Notifying Clients with the DriverCallback Function

Audio 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.

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.