DWORD modMessage(uDeviceID, uMsg, dwUser, dwParam1, dwParam2)
This user-supplied function is the entry point for a MIDI output device driver.
UINT uDeviceID
Specifies the ID of the target device.
UINT uMsg
Specifies the message being sent to the driver.
DWORD dwUser
For the MODM_OPEN message, the driver should fill this location with its instance data. For any other messages, the instance data is returned to the driver. Drivers supporting multiple clients can use this instance data to keep track of which client is associated with the message.
DWORD dwParam1
Specifies a message-dependent parameter.
DWORD dwParam2
Specifies a message-dependent parameter.
Unless specified otherwise under a specific message, the driver should return an MMSYSERR_ or MIDIERR_ error code.
The driver should return MMSYSERR_NOTSUPPORTED if it does not support the specified message.