This is elisp, produced by makeinfo version 4.0f from ./elisp.texi. INFO-DIR-SECTION Editors START-INFO-DIR-ENTRY * Elisp: (elisp). The Emacs Lisp Reference Manual. END-INFO-DIR-ENTRY This Info file contains edition 2.8 of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 21.2. Published by the Free Software Foundation 59 Temple Place, Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Copying", with the Front-Cover texts being "A GNU Manual", and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Back-Cover Text is: "You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development."  File: elisp, Node: Format Conversion, Prev: Magic File Names, Up: Files File Format Conversion ====================== The variable `format-alist' defines a list of "file formats", which describe textual representations used in files for the data (text, text-properties, and possibly other information) in an Emacs buffer. Emacs performs format conversion if appropriate when reading and writing files. - Variable: format-alist This list contains one format definition for each defined file format. Each format definition is a list of this form: (NAME DOC-STRING REGEXP FROM-FN TO-FN MODIFY MODE-FN) Here is what the elements in a format definition mean: NAME The name of this format. DOC-STRING A documentation string for the format. REGEXP A regular expression which is used to recognize files represented in this format. FROM-FN A shell command or function to decode data in this format (to convert file data into the usual Emacs data representation). A shell command is represented as a string; Emacs runs the command as a filter to perform the conversion. If FROM-FN is a function, it is called with two arguments, BEGIN and END, which specify the part of the buffer it should convert. It should convert the text by editing it in place. Since this can change the length of the text, FROM-FN should return the modified end position. One responsibility of FROM-FN is to make sure that the beginning of the file no longer matches REGEXP. Otherwise it is likely to get called again. TO-FN A shell command or function to encode data in this format--that is, to convert the usual Emacs data representation into this format. If TO-FN is a string, it is a shell command; Emacs runs the command as a filter to perform the conversion. If TO-FN is a function, it is called with two arguments, BEGIN and END, which specify the part of the buffer it should convert. There are two ways it can do the conversion: * By editing the buffer in place. In this case, TO-FN should return the end-position of the range of text, as modified. * By returning a list of annotations. This is a list of elements of the form `(POSITION . STRING)', where POSITION is an integer specifying the relative position in the text to be written, and STRING is the annotation to add there. The list must be sorted in order of position when TO-FN returns it. When `write-region' actually writes the text from the buffer to the file, it intermixes the specified annotations at the corresponding positions. All this takes place without modifying the buffer. MODIFY A flag, `t' if the encoding function modifies the buffer, and `nil' if it works by returning a list of annotations. MODE-FN A minor-mode function to call after visiting a file converted from this format. The function is called with one argument, the integer 1; that tells a minor-mode function to enable the mode. The function `insert-file-contents' automatically recognizes file formats when it reads the specified file. It checks the text of the beginning of the file against the regular expressions of the format definitions, and if it finds a match, it calls the decoding function for that format. Then it checks all the known formats over again. It keeps checking them until none of them is applicable. Visiting a file, with `find-file-noselect' or the commands that use it, performs conversion likewise (because it calls `insert-file-contents'); it also calls the mode function for each format that it decodes. It stores a list of the format names in the buffer-local variable `buffer-file-format'. - Variable: buffer-file-format This variable states the format of the visited file. More precisely, this is a list of the file format names that were decoded in the course of visiting the current buffer's file. It is always buffer-local in all buffers. When `write-region' writes data into a file, it first calls the encoding functions for the formats listed in `buffer-file-format', in the order of appearance in the list. - Command: format-write-file file format This command writes the current buffer contents into the file FILE in format FORMAT, and makes that format the default for future saves of the buffer. The argument FORMAT is a list of format names. - Command: format-find-file file format This command finds the file FILE, converting it according to format FORMAT. It also makes FORMAT the default if the buffer is saved later. The argument FORMAT is a list of format names. If FORMAT is `nil', no conversion takes place. Interactively, typing just for FORMAT specifies `nil'. - Command: format-insert-file file format &optional beg end This command inserts the contents of file FILE, converting it according to format FORMAT. If BEG and END are non-`nil', they specify which part of the file to read, as in `insert-file-contents' (*note Reading from Files::). The return value is like what `insert-file-contents' returns: a list of the absolute file name and the length of the data inserted (after conversion). The argument FORMAT is a list of format names. If FORMAT is `nil', no conversion takes place. Interactively, typing just for FORMAT specifies `nil'. - Variable: auto-save-file-format This variable specifies the format to use for auto-saving. Its value is a list of format names, just like the value of `buffer-file-format'; however, it is used instead of `buffer-file-format' for writing auto-save files. This variable is always buffer-local in all buffers.  