is a twin copy of the current buffer.

C-x 4 c
     Create an indirect buffer that is a twin copy of the current buffer, and select it in another window (clone-indirect-buffer-other-window).

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.  But in all other respects, the indirect buffer and its base buffer are completely separate.  They can have different names, different values of point, different narrowing, different markers, different major modes, and different 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 the base buffer effectively kills the indirect buffer, but killing an indirect buffer has no effect on its base buffer.

One way to use indirect buffers is to display multiple views of an outline.  See Outline Views.

A quick and handy way to make an indirect buffer is with the command M-x clone-indirect-buffer.  It creates and selects an indirect buffer whose base buffer is the current buffer.  With a numeric argument, it prompts for the name of the indirect buffer; otherwise it uses the name of the current buffer, with a <N> suffix added.  C-x 4 c (clone-indirect-buffer-other-window) works like M-x clone-indirect-buffer, but it selects the new buffer in another window.  These functions run the hook clone-indirect-buffer-hook after creating the indirect buffer.

The more general way to make an indirect buffer is with the command M-x make-indirect-buffer.  It creates an indirect buffer from buffer BASE-BUFFER, under the name INDIRECT-NAME.  It prompts for both BASE-BUFFER and INDIRECT-NAME using the minibuffer.

Convenience Features and Customization of Buffer Handling
=========================================================

This section describes several modes and features that make it more convenient to switch between buffers.

Making Buffer Names Unique
--------------------------

When several buffers visit identically-named files, Emacs must give the buffers distinct names.  The usual method for making buffer names unique adds <2>, <3>, etc. to the end of the buffer names (all but one of them).

Other methods work by adding parts of each file's directory to the buffer name.  To select one, customize the variable uniquify-buffer-name-style (see Easy Customization).

To begin with, the forward naming method includes part of the file's directory name at the beginning of the buffer name; using this method, buffers visiting the files /u/rms/tmp/Makefile and /usr/projects/zaphod/Makefile would be named tmp/Makefile and zaphod/Makefile, respectively (instead of Makefile and Makefile<2>).

In contrast, the post-forward naming method would call the buffers Makefile|tmp and Makefile|zaphod, and the reverse naming method would call them Makefile\tmp and Makefile\zaphod.  The nontrivial difference between post-forward and reverse occurs when just one directory name is not enough to distinguish two files; then reverse puts the directory names in reverse order, so that /top/middle/file becomes file\middle\top, while post-forward puts them in forward order after the file name, as in file|top/middle.

Which rule to follow for putting the directory names in the buffer name is not very important if you are going to look at the buffer names before you type one.  But as an experienced user, if you know the rule, you won't have to look.  And then you may find that one rule or another is easier for you to remember and apply quickly.

Switching Between Buffers using Substrings
------------------------------------------

Iswitchb global minor mode provides convenient switching between buffers using substrings of their names.  It replaces the normal definitions of C-x b, C-x 4 b, C-x 5 b, and C-x 4 C-o with alternative commands that are somewhat ``smarter.''

When one of these commands prompts you for a buffer name, you can type in just a substring of the name you want to choose.  As you enter the substring, Iswitchb mode continuously displays a list of buffers that match the substring you have typed.

At any time, you can type RET to select the first buffer in the list.  So the way to select a particular buffer is to make it the first in the list.  There are two ways to do this.  You can type more of the buffer name and thus narrow down the list, excluding unwanted buffers above the desired one.  Alternatively, you can use C-s and C-r to rotate the list until the desired buffer is first.

TAB while entering the buffer name performs completion on the string you have entered, based on the displayed list of buffers.

To enable Iswitchb mode, type M-x iswitchb-mode, or customize the variable iswitchb-mode to t (see Easy Customization).

Customizing Buffer Menus
------------------------

M-x bs-show
     Make a list of buffers similarly to M-x list-buffers but customizable.

M-x bs-show pops up a buffer list similar to the one normally displayed by C-x C-b but which you can customize.  If you prefer this to the usual buffer list, you can bind this command to C-x C-b.  To customize this buffer list, use the bs Custom group (see Easy Customization).

MSB global minor mode (``MSB'' stands for ``mouse select buffer'') provides a different and customizable mouse buffer menu which you may prefer.  It replaces the bindings of mouse-buffer-menu, normally on C-Down-Mouse-1, and the menu bar buffer menu.  You can customize the menu in the msb Custom group.