Here's the code from CUPS that replaces the mkstemp() functionality
with something that is fairly portable; feel free to rip out the
Windows code... :)

/*
 * 'cupsTempFile()' - Generate a temporary filename.
 */

char *                                  /* O - Filename */
cupsTempFile(char *filename,            /* I - Pointer to buffer */
             int  len)                  /* I - Size of buffer */
{
  int           fd;                     /* File descriptor for temp file */
#ifdef WIN32
  char          tmpdir[1024];           /* Windows temporary directory */
#else
  char          *tmpdir;                /* TMPDIR environment var */
#endif /* WIN32 */
  struct timeval curtime;               /* Current time */
  static char   buf[1024] = "";               /* Buffer if you pass in NULL and 0 */

 /*
  * See if a filename was specified...
  */

  if (filename == NULL)
  {
    filename = buf;
    len      = sizeof(buf);
  }

 /*
  * See if TMPDIR is defined...
  */

#ifdef WIN32
  GetTempPath(sizeof(tmpdir), tmpdir);
#else
  if ((tmpdir = getenv("TMPDIR")) == NULL)
  {
   /*
    * Put root temp files in restricted temp directory...
    */

    if (getuid() == 0)
      tmpdir = CUPS_REQUESTS "/tmp";
    else
      tmpdir = "/var/tmp";
  }
#endif /* WIN32 */

 /*
  * Make the temporary name using the specified directory...
  */

  do
  {
   /*
    * Get the current time of day...
    */

    gettimeofday(&curtime, NULL);

   /*
    * Format a string using the hex time values...
    */

    snprintf(filename, len - 1, "%s/%08x%05x", tmpdir,
             curtime.tv_sec, curtime.tv_usec);

   /*
    * Open the file in "exclusive" mode, making sure that we don't
    * stomp on an existing file or someone's symlink crack...
    */

#ifdef O_NOFOLLOW
    fd = open(filename, O_WRONLY | O_CREAT | O_EXCL | O_NOFOLLOW, 0600);
#else
    fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
#endif /* O_NOFOLLOW */
  }
  while (fd < 0);

 /*
  * Close the temp file - it'll be reopened later as needed...
  */

  close(fd);

 /*
  * Return the temp filename...
  */

  return (filename);

--
______________________________________________________________________
Michael Sweet, Easy Software Products                  m...@easysw.com
Printing Software for UNIX                       http://www.easysw.com