c# socket receive byte array length

Question

i'm trying to learn to use sockets in c# and i've a doubt, i'm using a code something like this:

byte[] data = new byte[64]; 
int length = 0;
length = sock.Receive(data);
//more code...

So, the byte[] data is filled with the recived data and the left space in the array is filled with 0s, Is the byte[] allocated into memory completely (all 64bytes)? If it's so, is there a way to make the byte[] the same size as the actual sent data?

Answer 1

You can check sock.Available to see what has already come in. (so far)

byte[] data = new byte[sock.Available]; 
int length = sock.Receive(data);
//more code...

Note: Since you may or may not know what is coming in next on the network it usually makes more sense to read only a header (with size info) first or to allocate more space than necessary, and call .Recieve() multiple times until the end of a record is reached.

Note: This code assumes you already know there is some data to receive and you've waited long enough for some useful amount of data to be ready.

If you do choose to use length headers, .Available can help you avoid reading a partial header and having to re-assemble it which is nice. (Only large messages may need manual reassembly in that case)

Answer 2

As noted normally read may return fewer bytes then it was told. See a workaround function below which ensures it reads as many bytes as it was told - basically size of the passed buffer. Function is from here .

/// Reads data into a complete array, throwing an EndOfStreamException
/// if the stream runs out of data first, or if an IOException
/// naturally occurs.
/// </summary>
/// <param name="stream">The stream to read data from</param>
/// <param name="data">The array to read bytes into. The array
/// will be completely filled from the stream, so an appropriate
/// size must be given.</param>
public static void ReadWholeArray (Stream stream, byte[] data)
{
    int offset=0;
    int remaining = data.Length;
    while (remaining > 0)
    {
        int read = stream.Read(data, offset, remaining);
        if (read <= 0)
            throw new EndOfStreamException 
                (String.Format("End of stream reached with {0} bytes left to read", remaining));
        remaining -= read;
        offset += read;
    }
}

You can use this method first to read say a 2 byte integer which should represent the number of bytes that will follow. Then you read once again however now read as many bytes as specified in that two byte integer.

But for this to work, clearly the sender first has to send a two byte integer which represents length of data that will follow - and then the data itself.

So basically you call above function on a byte array of size two first (to get data length), and then on a byte array with size as indicated in that 2 byte integer (to get data).

You can use this to read from NetworkStream . Some more reading on this topic.

Answer 3

You simply need to use the return value from Receive to understand how much data has arrived. You can shorten the buffer using Array.Resize if you want but that normally would be a sign that something is wrong.

Also note, that TCP is a stream of bytes and does not preserve message boundaries.