Microsoft Windows CE 3.0  


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This message requests an ACM driver to clear the preparation of the buffers associated with an ACMDRVSTREAMHEADERstructure.


Specifies a driver instance identifier. The driver returns this value in response to the ACM_Openfunction.
Handle to a driver.
Specifies this message.
Pointer to an ACMDRVSTREAMINSTANCEstructure.
Pointer to an ACMDRVSTREAMHEADERstructure.

Return Values

MMSYSERR_NOERROR indicates success. Otherwise, the driver returns one of the MMSYSERR error values declared in the Mmsystem.h header file or one of the ACMERR error values declared in the Msacm.h header file.


The ACM (Waveapi.dll) sends this message by calling the ACM driver's ACM_IOControlentry point through the DeviceIoControlfunction. The ACM sends this message when an application calls the acmStreamUnprepareHeaderfunction.

Support for this message is optional. However, if a driver supports ACMDM_STREAM_PREPARE, it must support this message.

If the driver returns MMSYSERR_NOTSUPPORTED, the ACM clears the buffer preparation. For most drivers, this behavior is sufficient. If the driver does clear buffer preparation, it returns MMSYSERR_NOERROR. In either case, the ACM clears ACMSTREAMHEADER_STATUSF_PREPARED in the fdwStatusmember of ACMDRVSTREAMHEADER. The driver never modifies this flag. If you want both your driver and the ACM to clear buffer preparations, the driver returns MMSYSERR_NOTSUPPORTED after clearing its preparation. The ACM can then clear its preparation.

Before calling the driver's ACM_IOControlfunction, the ACM verifies the following conditions: