MMIOProc

Article
12/10/2007

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.

LRESULT MMIOProc(
  LPSTR lpmmioinfo,  
  UINT uMsg,         
  LONG lParam1,      
  LONG lParam2);

Parameters

lpmmioinfo

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.

uMsg

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.

lParam1

Specifies an application-defined parameter for the message.

lParam2

Specifies an application-defined parameter for the message.

Return Values

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

Remarks

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 "one.xyz+two", the I/O procedure associated with the four-character code "XYZ" is called to open the "two" element of the file "one.xyz".

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.

Requirements

**  Windows NT/2000/XP:** Included in Windows NT 3.1 and later.
**  Windows 95/98/Me:** Included in Windows 95 and later.
**  Header:** Declared in Mmsystem.h; include Windows.h.

MMIOProc

Additional resources