The MODM_PREPARE message requests a MIDI output driver to prepare a system-exclusive data buffer for output.
Parameters
uDeviceId
Device identifier (0, 1, 2, and so on) for the target device.
uMsg
MODM_PREPARE
dwUser
Device instance identifier.
dwParam1
Pointer to a MIDIHDR structure identifying the buffer. (The MIDIHDR structure is described in the Win32 SDK.)
dwParam2
Size of the MIDIHDR structure.
Return Value
The driver should return MMSYSERR_NOERROR if the operation succeeds. Otherwise it should return one of the MMSYSERR or MIDIERR error codes defined in mmsystem.h. See midiOutPrepareHeader return values in the Win32 SDK.
Comments
A client sends the MODM_PREPARE message by calling the user-mode driver's modMessage entry point, passing the specified parameters.
Support for this message by user-mode drivers is optional. If the driver supports MODM_PREPARE, it must also support MODM_UNPREPARE.
If the driver returns MMSYSERR_NOTSUPPORTED, winmm.dll prepares the buffer for use. For most drivers, this behavior is sufficient. If the driver does perform buffer preparation, it must set MHDR_PREPARED in the dwFlags member of MIDIHDR and return MMSYSERR_NOERROR.
For additional information, see Transferring MIDI Output Data.