Is there a way to flush a POSIX socket?

Question

Is there a standard call for flushing the transmit side of a POSIX socket all the way through to the remote end or does this need to be implemented as part of the user level protocol? I looked around the usual headers but couldn't find anything.

Answer 1

What about setting TCP_NODELAY and than reseting it back? Probably it could be done just before sending important data, or when we are done with sending a message.

send(sock, "notimportant", ...); send(sock, "notimportant", ...); send(sock, "notimportant", ...); int flag = 1;  setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, (char *) &flag, sizeof(int)); send(sock, "important data or end of the current message", ...); flag = 0;  setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, (char *) &flag, sizeof(int)); 

As linux man pages says

TCP_NODELAY ... setting this option forces an explicit flush of pending output ...

So probably it would be better to set it after the message, but am not sure how it works on other systems

Answer 2

For Unix-domain sockets, you can use fflush(), but I'm thinking you probably mean network sockets. There isn't really a concept of flushing those. The closest things are:

1. At the end of your session, calling shutdown(sock, SHUT_WR) to close out writes on the socket.

2. On TCP sockets, disabling the Nagle algorithm with sockopt TCP_NODELAY, which is generally a terrible idea that will not reliably do what you want, even if it seems to take care of it on initial investigation.

It's very likely that handling whatever issue is calling for a 'flush' at the user protocol level is going to be the right thing.