Sent to a MIDI output device to send stream data.
DWORD dwParam1
Specifies a far pointer to MIDIHDR structure containing data in the stream buffer format.
DWORD dwParam2
Specifies the size, in bytes, of the MIDIHDR structure.
The dwStreamID member of the MIDIEVENT structure is documented as reserved in the SDK documentation. This member actually contains the destination of the given event. The device driver should process the event only if dwStreamID either matches one of the stream identifiers given in the array specified by the rgIds member of the MIDIOPENDESC structure (specified in the MODM_OPEN message), or if it contains a -1 (0xFFFFFFFFL).
If dwStreamID matches a relevant stream identifier from rgIds, the driver should process the event on the port which the stream identifier was bound to by means of a device identifier in the MODM_OPEN message.
If dwStreamID contains a -1, the driver should process the event for all ports which are open on this stream.