Notifying Clients from Audio Drivers

User-mode drivers are responsible for notifying clients upon the completion of various audio operations. Clients indicate the type of notification, if any, they expect when they open a driver instance for waveform, MIDI, or mixer operations. (Refer to MIDM_OPEN, MODM_OPEN, MXDM_OPEN, WIDM_OPEN, and WODM_OPEN messages.) Clients that request notification can specify any of the following notification targets:

• A callback function

• A window handle

• An event handle

• A thread identifier

Mixer drivers accept only window handles.

User-mode drivers notify clients by calling the DriverCallback function in winmm.dll. This function delivers a message to the client’s notification target. The function also delivers message parameters, if the target type accepts parameters.

Following are the messages user-mode drivers must send to a client if the client has requested notification:

Operations	Messages
Waveform Input	WIM_CLOSE WIM_DATA WIM_OPEN
Waveform Output	WOM_CLOSE WOM_DONE WOM_OPEN
MIDI Input	MIM_CLOSE MIM_DATA MIM_ERROR MIM_LONGDATA MIM_LONGERROR MIM_MOREDATA MIM_OPEN
MIDI Output	MOM_CLOSE MOM_DONE MOM_OPENMOM_POSITIONCB
Mixer	MM_MIXM_LINE_CHANGE MM_MIXM_CONTROL_CHANGE