The following code illustrates how to determine the header size.

   HMMIO  hmmio1;
   LONG   lHeaderLength;
   ULONG  ulReserved = 0L;
   ULONG  ulFlags = 0L;
   ULONG  rc;
    ...

   rc = mmioQueryHeaderLength( hmmio1,
                               &lHeaderLength,
                               ulReserved,
                               ulFlags);
   if (rc)
     /* error */
   else
    ...