This is Info file elisp, produced by Makeinfo-1.55 from the input file elisp.texi. This version is the edition 2.3 of the GNU Emacs Lisp Reference Manual. It corresponds to Emacs Version 19.23. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English.  File: elisp, Node: Making Backups, Next: Rename or Copy, Up: Backup Files Making Backup Files ------------------- - Function: backup-buffer This function makes a backup of the file visited by the current buffer, if appropriate. It is called by `save-buffer' before saving the buffer the first time. - Variable: buffer-backed-up This buffer-local variable indicates whether this buffer's file has been backed up on account of this buffer. If it is non-`nil', then the backup file has been written. Otherwise, the file should be backed up when it is next saved (if backups are enabled). This is a permanent local; `kill-local-variables' does not alter it. - User Option: make-backup-files This variable determines whether or not to make backup files. If it is non-`nil', then Emacs creates a backup of each file when it is saved for the first time. The following example shows how to change the `make-backup-files' variable only in the `RMAIL' buffer and not elsewhere. Setting it `nil' stops Emacs from making backups of the `RMAIL' file, which may save disk space. (You would put this code in your `.emacs' file.) (add-hook 'rmail-mode-hook (function (lambda () (make-local-variable 'make-backup-files) (setq make-backup-files nil)))) - Variable: backup-enable-predicate This variable's value is a function to be called on certain occasions to decide whether a file should have backup files. The function receives one argument, a file name to consider. If the function returns `nil', backups are disabled for that file. Otherwise, the other variables in this section say whether and how to make backups. The default value is this: (lambda (name) (or (< (length name) 5) (not (string-equal "/tmp/" (substring name 0 5))))) - Variable: backup-inhibited If this variable is non-`nil', backups are inhibited. It records the result of testing `backup-enable-predicate' on the visited file name. It can also coherently be used by other mechanisms that inhibit backups based on which file is visited. This is a permanent local, so that changing the major mode does not lose its value. Major modes should not set this variable--they should set `make-backup-files' instead.  File: elisp, Node: Rename or Copy, Next: Numbered Backups, Prev: Making Backups, Up: Backup Files Backup by Renaming or by Copying? --------------------------------- There are two ways that Emacs can make a backup file: * Emacs can rename the original file so that it becomes a backup file, and then write the buffer being saved into a new file. After this procedure, any other names (i.e., hard links) of the original file now refer to the backup file. The new file is owned by the user doing the editing, and its group is the default for new files written by the user in that directory. * Emacs can copy the original file into a backup file, and then overwrite the original file with new contents. After this procedure, any other names (i.e., hard links) of the original file still refer to the current version of the file. The file's owner and group will be unchanged. The first method, renaming, is the default. The variable `backup-by-copying', if non-`nil', says to use the second method, which is to copy the original file and overwrite it with the new buffer contents. The variable `file-precious-flag', if non-`nil', also has this effect (as a sideline of its main significance). *Note Saving Buffers::. - Variable: backup-by-copying If this variable is non-`nil', Emacs always makes backup files by copying. The following two variables, when non-`nil', cause the second method to be used in certain special cases. They have no effect on the treatment of files that don't fall into the special cases. - Variable: backup-by-copying-when-linked If this variable is non-`nil', Emacs makes backups by copying for files with multiple names (hard links). This variable is significant only if `backup-by-copying' is `nil', since copying is always used when that variable is non-`nil'. - Variable: backup-by-copying-when-mismatch If this variable is non-`nil', Emacs makes backups by copying in cases where renaming would change either the owner or the group of the file. The value has no effect when renaming would not alter the owner or group of the file; that is, for files which are owned by the user and whose group matches the default for a new file created there by the user. This variable is significant only if `backup-by-copying' is `nil', since copying is always used when that variable is non-`nil'.  File: elisp, Node: Numbered Backups, Next: Backup Names, Prev: Rename or Copy, Up: Backup Files Making and Deleting Numbered Backup Files ----------------------------------------- If a file's name is `foo', the names of its numbered backup versions are `foo.