Re: recvmmsg() behaviour with MSG_PEEK flag

On Fri, Jul 15, 2022 at 1:03 PM Alexander Babayants
[off-list ref] wrote:

The behaviour of recvmmsg() with MSG_PEEK flag confuses me. I'd expect
it to peek multiple messages at once, but it seems to peek only the
first one, filling each of the provided struct msghdr with a copy of
the first message. I do not see if it is documented anywhere, is it a
bug or intended design?

What I want to achieve is to first peek into the socket, get the size
of each message, then allocate appropriate buffers and read the
messages with the second recvmmsg() call. This seems to be a
relatively common pattern for reading single messages via recvmsg(),
and I naively expected it to work with recvmmsg() too.

You might want to look at SO_PEEK_OFF in the socket.7 manpage. (Or
even better, as suggested before in the thread, rework your code to
not peek at all.)