File I/O messages are sent to custom I/O procedures to request I/O operations on a file. I/O procedures must respond to all of the following messages:
MMIOM_CLOSE
Sent to an I/O procedure to request that a file be closed.
MMIOM_OPEN
Sent to an I/O procedure to request that a file be opened.
MMIOM_READ
Sent to an I/O procedure to request that data be read from a file.
MMIOM_SEEK
Sent to an I/O procedure to request that the current position for reading and writing be changed.
MMIOM_WRITE
Sent to an I/O procedure to request that data be written to a file.
MMIOM_WRITEFLUSH
Sent to an I/O procedure to request that an I/O buffer be flushed to disk.