File: elisp, Node: Backups and Auto-Saving, Next: Buffers, Prev: Files, Up: Top Backups and Auto-Saving *********************** Backup files and auto-save files are two methods by which Emacs tries to protect the user from the consequences of crashes or of the user's own errors. Auto-saving preserves the text from earlier in the current editing session; backup files preserve file contents prior to the current session. * Menu: * Backup Files:: How backup files are made; how their names are chosen. * Auto-Saving:: How auto-save files are made; how their names are chosen. * Reverting:: `revert-buffer', and how to customize what it does.  File: elisp, Node: Backup Files, Next: Auto-Saving, Up: Backups and Auto-Saving Backup Files ============ A "backup file" is a copy of the old contents of a file you are editing. Emacs makes a backup file the first time you save a buffer into its visited file. Normally, this means that the backup file contains the contents of the file as it was before the current editing session. The contents of the backup file normally remain unchanged once it exists. Backups are usually made by renaming the visited file to a new name. Optionally, you can specify that backup files should be made by copying the visited file. This choice makes a difference for files with multiple names; it also can affect whether the edited file remains owned by the original owner or becomes owned by the user editing it. By default, Emacs makes a single backup file for each file edited. You can alternatively request numbered backups; then each new backup file gets a new name. You can delete old numbered backups when you don't want them any more, or Emacs can delete them automatically. * Menu: * Making Backups:: How Emacs makes backup files, and when. * Rename or Copy:: Two alternatives: renaming the old file or copying it. * Numbered Backups:: Keeping multiple backups for each source file. * Backup Names:: How backup file names are computed; customization.  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-all-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--provided that `backup-inhibited' is `nil' (see below). The following example shows how to change the `make-backup-files' variable only in the Rmail buffers and not elsewhere. Setting it `nil' stops Emacs from making backups of these files, which may save disk space. (You would put this code in your init 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 `normal-backup-enable-predicate', which checks for files in `temporary-file-directory' and `small-temporary-file-directory'. - 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. For example, VC sets this variable non-`nil' to prevent making backups for files managed with a version control system. 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. - Variable: backup-directory-alist This variable's value is an alist of filename patterns and backup directory names. Each element looks like (REGEXP . DIRECTORY) Backups of files with names matching REGEXP will be made in DIRECTORY. DIRECTORY may be relative or absolute. If it is absolute, so that all matching files are backed up into the same directory, the file names in this directory will be the full name of the file backed up with all directory separators changed to `!' to prevent clashes. This will not work correctly if your filesystem truncates the resulting name. For the common case of all backups going into one directory, the alist should contain a single element pairing `"."' with the appropriate directory name. If this variable is `nil', or it fails to match a filename, the backup is made in the original file's directory. On MS-DOS filesystems without long names this variable is always ignored. - Variable: make-backup-file-name-function This variable's value is a function to use for making backups instead of the default `make-backup-file-name'. A value of nil gives the default `make-backup-file-name' behaviour. This could be buffer-local to do something special for specific files. If you define it, you may need to change `backup-file-name-p' and `file-name-sans-versions' too.  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 continue to refer to the current (updated) 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'. - Variable: backup-by-copying-when-privileged-mismatch This variable, if non-`nil', specifies the same behavior as `backup-by-copying-when-mismatch', but only for certain user-id values: namely, those less than or equal to a certain number. You set this variable to that number. Thus, if you set `backup-by-copying-when-privileged-mismatch' to 0, backup by copying is done for the superuser only, when necessary to prevent a change in the owner of the file. The default is 200.  