The mkstemp function replaces the six trailing Xs of the string
    pointed to by template with a unique set of characters, and
    returns a file descriptor for the file open for reading and
    writing.

    The string pointed to by template should look like a filename
    with six trailing X's. The mkstemp function replaces each X with
    a character from the portable file-name character set, making
    sure not to duplicate an existing filename.

    If the string pointed to by template does not contain six
    trailing Xs, -1 is returned.