The IOProc determines how to handle this message. All IOProcs must support this message because mmioOpen sends MMIOM_IDENTIFYFILE when it attempts to automatically identify a file.

For file format IOProcs, the header needs to be read and checked to see if it matches with what the IOProc expects. The lParm2 field contains the handle used for the mmioRead function. In the following example, the header is a M-Motion file that is compared with the expected string defined in the IOProc. If it compares correctly, the message returns TRUE; otherwise FALSE is returned.

case MMIOM_IDENTIFYFILE:
    {
    /************************************************************
     * Declare local variables.
     ************************************************************/
    MMOTIONHEADER   mmotHeader;   /* M-Motion structure variable */
    HMMIO           hmmioTemp;    /* MMIO File Handle            */
    ULONG           ulWidth;
    ULONG           ulHeight;
    ULONG           ulRequiredFileLength;
    ULONG           ulActualFileLength;
    BOOL             fValidMMotionFile = FALSE;

    ULONG           ulTempFlags = MMIO_READ | MMIO_DENYWRITE |
MMIO_NOIDENTIFY;
                                   /* Flags used for temp open  */
                                   /* and close                 */

    /************************************************************
     * We need either a file name (lParam1) or file handle (lParam2)
     ************************************************************/
    if (!lParam1 && !lParam2)
        return (MMIO_ERROR);

    /* Copy the file handle, assuming one was provided... */
    hmmioTemp = (HMMIO)lParam2;

    /************************************************************
     * If no handle, then open the file using the string name
     ************************************************************/
    if (!hmmioTemp)
       {
       if (!(hmmioTemp = mmioOpen ((PSZ) lParam1,
                                  NULL,
                                  ulTempFlags)))
           {
           return (MMIO_ERROR);
           }
       }

    /************************************************************
     * Read in enough bytes to check out file.
     ************************************************************/
    if (sizeof (MMOTIONHEADER) !=
                    mmioRead (hmmioTemp,
                              (PVOID) &mmotHeader,
                              (ULONG) sizeof (MMOTIONHEADER)))
        {
        /********************************************************
         * Fail so close file and then return.
         ********************************************************/
        if (!lParam2) /* Don't close handle if provided to us  */
           mmioClose (hmmioTemp, 0);
        return (MMIO_ERROR);
        }

    /************************************************************
     * Close file before returning.
     ************************************************************/
    if (!lParam2) /* Don't close handle if provided to us  */
       mmioClose (hmmioTemp, 0);

    /************************************************************
     * Check validity of file and return result.
     ************************************************************/
    if (memcmp (mmotHeader.mmID, "YUV12C", 6) == 0)
        {
        ulWidth = mmotHeader.mmXlen;
        ulHeight = mmotHeader.mmYlen;

        /* Calculate what the length of the file SHOULD be based on the */
        /*   header contents                                            */
        ulRequiredFileLength = (((ulWidth >> 2) * 6) * ulHeight) +
                                               sizeof (MMOTIONHEADER);

        /* Query what the ACTUAL length of the file is                  */
        ulActualFileLength = (ULONG)mmioSeek (hmmioTemp, 0, SEEK_END);

        /* If these don't match, then it isn't a VALID M-Motion file    */
        /*     - regardless of what the header says.                    */
        if (ulRequiredFileLength == ulActualFileLength)
           fValidMMotionFile = TRUE;
        else
           fValidMMotionFile = FALSE;
        }  /* end header check block */

    /************************************************************
     * Close file before returning.
     ************************************************************/
    if (!lParam2)  /* Don't close handle if provided to us      */
       mmioClose (hmmioTemp, 0);

    if (fValidMMotionFile)
       return (MMIO_SUCCESS);
    else
       return (MMIO_ERROR);
    } /* end case of MMIOM_IDENTIFYFILE */