Receiving a part of packet via recvfrom (UDP)

Question

I'm trying to receive a part of a packet via recvfrom. It actually works like this:

recvfrom(sockfd, serialised_meta, 12, flags, src_addr, addrlen);
recvfrom(sockfd, serialised_buf, BUFLEN, flags, src_addr, addrlen);

The data is sent like this:

 bufd->Serialise(serialised_buf, BUFLEN+12);
 sendto(sockfd, serialised_buf, BUFLEN+12, flags, dest_addr, addrlen);

So the idea is to read some meta data first and then decide whether to receive something else. The problem is that I receive 4 '/0' bytes in the beginning if second buffer (serialised_buf). It doesn't seem to be serialisation issue, I used my serialisation before, and everything was cool, while I was receiving the whole packet (meta and data) at once. Any ideas on how it could be fixed?

PS. I understand I can just skip unnecessary bytes) But anyway, why it might be happening?

Answer 1

UDP isn't a "stream" protocol... once you do the initial recvfrom, the remainder of the packet is discarded. The second recvfrom is awaiting the next packet...

Answer 2

UDP operates on messages, not streams like TCP does. There is a 1-to-1 relationship between sendto() and recvfrom() when using UDP. There is no option to receive partial data in UDP, it is an all-or-nothing type of transport. You have to recvfrom() the entire BUFLEN+12 message in one go, then decide whether you are going to actually use it or not. That is just the way UDP works.