Multimedia file I/O services use I/O procedures to handle the physical input and output associated with reading and writing to different types of storage systems, such as file-archival systems and database-storage systems. There are predefined I/O procedures for standard MS-DOS files and for memory files. In the future, there will be a predefined I/O procedure for accessing elements of compound files. Compound files consist of a number of individual files, called file elements, bound together in one physical file.
You can supply a custom I/O procedure for accessing a unique storage system such as a database or file archive. This I/O procedure can be private to your application or it can be shared with other applications.
The multimedia file I/O services provide the following functions to support custom I/O procedures:
| Function | Description |
| mmioInstallIOProc | Installs, removes, or locates an I/O procedure. |
| mmioSendMessage | Sends a custom message to the I/O procedure associated with a file. |