Custom Services - Win32 Multimedia Programmer's Reference

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Custom Services

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. Predefined I/O procedures exist for the standard file systems and for memory files, but you can supply a custom I/O procedure for accessing a unique storage system by using the mmioInstallIOProc function.

To open a file by using a custom I/O procedure, use the mmioOpen function. Include a plus sign (+) or the CFSEPCHAR constant in the filename to separate the name of the physical file from the name of the element of the file you want to open. The following example opens a file element named "element" from a file named FILENAME.ARC:

mmioOpen("filename.arc+element", NULL, MMIO_READ);

When the file I/O manager encounters a plus sign in a filename, it examines the filename extension to determine which I/O procedure to associate with the file. In the previous example, the file I/O manager would attempt to use the I/O procedure associated with the .ARC filename extension; this I/O procedure would have been installed by using mmioInstallIOProc. If no I/O procedure is installed, mmioOpen returns an error.

I/O procedures must respond to the following messages:

MMIOM_CLOSE
MMIOM_OPEN
MMIOM_READ
MMIOM_WRITE
MMIOM_SEEK
MMIOM_RENAME
MMIOM_WRITEFLUSH

You can also create custom messages and send them to your I/O procedure by using the mmioSendMessage function. If you define your own messages, make sure they are defined at or above the value defined by the MMIOM_USER constant.

In addition to processing messages, an I/O procedure must maintain the lDiskOffset member of the MMIOINFO structure (pointed to by the lpmmioinfo parameter of the mmioOpen function). The lDiskOffset member must always contain the file offset to the location that the next MMIOM_READ or MMIOM_WRITE message will access. The offset is specified in bytes and is relative to the beginning of the file. The I/O procedure can use the adwInfo member to maintain any required state information. The I/O procedure should not modify any other members in the MMIOINFO structure.

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