What does FIONREAD of UDP (datagram) sockets return?
datagram, ioctl, networking, udp
Solution
man udp (7) states:
FIONREAD (SIOCINQ)
Gets a pointer to an integer as argument. Returns the size of
the next pending datagram in the integer in bytes, or 0 when no
datagram is pending. Warning: Using FIONREAD, it is impossible
to distinguish the case where no datagram is pending from the
case where the next pending datagram contains zero bytes of
data. It is safer to use select(2), poll(2), or epoll(7) to
distinguish these cases.
So, the answer to your question is: FIONREAD returns the size of the next (first) pending datagram.
Problem
Which one does `ioctl` of `FIONREAD` return, the next packet's length, or the length of all data in the buffer? Suppose there is a `UDP` server that receives 2 packets from a client 1 and another 2 packets from client 2 after client 1's packet. Then, what is the value of `ioctl` of `FIONREAD`, and what does `readfrom` return in that case? Client 1 : v two packets ++UDP Server got 4 packets <- FIONREAD? Client 2 : ^ two packets FIONREAD? (server) - length of client 1's first packet - length of client 1's two packets - length of client 1's two packets + client 2's two packets - length of client 1's first packet + client 2's first packet - other