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*.

At any time, one and only one buffer is selected.  It is also called the current buffer.  Often we say that a command operates on ``the buffer'' as if there were only one; but really this means that the command operates on the selected buffer (most commands do).

When Emacs has multiple windows, each window has its own chosen buffer and displays it; at any time, only one of the windows is selected, and its chosen buffer is the selected buffer.  Each window's mode line normally displays the name of the window's chosen buffer (see Windows).

Each buffer has a name, which can be of any length, and you can select any buffer by giving its name.  Most buffers are made by visiting files, and their names are derived from the files' names.  But you can also create an empty buffer with any name you want.  A newly started Emacs has a buffer named *scratch* which can be used for evaluating Lisp expressions in Emacs.  The distinction between upper and lower case matters in buffer names.

Each buffer records individually what file it is visiting, whether it is modified, and what major mode and minor modes are in effect in it (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 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 128 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).

To select the buffer named BUFNAME, type C-x b BUFNAME RET.  This runs the command switch-to-buffer with argument BUFNAME.  You can use completion on an abbreviation for the buffer name you want (see Completion).  An empty argument to C-x b specifies the most recently selected buffer that is not displayed in any window.

To select a buffer in a window other than the current one, type C-x 4 b BUFNAME RET.  This runs the command switch-to-buffer-other-window which displays the buffer BUFNAME in another 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 BUFFER RET runs the command switch-to-buffer-other-frame which selects a buffer in another frame.

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 raise that frame.

Most buffers are created by visiting files, or by Emacs commands that want to display some text, but you can also create a buffer explicitly by typing C-x b BUFNAME RET.  This makes a new, empty buffer that is not visiting any file, and selects it for editing.  Such buffers are used for making notes to yourself.  If you try to save one, you are asked for the file name to use.  The new buffer's major mode is determined by the value of default-major-mode (see Major Modes).

Note that 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.

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 all the buffers that exist, 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.

* at the beginning of a line indicates the buffer is ``modified.'' If several buffers are modified, it may be time to save some with C-x s (see Saving).  % indicates a read-only buffer.  . marks the selected buffer.  Here is an example of a buffer list:

      MR Buffer         Size  Mode           File
      -- ------         ----  ----           ----
     .*  emacs.tex      383402 Texinfo       /u2/emacs/man/emacs.tex
         *Help*         1287  Fundamental
         files.el       23076 Emacs-Lisp     /u2/emacs/lisp/files.el
       % RMAIL          64042 RMAIL          /u/rms/RMAIL
      *% man            747   Dired          /u2/emacs/man/
         net.emacs      343885 Fundamental   /u/rms/net.emacs
         fileio.c       27691 C              /u2/emacs/src/fileio.c
         NEWS           67340 Text           /u2/emacs/etc/NEWS
         *scratch*	   0	 Lisp Interaction

Note that the buffer *Help* was made by a help request; it is not visiting any file.  The buffer man was made by Dired on the directory /u2/emacs/man/.  You can list only buffers that are visiting files by giving the command a prefix; for instance, by typing C-u C-x C-b.

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

C-x C-q
     Toggle read-only status of buffer (vc-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 (vc-toggle-read-only).  It makes a read-only buffer writable, and makes a writable buffer read-only.  In most cases, 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 the file is maintained with version control, C-x C-q works through the version control system to change the read-only status of the file as well as the buffer.  See Version Control.

M-x rename-buffer changes the name of the current buffer.  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.

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.

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 is selected; one that has been selected recently but does not appear in any window now.  If you ask to kill a file-visiting buffer that is modified (has unsaved editing), 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.  Killing the current buffer or a buffer containing unsaved changes selects a new buffer or asks for confirmation just like kill-buffer.

The buffer menu feature (see Several Buffers) is also convenient for killing various 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
============================

The buffer-menu facility is like a ``Dired for buffers''; it allows you to request operations on various Emacs buffers by editing an Emacs buffer containing a list of them.  You can save buffers, kill them (here called deleting them, for consistency with Dired), or display them.

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

The command buffer-menu writes a list of all Emacs buffers into the buffer *Buffer List*, and selects that buffer in Buffer Menu mode.  The buffer is read-only, and can be changed only through the special commands described in this section.  The usual Emacs cursor motion commands can be used in the *Buffer List* 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 in one, and the previously selected buffer (aside from the buffer *Buffer List*) 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.

All that buffer-menu does directly is create and switch to a suitable buffer, and turn on Buffer Menu mode.  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.

The only difference between buffer-menu and list-buffers is that buffer-menu switches to the *Buffer List* buffer in the selected window; list-buffers displays it in another window.  If you run list-buffers (that is, type C-x C-b) and select the buffer list manually, you can use all of the commands described here.

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) or repeat the buffer-menu command.

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 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 defaults to the name of the current buffer, modifying it by adding a <N> prefix if required.  C-x 4 c (clone-indirect-buffer-other-window) works like M-x clone-indirect-buffer, but it selects the cloned buffer in another window.  These commands come in handy if you want to create new *info* or *Help* buffers, for example.

The more general way 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).

For instance, the forward naming method puts part of the directory name at the beginning of the buffer name; using this method, buffers visiting /u/mernst/tmp/Makefile and /usr/projects/zaphod/Makefile would be named tmp/Makefile and zaphod/Makefile, respectively (instead of Makefile and Makefile<2>).

By 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 utilize fast.

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.