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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-file name-of-application)

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 suffix text

This function creates a temporary file and returns its name. Emacs creates the temporary file’s name by adding to prefix some random characters that are different in each Emacs job. The result is guaranteed to be a newly created file, containing text if that’s given as a string and empty otherwise. On MS-DOS, this function can truncate prefix to fit into the 8+3 file-name limits. 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. It returns the file name, not the directory name, of that directory. See Directory Names.

If suffix is non-nil, make-temp-file adds it at the end of the file name.

If text is a string, make-temp-file inserts it in the 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.

User Option: 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-file to create the temporary file, 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.

User Option: small-temporary-file-directory

This variable 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)))

Function: make-temp-name base-name

This function generates a string that might be a unique file name. The name starts with base-name, and has several random characters appended to it, which are different in each Emacs job. It is like make-temp-file except that (i) it just constructs a name and does not create a file, (ii) base-name should be an absolute file name that is not magic, and (iii) if the returned file name is magic, it might name an existing file. See Magic File Names.

Warning: In most cases, you should not use this function; use make-temp-file instead! This function is susceptible to a race condition, between the make-temp-name call and the creation of the file, which in some cases may cause a security hole.

Sometimes, it is necessary to create a temporary file on a remote host or a mounted directory. The following two functions support this.

Function: make-nearby-temp-file prefix &optional dir-flag suffix

This function is similar to make-temp-file, but it creates a temporary file as close as possible to default-directory. If prefix is a relative file name, and default-directory is a remote file name or located on a mounted file systems, the temporary file is created in the directory returned by the function temporary-file-directory. Otherwise, the function make-temp-file is used. prefix, dir-flag and suffix have the same meaning as in make-temp-file.

(let ((default-directory "/ssh:remotehost:"))
  (make-nearby-temp-file "foo"))
     ⇒ "/ssh:remotehost:/tmp/foo232J6v"

Function: temporary-file-directory

The directory for writing temporary files via make-nearby-temp-file. In case of a remote default-directory, this is a directory for temporary files on that remote host. If such a directory does not exist, or default-directory ought to be located on a mounted file system (see mounted-file-systems), the function returns default-directory. For a non-remote and non-mounted default-directory, the value of the variable temporary-file-directory is returned.

In order to extract the local part of the file’s name of a temporary file, use file-local-name (see Magic File Names).