C# Sockets Buffer Overflow No Error

Posted by Michael Covelli on Stack Overflow See other posts from Stack Overflow or by Michael Covelli
Published on 2010-04-04T22:55:00Z Indexed on 2010/04/04 23:03 UTC
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I have one thread that is receiving data over a socket like this:

while (sock.Connected)
{
    // Receive Data (Block if no data)
    recvn = sock.Receive(recvb, 0, rlen, SocketFlags.None, out serr);

    if (recvn <= 0 || sock == null || !sock.Connected)
    {
        OnError("Error In Receive, recvn <= 0 || sock == null || !sock.Connected");
        return;
    }
    else if (serr != SocketError.Success)
    {
         OnError("Error In Receive, serr = " + serr);
         return;
    }

    // Copy Data Into Tokenizer
    tknz.Read(recvb, recvn);

    // Parse Data
    while (tknz.MoveToNext())
    {
        try
        {
            ParseMessageAndRaiseEvents(tknz.Buffer(), tknz.Length);
        }
        catch (System.Exception ex)
        {
            string BadMessage = ByteArrayToStringClean(tknz.Buffer(), tknz.Length);
            string msg = string.Format("Exception in MDWrapper Parsing Message, 
                           Ex = {0}, Msg = {1}", ex.Message, BadMessage);
            OnError(msg);
        }
    }
}

And I kept seeing occasional errors in my parsing function indicating that the message wasn't valid. At first, I thought that my tokenizer class was broken. But after logging all the incoming bytes to the tokenizer, it turns out that the raw bytes in recvb weren't a valid message. I didn't think that corrupted data like this was possible with a tcp data stream.

I figured it had to be some type of buffer overflow so I set

sock.ReceiveBufferSize = 1024 * 1024 * 8;

and the parsing error never, ever occurs in testing (it happens often enough to replicate if I don't change the ReceiveBufferSize).

But my question is: why wasn't I seeing an exception or an error state or something if the socket's internal buffer was overflowing before I changed this buffer size?

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