By calling a client-specified callback functionAudio device drivers need to notify clients when certain events occur, such as when a waveform data block has been played or when a MIDI input event is received. When the client opens a device, the client specifies how it wants to be notified. There are three ways a driver can notify a client application:
By calling a client-specified callback function
By sending a message to a client-specified window
By unblocking a client-specified task
To make the job of notifying clients easier, MMSYSTEM provides the DriverCallback function. DriverCallback takes information supplied by the client when it opened a device and notifies the client accordingly. The Sound Blaster MIDI driver uses the function shown in the following example to call DriverCallback:
void FAR PASCAL midiCallback(
NPPORTALLOC pPortalloc,
WORD msg,
DWORD dwParam1,
DWORD dwParam2)
{
if (pPortalloc->dwCallback)
( pPortalloc->dwCallback, /* users callback DWORD */
HIWORD(pPortalloc->dwFlags),/* callback flags */
pPortalloc->hMidi,/* handle to the wave device */
msg, /* message from the driver */
Portalloc->dwInstance, /* user's instance data */
dwParam1,/* message-dependent parameter */
dwParam2 /* message-dependent parameter */
);
}
The parameter pPortalloc points to a structure containing the MIDI device handle, callback flags, and user instance data passed to the driver with the MODM_OPEN message. See “Opening and Closing Devices,” earlier in this chapter, for an example of how to handle the MODM_OPEN message. The parameters msg, dwParam1, and dwParam2 specify the message and message-dependent parameters the driver is sending to the client.
Clients must be notified when the following conditions occur:
When a device is opened.
When a device is closed.
When the driver is finished with a data block.
When the driver receives data that should be passed directly back to the application, such as when a MIDI input driver receives a MIDI event from an input port.