IMFMediaSource::Start method (mfidl.h)

Starts, seeks, or restarts the media source by specifying where to start playback.


  [in] IMFPresentationDescriptor *pPresentationDescriptor,
  [in] const GUID                *pguidTimeFormat,
  [in] const PROPVARIANT         *pvarStartPosition


[in] pPresentationDescriptor

Pointer to the IMFPresentationDescriptor interface of the media source's presentation descriptor. To get the presentation descriptor, call IMFMediaSource::CreatePresentationDescriptor. You can modify the presentation descriptor before calling Start, to select or deselect streams or change the media types.

[in] pguidTimeFormat

Pointer to a GUID that specifies the time format. The time format defines the units for the pvarStartPosition parameter. If the value is GUID_NULL, the time format is 100-nanosecond units. Some media sources might support additional time format GUIDs. This parameter can be NULL. If the value is NULL, it is equivalent to GUID_NULL.

[in] pvarStartPosition

Specifies where to start playback. The units of this parameter are indicated by the time format given in pguidTimeFormat. If the time format is GUID_NULL, the variant type must be VT_I8 or VT_EMPTY. Use VT_I8 to specify a new starting position, in 100-nanosecond units. Use VT_EMPTY to start from the current position. Other time formats might use other PROPVARIANT types.

Return value

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

Return code Description
The method succeeded.
The start position is past the end of the presentation (ASF media source).
A hardware device was unable to start streaming. This error code can be returned by a media source that represents a hardware device, such as a camera. For example, if the camera is already being used by another application, the method might return this error code.
The start request is not valid. For example, the start position is past the end of the presentation.
The media source's Shutdown method has been called.
The media source does not support the time format specified in pguidTimeFormat.


This method is asynchronous. If the operation succeeds, the media source sends the following events:

  • For each new stream, the source sends an MENewStream event. This event is sent for the first Start call in which the stream appears. The event data is a pointer to the stream's IMFMediaStream interface.
  • For each updated stream, the source sends an MEUpdatedStream event. A stream is updated if the stream already existed when Start was called (for example, if the application seeks during playback). The event data is a pointer to the stream's IMFMediaStream interface.
  • If the previous state was stopped, the source sends an MESourceStarted event.
  • If the previous state was started or paused and the starting position is the current position (VT_EMPTY), the source sends an MESourceStarted event.
  • If the previous state was started or paused, and a new starting position is specified, the source sends an MESourceSeeked event.
  • If the source sends an MESourceStarted event, each media stream sends an MEStreamStarted event. If the source sends an MESourceSeeked event, each stream sends an MEStreamSeeked event.
If the start operation fails asynchronously (after the method returns S_OK), the media source sends an MESourceStarted event that contains a failure code, without sending any of the other events listed here. If the method fails synchronously (returns an error code), no events are raised.

A call to Start results in a seek if the previous state was started or paused, and the new starting position is not VT_EMPTY. Not every media source can seek. If a media source can seek, the IMFMediaSource::GetCharacteristics method returns the MFMEDIASOURCE_CAN_SEEK flag.

Events from the media source are not synchronized with events from the media streams. If you seek a media source, therefore, you can still receive samples from the earlier position after getting the MESourceSeeked event. If you need to synchronize the operations, wait for the stream event, MEStreamSeeked, which marks the exact point in the stream where the seek occurs.

End of Stream

When a stream plays to the end, the stream sends an MEEndOfStream event. When all of the selected streams have reached the end, the media source sends an MEEndOfPresentation event.

If the starting position is past the end of a selected stream (but before the end of the presentation), the stream should send MEEndOfStream immediately after MEStreamStarted/MEStreamSeeked. If playback reaches the end of the presentation and Start is called again from the current position, the streams re-send the MEEndOfStream event and the media source re-sends the MEEndOfPresentation event. These events inform the pipeline not to request any more data.

During reverse playback, the start of the file is considered the end of the stream. For more information, see Implementing Rate Control.

Implementing Start

When a media source executes a seek, it should start at the first key frame before the seek time, so that the decoder can decode the samples for the target start time. The pipeline will discard any decoded samples that are too early.

If the start time is VT_EMPTY and the previous state was started or paused, the source should resume from its current position. In this case, it is not necessary to resend the previous key frame, because the decoder will still have the data that was previously sent.

When validating the pPresentationDescriptor parameter, the media source should check only for the information that it needs to function correctly. In particular, the client can add private attributes to the presentation descriptor. The presence of additional attributes should not cause the Start method to fail.

After Start is called, each stream on the media source must do one of the following:

For more information, see Writing a Custom Media Source.


The following example starts playback at 1 second into the presentation.

var.vt = VT_I8;
var.hVal.QuadPart = 10000000; // 10^7 = 1 second.

hr = pSource->Start(pPresentationDescriptor, NULL, &var);


Requirement Value
Minimum supported client Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2008 [desktop apps | UWP apps]
Target Platform Windows
Header mfidl.h
Library Mfuuid.lib

See also


Media Sources