The MIDM_UNPREPARE message requests a MIDI input driver to remove the buffer preparation that was performed in response to a MIDM_PREPARE message.
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 midiInUnprepareHeader return values in the Win32 SDK.
A client sends the MIDM_UNPREPARE message by calling the user-mode driver’s midMessage entry point, passing the specified parameters.
Support for this message by user-mode drivers is optional. If the driver supports MIDM_PREPARE, it must also support MIDM_UNPREPARE.
If the driver returns MMSYSERR_NOTSUPPORTED, winmm.dll removes the buffer preparation. For most drivers, this behavior is sufficient. If the driver does support MIDM_UNPREPARE, it must clear MHDR_PREPARED in the dwFlags member of MIDIHDR and return MMSYSERR_NOERROR.
For additional information, see Transferring MIDI Input Data.