How to read UDP packet with variable length in C

Question

I'm sending a C struct over UDP

struct packet{
    int numInt;
    int* intList; //malloc'ed as (sizeof(int)*numInt)
}

It will be serialized as [numInt][intList[0]]...[intList[numInt-1]].

My understanding is that calling recvfrom on UDP will read the entire packet, even if the buffer doesn't hold that many bytes. Is using a really large buffer the only option I have?

Answer 1

You could pass MSG_PEEK to recvfrom to find out exactly how big the buffer needs to be. So just recvfrom a few bytes with MSG_PEEK to find numInt and then recvfrom the real thing (this time without MSG_PEEK).

The standard says something about MSG_PEEK, but kernel.org spells it better:

MSG_PEEK

This flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data.

Obviously at some point you will start wondering if doubling the number of system calls to save memory is worth it. I think it isn't.

Answer 2

UDP packets are sent and received as a whole. if you receive it, the size is right. The only thing you have to do is to supply a big enough buffer on read() or recv() or recfrom(). The length field inside the payload is redundant, since the read() will tell you the correct size. It is also dangerous, since it relies on the sender and reciever having the same byte order.

Answer 3

You could try using a small buffer, just large enough to get numInt, with the MSG_PEEK flag set. Then you can find out the size you actually need, and receive again without MSG_PEEK to get the whole thing.

Answer 4

I'm pretty sure recvfrom will read up to as many bytes as is told to it by its 3rd argument, len. If there are fewer bytes available, it will return what is there. If there are more, it will return up to len bytes. You may have to make additional calls to obtain all the data your are expecting.