C - How to add multiple flags

Question

Is this the right way to add multiple flags into a function?

recv(sfd, &buf, sizeof(buf), MSG_DONTWAIT | MSG_ERRQUEUE);

I don't get an error message in my buf.
recv() does not block.
I get errno: 11, which says "try again".

Does this answer your question? [Meaning of FLAG in socket send and recv](https://stackoverflow.com/questions/24430564/meaning-of-flag-in-socket-send-and-recv) — Tarek Dakhran, Feb 22 '20 at 18:50
What does `recv` *return*? Please also try to create a [mcve] to show us. — Some programmer dude, Feb 22 '20 at 18:50
"[C]onnect returned 0" is odd on just about any system. The only place that would happen is for Windows systems (unless you're on a POSIX system and close the standard input/output/error). — Some programmer dude, Feb 22 '20 at 22:46

jxh · Accepted Answer · 2020-02-22T20:55:53.750

The way you have added flags to the last parameter to recv() is fine. It seems you do not understand what the MSG_DONTWAIT will do.

The MSG_DONTWAIT flag will cause the recv() call to be performed as a non-blocking operation. That means it will return -1 with errno set to EAGAIN or EWOULDBLOCK if there is no data to be returned.

MSG_DONTWAIT (since Linux 2.2)

Enables nonblocking operation; if the operation would block, the call fails with the error EAGAIN or EWOULDBLOCK. This provides similar behavior to setting the O_NONBLOCK flag (via the fcntl(2) F_SETFL operation), but differs in that MSG_DONTWAIT is a per-call option ...

_{man 2 recv}

If you want recv() to block until there is data returned, remove the MSG_DONTWAIT flag, and make sure your socket is does not set O_NONBLOCK.

C - How to add multiple flags

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