This message requests the IOProc to return header-specific information about the current file or file element opened for reading by mmioOpen; for example, media type, media structure, and the size of the media structure. When you call mmioRead to read in the file header, use the size of the header, which is passed in by the lParam2 parameter.

When the translate header is TRUE, the IOProc is expected to return the standard presentation header structure for that media type. The IOProc must transpose the file's native header data into that structure.

The following example shows an example of how the M-Motion IOProc supports the MMIOM_GETHEADER message.

case MMIOM_GETHEADER:
    {
    /************************************************************
     * Declare local variables.
     ************************************************************/
    PMMFILESTATUS       pVidInfo;

    /************************************************************
     * Check for valid MMIOINFO block.
     ************************************************************/
    if (!pmmioinfo)
        return (0);

    /************************************************************
     * Set up our working file status variable.
     ************************************************************/
    pVidInfo = (PMMFILESTATUS)pmmioinfo->pExtraInfoStruct;

    /**************************************************
     * Getheader only valid in READ or READ/WRITE mode.
     * There is no header to get in WRITE mode.  We
     * must also have a valid file handle to read from
     **************************************************/
    if ((pmmioinfo->ulFlags & MMIO_WRITE) ||
        (!(pVidInfo->hmmioSS)))
       return (0);

    /************************************************************
     * Check for Translation mode.
     ************************************************************/
    if (!(pmmioinfo->ulTranslate & MMIO_TRANSLATEHEADER))
        {
        /********************************************************
         * Translation is off.
         ********************************************************/
        if (lParam2 < sizeof (MMOTIONHEADER))
            {
            pmmioinfo->ulErrorRet = MMIOERR_INVALID_BUFFER_LENGTH;
            return (0);
            }

        if (!lParam1)
            {
            pmmioinfo->ulErrorRet = MMIOERR_INVALID_STRUCTURE;
            return (0);
            }

        /********************************************************
         * Read in first 16 bytes to fill up M-Motion header.
         ********************************************************/
        memcpy ((PVOID) lParam1,
                (PVOID)&pVidInfo->mmotHeader,
                sizeof (MMOTIONHEADER));

        /* Indicate that the header has been set, which    */
        /* is meaningless in read mode, but allows the     */
        /* application to do writes in read/write mode     */
        pVidInfo->bSetHeader = TRUE;

        return (sizeof (MMOTIONHEADER));
        }   /* end IF NOT TRANSLATED block */

    /******************
     * TRANSLATION IS ON
     ******************/
    if (lParam2 < sizeof (MMIMAGEHEADER))
        {
        pmmioinfo->ulErrorRet = MMIOERR_INVALID_BUFFER_LENGTH;
        return (0);
        }

    if (!lParam1)
        {
        pmmioinfo->ulErrorRet = MMIOERR_INVALID_STRUCTURE;
        return (0);
        }

    memcpy ((PVOID)lParam1,
            (PVOID)&pVidInfo->mmImgHdr,
            sizeof (MMIMAGEHEADER));

    return (sizeof (MMIMAGEHEADER));
    } /* end case of MMIOM_GETHEADER */