The lDiskOffset field of the MMIOINFO structure is initialized to 0 by mmioOpen before MMIOM_OPEN is called. If this value is incorrect, the IOProc must correct it.

See mmioOpen for a description of the file ulFlags field of pmmioinfo (which is passed to mmioOpen as ulOpenFlags). In particular, if the MMIO_DELETE flag is present, the file must be deleted.