Reading the ASF Header Object of an Existing File

The ASF ContentInfo object stores information that represents the ASF Header Objects of a media file. A populated ContentInfo object is required in order to read and parse an existing ASF file.

After creating the ContentInfo object by calling the MFCreateASFContentInfo function, the application must initialize it with header information from the ASF file that is to be read. To populate the object, call IMFASFContentInfo::ParseHeader.

ParseHeader requires a media buffer that contains the Header Object of the ASF file. One option is to fill a media buffer with the Header Object to create a byte stream for the file and then read the first 30 bytes of data from the byte stream into a media buffer. You can then get the size by passing the first 24 bytes of the Header Object to the IMFASFContentInfo::GetHeaderSize method. After getting the size, you can read the entire Header Object in a media buffer and pass it to ParseHeader. The method starts parsing at the offset from the start of the media buffer specified in the cbOffsetWithinHeader parameter.

The following example code creates and initializes a ContentInfo object for reading an existing ASF file contained in a byte stream. First, we define a helper function that reads data from a byte stream and allocates a media buffer to hold the data:

// Read the ASF Header Object from a byte stream and return a pointer to the 
// populated ASF ContentInfo object.
//
// The current read position of the byte stream must be at the start of the
// ASF Header Object.

HRESULT CreateContentInfo(IMFByteStream *pStream, 
    IMFASFContentInfo **ppContentInfo)
{
    const DWORD MIN_ASF_HEADER_SIZE = 30;
    
    QWORD cbHeader = 0;
    DWORD cbBuffer = 0;

    IMFASFContentInfo *pContentInfo = NULL;
    IMFMediaBuffer *pBuffer = NULL;

    // Create the ASF ContentInfo object.
    HRESULT hr = MFCreateASFContentInfo(&pContentInfo);
    
    // Read the first 30 bytes to find the total header size.

    if (SUCCEEDED(hr))
    {
        hr = MFCreateMemoryBuffer(MIN_ASF_HEADER_SIZE, &pBuffer);
    }
    if (SUCCEEDED(hr))
    {
        hr = ReadFromByteStream(pStream, pBuffer,MIN_ASF_HEADER_SIZE);
    }
    if (SUCCEEDED(hr))
    {
        hr = pContentInfo->GetHeaderSize(pBuffer, &cbHeader);
    }

    // Pass the first 30 bytes to the ContentInfo object.
    if (SUCCEEDED(hr))
    {
        hr = pContentInfo->ParseHeader(pBuffer, 0);
    }

    SafeRelease(&pBuffer);

    if (SUCCEEDED(hr))
    {
        cbBuffer = (DWORD)(cbHeader - MIN_ASF_HEADER_SIZE);

        hr = MFCreateMemoryBuffer(cbBuffer, &pBuffer);
    }

    // Read the rest of the header and finish parsing the header.
    if (SUCCEEDED(hr))
    {
        hr = ReadFromByteStream(pStream, pBuffer, cbBuffer);
    }
    if (SUCCEEDED(hr))
    {
        hr = pContentInfo->ParseHeader(pBuffer, MIN_ASF_HEADER_SIZE);
    }
    if (SUCCEEDED(hr))
    {
        // Return the pointer to the caller.
        *ppContentInfo = pContentInfo;
        (*ppContentInfo)->AddRef();
    }
    SafeRelease(&pBuffer);
    SafeRelease(&pContentInfo);
    return hr;
} 

The next example reads the ASF Header Object from a byte stream and populates an ASF ContentInfo object.

// Read the ASF Header Object from a byte stream and return a pointer to the 
// populated ASF ContentInfo object.
//
// The current read position of the byte stream must be at the start of the
// ASF Header Object.

HRESULT CreateContentInfo(IMFByteStream *pStream, 
    IMFASFContentInfo **ppContentInfo)
{
    const DWORD MIN_ASF_HEADER_SIZE = 30;
    
    QWORD cbHeader = 0;
    DWORD cbBuffer = 0;

    IMFASFContentInfo *pContentInfo = NULL;
    IMFMediaBuffer *pBuffer = NULL;

    // Create the ASF ContentInfo object.
    HRESULT hr = MFCreateASFContentInfo(&pContentInfo);
    
    // Read the first 30 bytes to find the total header size.

    if (SUCCEEDED(hr))
    {
        hr = MFCreateMemoryBuffer(MIN_ASF_HEADER_SIZE, &pBuffer);
    }
    if (SUCCEEDED(hr))
    {
        hr = ReadFromByteStream(pStream, pBuffer,MIN_ASF_HEADER_SIZE);
    }
    if (SUCCEEDED(hr))
    {
        hr = pContentInfo->GetHeaderSize(pBuffer, &cbHeader);
    }

    // Pass the first 30 bytes to the ContentInfo object.
    if (SUCCEEDED(hr))
    {
        hr = pContentInfo->ParseHeader(pBuffer, 0);
    }

    SafeRelease(&pBuffer);

    if (SUCCEEDED(hr))
    {
        cbBuffer = (DWORD)(cbHeader - MIN_ASF_HEADER_SIZE);

        hr = MFCreateMemoryBuffer(cbBuffer, &pBuffer);
    }

    // Read the rest of the header and finish parsing the header.
    if (SUCCEEDED(hr))
    {
        hr = ReadFromByteStream(pStream, pBuffer, cbBuffer);
    }
    if (SUCCEEDED(hr))
    {
        hr = pContentInfo->ParseHeader(pBuffer, MIN_ASF_HEADER_SIZE);
    }
    if (SUCCEEDED(hr))
    {
        // Return the pointer to the caller.
        *ppContentInfo = pContentInfo;
        (*ppContentInfo)->AddRef();
    }
    SafeRelease(&pBuffer);
    SafeRelease(&pContentInfo);
    return hr;
} 

Note

These examples use the SafeRelease function to release interface pointers.

 

Related topics

ASF ContentInfo Object

ASF Header Object

ASF Support in Media Foundation

Getting Information from ASF Header Objects