The tempnam function generates filenames for temporary files. It
    allows you to control the choice of a directory.

    If the directory argument is null or points to a string that is
    not a pathname for an appropriate directory, the pathname defined
    as P_tmpdir in the <stdio.h> header file is used. For programs
    running under a detached process, the directory argument cannot
    be null.

    You can bypass the selection of a pathname by providing the
    TMPDIR environment variable in the user environment. The value
    of the TMPDIR variable is a pathname for the desired temporary
    file directory.

    Use the prefix argument to specify a prefix of up to five
    characters for the temporary filename.

    The tempnam function returns a pointer to the generated pathname,
    suitable for use in a subsequent call to the free function.

    See also free.

                                   NOTE

       In contrast to tmpnam, tempnam does not have to generate
       a different filename on each call. tempnam generates a new
       filename only if the file with the specified name exists. If
       you need a unique filename on each call, use tmpnam instead
       of tempnam.