KsPinGetLeadingEdgeStreamPointer

对于pin-center的ks架构，KsPinGetLeadingEdgeStreamPointer用于获取pin关联队列的前导流指针。
对于pin的数组结构组成由KSPPROCESSPIPESECTION组成。

KSDDKAPI
PKSSTREAM_POINTER
NTAPI
KsPinGetLeadingEdgeStreamPointer(
    IN PKSPIN Pin,
    IN KSSTREAM_POINTER_STATE State
    )
{
    PKSPPROCESSPIPESECTION pipeSection = CONTAINING_RECORD(Pin,KSPIN_EXT,Public)->ProcessPin->PipeSection;

    PKSPSTREAM_POINTER result = NULL;
    if (pipeSection && pipeSection->Queue) 
    {
        result = pipeSection->Queue->GetLeadingStreamPointer(State);
    }

    return result ? &result->Public : NULL;
}

GetLeadingStreamPointer的实现如下：

CKsQueue::
GetLeadingStreamPointer(
    IN KSSTREAM_POINTER_STATE State
    )
{
    if ((State == KSSTREAM_POINTER_STATE_LOCKED) && !LockStreamPointer(m_Leading))
    {
        return NULL;
    }

    return m_Leading;
}

在CKsQueue队列中，头指针由m_Leading成员保留。

class CKsQueue:
    public IKsQueue,
    public CBaseUnknown
{
    PKSPSTREAM_POINTER m_Leading;
    PKSPSTREAM_POINTER m_Trailing;
}

关于m_Leading
在CKsQueue::Init时，会创建

status = CreateStreamPointer(&m_Leading);
if (NT_SUCCESS(status) && (Flags & KSPIN_FLAG_DISTINCT_TRAILING_EDGE)) {
    status = CreateStreamPointer(&m_Trailing);
}

CKsQueue::CreateStreamPointer的原理就比较简单，就是申请内存空间并初始化。

NTSTATUS
CKsQueue::
CreateStreamPointer(
    OUT PKSPSTREAM_POINTER* StreamPointer
    )
{
    NTSTATUS status;
    PKSPSTREAM_POINTER streamPointer = (PKSPSTREAM_POINTER) ExAllocatePoolWithTag(NonPagedPool,sizeof(KSPSTREAM_POINTER),POOLTAG_STREAMPOINTER);
    if (streamPointer) 
    {
        //InterlockedIncrement(&m_StreamPointersPlusOne);
        RtlZeroMemory(streamPointer,sizeof(KSPSTREAM_POINTER));
        streamPointer->State = KSPSTREAM_POINTER_STATE_UNLOCKED;//默认未锁定
        streamPointer->Type = KSPSTREAM_POINTER_TYPE_NORMAL; //非内部
        streamPointer->Stride = m_MappingTableStride;
        streamPointer->Queue = this;
        streamPointer->Public.Pin = m_MasterPin;
        if (m_InputData) {
            streamPointer->Public.Offset = &streamPointer->Public.OffsetIn;
        } else {
            streamPointer->Public.Offset = &streamPointer->Public.OffsetOut;
        }

        *StreamPointer = streamPointer;
        status = STATUS_SUCCESS;
    } else {
        status = STATUS_INSUFFICIENT_RESOURCES;
    }

    return status;
}