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: delete-old-versions If this variable is `t', then saving a file deletes excess backup versions silently. If it is `nil', that means to ask for confirmation before deleting excess backups. Otherwise, they are not deleted at all. - 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, on most operating systems, 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) (expand-file-name (concat "." (file-name-nondirectory filename) "~") (file-name-directory filename))) (make-backup-file-name "backups.texi") => ".backups.texi~" Some parts of Emacs, including some Dired commands, assume that backup file names end with `~'. If you do not follow that convention, it will not cause serious problems, but these commands may give less-than-desirable results. - 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/#backups.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 assumes the usual 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/#backups.texi#" (auto-save-file-name-p "#backups.texi#") => 0 (auto-save-file-name-p "backups.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 (`#') prepended and appended to it. This function does not look at the variable `auto-save-visited-file-name' (described below); callers of this function should check that variable first. (make-auto-save-file-name) => "/xcssun/users/rms/lewis/#backups.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 new value does not take effect in an existing buffer until the next time auto-save mode is reenabled in it. 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 specifies how often to do auto-saving, in terms of number of input events. Each time this many additional input events are read, Emacs does auto-saving for all buffers in which that 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 does auto-saving for all buffers in which that is enabled. (If the current buffer is large, the specified timeout is multiplied by a factor that increases as the size increases; for a million-byte buffer, the factor is almost 4.) If the value is zero or nil, then auto-saving is not done as a result of idleness, only after a certain number of input events as specified by `auto-save-interval'. - 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 current-only 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. If CURRENT-ONLY is non-`nil', only the current buffer is auto-saved. - 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, when it was last 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 decrease in size. Explicitly saving the buffer stores a positive value in this variable, thus reenabling auto-saving. Turning auto-save mode off or on also updates this variable, so that the substantial decrease in size is forgotten. - Variable: auto-save-list-file-name This variable (if non-`nil') specifies a file for recording the names of all the auto-save files. Each time Emacs does auto-saving, it writes two lines into this file for each buffer that has auto-saving enabled. The first line gives the name of the visited file (it's empty if the buffer has none), and the second gives the name of the auto-save file. When Emacs exits normally, it deletes this file; if Emacs crashes, you can look in the file to find all the auto-save files that might contain work that was otherwise lost. The `recover-session' command uses this file to find them. The default name for this file specifies your home directory and starts with `.saves-'. It also contains the Emacs process ID and the host name. - Variable: auto-save-list-file-prefix After Emacs reads your init file, it initializes `auto-save-list-file-name' (if you have not already set it non-`nil') based on this prefix, adding the host name and process ID. If you set this to `nil' in your init file, then Emacs does not initialize `auto-save-list-file-name'.  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 ignore-auto 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. By default, if the latest auto-save file is more recent than the visited file, and the argument IGNORE-AUTO is `nil', `revert-buffer' asks the user whether to use that auto-save instead. When you invoke this command interactively, IGNORE-AUTO is `t' if there is no numeric prefix argument; thus, the interactive default is not to check the auto-save file. 