Using the Command Message Interface

To use the command message interface to communicate with a device, an application calls mciSendCommand and passes the command message MCI_OPEN. If the request is successful, a device handle for access to the device context is returned in the usDeviceID field of the MCI_OPEN_PARMS data structure. This handle is retained for use in subsequent message commands.

An alias can be specified with the MCI_OPEN_ALIAS flag in the command message MCI_OPEN. Commands can then be issued to the device context by means of the string interface.

The following code fragment shows the opening of devices in the Duet Player I sample program. The hwndCallback field contains the application's main window procedure so that the MM_MCIPASSDEVICE messages are sent to it when the duet player gains or passes control of the device. The device ID and type fields of the structure are not needed because the audio file name is specified as the element field of the structure. This causes the Media Device Manager (MDM) to open the appropriate device based on the file name extension. Once the MCI_OPEN_PARMS structure is initialized, an MCI_OPEN command is specified with the mciSendCommand function for each separate part of the duet.

 /*
  * Open one part of the duet. The first step is to initialize an
  * MCI_OPEN_PARMS data structure with the appropriate information,
  * then issue the MCI_OPEN command with the mciSendCommand function.
  * We will be using an open with only the element name specified.
  * This will cause the default connection, as specified in the
  * MMPM.INI file, for the data type.
  */
mopDuetPart.hwndCallback   = (ULONG)  hwnd; /* For MM_MCIPASSDEVICE */
mopDuetPart.usDeviceID     = (USHORT) NULL; /* this is returned    */
mopDuetPart.pszDeviceType  = (PSZ)    NULL; /* using default conn. */
mopDuetPart.pszElementName = (PSZ)   aDuet[sDuet].achPart1;


     ulError = mciSendCommand( (USHORT) 0,
                               MCI_OPEN,
                               MCI_WAIT | MCI_OPEN_ELEMENT |
                               MCI_OPEN_SHAREABLE | MCI_READONLY,
                               (PVOID) &mopDuetPart,
                               UP_OPEN);


     if (!ulError)  /* if we opened part 1 */
     {
        usDuetPart1ID = mopDuetPart.usDeviceID;

        /*
         * Now, open the other part
         */
        mopDuetPart.pszElementName    = (PSZ)   aDuet[sDuet]achPart2;

        ulError = mciSendCommand( (USHORT) 0,
                                  MCI_OPEN,
                                  MCI_WAIT | MCI_OPEN_ELEMENT |
                                  MCI_OPEN_SHAREABLE | MCI_READONLY,
                                  (PVOID) &mopDuetPart,
                                  UP_OPEN);

        if (!ulError)  /* if we opened part 2 */
        {
           usDuetPart2ID = mopDuetPart.usDeviceID;