Because its primary purpose is to play and record waveform data, the waveform audio MCD routes an MCI_SET application request to set audio attributes. The waveform audio MCD can route requests to the amp mixer because a default logical connection exists from the waveform audio MCD to the ampmixer MCD.

A default connection is the name of a connected device, while a device-context connection is the actual handle to a particular instance of an opened device. The waveaudio device has a default connection to an ampmixer device. When the waveaudio device is opened, it automatically opens the ampmix device creating an instance of each device. Since devices may be shared in OS/2 multimedia, the waveaudio device can be opened again by another application and two new instances are created. While the default connection is the same in both cases, the device context connections are different.

The code fragment in the following example illustrates how the waveaudio device gets a default connection and opens an associated amp mixer. The waveform audio MCD uses the device handle of the amp mixer to route any requests to change the volume.

An application can retrieve the amp mixer handle by issuing the MCI_CONNECTION message. This is necessary if the application needs to use any advanced audio shaping functions such as treble, bass, balance, and so on.

  ulpInstance->usWaveDeviceID = pDrvOpenParams->usDeviceID;

  ulDeviceTypeID = MAKEULONG ( MCI_DEVTYPE_WAVEFORM_AUDIO,
                               pDrvOpenParams->usDeviceOrd);
  /******************************************************
  * Ensure that the INI file contains the right device id
  ******************************************************/

  if ( pDrvOpenParams->usDeviceType != MCI_DEVTYPE_WAVEFORM_AUDIO )
     {
     return ( MCIERR_INI_FILE );
     }

  usConnLength = sizeof(DEFAULTCONNECTIONS2);

  ulrc =  mciQueryDefaultConnections ( ulDeviceTypeID,
                                       &DefCon,
                                       &usConnLength);
  /******************************************************
  * Ensure that the INI file says that we are connected
  * to an amp mixer.  If it says that we are connected
  * to ourselves, return an error.
  ******************************************************/
  if ( ULONG_LOWD( DefCon.dwDeviceTypeID2 ) == MCI_DEVTYPE_WAVEFORM_AUDIO )
     {
     return ( MCIERR_INI_FILE );
     }