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#### 24.8.7 Standard File Names

Sometimes, an Emacs Lisp program needs to specify a standard file name for a particular use—typically, to hold configuration data specified by the current user. Usually, such files should be located in the directory specified by `user-emacs-directory`, which is `~/.emacs.d` by default (see Init File). For example, abbrev definitions are stored by default in `~/.emacs.d/abbrev_defs`. The easiest way to specify such a file name is to use the function `locate-user-emacs-file`.

Function: locate-user-emacs-file base-name &optional old-name

  This function returns an absolute file name for an Emacs-specific configuration or data file. The argument `base-name` should be a relative file name. The return value is the absolute name of a file in the directory specified by `user-emacs-directory`; if that directory does not exist, this function creates it.

  If the optional argument old-name is non-`nil`, it specifies a file in the user’s home directory, `~/old-name`. If such a file exists, the return value is the absolute name of that file, instead of the file specified by base-name. This argument is intended to be used by Emacs packages to provide backward compatibility. For instance, prior to the introduction of `user-emacs-directory`, the abbrev file was located in `~/.abbrev_defs`. Here is the definition of `abbrev-file-name`:

      (defcustom abbrev-file-name
        (locate-user-emacs-file "abbrev_defs" ".abbrev_defs")
        "Default name of file from which to read abbrevs."
        …
        :type 'file)

A lower-level function for standardizing file names, which `locate-user-emacs-file` uses as a subroutine, is `convert-standard-filename`.

Function: convert-standard-filename filename

  This function returns a file name based on filename, which fits the conventions of the current operating system.

  On GNU and Unix systems, this simply returns filename. On other operating systems, it may enforce system-specific file name conventions; for example, on MS-DOS this function performs a variety of changes to enforce MS-DOS file name limitations, including converting any leading ‘`.`’ to ‘`_`’ and truncating to three characters after the ‘`.`’.

  The recommended way to use this function is to specify a name which fits the conventions of GNU and Unix systems, and pass it to `convert-standard-filename`.

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