EVT_ACX_STREAM_GET_PRESENTATION_POSITION callback function (acxstreams.h)

Article
02/22/2024

EvtAcxStreamGetPresentationPosition tells the driver to indicate the current position along with the QPC value at the time the current position was calculated.

Syntax

EVT_ACX_STREAM_GET_PRESENTATION_POSITION EvtAcxStreamGetPresentationPosition;

NTSTATUS EvtAcxStreamGetPresentationPosition(
  ACXSTREAM Stream,
  PULONGLONG PositionInBlocks,
  PULONGLONG QPCPosition
)
{...}

Parameters

Stream

An ACXSTREAM object represents an audio stream created by a circuit. The stream is composed of a list of elements created based on the parent circuit's elements. For more information, see ACX - Summary of ACX Objects.

PositionInBlocks

Specifies the block offset from the start of the stream to the current post-decoded, uncompressed position in the stream. A "block" refers to the group of channels in the same sample. So, for example, in a PCM stream a block is the same as a frame. However, for compressed formats a block is a single sample within a frame. This means that for a typical MP3 stream that has 1152 samples in a frame, there are 1152 blocks.

QPCPosition

Specifies the value of the performance counter at the time that the audio driver reads the presentation position in response to the KSAUDIO_PRESENTATION_POSITION call. A driver writes to this field with the value read from calling KeQueryPerformanceCounter when a snapshot is taken of the presentation position.

Return value

Returns STATUS_SUCCESS if the call was successful. Otherwise, it returns an appropriate error code. For more information, see Using NTSTATUS Values.

Remarks

Example

Example usage is shown below.

    //
    // Init streaming callbacks.
    //
    ACX_RT_STREAM_CALLBACKS rtCallbacks;
    ACX_RT_STREAM_CALLBACKS_INIT(&rtCallbacks);

    rtCallbacks.EvtAcxStreamGetPresentationPosition = EvtStreamGetPresentationPosition;

    status = AcxStreamInitAssignAcxRtStreamCallbacks(StreamInit, &rtCallbacks);

PAGED_CODE_SEG
NTSTATUS
EvtStreamGetPresentationPosition(
    _In_ ACXSTREAM      Stream,
    _Out_ PULONGLONG    PositionInBlocks,
    _Out_ PULONGLONG    QPCPosition
)
{
    PSTREAM_CONTEXT ctx;
    ULONG               blockAlign;
    LARGE_INTEGER       qpc;

    PAGED_CODE();

    ctx = GetStreamContext(Stream);

    blockAlign = AcxDataFormatGetBlockAlign(ctx->StreamFormat);

    // Recalculate the stream position that is stored in ctx->StreamPosition
    UpdateStreamPosition(Stream);
    qpc = KeQueryPerformanceCounter(NULL);

    *PositionInBlocks = ctx->StreamPosition / blockAlign;
    *QPCPosition = qpc;

    return STATUS_SUCCESS;
}

ACX requirements

Minimum ACX version: 1.0

For more information about ACX versions, see ACX version overview.

Requirements

Requirement	Value
Header	acxstreams.h
IRQL	PASSIVE_LEVEL

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