Description

   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.