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; in that case, 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 the variables described in the rest of this section. - Variable: revert-without-query This variable holds a list of files that should be reverted without query. The value is a list of regular expressions. If the visited file name matches one of these regular expressions, and the file has changed on disk but the buffer is not modified, then `revert-buffer' reverts the file without asking the user for confirmation. Some major modes customize `revert-buffer' by making buffer-local bindings for these 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, can 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', specifies the function to use to insert the updated 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. The reason for a mode to set this variable instead of `revert-buffer-function' is to avoid duplicating or replacing the rest of what `revert-buffer' does: asking for confirmation, clearing the undo list, deciding the proper major mode, and running the hooks listed below. - Variable: before-revert-hook This normal hook is run by `revert-buffer' before inserting the modified contents--but only if `revert-buffer-function' is `nil'. - Variable: after-revert-hook This normal hook is run by `revert-buffer' after inserting the modified contents--but only if `revert-buffer-function' is `nil'.  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 that are not visiting files. While several buffers may exist at one time, only one buffer is designated the "current buffer" at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows. * Menu: * Buffer Basics:: What is a buffer? * Current Buffer:: Designating a buffer as current so that primitives will access its contents. * 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. * Indirect Buffers:: An indirect buffer shares text with some other buffer. * Buffer Gap:: The gap in the buffer.  File: elisp, Node: Buffer Basics, Next: Current Buffer, 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 that are not visiting files. Although several buffers normally exist, only one buffer is designated the "current buffer" at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows. Buffers in Emacs editing are objects that have distinct names and hold text that can be edited. Buffers appear to Lisp programs as a special data type. You can think of the contents of a buffer as a string that you can extend; insertions and deletions may occur in any part of the buffer. *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 accessible only through special-purpose functions. For example, the visited file name is directly accessible through a variable, while the value of point is accessible only through a primitive function. Buffer-specific information that is directly accessible is stored in "buffer-local" variable bindings, which are variable values that are effective only in a particular buffer. This feature allows each buffer to override the values of certain variables. Most major modes override variables such as `fill-column' or `comment-column' in this way. For more information about buffer-local variables and functions related to them, see *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: Current Buffer, Next: Buffer Names, Prev: Buffer Basics, Up: Buffers The Current Buffer ================== There are, in general, many buffers in an Emacs session. At any time, one of them is designated as the "current buffer". This is the buffer in which most editing takes place, because most of the primitives for examining or changing text in a buffer operate implicitly on the current buffer (*note Text::). Normally the buffer that is displayed on the screen in the selected window is the current buffer, but this is not always so: a Lisp program can temporarily designate any buffer as current in order to operate on its contents, without changing what is displayed on the screen. The way to designate a current buffer in a Lisp program is by calling `set-buffer'. The specified buffer remains current until a new one is designated. When an editing command returns to the editor command loop, the command loop designates the buffer displayed in the selected window as current, to prevent confusion: the buffer that the cursor is in when Emacs reads a command is the buffer that the command will apply to. (*Note Command Loop::.) Therefore, `set-buffer' is not the way to switch visibly to a different buffer so that the user can edit it. For that, you must use the functions described in *Note Displaying Buffers::. *Note:* Lisp functions that change to a different current buffer should not depend on the command loop to set it back afterwards. Editing commands written in Emacs Lisp can be called from other programs as well as from the command loop; it is convenient for the caller if the subroutine does not change which buffer is current (unless, of course, that is the subroutine's purpose). Therefore, you should normally use `set-buffer' within a `save-current-buffer' or `save-excursion' (*note Excursions::) form that will restore the current buffer when your function is done. Here is an example, the code for the command `append-to-buffer' (with the documentation string abridged): (defun append-to-buffer (buffer start end) "Append to specified buffer the text of the region. ..." (interactive "BAppend to buffer: \nr") (let ((oldbuf (current-buffer))) (save-current-buffer (set-buffer (get-buffer-create buffer)) (insert-buffer-substring oldbuf start end)))) This function binds a local variable to record the current buffer, and then `save-current-buffer' arranges to make it current again. Next, `set-buffer' makes the specified buffer