The MMSYSTEM.H header file defines data types and function prototypes for all multimedia file I/O functions. You must include this header file in any source module that uses these functions. MMSYSTEM.H depends on declarations made in WINDOWS.H, so you must first include the WINDOWS.H header file. MMSYSTEM.H defines the following data types for the multimedia file I/O functions:
| Type | Description |
| FOURCC | A four-character code identifying an element of a RIFF file. |
| HMMIO | A handle to an open file. |
| MMCKINFO | A data structure containing information about a chunk in a RIFF file. |
| MMIOINFO | A data structure used to maintain the current state of a file accessed using the multimedia file I/O services. |
| MMIOPROC | A custom multimedia file I/O procedure. |