Unique File Names - GNU Emacs Lisp Reference Manual

Some programs need to write temporary files. Here is the usual way to construct a name for such a file, starting in Emacs 21:

The job of make-temp-file is to prevent two different users or two different jobs from trying to use the exact same file name.

— Function: make-temp-file prefix &optional dir-flag

This function creates a temporary file and returns its name. The name starts with prefix; it also contains a number that is different in each Emacs job. If prefix is a relative file name, it is expanded against temporary-file-directory.
          (make-temp-file "foo")
               => "/tmp/foo232J6v"
     
When make-temp-file returns, the file has been created and is empty. At that point, you should write the intended contents into the file.
If dir-flag is non-nil, make-temp-file creates an empty directory instead of an empty file.
To prevent conflicts among different libraries running in the same Emacs, each Lisp program that uses make-temp-file should have its own prefix. The number added to the end of prefix 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.

The default directory for temporary files is controlled by the variable temporary-file-directory. This variable gives the user a uniform way to specify the directory for all temporary files. Some programs use small-temporary-file-directory instead, if that is non-nil. To use it, you should expand the prefix against the proper directory before calling make-temp-file.

In older Emacs versions where make-temp-file does not exist, you should use make-temp-name instead:

— 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. It is like make-temp-file except that it just constructs a name, and does not create a file. On MS-DOS, the string prefix can be truncated to fit into the 8+3 file-name limits.

— Variable: temporary-file-directory

This variable specifies the directory name for creating temporary files. Its value should be a directory name (see 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; it is based on the TMPDIR, TMP and TEMP environment variables, with a fall-back to a system-dependent name if none of these variables is defined.
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. However, if you expect the file to be small, you should use small-temporary-file-directory first if that is non-nil.

— Variable: small-temporary-file-directory

This variable (new in Emacs 21) specifies the directory name for creating certain temporary files, which are likely to be small.
If you want to write a temporary file which is likely to be small, you should compute the directory like this:
          (make-temp-file
            (expand-file-name prefix
                              (or small-temporary-file-directory
                                  temporary-file-directory)))
     

25.8.5 Generating Unique File Names