IWMReader::Start method (wmsdkidl.h)

Artikel
4/26/2023

[The feature associated with this page, Windows Media Format 11 SDK, is a legacy feature. It has been superseded by Source Reader and Sink Writer. Source Reader and Sink Writer have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use Source Reader and Sink Writer instead of Windows Media Format 11 SDK, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The Start method causes the reader object to start reading from the specified starting time offset. As data is read, it is passed to the application through the application's IWMReaderCallback::OnSample callback method.

Syntax

HRESULT Start(
  [in] QWORD cnsStart,
  [in] QWORD cnsDuration,
  [in] float fRate,
  [in] void  *pvContext
);

Parameters

[in] cnsStart

Time within the file at which to start reading, in 100-nanosecond units. If cnsStart is set to WM_START_CURRENTPOSITION, playback starts from the current position.

[in] cnsDuration

Duration of the read in 100-nanosecond units, or zero to read to the end of the file.

[in] fRate

Playback speed. Normal speed is 1.0. Higher numbers cause faster playback, and numbers less than zero indicate reverse rate (rewinding). The valid ranges are 1.0 through 10.0, and -1.0 through -10.0.

[in] pvContext

Generic pointer, for use by the application. This pointer is passed back to the OnSample method.

Return value

The method returns an HRESULT. Possible values include, but are not limited to, those in the following table.

Return code	Description
S_OK	The method succeeded.
E_OUTOFMEMORY	There is not enough available memory.
NS_E_INVALID_REQUEST	The value for fRate is not within the valid ranges, or the file is not seekable and a non-zero start position has been specified.
E_UNEXPECTED	The method failed for an unspecified reason.

Remarks

This method sends a WMT_STARTED status notification to the application's IWMReaderCallback::OnStatus function.

To change the rate but not the current file position, use the Start method with the WM_START_CURRENTPOSITION value.

Any call to Start while paused is treated as a seek through the file, and incurs a buffering penalty from network files. This is true even for calls to Start with the WM_START_CURRENTPOSITION value. To continue playing from the current paused position with no buffering penalty, call Resume.

If the application is providing the clock (by calling IWMReaderAdvanced::SetUserProvidedClock), it should usually set the cnsDuration parameter to zero. If the application specifies a non-zero value, then it must call the IWMReaderAdvanced::DeliverTime method exactly once, and the value passed to DeliverTime must be either the stop time or (QWORD)-1. The reader object will then deliver samples up to the specified duration.

This method is very similar to the IWMReaderAdvanced2::StartAtMarker method, but that method uses a marker instead of a start time.

Requirements


Minimum supported client	Windows 2000 Professional [desktop apps only],Windows Media Format 7 SDK, or later versions of the SDK
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	wmsdkidl.h (include Wmsdk.h)
Library	Wmvcore.lib; WMStubDRM.lib (if you use DRM)