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.