~V~', for various integers V, like this: `foo.~1~', `foo.~2~', `foo.~3~', ..., `foo.~259~', and so on. - User Option: version-control This variable controls whether to make a single non-numbered backup file or multiple numbered backups. `nil' Make numbered backups if the visited file already has numbered backups; otherwise, do not. `never' Do not make numbered backups. ANYTHING ELSE Make numbered backups. The use of numbered backups ultimately leads to a large number of backup versions, which must then be deleted. Emacs can do this automatically or it can ask the user whether to delete them. - User Option: kept-new-versions The value of this variable is the number of newest versions to keep when a new numbered backup is made. The newly made backup is included in the count. The default value is 2. - User Option: kept-old-versions The value of this variable is the number of oldest versions to keep when a new numbered backup is made. The default value is 2. If there are backups numbered 1, 2, 3, 5, and 7, and both of these variables have the value 2, then the backups numbered 1 and 2 are kept as old versions and those numbered 5 and 7 are kept as new versions; backup version 3 is excess. The function `find-backup-file-name' (*note Backup Names::.) is responsible for determining which backup versions to delete, but does not delete them itself. - User Option: trim-versions-without-asking If this variable is non-`nil', then saving a file deletes excess backup versions silently. Otherwise, it asks the user whether to delete them. - User Option: dired-kept-versions This variable specifies how many of the newest backup versions to keep in the Dired command `.' (`dired-clean-directory'). That's the same thing `kept-new-versions' specifies when you make a new backup file. The default value is 2.  File: elisp, Node: Backup Names, Prev: Numbered Backups, Up: Backup Files Naming Backup Files ------------------- The functions in this section are documented mainly because you can customize the naming conventions for backup files by redefining them. If you change one, you probably need to change the rest. - Function: backup-file-name-p FILENAME This function returns a non-`nil' value if FILENAME is a possible name for a backup file. A file with the name FILENAME need not exist; the function just checks the name. (backup-file-name-p "foo") => nil (backup-file-name-p "foo~") => 3 The standard definition of this function is as follows: (defun backup-file-name-p (file) "Return non-nil if FILE is a backup file \ name (numeric or not)..." (string-match "~$" file)) Thus, the function returns a non-`nil' value if the file name ends with a `~'. (We use a backslash to split the documentation string's first line into two lines in the text, but produce just one line in the string itself.) This simple expression is placed in a separate function to make it easy to redefine for customization. - Function: make-backup-file-name FILENAME This function returns a string that is the name to use for a non-numbered backup file for file FILENAME. On Unix, this is just FILENAME with a tilde appended. The standard definition of this function is as follows: (defun make-backup-file-name (file) "Create the non-numeric backup file name for FILE. ..." (concat file "~")) You can change the backup-file naming convention by redefining this function. The following example redefines `make-backup-file-name' to prepend a `.' in addition to appending a tilde: (defun make-backup-file-name (filename) (concat "." filename "~")) (make-backup-file-name "backups.texi") => ".backups.texi~" - Function: find-backup-file-name FILENAME This function computes the file name for a new backup file for FILENAME. It may also propose certain existing backup files for deletion. `find-backup-file-name' returns a list whose CAR is the name for the new backup file and whose CDR is a list of backup files whose deletion is proposed. Two variables, `kept-old-versions' and `kept-new-versions', determine which backup versions should be kept. This function keeps those versions by excluding them from the CDR of the value. *Note Numbered Backups::. In this example, the value says that `~rms/foo.~5~' is the name to use for the new backup file, and `~rms/foo.~3~' is an "excess" version that the caller should consider deleting now. (find-backup-file-name "~rms/foo") => ("~rms/foo.~5~" "~rms/foo.~3~") - Function: file-newest-backup FILENAME This function returns the name of the most recent backup file for FILENAME, or `nil' if that file has no backup files. Some file comparison commands use this function so that they can automatically compare a file with its most recent backup.  