The mmioInstallIOProc function installs or removes a custom I/O procedure. This function also locates an installed I/O procedure, using its corresponding four-character code.

LPMMIOPROC mmioInstallIOProc(
  FOURCC fccIOProc,    
  DWORD dwFlags        



Four-character code identifying the I/O procedure to install, remove, or locate. All characters in this code should be uppercase.


Pointer to the I/O procedure to install. To remove or locate an I/O procedure, set this parameter to NULL. For more information about the I/O procedure, see MMIOProc.


Flag indicating whether the I/O procedure is being installed, removed, or located. The following values are defined.

Value Meaning
MMIO_FINDPROC Searches for the specified I/O procedure.
MMIO_GLOBALPROC This flag is a modifier to the MMIO_INSTALLPROC flag and indicates the I/O procedure should be installed for global use. This flag is ignored if MMIO_FINDPROC or MMIO_REMOVEPROC is specified.
MMIO_INSTALLPROC Installs the specified I/O procedure.
MMIO_REMOVEPROC Removes the specified I/O procedure.

Return Values

Returns the address of the I/O procedure installed, removed, or located. Returns NULL if there is an error.


  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.
  Library: Use Winmm.lib.
  Unicode: Implemented as Unicode and ANSI versions on Windows NT/2000/XP.

See Also