Using
a Callback Function to Process Driver Messages
You can write
your own callback function to process messages sent by the device driver. To
use a callback function, specify the CALLBACK_FUNCTION flag in the fdwOpen
parameter and the address of the callback in the dwCallback parameter of
the waveInOpen
or waveOutOpen
function.
Messages sent
to a callback function are similar to messages sent to a window, except they
have two DWORD parameters instead of a UINT and a DWORD
parameter. For details on these messages, see Playing Waveform-Audio Files.
To pass
instance data from an application to a callback function, use one of the
following techniques:
Pass the instance data using
the dwInstance parameter of the function that opens the device driver.
Pass the instance data using
the dwUser member of the WAVEHDR structure that identifies an audio data block
being sent to a device driver.
If you need
more than 32 bits of instance data, pass a pointer to a structure containing
the additional information.