The following code illustrates how to install an IOProc.

  FOURCC     fourcc1 ;
  PMMIOPROC  pIOProc1;
  ULONG      ulFlags;
       ....

             fourcc1= FOURCC_WAVE;
             ulFlags= MMIO_FINDPROC;
             pIOProc1= mmioInstallIOProc (fourcc1, pIOProc1, ulFlags);
             if (!pIOProc1)
             /* WAVE I/O Procedure NOT FOUND */
             else
             /* WAVE I/O Procedure FOUND */
       ....

The following is a procedure prototype for a standard I/O procedure call.

LONG APIENTRY MMIOPROC (PVOID pmmioinfo, USHORT usMsg,
                        LONG lParam1, LONG lParam2);

The following code illustrates how to call a custom I/O procedure and what parameters are required.

(LONG) lrc= IOProc(pmmioinfo, usMsg, lParam1, lParam2);

The I/O procedure must return MMIOERR_UNSUPPORTED_MESSAGE if it or subsequent I/O procedures it has called through mmioSendMessage does not understand usMsg.