27.2 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. See Auto-Saving: Protection Against Disasters in The GNU Emacs Manual, 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.

     ⇒ "/xcssun/users/rms/lewis/#backups.texi#"
Command: auto-save-mode arg

This is the mode command for Auto Save mode, a buffer-local minor mode. When Auto Save mode is enabled, auto-saving is enabled in the buffer. The calling convention is the same as for other minor mode commands (see Conventions for Writing Minor Modes).

Unlike most minor modes, there is no auto-save-mode variable. Auto Save mode is enabled if buffer-auto-save-file-name is non-nil and buffer-saved-size (see below) is non-zero.

Variable: auto-save-file-name-transforms

This variable lists transforms to apply to buffer’s file name before making the auto-save file name.

Each transform is a list of the form (regexp replacement [uniquify]). regexp is a regular expression to match against the file name; if it matches, replace-match is used to replace the matching part with replacement. If the optional element uniquify is non-nil, the auto-save file name is constructed by concatenating the directory part of the transformed file name with the buffer’s file name in which all directory separators were changed to ‘!’ to prevent clashes. (This will not work correctly if your filesystem truncates the resulting name.)

If uniquify is one of the members of secure-hash-algorithms, Emacs constructs the nondirectory part of the auto-save file name by applying that secure-hash to the buffer file name. This avoids any risk of excessively long file names.

All the transforms in the list are tried, in the order they are listed. When one transform applies, its result is final; no further transforms are tried.

The default value is set up to put the auto-save files of remote files into the temporary directory (see Generating Unique File Names).

On MS-DOS filesystems without long names this variable is always ignored.

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.

     ⇒ "/xcssun/users/rms/lewis/#backups.texi#"
(auto-save-file-name-p "#backups.texi#")
     ⇒ 0
(auto-save-file-name-p "backups.texi")
     ⇒ nil
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.

     ⇒ "/xcssun/users/rms/lewis/#backups.texi#"
User Option: 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.

Note that setting this variable to a non-nil value does not change the fact that auto-saving is different from saving the buffer; e.g., the hooks described in Saving Buffers are not run when a buffer is auto-saved.

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. Setting this to zero disables autosaving based on the number of characters typed.

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.

If any buffers are auto-saved, do-auto-save normally displays a message saying ‘Auto-saving...’ 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 &optional force

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.

Unless force is non-nil, this function only deletes the file if it was written by the current Emacs session since the last true save.

User Option: 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 it was made in the current Emacs session. 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.

If it is −2, that means this buffer should disregard changes in buffer size; in particular, it should not shut off auto-saving temporarily due to changes in buffer size.

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.

User Option: 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.