File: elisp, Node: Auto-Saving, Next: Reverting, Prev: Backup Files, Up: Backups and Auto-Saving Auto-Saving =========== Emacs periodically saves all files that you are visiting; this is called "auto-saving". Auto-saving prevents you from losing more than a limited amount of work if the system crashes. By default, auto-saves happen every 300 keystrokes, or after around 30 seconds of idle time. *Note Auto-Save: (emacs)Auto-Save, for information on auto-save for users. Here we describe the functions used to implement auto-saving and the variables that control them. - Variable: buffer-auto-save-file-name This buffer-local variable is the name of the file used for auto-saving the current buffer. It is `nil' if the buffer should not be auto-saved. buffer-auto-save-file-name => "/xcssun/users/rms/lewis/#files.texi#" - Command: auto-save-mode ARG When used interactively without an argument, this command is a toggle switch: it turns on auto-saving of the current buffer if it is off, and vice-versa. With an argument ARG, the command turns auto-saving on if the value of ARG is `t', a nonempty list, or a positive integer. Otherwise, it turns auto-saving off. - Function: auto-save-file-name-p FILENAME This function returns a non-`nil' value if FILENAME is a string that could be the name of an auto-save file. It works based on knowledge of the naming convention for auto-save files: a name that begins and ends with hash marks (`#') is a possible auto-save file name. The argument FILENAME should not contain a directory part. (make-auto-save-file-name) => "/xcssun/users/rms/lewis/#files.texi#" (auto-save-file-name-p "#files.texi#") => 0 (auto-save-file-name-p "files.texi") => nil The standard definition of this function is as follows: (defun auto-save-file-name-p (filename) "Return non-nil if FILENAME can be yielded by..." (string-match "^#.*#$" filename)) This function exists so that you can customize it if you wish to change the naming convention for auto-save files. If you redefine it, be sure to redefine the function `make-auto-save-file-name' correspondingly. - Function: make-auto-save-file-name This function returns the file name to use for auto-saving the current buffer. This is just the file name with hash marks (`#') appended and prepended to it. This function does not look at the variable `auto-save-visited-file-name' (described below); you should check that before calling this function. (make-auto-save-file-name) => "/xcssun/users/rms/lewis/#backup.texi#" The standard definition of this function is as follows: (defun make-auto-save-file-name () "Return file name to use for auto-saves \ of current buffer. ..." (if buffer-file-name (concat (file-name-directory buffer-file-name) "#" (file-name-nondirectory buffer-file-name) "#") (expand-file-name (concat "#%" (buffer-name) "#")))) This exists as a separate function so that you can redefine it to customize the naming convention for auto-save files. Be sure to change `auto-save-file-name-p' in a corresponding way. - Variable: auto-save-visited-file-name If this variable is non-`nil', Emacs auto-saves buffers in the files they are visiting. That is, the auto-save is done in the same file that you are editing. Normally, this variable is `nil', so auto-save files have distinct names that are created by `make-auto-save-file-name'. When you change the value of this variable, the value does not take effect until the next time auto-save mode is reenabled in any given buffer. If auto-save mode is already enabled, auto-saves continue to go in the same file name until `auto-save-mode' is called again. - Function: recent-auto-save-p This function returns `t' if the current buffer has been auto-saved since the last time it was read in or saved. - Function: set-buffer-auto-saved This function marks the current buffer as auto-saved. The buffer will not be auto-saved again until the buffer text is changed again. The function returns `nil'. - User Option: auto-save-interval The value of this variable is the number of characters that Emacs reads from the keyboard between auto-saves. Each time this many more characters are read, auto-saving is done for all buffers in which it is enabled. - User Option: auto-save-timeout The value of this variable is the number of seconds of idle time that should cause auto-saving. Each time the user pauses for this long, Emacs auto-saves any buffers that need it. (Actually, the specified timeout is multiplied by a factor depending on the size of the current buffer.) - Variable: auto-save-hook This normal hook is run whenever an auto-save is about to happen. - User Option: auto-save-default If this variable is non-`nil', buffers that are visiting files have auto-saving enabled by default. Otherwise, they do not. - Command: do-auto-save &optional NO-MESSAGE This function auto-saves all buffers that need to be auto-saved. It saves all buffers for which auto-saving is enabled and that have been changed since the previous auto-save. Normally, if any buffers are auto-saved, a message that says `Auto-saving...' is displayed in the echo area while auto-saving is going on. However, if NO-MESSAGE is non-`nil', the message is