function accesses a unique storage system, such as a database or file
archive. You can install or remove this callback function by using the mmioInstallIOProc
IOProc is a placeholder for the application-defined function name. The actual name
must be exported by including it in a EXPORTS statement in the application's
LRESULT PASCAL IOProc(
| LPSTR lpmmioinfo,
| UINT wMsg,
| LPARAM lParam1,
| LPARAM lParam2
Address of an MMIOINFO
structure containing information about the open file. The I/O procedure must
maintain the lDiskOffset
member in this structure to indicate the file offset to the next read or
write location. The I/O procedure can use the adwInfo
member to store state information. The I/O procedure should not modify any
other members of the MMIOINFO
Message indicating the requested I/O operation. Messages that can be received
, and MMIOM_SEEK
Parameters for the message.
Returns a value that corresponds to the message specified by wMsg. If the I/O
procedure does not recognize a message, it should return zero.
The four-character code specified by the fccIOProc
member of the MMIOINFO
structure associated with a file identifies a filename extension for a custom
storage system. When an application calls the mmioOpen
function with a filename such as EXAMPLE.XYZ! ABC
, the I/O procedure associated with the four-character code "XYZ" is called to
open the ABC element of the file EXAMPLE.XYZ.
function maintains a separate list of installed I/O procedures for each
Windows application. Therefore, different applications can use the same I/O
procedure identifier for different I/O procedures without conflict.
If an application calls mmioInstallIOProc
more than once to register the same I/O procedure, it must call this function
to remove the procedure once for each time it installed the procedure.
function does not prevent an application from installing two different I/O
procedures with the same identifier, or installing an I/O procedure with one of
the predefined identifiers (DOS or MEM). The most recently installed procedure
takes precedence, and the most recently installed procedure is the first one to
When searching for a specified I/O procedure, local procedures are searched
first, then global procedures.
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