18.6.1 Auto-Save Files
Auto-saving does not normally save in the files that you visited,
because it can be very undesirable to save a change that you did not
want to make permanent. Instead, auto-saving is done in a different
file called the auto-save file, and the visited file is changed
only when you request saving explicitly (such as with C-x C-s).
Normally, the auto-save file name is made by appending ‘ #’ to the
front and rear of the visited file name. Thus, a buffer visiting file
foo.c is auto-saved in a file #foo.c#. Most buffers that
are not visiting files are auto-saved only if you request it explicitly;
when they are auto-saved, the auto-save file name is made by appending
‘ #’ to the front and rear of buffer name, then
adding digits and letters at the end for uniqueness. For
example, the *mail* buffer in which you compose messages to be
sent might be auto-saved in a file named #*mail*#704juu. Auto-save file
names are made this way unless you reprogram parts of Emacs to do
something different (the functions make-auto-save-file-name and
auto-save-file-name-p). The file name to be used for auto-saving
in a buffer is calculated when auto-saving is turned on in that buffer.
The variable auto-save-file-name-transforms allows a degree
of control over the auto-save file name. It lets you specify a series
of regular expressions and replacements to transform the auto save
file name. The default value puts the auto-save files for remote
files (see Remote Files) into the temporary file directory on the
local machine.
When you delete a substantial part of the text in a large buffer, auto
save turns off temporarily in that buffer. This is because if you
deleted the text unintentionally, you might find the auto-save file more
useful if it contains the deleted text. To reenable auto-saving after
this happens, save the buffer with C-x C-s, or use C-u 1 M-x auto-save-mode.
If you want auto-saving to be done in the visited file rather than
in a separate auto-save file, enable the global minor mode
auto-save-visited-mode. In this mode, auto-saving is identical
to explicit saving. Note that this mode is orthogonal to the
auto-save mode described above; you can enable both at the same
time. However, if auto-save mode is active in some buffer and
the obsolete auto-save-visited-file-name variable is set to a
non- nil value, that buffer won’t be affected by
auto-save-visited-mode.
You can use the variable auto-save-visited-interval to
customize the interval between auto-save operations in
auto-save-visited-mode; by default it’s five seconds.
auto-save-interval and auto-save-timeout have no effect
on auto-save-visited-mode. See Controlling Auto-Saving, for
details on these variables.
A buffer’s auto-save file is deleted when you save the buffer in its
visited file. (You can inhibit this by setting the variable
delete-auto-save-files to nil.) Changing the visited
file name with C-x C-w or set-visited-file-name renames
any auto-save file to go with the new visited name.
Killing a buffer, by default, doesn’t remove the buffer’s auto-save
file. If kill-buffer-delete-auto-save-files is non- nil,
killing a buffer that has an auto-save file will make Emacs prompt the
user for whether the auto-save file should be deleted. (This is
inhibited if delete-auto-save-files is nil.)