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#### 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, set the variable `auto-save-visited-file-name` to a non-`nil` value. In this mode, there is no real difference between auto-saving and explicit saving.

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.

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