Thread: Winsock, weird sideeffect

I got this odd error where if I send some data from a client to a server the message is received but I also get a number of empty messages, the same amount as the nr of letters in the message (in the example below, the string "Hello" gets 5 empty messages.
It's a non-blocking system, the client sends simply using send() and the server's recieve-part looks like this:

Code:

case FD_READ:
{
	CHAR Char;
	INT Result;
	std::string String;
	std::vector<CHAR> Buffer;

	while(TRUE)
	{
		Result = recv(W, &Char, 1, 0);
				
		if(Result == SOCKET_ERROR)
		{
			if(WSAGetLastError() == WSAEWOULDBLOCK) break;
			ErrorMessage = "recv() failed!";
			return FALSE;
		}

		if(Result < 1) break;

		Buffer.push_back(Char);
	}

	Buffer.push_back('\0');
	String = "MSG: ";
	String += &Buffer[0];
	AddMessage(String);

	return TRUE;
}

As an example, if the client sends the string "Hello" to the server it would receive these messages:

Code:

MSG: Hello
MSG:
MSG:
MSG:
MSG:
MSG:

Prolly some stupid error, but I just can't find it...

It seems to me you add a string even if your while loop exits on the first pass.

I could add:

Code:

if(String != "") AddMessage(String);

or some other test to prevent this, but that doesn't remove the fact that the FD_READ message is sent 1000 times for a 1000-byte message, which is insane...

Are you saying that you read a char, then the loop breaks out because of this
if(WSAGetLastError() == WSAEWOULDBLOCK) break;

That's because you read it char at a time and FD_READ is sent to indicate that data is still available.

That seems like a logical reason for my "error". Would it be better to read it it chunks (say 1024 bytes) and then check by the return value how much of the buffer is valid and use that?

Is there some specific API you can use to find the amount of data available at the moment?

At the moment, you can only guarantee with 'data available' that there is at least one byte.

If you try and read 10 say, and there is only 9 available, then I think you're going to get "WOULDBLOCK" back straight away.