Each message received by the midMessage function comes with two DWORD parameters. For some messages, one of these parameters points to a structure containing additional information from the client, or to an empty structure for the driver to fill with information for the client.
MIDI input device drivers use the structures listed in the following table. For detailed information on these structures, see the reference section later in this chapter.
| MIDIHDR | A client-filled structure identifying a MIDI data block. |
| MIDIINCAPS | A driver-filled structure providing a client with information about the capabilities of a MIDI input device. |
| MIDIOPENDESC | A client-filled structure providing information about opening a MIDI device. |