(Buffers): Document the maximum buffer size.

[gnu-emacs] / man / buffers.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 97, 2000, 2001
@c   Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Buffers, Windows, Files, Top
@chapter Using Multiple Buffers

@cindex buffers
  The text you are editing in Emacs resides in an object called a
@dfn{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 @kbd{C-x m}, a buffer named
@samp{*mail*} is used to hold the text of the message.  When you ask for a
command's documentation, that appears in a buffer called @samp{*Help*}.

@cindex selected buffer
@cindex current buffer
  At any time, one and only one buffer is @dfn{selected}.  It is also
called the @dfn{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
(@pxref{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 @samp{*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
(@pxref{Major Modes}).  Any Emacs variable can be made @dfn{local to} a
particular buffer, meaning its value in that buffer can be different from
the value in other buffers.  @xref{Locals}.

@cindex buffer size, maximum
  A buffer's size cannot be larger than some maximum, which is defined
by largest buffer position representable by the @dfn{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.

@menu
* Select Buffer::       Creating a new buffer or reselecting an old one.
* List Buffers::        Getting a list of buffers that exist.
* Misc Buffer::         Renaming; changing read-onlyness; copying text.
* Kill Buffer::         Killing buffers you no longer need.
* Several Buffers::     How to go through the list of all buffers
                          and operate variously on several of them.
* Indirect Buffers::    An indirect buffer shares the text of another buffer. 
* Buffer Convenience::  Convenience and customization features for
                          buffer handling.
@end menu

@node Select Buffer
@section Creating and Selecting Buffers
@cindex change buffers
@cindex switch buffers

@table @kbd
@item C-x b @var{buffer} @key{RET}
Select or create a buffer named @var{buffer} (@code{switch-to-buffer}).
@item C-x 4 b @var{buffer} @key{RET}
Similar, but select @var{buffer} in another window
(@code{switch-to-buffer-other-window}).
@item C-x 5 b @var{buffer} @key{RET}
Similar, but select @var{buffer} in a separate frame
(@code{switch-to-buffer-other-frame}).
@end table

@kindex C-x b
@findex switch-to-buffer
  To select the buffer named @var{bufname}, type @kbd{C-x b @var{bufname}
@key{RET}}.  This runs the command @code{switch-to-buffer} with argument
@var{bufname}.  You can use completion on an abbreviation for the buffer
name you want (@pxref{Completion}).  An empty argument to @kbd{C-x b}
specifies the most recently selected buffer that is not displayed in any
window.@refill

@kindex C-x 4 b
@findex switch-to-buffer-other-window
@vindex even-window-heights
  To select a buffer in a window other than the current one, type
@kbd{C-x 4 b @var{bufname} @key{RET}}.  This runs the command
@code{switch-to-buffer-other-window} which displays the buffer
@var{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 @code{even-window-heights} to
@code{nil}.

@kindex C-x 5 b
@findex switch-to-buffer-other-frame
  Similarly, @kbd{C-x 5 b @var{buffer} @key{RET}} runs the command
@code{switch-to-buffer-other-frame} which selects a buffer in another
frame.

@vindex display-buffer-reuse-frames
  You can control how certain buffers are handled by these commands by
customizing the variables @code{special-display-buffer-names},
@code{special-display-regexps}, @code{same-window-buffer-names}, and
@code{same-window-regexps}.  See @ref{Force Same Window}, and
@ref{Special Buffer Frames}, for more about these variables.  In
addition, if the value of @code{display-buffer-reuse-frames} is
non-@code{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 @kbd{C-x b @var{bufname} @key{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 @code{default-major-mode} (@pxref{Major
Modes}).

  Note that @kbd{C-x C-f}, and any other command for visiting a file,
can also be used to switch to an existing file-visiting buffer.
@xref{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.

@node List Buffers
@section Listing Existing Buffers

@table @kbd
@item C-x C-b
List the existing buffers (@code{list-buffers}).
@end table

@cindex listing current buffers
@kindex C-x C-b
@findex list-buffers
  To display a list of all the buffers that exist, type @kbd{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.

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

@smallexample
 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
@end smallexample

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

@need 2000
@node Misc Buffer
@section Miscellaneous Buffer Operations

@table @kbd
@item C-x C-q
Toggle read-only status of buffer (@code{vc-toggle-read-only}).
@item M-x rename-buffer @key{RET} @var{name} @key{RET}
Change the name of the current buffer.
@item M-x rename-uniquely
Rename the current buffer by adding @samp{<@var{number}>} to the end.
@item M-x view-buffer @key{RET} @var{buffer} @key{RET}
Scroll through buffer @var{buffer}.
@end table

@kindex C-x C-q
@c Don't index vc-toggle-read-only here, it is indexed in files.texi,
@c in the node "Basic VC Editing".
@c @findex vc-toggle-read-only
@vindex buffer-read-only
@cindex read-only buffer
  A buffer can be @dfn{read-only}, which means that commands to change
its contents are not allowed.  The mode line indicates read-only
buffers with @samp{%%} or @samp{%*} 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
@kbd{C-x C-q} (@code{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 @code{buffer-read-only}, which has a local
value in each buffer and makes the buffer read-only if its value is
non-@code{nil}.  If the file is maintained with version control,
@kbd{C-x C-q} works through the version control system to change the
read-only status of the file as well as the buffer.  @xref{Version
Control}.

