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Old 10-21-2006, 03:19 PM   #1
Registered: Oct 2003
Location: Ireland
Distribution: Slackware 9.1, Ubuntu
Posts: 192

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connect() with timeout

Im trying to put a timeout on a connect call, and it works fine if I try to contact on say port 81, but for some reason if I try to connect to an unopen port on or my router's IP, it succesds all the time.

The method I used is : first I set the socket to non-blocking, tried connecting with connect(), not bothering to check the return value because its always -1, then using select() to check if the socket could be written to.

heres my code:
bool TcpClient::Connect(	const char* host,
				const unsigned short port,
				const unsigned short timeOut)
	// get the IP address
	struct hostent* h=gethostbyname(host);
	if (!h) return false;

	// create socket and set it to non-blocking

	// set up address structure

	// attempt to connect
	connect(sockfd,(struct sockaddr*)&remoteAddr,sizeof(struct sockaddr));

	// set up file descriptor set

	// set up timeval struct

	// monitor the socket
	if (select(sockfd+1,NULL,&fds,NULL,&tv)<=0) 
		return false;

	return true;
Thanks for any help.

Last edited by nodger; 10-21-2006 at 03:23 PM.
Old 10-21-2006, 03:34 PM   #2
Registered: Oct 2003
Location: Ireland
Distribution: Slackware 9.1, Ubuntu
Posts: 192

Original Poster
Rep: Reputation: 30
hmm.. i think Ive found a solution. if I just call send(sockfd,0,0,0) and I get a return value of 0, then its connected.
Old 10-21-2006, 08:58 PM   #3
Registered: Aug 2003
Location: Austin, TX
Distribution: Linux from Scratch
Posts: 52

Rep: Reputation: 15
A method I have used before is to create a signal handler function for SIGALRM, and before calling connect call alarm(timeout); and after the connect call alarm(0);

When the alarm goes off and your signal handler returns, connect will exit, and you just turn off the alarm with the alarm(0).
Old 10-21-2006, 11:14 PM   #4
LQ Guru
Registered: Mar 2004
Distribution: SusE 8.2
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Hi -

There are several ways for a sockets client to "connect() with timeout":

1. Timeout with "select()"
I really wouldn't recommend this one for your situation. At best, if I was using UDP "sendto()" and "recvfrom ()", I might use "select()" to verify that the socket became writeable or readable within a specific time before I used it.
2. Sigalarm
_john_i_'s suggestion is a good one. For example:
  /* Adapted from W. Richard Stevens, "Unix Network Programming" */
  oldsigfunc = signam (SIGALRM, mynewfunc);
  if (alarm (nsec) != 0) {
    printf ("WARNING: Alarm already set!\n");

  /* Try to connect */
  iretval = connect (sockfd, &sa, slen);
  if (iretval  < 0) {
    close (sockfd);
    if (errno == EINTR)
      printf ("WARNING: timeout occurred...\n");

  /* Clear Alarm/reset original SIGALRM handler */
  alarm (0);
  signal (SIGALRM, oldsigfunc);

  /* ... continue or not, based on "iretval" ... */
3. "setsockopt (SO_RCVTIMEO)"
This is the best option of all ... if your stack supports it.

My version of Stevens (the 2nd Edition, 1998) states that you CANNOT use "setsockopt(SO_RCVTIMEO)" to set a connect timeout. This is no longer true on most OS's, and it has been available on Linux since kernel 2.3.41.
<= If at all possible, I'd encourage you to consider SO_RECVTIMEO
'Hope that helps .. PSM

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