MMIOPROC callback function

The MMIOProc function is a custom input/output (I/O) procedure installed by the mmioInstallIOProc function. MMIOProc is a placeholder for the application-defined function name. The address of this function can be specified in the callback-address parameter of mmioInstallIOProc.


MMIOPROC Mmioproc;

LRESULT Mmioproc(
  LPSTR lpmmioinfo,
  UINT uMsg,
  LPARAM lParam1,
  LPARAM lParam2



Points to 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 structure.


Specifies a message indicating the requested I/O operation. Messages that can be received include MMIOM_OPEN, MMIOM_CLOSE, MMIOM_READ, MMIOM_SEEK, MMIOM_WRITE, and MMIOM_WRITEFLUSH.


Specifies an application-defined parameter for the message.


Specifies an application-defined parameter for the message.

Return Value

The return value depends on the message specified by uMsg. If the I/O procedure does not recognize a message, it should return zero.


The four-character code specified by the fccMMIOProc member in the MMIOINFO structure associated with a file identifies a file name extension for a custom storage system. When an application calls mmioOpen with a file name such as "", the I/O procedure associated with the four-character code "XYZ" is called to open the "two" element of the file "".

The mmioInstallIOProc function maintains a separate list of installed I/O procedures for each Windows-based application. Therefore, different applications can use the same I/O procedure identifier for different I/O procedures without conflict. However, installing an I/O procedure globally enables any process to use the procedure.

If an application calls mmioInstallIOProc more than once to register the same I/O procedure, then it must call mmioInstallIOProc to remove the procedure once for each time it installed the procedure.

mmioInstallIOProc will 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 ", "MEM "). The most recently installed procedure takes precedence, and the most recently installed procedure is the first one to be removed.

When searching for a specified I/O procedure, local procedures are searched first, then global procedures.


Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header mmiscapi.h (include Mmiscapi.h, Windows.h)