Generating Unique File Names

Some programs need to write temporary files. Here is the usual way to construct a name for such a file:

(make-temp-name
 (expand-file-name name-of-application
                   temporary-file-directory))

The job of make-temp-name is to prevent two different users or two different jobs from trying to use the exact same file name. This example uses the variable temporary-file-directory to decide where to put the temporary file. All Emacs Lisp programs should use temporary-file-directory for this purpose, to give the user a uniform way to specify the directory for all temporary files.

Function: make-temp-name string

This function generates a string that can be used as a unique file name. The name starts with string, and contains a number that is different in each Emacs job.

(make-temp-name "/tmp/foo")
     => "/tmp/foo232J6v"

To prevent conflicts among different libraries running in the same Emacs, each Lisp program that uses make-temp-name should have its own string. The number added to the end of string distinguishes between the same application running in different Emacs jobs. Additional added characters permit a large number of distinct names even in one Emacs job.

Variable: temporary-file-directory

This variable specifies the directory name for creating temporary files. Its value should be a directory name (see section Directory Names), but it is good for Lisp programs to cope if the value is a directory's file name instead. Using the value as the second argument to expand-file-name is a good way to achieve that.

The default value is determined in a reasonable way for your operating system; on GNU and Unix systems it is based on the TMP and TMPDIR environment variables.

Even if you do not use make-temp-name to choose the temporary file's name, you should still use this variable to decide which directory to put the file in.

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