current. Finally, `insert-buffer-substring' copies the string from the original current buffer to the specified (and now current) buffer. If the buffer appended to happens to be displayed in some window, the next redisplay will show how its text has changed. Otherwise, you will not see the change immediately on the screen. The buffer becomes current temporarily during the execution of the command, but this does not cause it to be displayed. If you make local bindings (with `let' or function arguments) for a variable that may also have buffer-local bindings, make sure that the same buffer is current at the beginning and at the end of the local binding's scope. Otherwise you might bind it in one buffer and unbind it in another! There are two ways to do this. In simple cases, you may see that nothing ever changes the current buffer within the scope of the binding. Otherwise, use `save-current-buffer' or `save-excursion' to make sure that the buffer current at the beginning is current again whenever the variable is unbound. Do not rely on using `set-buffer' to change the current buffer back, because that won't do the job if a quit happens while the wrong buffer is current. Here is what _not_ to do: (let (buffer-read-only (obuf (current-buffer))) (set-buffer ...) ... (set-buffer obuf)) Using `save-current-buffer', as shown here, handles quitting, errors, and `throw', as well as ordinary evaluation. (let (buffer-read-only) (save-current-buffer (set-buffer ...) ...)) - Function: current-buffer This function returns the current buffer. (current-buffer) => # - Function: set-buffer buffer-or-name This function makes BUFFER-OR-NAME the current buffer. This does not display the buffer in any window, so the user cannot necessarily see the buffer. But Lisp programs will now operate on it. This function returns the buffer identified by BUFFER-OR-NAME. An error is signaled if BUFFER-OR-NAME does not identify an existing buffer. - Special Form: save-current-buffer body... The `save-current-buffer' macro saves the identity of the current buffer, evaluates the BODY forms, and finally restores that buffer as current. The return value is the value of the last form in BODY. The current buffer is restored even in case of an abnormal exit via `throw' or error (*note Nonlocal Exits::). If the buffer that used to be current has been killed by the time of exit from `save-current-buffer', then it is not made current again, of course. Instead, whichever buffer was current just before exit remains current. - Macro: with-current-buffer buffer body... The `with-current-buffer' macro saves the identity of the current buffer, makes BUFFER current, evaluates the BODY forms, and finally restores the buffer. The return value is the value of the last form in BODY. The current buffer is restored even in case of an abnormal exit via `throw' or error (*note Nonlocal Exits::). - Macro: with-temp-buffer body... The `with-temp-buffer' macro evaluates the BODY forms with a temporary buffer as the current buffer. It saves the identity of the current buffer, creates a temporary buffer and makes it current, evaluates the BODY forms, and finally restores the previous current buffer while killing the temporary buffer. The return value is the value of the last form in BODY. You can return the contents of the temporary buffer by using `(buffer-string)' as the last form. The current buffer is restored even in case of an abnormal exit via `throw' or error (*note Nonlocal Exits::). See also `with-temp-file' in *Note Writing to Files::.  File: elisp, Node: Buffer Names, Next: Buffer File Name, Prev: Current Buffer, 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' and `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 NEWNAME. Ordinarily, `rename-buffer' signals an error if NEWNAME is already in use. However, if UNIQUE is non-`nil', it modifies NEWNAME to make a name that is not in use. Interactively, you can make UNIQUE non-`nil' with a numeric prefix argument. (This is how the command `rename-uniquely' is implemented.) - Function: get-buffer buffer-or-name This function returns the buffer specified by BUFFER-OR-NAME. If BUFFER-OR-NAME is a string and there is no buffer with that name, the value is `nil'. If BUFFER-OR-NAME is a buffer, it is returned as given; that is not very useful, so the argument is usually a name. For example: (setq b (get-buffer "lewis")) => # (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 &rest ignore This function returns a name that would be unique for a new buffer--but does not create the buffer. It starts with STARTING-NAME, and produces a name not currently in use for any buffer by appending a number inside of `<...>'. If the optional second argument IGNORE is non-`nil', it should be a string; it makes a difference if it is a name in the sequence of names to be tried. That name will be considered acceptable, if it is tried, even if a buffer with that name exists. Thus, if buffers named `foo', `foo<2>', `foo<3>' and `foo<4>' exist, (generate-new-buffer-name "foo") => "foo<5>" (generate-new-buffer-name "foo" "foo<3>") => "foo<3>" (generate-new-buffer-name "foo" "foo<6>") => "foo<5>" See the related function `generate-new-buffer' in *Note Creating Buffers::.