MMIOM_SEEK

This message is handled differently by each IOProc. Because of the requirements of different file formats, you will need to determine if this message can be supported, and if so, how to implement the three different types of seeks. A file format IOProc can use some type of calculation to determine the seek distance, and what those distance units are. The file format IOProc should use mmioSeek to call the storage system IOProc. A storage system IOProc can call the DOSSetFilePtr function to set the file's position. The bounds checking that must occur for compound file elements is handled by the mmioSeek function for BND file elements. Any other compound file IOProcs of this nature need to implement this checking in its own code.

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

case MMIOM_SEEK:
     {
     /************************************************************
      * Set up locals.
      ************************************************************/
     PMMFILESTATUS   pVidInfo;
     LONG            lNewFilePosition;
     LONG            lPosDesired;
     SHORT           sSeekMode;

     /************************************************************
      * Check to make sure MMIOINFO block is valid.
      ************************************************************/
     if (!pmmioinfo)
         return (MMIO_ERROR);

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

     lPosDesired = lParam1;
     sSeekMode = (SHORT)lParam2;

     /************************************************************
      * Is Translate Data on?
      ************************************************************/
     if (pmmioinfo->ulTranslate & MMIO_TRANSLATEDATA)
         {
         /********************************************************
          * Attempt to move the Image buffer pointer to the
          * desired location.  App sends SEEK requests in
          * positions relative to the image planes & bits/pel
          * We must also convert this to RGB positions
          ********************************************************/
         switch (sSeekMode)
             {
             case SEEK_SET:
                 {
                 lNewFilePosition = lPosDesired;
                 break;
                 }
             case SEEK_CUR:
                 {
                 lNewFilePosition = pVidInfo->lImgBytePos + lPosDesired;
                 break;
                 }
             case SEEK_END:
                 {

                 lNewFilePosition =
                              pVidInfo->ulImgTotalBytes += lPosDesired;
                 break;
                 }

             default :
                return (MMIO_ERROR);
             }

         /********************************************************
          * Make sure seek did not go before start of file.
          * If so, then don't change anything, just return an error
          ********************************************************/
         if (lNewFilePosition < 0)
            {
            return (MMIO_ERROR);
            }

         /********************************************************
          * Make sure seek did not go past the end of file.
          ********************************************************/
         if (lNewFilePosition > (LONG)pVidInfo->ulImgTotalBytes)
             lNewFilePosition = pVidInfo->ulImgTotalBytes;

         pVidInfo->lImgBytePos = lNewFilePosition;

         return (pVidInfo->lImgBytePos);
         } /* end  IF DATA TRANSLATED */

     /************************************************************
      * Translate Data is OFF...
      ************************************************************
      * if this is a seek from the beginning of the file,
      *    we must account for and pass the header
      ************************************************************/
     if (lParam2==SEEK_SET)
        lPosDesired += MMOTION_HEADER_SIZE;

     lNewFilePosition = mmioSeek (pVidInfo->hmmioSS,
                                  lPosDesired,
                                  sSeekMode);

     /********************************************
      * Ensure we did not move to within the header
      ********************************************/
     if ((lNewFilePosition != MMIO_ERROR) &&
         (lNewFilePosition < MMOTION_HEADER_SIZE))
        {
        lNewFilePosition = mmioSeek (pVidInfo->hmmioSS,
                                      (LONG)MMOTION_HEADER_SIZE,
                                      SEEK_SET);
        }

     /************************************************************
      * Return new position.  Always remove the length of the
      *    header from the this position value
      ************************************************************/
     if (lNewFilePosition != MMIO_ERROR)
        lNewFilePosition -= MMOTION_HEADER_SIZE;

     return (lNewFilePosition);
     }  /* end case of MMIOM_SEEK */