gen_tcp：send / 2是否阻塞？

Question

Is gen_tcp:send() asynchronous? Assume I'll send some byte array using gen_tcp:send/2. Will process continue to work:

a) Immediately
b) At the time data will arrive in target's inner buffer
c) When the target gets the data from buffer

Thank You in advance.

Answer 1

gen_tcp:send/2 is synchronous. It means that the call returns only after the given packet is really sent. Usually it happens immediately, however if TCP window is full gen_tcp:send/2 blocks until the data is sent. So it means that the call can theoretically block infinitely (for example when receiver does not read data from socket on its side). Fortunately there are some options to avoid such situation. There are two options {send_timeout, Integer} and {send_timeout_close, Boolean} for sockets which can be specified by the call inet:setopts/2 . The first one allows to specify a longest time to wait for a send operation.

When the limit is exceeded, the send operation will return {error, timeout} . Default value of that option is infinity (and it is the reason of infinite block). Also unfortunately it is unknown how much of data was sent if {error, timeout} was returned. In that case it is better to close the socket. If the second option {send_timeout_close, Boolean} is set to true then the socket will be close automatically if {error, timeout} occurs.