The mmioSetBuffer function enables or disables buffered I/O, or changes the buffer or buffer size for a file opened by using the mmioOpen function.

MMRESULT mmioSetBuffer(
  HMMIO hmmio,     
  LPSTR pchBuffer, 
  LONG cchBuffer,  
  UINT wFlags      



File handle of the file.


Pointer to an application-defined buffer to use for buffered I/O. If this parameter is NULL, mmioSetBuffer allocates an internal buffer for buffered I/O.


Size, in characters, of the application-defined buffer, or the size of the buffer for mmioSetBuffer to allocate.


Reserved; must be zero.

Return Values

Returns zero if successful or an error otherwise. If an error occurs, the file handle remains valid. The following values are defined.

Value Description
MMIOERR_CANNOTWRITE The contents of the old buffer could not be written to disk, so the operation was aborted.
MMIOERR_OUTOFMEMORY The new buffer could not be allocated, probably due to a lack of available memory.


To enable buffering using an internal buffer, set pchBuffer to NULL and cchBuffer to the desired buffer size.

To supply your own buffer, set pchBuffer to point to the buffer, and set cchBuffer to the size of the buffer.

To disable buffered I/O, set pchBuffer to NULL and cchBuffer to zero.

If buffered I/O is already enabled using an internal buffer, you can reallocate the buffer to a different size by setting pchBuffer to NULL and cchBuffer to the new buffer size. The contents of the buffer can be changed after resizing.


  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.

See Also