14.5.1 Auto-Save Files

Auto-saving does not normally write to the files you visited, because it can be undesirable to save a program that is in an inconsistent state when you have made only half of a planned change. Instead, auto-saving is done in a different file called the auto-save file, and the visited file is changed only when you save explicitly, for example, with C-x C-s.

Normally, the name of the auto-save file is generated by appending ‘#’ to the front and back of the visited file name. Thus, a buffer visiting file foo.c would be 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 generated by appending ‘#%’ to the front and ‘#’ to the back of buffer name. For example, the ‘*mail*’ buffer in which you compose messages to be sent is auto-saved in a file named #%*mail*#. Names of auto-save files are generated this way unless you customize the functions make-auto-save-file-name and auto-save-file-name-p to do something different. The file name to be used for auto-saving a buffer is calculated at the time auto-saving is turned on in that buffer.

If you want auto-saving to be done in the visited file, set the variable auto-save-visited-file-name to be non-nil. In this mode, there is really no difference between auto-saving and explicit saving.

Emacs deletes a buffer’s auto-save file when you explicitly save the buffer. To inhibit the deletion, set 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 correspond to the new visited name.