What socket error do I get when TCP keep-alive breaks the connection?

Question

I have a set of TCP sockets with keep-alive (interval 1 min), controlled by a select(2) loop (selecting for read).

• Will select(2) return an erro

Answer 1

select() sets a bit in the FDSET that indicates which socket has triggered. Use FD_ISSET macro to determine which socket asked for service.

Answer 2

• select() itself does not return an error if an error is signalled for one of the sockets it is selecting for. [Indeed, the API can't indicate per-socket errors this way, because two different sockets could each acquire a pending error during a single call of select(). Which one would select() return?]
• After each iteration of the select() loop, you instead use the FD_ISSET macro to attempt a read() on each socket marked readable.
• Any time a socket has a pending error set, its read event (and write event) are signalled, and select() returns, allowing you to pick up timed-out errors due to keep-alive immediately. Note that select marking a socket for read does not indicate that there is data to read, only that an attempt to read will not block. If the socket has a pending error to retrieve, reading will not block. Both read(2) and write(2) first retrieve any pending error on the socket before even attempting to handle any data.

  A descriptor shall be considered ready for reading when a call to an input function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully. (The function might return data, an end-of-file indication, or an error other than one indicating that it is blocked, and in each of these cases the descriptor shall be considered ready for reading.) [POSIX:select()]

• Finally, what error is returned? Crucially, it depends on how the keepalive failed. You'll get ETIMEDOUT if the other end vanishes totally. If a packet delivery error occurs, you'll get that through instead (so if the keep-alive packet gets an ICMP error reply, like "host unreachable", you'll have EHOSTUNREACH delivered). [For more details on these cases, see Stevens, "Unix Network Programming, vol 1".]