IMFByteStream::Read method (mfobjects.h)

Reads data from the stream.

Syntax

HRESULT Read(
  [in]  BYTE  *pb,
  [in]  ULONG cb,
  [out] ULONG *pcbRead
);

Parameters

[in] pb

Pointer to a buffer that receives the data. The caller must allocate the buffer.

[in] cb

Size of the buffer in bytes.

[out] pcbRead

Receives the number of bytes that are copied into the buffer. This parameter cannot be NULL.

Return value

If this method succeeds, it returns S_OK. Otherwise, it returns an HRESULT error code.

Remarks

This method reads at most cb bytes from the current position in the stream and copies them into the buffer provided by the caller. The number of bytes that were read is returned in the pcbRead parameter. The method does not return an error code on reaching the end of the file, so the application should check the value in pcbRead after the method returns.

This method is synchronous. It blocks until the read operation completes.

Implementation notes:This method should update the current position in the stream by adding the number of bytes that were read, which is specified by the value returned in the pcbRead parameter, to the current position. Other methods that can update the current position are Read, Write, BeginWrite, Seek, and SetCurrentPosition.

This interface is available on the following platforms if the Windows Media Format 11 SDK redistributable components are installed:

• Windows XP with Service Pack 2 (SP2) and later.
• Windows XP Media Center Edition 2005 with KB900325 (Windows XP Media Center Edition 2005) and KB925766 (October 2006 Update Rollup for Windows XP Media Center Edition) installed.

Examples

The following example reads data from a byte stream into a caller-allocated media buffer. For more information about media buffers, see Media Buffers.

// Read data from a byte stream into a media buffer.
//
// This function reads a maximum of cbMax bytes, or up to the size of the 
// buffer, whichever is smaller. If the end of the byte stream is reached, the 
// actual amount of data read might be less than either of these values.
//
// To find out how much data was read, call IMFMediaBuffer::GetCurrentLength.

HRESULT ReadFromByteStream(
    IMFByteStream *pStream,     // Pointer to the byte stream.
    IMFMediaBuffer *pBuffer,    // Pointer to the media buffer.
    DWORD cbMax                 // Maximum amount to read.
    )
{
    DWORD cbBufferMax = 0;
    DWORD cbRead = 0;
    BYTE *pData= NULL;

    HRESULT hr = pBuffer->Lock(&pData, &cbBufferMax, NULL);

    // Do not exceed the maximum size of the buffer.
    if (SUCCEEDED(hr))
    {
        if (cbMax > cbBufferMax)
        {
            cbMax = cbBufferMax;
        }

        // Read up to cbMax bytes.
        hr = pStream->Read(pData, cbMax, &cbRead);
    }

    // Update the size of the valid data in the buffer.
    if (SUCCEEDED(hr))
    {
        hr = pBuffer->SetCurrentLength(cbRead);
    }
    if (pData)
    {
        pBuffer->Unlock();
    }
    return hr;
}

The next example is similar, but allocates a new media buffer to hold the data.

//-------------------------------------------------------------------
// AllocReadFromByteStream
//
// Reads data from a byte stream and returns a media buffer that
// contains the data.
//-------------------------------------------------------------------

HRESULT AllocReadFromByteStream(
    IMFByteStream *pStream,         // Pointer to the byte stream.
    DWORD cbToRead,                 // Number of bytes to read.
    IMFMediaBuffer **ppBuffer       // Receives a pointer to the media buffer. 
    )
{
    HRESULT hr = S_OK;
    BYTE *pData = NULL;
    DWORD cbRead = 0;   // Actual amount of data read.

    IMFMediaBuffer *pBuffer = NULL;

    // Create the media buffer. 
    // This function allocates the memory for the buffer.
    hr = MFCreateMemoryBuffer(cbToRead, &pBuffer);

    // Get a pointer to the memory buffer.
    if (SUCCEEDED(hr))
    {
        hr = pBuffer->Lock(&pData, NULL, NULL);
    }

    // Read the data from the byte stream.
    if (SUCCEEDED(hr))
    {
        hr = pStream->Read(pData, cbToRead, &cbRead);
    }

    // Update the size of the valid data in the buffer.
    if (SUCCEEDED(hr))
    {
        hr = pBuffer->SetCurrentLength(cbRead);
    }

    // Return the pointer to the caller.
    if (SUCCEEDED(hr))
    {
        *ppBuffer = pBuffer;
        (*ppBuffer)->AddRef();
    }

    if (pData)
    {
        pBuffer->Unlock();
    }
    SafeRelease(&pBuffer);
    return hr;
}

Requirements

See also

IMFByteStream