An IOProc is a message-based handler. Applications and the MMIO subsystem communicate to IOProcs through the use of MMIO messages. When MMIO receives a request from an application, the MMIO Manager sends a message for that operation to the IOProc that is responsible for that particular file format or storage system. In turn, the I/O procedure performs operations based on the messages it receives from the MMIO Manager or an application.

MMIO messages can be either pre-defined or user-defined messages:

Pre-defined Message

This is a system message that is sent by the MMIO Manager for its associated function. For example, an application issuing an mmioOpen function causes the MMIO Manager to send an MMIOM_OPEN message to an I/O procedure to open a specific file. These messages enable applications to manage media files in a format-independent manner. The MMIO Manager determines the correct I/O procedure to process the message, based on an I/O procedure identifier and I/O procedure type specified in the function. See I/O Procedure Identifier (FOURCC) and I/O Procedure Type.
User-defined Message
This is a private message sent directly to an I/O procedure from an application through the use of the mmioSendMessage function. This function enables a program to call an I/O procedure directly (unlike system messages, which are sent by the MMIO Manager). MMIOOS2.H in the \TOOLKIT\H subdirectory defines the identifier MMIO_USER so that you can create your own messages. The mmioSendMessage function requires that your custom messages be defined at or above the MMIOM_USER value defined in the MMIOOS2.H file.

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