@findex rename-buffer
  @kbd{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.

  @kbd{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 @samp{*Shell*} buffer, then do @kbd{M-x
shell} again, it makes a new shell buffer named @samp{*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.

@findex view-buffer
  @kbd{M-x view-buffer} is much like @kbd{M-x view-file} (@pxref{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
@kbd{q}, that switches back to the buffer (and the position) which was
previously displayed in the window.  Alternatively, if you exit View
mode with @kbd{e}, the buffer and the value of point that resulted from
your perusal remain in effect.

  The commands @kbd{M-x append-to-buffer} and @kbd{M-x insert-buffer}
can be used to copy text from one buffer to another.  @xref{Accumulating
Text}.@refill

@node Kill Buffer
@section Killing Buffers

@cindex 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 @dfn{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 WideCommands
@table @kbd
@item C-x k @var{bufname} @key{RET}
Kill buffer @var{bufname} (@code{kill-buffer}).
@item M-x kill-some-buffers
Offer to kill each buffer, one by one.
@end table

@findex kill-buffer
@findex kill-some-buffers
@kindex C-x k

  @kbd{C-x k} (@code{kill-buffer}) kills one buffer, whose name you
specify in the minibuffer.  The default, used if you type just @key{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 @kbd{yes} before the buffer is killed.

  The command @kbd{M-x kill-some-buffers} asks about each buffer, one by
one.  An answer of @kbd{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 @code{kill-buffer}.

  The buffer menu feature (@pxref{Several Buffers}) is also convenient
for killing various buffers.

@vindex kill-buffer-hook
  If you want to do something special every time a buffer is killed, you
can add hook functions to the hook @code{kill-buffer-hook} (@pxref{Hooks}).

@findex clean-buffer-list
  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
@kbd{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.

@cindex Midnight mode
@vindex midnight-mode
@vindex midnight-hook
  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 @code{clean-buffer-list}, or whichever
functions you have placed in the normal hook @code{midnight-hook}
(@pxref{Hooks}).

  To enable Midnight mode, use the Customization buffer to set the
variable @code{midnight-mode} to @code{t}.  @xref{Easy Customization}.

@node Several Buffers
@section Operating on Several Buffers
@cindex buffer menu

  The @dfn{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 @dfn{deleting} them, for consistency with Dired), or display
them.

@table @kbd
@item M-x buffer-menu
Begin editing a buffer listing all Emacs buffers.
@end table

@findex buffer-menu
  The command @code{buffer-menu} writes a list of all Emacs buffers into
the buffer @samp{*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 @samp{*Buffer List*} buffer.  The
following commands apply to the buffer described on the current line.

@table @kbd
@item d
Request to delete (kill) the buffer, then move down.  The request
shows as a @samp{D} on the line, before the buffer name.  Requested
deletions take place when you type the @kbd{x} command.
@item C-d
Like @kbd{d} but move up afterwards instead of down.
@item s
Request to save the buffer.  The request shows as an @samp{S} on the
line.  Requested saves take place when you type the @kbd{x} command.
You may request both saving and deletion for the same buffer.
@item x
Perform previously requested deletions and saves.
@item u
Remove any request made for the current line, and move down.
@item @key{DEL}
Move to previous line and remove any request made for that line.
@end table

  The @kbd{d}, @kbd{C-d}, @kbd{s} and @kbd{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:

@table @kbd
@item ~
Mark the buffer ``unmodified.''  The command @kbd{~} does this
immediately when you type it.
@item %
Toggle the buffer's read-only flag.  The command @kbd{%} does
this immediately when you type it.
@item t
Visit the buffer as a tags table.  @xref{Select Tags Table}.
@end table

  There are also commands to select another buffer or buffers:

@table @kbd
@item q
Quit the buffer menu---immediately display the most recent formerly
visible buffer in its place.
@item @key{RET}
@itemx f
Immediately select this line's buffer in place of the @samp{*Buffer
List*} buffer.
@item o
Immediately select this line's buffer in another window as if by
@kbd{C-x 4 b}, leaving @samp{*Buffer List*} visible.
@item C-o
Immediately display this line's buffer in another window, but don't
select the window.
@item 1
Immediately select this line's buffer in a full-screen window.
@item 2
Immediately set up two windows, with this line's buffer in one, and the
previously selected buffer (aside from the buffer @samp{*Buffer List*})
in the other.
@item b
Bury the buffer listed on this line.
@item m
Mark this line's buffer to be displayed in another window if you exit
with the @kbd{v} command.  The request shows as a @samp{>} at the
beginning of the line.  (A single buffer may not have both a delete
request and a display request.)
@item v
Immediately select this line's buffer, and also display in other windows
any buffers previously marked with the @kbd{m} command.  If you have not
marked any buffers, this command is equivalent to @kbd{1}.
@end table

  All that @code{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 @samp{*Buffer List*} buffer to another Emacs buffer, and edit there.
You can reselect the @samp{*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 @code{buffer-menu} and @code{list-buffers}
is that @code{buffer-menu} switches to the @samp{*Buffer List*} buffer
in the selected window; @code{list-buffers} displays it in another
window.  If you run @code{list-buffers} (that is, type @kbd{C-x C-b})
and select the buffer list manually, you can use all of the commands
described here.

  The buffer @samp{*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 @samp{*Buffer
List*} to show what you have done is to type @kbd{g}
(@code{revert-buffer}) or repeat the @code{buffer-menu} command.

@node Indirect Buffers
@section Indirect Buffers
@cindex indirect buffer
@cindex base buffer

  An @dfn{indirect buffer} shares the text of some other buffer, which
is called the @dfn{base buffer} of the indirect buffer.  In some ways it
is the analogue, for buffers, of a symbolic link between files.

@table @kbd
@findex make-indirect-buffer
@item M-x make-indirect-buffer @key{RET} @var{base-buffer} @key{RET} @var{indirect-name} @key{RET}
Create an indirect buffer named @var{indirect-name} whose base buffer
is @var{base-buffer}.
@findex clone-indirect-buffer
@item M-x clone-indirect-buffer @key{RET}
Create an indirect buffer that is a twin copy of the current buffer.
@item C-x 4 c
@kindex C-x 4 c
@findex clone-indirect-buffer-other-window
Create an indirect buffer that is a twin copy of the current buffer, and
select it in another window (@code{clone-indirect-buffer-other-window}).
@end table

  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.  @xref{Outline Views}.

@cindex multiple @samp{*info*} and @samp{*Help*} buffers
  A quick and handy way to make an indirect buffer is with the command
@kbd{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
@samp{<@var{n}>} prefix if required.  @kbd{C-x 4 c}
(@code{clone-indirect-buffer-other-window}) works like @kbd{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
@samp{*info*} or @samp{*Help*} buffers, for example.

  The more general way is with the command @kbd{M-x
make-indirect-buffer}.  It creates an indirect buffer from buffer
@var{base-buffer}, under the name @var{indirect-name}.  It prompts for
both @var{base-buffer} and @var{indirect-name} using the minibuffer.

@node Buffer Convenience
@section Convenience Features and Customization of Buffer Handling

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

@menu
* Uniquify::               Buffer names can contain directory parts.
* Iswitchb::               Switching between buffers with substrings.
* Buffer Menus::           Configurable buffer menu.     
@end menu

@node Uniquify
@subsection Making Buffer Names Unique

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

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

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

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

@node Iswitchb
@subsection Switching Between Buffers using Substrings

@findex iswitchb-mode
@cindex Iswitchb mode
@cindex mode, Iswitchb
@kindex C-x b @r{(Iswitchb mode)}
@kindex C-x 4 b @r{(Iswitchb mode)}
@kindex C-x 5 b @r{(Iswitchb mode)}
@kindex C-x 4 C-o @r{(Iswitchb mode)}

  Iswitchb global minor mode provides convenient switching between
buffers using substrings of their names.  It replaces the normal
definitions of @kbd{C-x b}, @kbd{C-x 4 b}, @kbd{C-x 5 b}, and @kbd{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 @key{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 @kbd{C-s}
and @kbd{C-r} to rotate the list until the desired buffer is first.

  @key{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 @kbd{M-x iswitchb-mode}, or customize
the variable @code{iswitchb-mode} to @code{t} (@pxref{Easy
Customization}).

@node Buffer Menus
@subsection Customizing Buffer Menus

@findex bs-show
@cindex buffer list, customizable
@table @kbd
@item M-x bs-show
Make a list of buffers similarly to @kbd{M-x list-buffers} but
customizable.
@end table

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

@findex msb-mode
@cindex mode, MSB
@cindex MSB mode
@cindex buffer menu
@findex mouse-buffer-menu
@kindex C-Down-Mouse-1
  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 @code{mouse-buffer-menu},
normally on @kbd{C-Down-Mouse-1}, and the menu bar buffer menu.  You
can customize the menu in the @code{msb} Custom group.