22.11 Coding Systems for File Names

C-x RET F coding RET

Use coding system coding for encoding and decoding file names (set-file-name-coding-system).

The command C-x RET F (set-file-name-coding-system) specifies a coding system to use for encoding file names. It has no effect on reading and writing the contents of files.

In fact, all this command does is set the value of the variable file-name-coding-system. If you set the variable to a coding system name (as a Lisp symbol or a string), Emacs encodes file names using that coding system for all file operations. This makes it possible to use non-ASCII characters in file names—or, at least, those non-ASCII characters that the specified coding system can encode.

If file-name-coding-system is nil, Emacs uses a default coding system determined by the selected language environment, and stored in the default-file-name-coding-system variable. In the default language environment, non-ASCII characters in file names are not encoded specially; they appear in the file system using the internal Emacs representation.

When Emacs runs on MS-Windows versions that are descendants of the NT family (Windows 2000, XP, Vista, Windows 7, and Windows 8), the value of file-name-coding-system is largely ignored, as Emacs by default uses APIs that allow passing Unicode file names directly. By contrast, on Windows 9X, file names are encoded using file-name-coding-system, which should be set to the codepage (see codepage) pertinent for the current system locale. The value of the variable w32-unicode-filenames controls whether Emacs uses the Unicode APIs when it calls OS functions that accept file names. This variable is set by the startup code to nil on Windows 9X, and to t on newer versions of MS-Windows.

Warning: if you change file-name-coding-system (or the language environment) in the middle of an Emacs session, problems can result if you have already visited files whose names were encoded using the earlier coding system and cannot be encoded (or are encoded differently) under the new coding system. If you try to save one of these buffers under the visited file name, saving may use the wrong file name, or it may encounter an error. If such a problem happens, use C-x C-w to specify a new file name for that buffer.

If a mistake occurs when encoding a file name, use the command M-x recode-file-name to change the file name’s coding system. This prompts for an existing file name, its old coding system, and the coding system to which you wish to convert.