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26.2 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 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

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 Minor Mode Conventions).

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.

— 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#"

Here is a simplified version of the standard definition of this function:

          (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.

— 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.

— 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.