inhibited. - Function: delete-auto-save-file-if-necessary This function deletes the current buffer's auto-save file if `delete-auto-save-files' is non-`nil'. It is called every time a buffer is saved. - Variable: delete-auto-save-files This variable is used by the function `delete-auto-save-file-if-necessary'. If it is non-`nil', Emacs deletes auto-save files when a true save is done (in the visited file). This saves disk space and unclutters your directory. - Function: rename-auto-save-file This function adjusts the current buffer's auto-save file name if the visited file name has changed. It also renames an existing auto-save file. If the visited file name has not changed, this function does nothing. - Variable: buffer-saved-size The value of this buffer-local variable is the length of the current buffer as of the last time it was read in, saved, or auto-saved. This is used to detect a substantial decrease in size, and turn off auto-saving in response. If it is -1, that means auto-saving is temporarily shut off in this buffer due to a substantial deletion. Explicitly saving the buffer stores a positive value in this variable, thus reenabling auto-save. Turning auto-save mode off or on also alters this variable.  File: elisp, Node: Reverting, Prev: Auto-Saving, Up: Backups and Auto-Saving Reverting ========= If you have made extensive changes to a file and then change your mind about them, you can get rid of them by reading in the previous version of the file with the `revert-buffer' command. *Note Reverting a Buffer: (emacs)Reverting. - Command: revert-buffer &optional CHECK-AUTO-SAVE NOCONFIRM This command replaces the buffer text with the text of the visited file on disk. This action undoes all changes since the file was visited or saved. If the argument CHECK-AUTO-SAVE is non-`nil', and the latest auto-save file is more recent than the visited file, `revert-buffer' asks the user whether to use that instead. Otherwise, it always uses the text of the visited file itself. Interactively, CHECK-AUTO-SAVE is set if there is a numeric prefix argument. Normally, `revert-buffer' asks for confirmation before it changes the buffer; but if the argument NOCONFIRM is non-`nil', `revert-buffer' does not ask for confirmation. Reverting tries to preserve marker positions in the buffer by using the replacement feature of `insert-file-contents'. If the buffer contents and the file contents are identical before the revert operation, reverting preserves all the markers. If they are not identical, reverting does change the buffer; then it preserves the markers in the unchanged text (if any) at the beginning and end of the buffer. Preserving any additional markers would be problematical. You can customize how `revert-buffer' does its work by setting these variables--typically, as buffer-local variables. - Variable: revert-buffer-function The value of this variable is the function to use to revert this buffer. If non-`nil', it is called as a function with no arguments to do the work of reverting. If the value is `nil', reverting works the usual way. Modes such as Dired mode, in which the text being edited does not consist of a file's contents but can be regenerated in some other fashion, give this variable a buffer-local value that is a function to regenerate the contents. - Variable: revert-buffer-insert-file-contents-function The value of this variable, if non-`nil', is the function to use to insert contents when reverting this buffer. The function receives two arguments, first the file name to use; second, `t' if the user has asked to read the auto-save file. - Variable: before-revert-hook This normal hook is run by `revert-buffer' before actually inserting the modified contents--but only if `revert-buffer-function' is `nil'. Font Lock mode uses this hook to record that the buffer contents are no longer fontified. - Variable: after-revert-hook This normal hook is run by `revert-buffer' after actually inserting the modified contents--but only if `revert-buffer-function' is `nil'. Font Lock mode uses this hook to recompute the fonts for the updated buffer contents. - Command: recover-file FILENAME This function visits FILENAME, but gets the contents from its last auto-save file. This is useful after the system has crashed, to resume editing the same file without losing all the work done in the previous session. An error is signaled if there is no auto-save file for FILENAME, or if FILENAME is newer than its auto-save file. If FILENAME does not exist, but its auto-save file does, then the auto-save file is read as usual. This last situation may occur if you visited a nonexistent file and never actually saved it.  