Can a Non Blocking UDP write return with fewer bytes than requested?

Question

I have an application that sends data point to point from a sender to the receiver over an link that can operate in simplex (one way transmission) or duplex modes (two way). In simplex mode, the application sends data using UDP, and in duplex it uses TCP. Since a write on TCP socket may block, we are using Non Blocking IO (ioctl with FIONBIO - O_NONBLOCK and fcntl are not supported on this distribution) and the select() system call to determine when data can be written. NIO is used so that we can abort out of send early after a timeout if needed should network conditions deteriorate. I'd like to use the same basic code to do the sending but instead change between TCP/UDP at a higher abstraction. This works great for TCP.

However I am concerned about how Non Blocking IO works for a UDP socket. I may be reading the man pages incorrectly, but since write() may return indicating fewer bytes sent than requested, does that mean that a client will receive fewer bytes in its datagram? To send a given buffer of data, multiple writes may be needed, which may be the case since I am using non blocking IO. I am concerned that this will translate into multiple UDP datagrams received by the client.

I am fairly new to socket programming so please forgive me if have some misconceptions here. Thank you.

Answer 1

Assuming a correct (not broken) UDP implementation, then each send/sendmsg/sendto will correspond to exactly one whole datagram sent and each recv/recvmsg/recvfrom will correspond to exactly one whole datagram received.

If a UDP message cannot be transmitted in its entirety, you should receive an EMSGSIZE error. A sent message might still fail due to size at some point in the network, in which case it will simply not arrive. But it will not be delivered in pieces (unless the IP stack is severely buggy).

A good rule of thumb is to keep your UDP payload size to at most 1400 bytes. That is very approximate and leaves a lot of room for various forms of tunneling so as to avoid fragmentation.