The MMSYSTEM.H file defines data types and function prototypes for all of the low-level audio functions. MMSYSTEM.H defines the following data types for low-level MIDI input functions:
| Function | Description |
| HMIDIIN | A handle to a MIDI input device. |
| MIDIHDR | A data structure that is a header for a block of MIDI system-exclusive data. This data structure is used for input as well as output. |
| MIDIINCAPS | A data structure used to inquire about the capabilities of a MIDI input device. |