File: elisp, Node: Buffers, Next: Windows, Prev: Backups and Auto-Saving, Up: Top 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 which are not visiting files. While several buffers may exist at one time, exactly 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. * Menu: * Buffer Basics:: What is a buffer? * Buffer Names:: Accessing and changing buffer names. * Buffer File Name:: The buffer file name indicates which file is visited. * Buffer Modification:: A buffer is "modified" if it needs to be saved. * Modification Time:: Determining whether the visited file was changed "behind Emacs's back". * Read Only Buffers:: Modifying text is not allowed in a read-only buffer. * The Buffer List:: How to look at all the existing buffers. * Creating Buffers:: Functions that create buffers. * Killing Buffers:: Buffers exist until explicitly killed. * Current Buffer:: Designating a buffer as current so primitives will access its contents.  File: elisp, Node: Buffer Basics, Next: Buffer Names, Up: Buffers 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 which are not visiting files. While several buffers may exist at one time, exactly 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 which have distinct names and hold text that can be edited. Buffers appear to Lisp programs as a special data type. The contents of a buffer may be viewed as an extendable string; insertions and deletions may occur in any part of the buffer. *Note 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 only accessible 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 *Note Buffer-Local Variables::. For functions and variables related to visiting files in buffers, see *Note Visiting Files:: and *Note Saving Buffers::. For functions and variables related to the display of buffers in windows, see *Note Buffers and Windows::. - Function: bufferp OBJECT This function returns `t' if OBJECT is a buffer, `nil' otherwise.  File: elisp, Node: Buffer Names, Next: Buffer File Name, Prev: Buffer Basics, Up: Buffers 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' or `buffer-menu' commands don't mention them. A name starting with space also initially disables recording undo information; see *Note 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. *Note Killing Buffers::. (buffer-name) => "buffers.texi" (setq foo (get-buffer "temp")) => # (kill-buffer foo) => nil (buffer-name foo) => nil foo => # - 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 `nil'. 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. One application of this command is to rename the `*shell*' buffer to some other name, thus making it possible to create a second shell buffer under the name `*shell*'. - 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")) => # (get-buffer b) => # (get-buffer "Frazzle-nots") => nil See also the function `get-buffer-create' in *Note Creating Buffers::. - Function: generate-new-buffer-name STARTING-NAME 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 `<...>'. See the related function `generate-new-buffer' in *Note Creating Buffers::.  File: elisp, Node: Buffer File Name, Next: Buffer Modification, Prev: Buffer Names, Up: Buffers 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. *Note 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, unaffected by `kill-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. See the definition of `set-visited-file-name' in `files.el'; 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-local-variables'. *Note 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-local-variables'. *Note Truenames::. 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 *Note File Attributes::, 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 (*note File Name Expansion::.), then compared against the visited file names of all live buffers. (get-file-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 If FILENAME is a non-empty string, this function changes the name of the file visited in 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. When the function `set-visited-file-name' is called interactively, it prompts for FILENAME in the minibuffer. See also `clear-visited-file-modtime' and `verify-visited-file-modtime' in *Note Buffer Modification::. - Variable: list-buffers-directory This buffer-local variable records a string to display in a buffer listing in place of the visited file name, for buffers that don't have a visited file name. Dired buffers use this variable.  File: elisp, Node: Buffer Modification, Next: Modification Time, Prev: Buffer File Name, Up: Buffers 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 (*note Mode Line Variables::.), and controls saving (*note Saving Buffers::.) and auto-saving (*note 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 *Note 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. 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.  File: elisp, Node: Modification Time, Next: Read Only Buffers, Prev: Buffer Modification, Up: Buffers 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 *Note File Attributes::.) - 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 FN This function is used to ask a user how to proceed after an attempt to modify an obsolete buffer. 