Question

I’m trying to copy some binary data from my IStream instance (since Gdiplus::Image only saves to IStream-deriving objects, or a file path) to a char pointer from which I can read simply by knowing the allocated binary size and have access to the pointer.

My class is as follows:

Upload::Upload(Gdiplus::Bitmap* bitmap, CLSID clsEncoderId)
{
    int result;
    STATSTG statResult;

    result = CreateStreamOnHGlobal(0, TRUE, &m_hBufferStream);

    if (result != S_OK)
        MessageBoxW(NULL, _T("Upload::Upload(): Could not create stream"), _T("Point"), MB_OK | MB_ICONERROR);
    else
    {
        if (bitmap->Save(m_hBufferStream, &clsEncoderId, NULL) != Gdiplus::Ok)
            MessageBoxW(NULL, _T("Upload::Upload(): Could not save() image"), _T("Point"), MB_OK | MB_ICONERROR);
    }

    if (m_hBufferStream->Stat(&statResult, STATFLAG_NONAME) != S_OK)
        return;

    Gdiplus::Image test(m_hBufferStream, TRUE);
    test.Save(_T("hejs.png"), &clsEncoderId, NULL);

    m_iSize = statResult.cbSize.LowPart;
}

char* Upload::GetBinaryData()
{
    char* buffer = (char*)malloc(m_iSize);
    ULONG size = 0;

    m_hBufferStream->Read(buffer, m_iSize, &size);

    return buffer;
}

In my function that processes the Upload instance I do this:

char* pBuffer = upload->GetBinaryData();
buffer.write(pBuffer, upload->GetSize());

But the memory stored is wrong (oddly it seems like a pattern though).

What am I doing wrong?

Thanks in advance.

P.S.:
The test Image-instance successfully saves to the file after reading from m_hBufferStream.

Answer 1

First of all, IStream::Read() is not required to read exactly the specified number of bytes – it is required to read no more than that number. Actual number is stored inside the veriable pointed to by the third parameter.

Second, you don’t check the HRESULT returned by Read().

A much better strategy would be to call Read() in a loop, check its return value and adjust the pointer to the buffer according to how many bytes have been actually read.