ACMDM_STREAM_SIZE

This message requests an ACM driver to return the size required for a source or destination buffer with a specified destination or source buffer size, along with source and destination format descriptions.

Parameters

dwDriverID

Specifies a driver instance identifier. The driver returns this value in response to the ACM_Open function.

hDriver

Handle to a driver.

uMsg

Specifies this message.

lParam1

Pointer to an ACMDRVSTREAMINSTANCE structure.

lParam2

Pointer to an ACMDRVSTREAMSIZE structure.

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.

Remarks

The ACM (Waveapi.dll) sends the ACMDM_STREAM_SIZE message by calling the ACM driver’s ACM_IOControl entry point through the DeviceIoControl function. The ACM sends this message when an application calls the acmStreamSize function.

All ACM drivers that provide stream conversions must support this message. For more information about stream conversions, see the Windows NT DDK.

Based on the input parameters, the driver must return the following information:

• How large the destination buffer must be in order to hold all the converted data for a source buffer of a specified size.
• The largest amount of source data that will not overflow such a destination buffer.

The ACMDRVSTREAMINSTANCE structure received with this message is the same structure that was received with a previous ACMDM_STREAM_OPEN message. The driver does not need to validate the structure’s contents again.

The driver examines ACMDRVSTREAMSIZE to determine which buffer, source, or destination the client has supplied. ACMDRVSTREAMINSTANCE contains structures that describe the source and destination formats and possibly a filter specification. The driver uses this information to determine the size of the requested buffer.

If the driver returns a buffer length of 0, the ACM provides an error return value of ACMERR_NOTPOSSIBLE to acmStreamSize.