In my scenario that buffer overrun never happens, maybe because I use messages that are not big enough to overflow the default recv() buffer size.

But I think I can confirm that multiple messages are never received in an atomic read, even if they are being issued intensively in short millisecond intervals. Even more, I think there is a recvmmsg() call specific for that purpose if you want to receive multiple reads at once.

As I said I do not have the answers, I rely on the high-level definitions and have little knowledge about how it works at low level.

But I think your question may be extended also to recvmsg(). What is its behaviour if it fills all the passed iovec structs?

Probably an answer can be found where you found the recommendation of using recvmsg() over recv(). There should be a reason for that recommendation.