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. 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. 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 `(FN)', in which case the proposed buffer modification is not allowed. See also the file locking mechanism in *Note File Locks::.  File: elisp, Node: Read Only Buffers, Next: The Buffer List, Prev: Modification Time, Up: Buffers 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 show 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-M-q'. * Modes such as Dired and Rmail make buffers read-only when altering the contents with the usual editing commands is probably 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 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. The value of `buffer-read-only' does not matter when `inhibit-read-only' is non-`nil'. If `inhibit-read-only' is `t', all `read-only' text properties have no effect (*note Special Properties::.). If `inhibit-read-only' is a list, then `read-only' text properties are ignored 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; don't 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. *Note Interactive Call::, for another way to signal an error if the current buffer is read-only.  File: elisp, Node: The Buffer List, Next: Creating Buffers, Prev: Read Only Buffers, Up: Buffers 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 deletes 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. Several functions, notably `other-buffer', use this ordering. A buffer list displayed for the user also follows this order. - Function: buffer-list This function returns a list of all buffers, including those whose names begin with a space. The elements are actual buffers, not their names. (buffer-list) => (# # # # #) ;; 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") This list is a copy of a list used inside Emacs; modifying it has no effect on the ordering of buffers. - Function: other-buffer &optional BUFFER-OR-NAME VISIBLE-OK This function returns the first buffer in the buffer list other than BUFFER-OR-NAME. Usually this is the buffer most recently shown in the selected window, aside from BUFFER-OR-NAME. Buffers whose names start with a space are not considered. If BUFFER-OR-NAME is not supplied (or if it is not a buffer), then `other-buffer' returns the first buffer on the buffer list that is not visible in any window in a visible frame. 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. If BUFFER-OR-NAME is `nil' or omitted, this means to bury the current buffer. In addition, this switches to some other buffer (obtained using `other-buffer') in the selected window. If the buffer is displayed in a window other than the selected one, it remains there. If you wish to replace a buffer in all the windows that display it, use `replace-buffer-in-windows'. *Note Buffers and Windows::.  File: elisp, Node: Creating Buffers, Next: Killing Buffers, Prev: The Buffer List, Up: Buffers Creating Buffers ================ This section describes the two primitives for creating buffers. `get-buffer-create' creates a buffer if it finds no existing buffer; `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' (*note Temporary Displays::.) and `create-file-buffer' (*note Visiting Files::.). Starting a subprocess can also create a buffer (*note 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") => # The major mode for the new buffer is set according to the variable `default-major-mode'. *Note Auto 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 `' are added 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") => # (generate-new-buffer "bar") => #> (generate-new-buffer "bar") => #> The major mode for the new buffer is set by the value of `default-major-mode'. *Note Auto Major Mode::. See the related function `generate-new-buffer-name' in *Note Buffer Names::.  File: elisp, Node: Killing Buffers, Next: Current Buffer, Prev: Creating Buffers, Up: Buffers Killing Buffers =============== "Killing a buffer" makes its name unknown to Emacs and makes its space available for other use. The buffer object for the buffer which 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; two distinct buffers, when killed, remain distinct according to `eq'. 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). *Note Current Buffer::. 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 use as space for other buffers. (Emacs version 18 and older was unable to return the memory 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.) *Note 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'. *Note 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 is that these functions ask for confirmation from the user for various nonstandard reasons. If any of them returns non-`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. *Note 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. *Note Buffer-Local Variables::.