The acmStreamUnprepareHeader function cleans up the preparation performed by the acmStreamPrepareHeader function for an ACM stream. This function must be called after the ACM is finished with the given buffers. An application must call this function before freeing the source and destination buffers.

MMRESULT acmStreamUnprepareHeader(
  HACMSTREAM has,          
  DWORD fdwUnprepare       



Handle to the conversion steam.


Pointer to an ACMSTREAMHEADER structure that identifies the source and destination buffers to be unprepared.


Reserved; must be zero.

Return Values

Returns zero if successful or an error otherwise. Possible error values include the following.

Value Description
ACMERR_BUSY The stream header specified in pash is currently in use and cannot be unprepared.
ACMERR_UNPREPARED The stream header specified in pash is currently not prepared by the acmStreamPrepareHeader function.
MMSYSERR_INVALFLAG At least one flag is invalid.
MMSYSERR_INVALHANDLE The specified handle is invalid.
MMSYSERR_INVALPARAM At least one parameter is invalid.


Unpreparing a stream header that has already been unprepared is an error. An application must specify the source and destination buffer lengths (cbSrcLength and cbDstLength, respectively) that were used during a call to the corresponding acmStreamPrepareHeader. Failing to reset these member values will cause acmStreamUnprepareHeader to fail with an MMSYSERR_INVALPARAM error.

The ACM can recover from some errors. The ACM will return a nonzero error, yet the stream header will be properly unprepared. To determine whether the stream header was actually unprepared, an application can examine the ACMSTREAMHEADER_STATUSF_PREPARED flag. If acmStreamUnprepareHeader returns success, the header will always be unprepared.


  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 Msacm.h.
  Library: Use Msacm32.lib.

See Also

Audio Compression Manager, Audio Compression Functions, acmStreamPrepareHeader, ACMSTREAMHEADER