Can send() become non-blocking automatically?

Question

Do I have to wait for signals with select() in order to send something in non-blocking sockets ? What if I have always have something to send and then I call send() function ? I mean, everytime i call send(), there is definitely some data to send with fixed length. Does it mean that send will not block ?

Answer 1

A call to send will never block on a non-blocking socket. If the data could not be sent (for example if the send buffer is full), then send will return immediately with a value of SOCKET_ERROR .

Any call that would block is signaled by an error code of WSAEWOULDBLOCK (by calling WSAGetLastError ).

The same is true for a call to receive , if there is no data in the receive buffer, the call still returns immediately, with an error.

Answer 2

That depends on the protocol. With regular blocking TCP socket send() might block since in-kernel socket buffer might be full. UDP, on the other hand, will either push the packet down the stack, or discard it if it's too big, without blocking.

Edit 0:

Ok, thanks to @Cicada, I see I missed that we are talking about non-blocking sockets, so ...

It still depends on the protocol - with UDP your packet is either sent or discarded; with TCP you try to send at the time you have data, and if you get EWOULDBLOCK you register your socket with select() / poll() , and re-try sending when kernel tells you the socket is writable again.