reading partially from sockets

Question

I'm having a little test program that sends a lot of udp packets between client->server->client (ping/pong test). The packets are fixed size on each run(last run is max allowable size of udp packet) I'm filling the packets with random data except for the beginning of each packet that contains the packet number. So I'm only interested to see if I receive all the packets back at the client.

I'm using sendto() and recvfrom() and I only read the sizeof(packet_number) (which in this case is an int). What happens to the rest of the data? Does it end up in fairyland (gets discarded)? or does the new packet that arrives gets appended to this "old" data?

(using linux)

Answer 1

Each read from UDP socket de-queues one whole datagram off kernel socket receive buffer no matter what's your userland buffer size. That is:

• If your buffer is bigger then the next pending datagram, you'll read less then your buffer size.
• If your buffer is smaller, you'll read your buffer size worth and the rest of the data is discarded.
• You can set MSG_TRUNC option in the flags, so recv(2) will return the whole datagram length, not just the part you read into your userland buffer.

Hope this helps.