IWMReaderAdvanced2::Preroll method (wmsdkidl.h)

Article
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 It has been superseded by Source Reader and Sink Writer.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 Preroll method is used to begin prerolling (buffering data) for the reader.

Syntax

HRESULT Preroll(
  [in] QWORD cnsStart,
  [in] QWORD cnsDuration,
  [in] float fRate
);

Parameters

[in] cnsStart

Specifies the start time in 100-nanosecond units.

[in] cnsDuration

Specifies the duration in 100-nanosecond units.

[in] fRate

Specifies the data rate.

Return value

If the method succeeds, it returns S_OK. If it fails, it returns an HRESULT error code.

Remarks

This method can be called before the application calls Start to begin buffering data in advance. The parameters here must be set to the same values as those that are passed to Start when it is called. If the parameters are different, Start will do rebuffering.

It is important to allow sufficient time for the prerolling (buffering data) for the reader to be completed before calling Start. When prerolling local files, 6 seconds normally is sufficient. When prerolling files over the Internet, allow more time before calling Start. If insufficient time is allowed, the effect will be a longer Start time when Start is called.

Requirements

Requirement	Value
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)

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