Set connect timout using setsockopt in Linux

Question

I am writing a linux Qt5/c++ app that tries to connect to a peer using a QTcpSocket. I call

tcpsocket->connectToHost(address,port,options)

When the peer is available it works great and connects immediately. However, when the peer is not available: The first time I call the above, the connect waits 1 minute before I receive a SocketTimeoutError (5). Then, every subsequent call to connect might wait a second before I receive a ConnectionRefusedError (0), or might wait a full minute (depending on the system tested).

Is there a setsockopt I can use to reduce the time waiting for initial connect?

I should point out that I already set some socket options in order to quickly notify me of a lost connection (see below). Hopefully these aren't causing the 1 minute initial connection error delay:

int enableKeepAlive = 1;
setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &enableKeepAlive, sizeof(enableKeepAlive));
int maxIdle = 5; /* seconds */
setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &maxIdle, sizeof(maxIdle));
int count = 3;  // send up to 3 keepalive packets out, then disconnect if no response
setsockopt(fd, SOL_TCP, TCP_KEEPCNT, &count, sizeof(count));
int interval = 2;   // send a keepalive packet out every 2 seconds (after the 5 second idle period)
setsockopt(fd, SOL_TCP, TCP_KEEPINTVL, &interval, sizeof(interval));

I found one reference on google to setsockopt(socket_handle, SOL_TCP, OPT_CONNECT_TIMEOUT, &option, sizeof(option)) but it won't compile as there is no OP_CONNECT_TIMEOUT (at least not in my headers) — TSG

Void Void · Accepted Answer · 2014-09-19T16:49:43

Rather than rely on setsockopt(), why don't you instead set your socket to non-blocking mode and perform an asynchronous connect(). You'd then block on select(), poll() or whatever event demultiplexing mechanism you are using, setting the timeout to whatever you desire. Once it becomes writable you know the connection is complete.

Set connect timout using setsockopt in Linux

1 Answers