Confusion with recv() in C, is there an underlying input buffer? Where is data coming from?

Question

I'm just learning socket/network programming in C, and have run into a situation where the recv() function is resulting in some unexpected behaviour (unexpected to me at least).

I have a server program with a socket set up, and a call to recv() like so:

bytes_received = recv(connected, recv_data, 5, 0); 

recv_data is a 5 byte buffer.

My client program uses send() to send data, its sending buffer is 1024 bytes, though since it is reading from standard input, I've only been giving it between 1-10 bytes during this experiment (characters).

I'm seeing that even if my recv() call uses a 5 byte buffer and imposes a limit of 5 bytes, I can still send more than 5 bytes and it seems to just queue somewhere on the server side, such that the next time recv() is called (in a loop for instance), it receives the next 5 bytes of incoming data.

My understanding was that send() would fire off its data (lets say all 12 bytes), but since recv() is only "set up" to handle 5 bytes, the final 7 would be lost. Where is it that the server is getting these from? Is there an underlying input buffer at the socket level that recv() is poping data from?

Answer 1

The implementation of the socket (within the kernel) handles this automatically for you.

As long as it is a TCP socket you would not lose data. If it is a UDP socket you might lose data if your buffers overflow..