set connect() timeout on unix domain socket - c

Question

Is using alarm() is the only to set connect() timeout on unix domain socket? I've tried select() which is described here but seems like select() returns ok immediately on unix domain socket every time and
no error occurred by calling getsockopt(SO_ERROR), but a send() on the fd returns an error says Transport endpoint is not connected. I paste the select() code below.
I think using alarm would meet the case, but seems it's considered as an old-fashion way. So I'm here to see if there's any other solutions for this. Thanks in advance.
if ((flags = fcntl(fd, F_GETFL, 0)) == -1) {
syslog(LOG_USER|LOG_ERR, "fcntl get failed: %s", strerror(errno));
close(fd);
return -1;
}
if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1) {
syslog(LOG_USER|LOG_ERR, "set fd nonblocking failed: %s", strerror(errno));
close(fd);
return -1;
}
if(connect(fd, (struct sockaddr *) &address, sizeof(struct sockaddr_un)) != 0) {
if (errno != EAGAIN && errno != EWOULDBLOCK && errno != EINPROGRESS) {
close(fd);
return -1;
}
FD_ZERO(&set);
FD_SET(fd, &set);
if(select(fd + 1, NULL, &set, NULL, &timeout) <= 0) {
close(fd);
return -1;
}
/*
if(connect(fd, (struct sockaddr *) &address, sizeof(struct sockaddr_un)) != 0) {
close(fd);
return -1;
}
*/
if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &error, (socklen_t *)&len) < 0) {
syslog(LOG_USER|LOG_ERR, "getsockopt failed: %s", strerror(errno));
close(fd);
return -1;
}
if(error != 0) {
syslog(LOG_USER|LOG_ERR, "getsockopt return error: %d", error);
close(fd);
return -1;
}
}
if (fcntl(fd, F_SETFL, flags & ~O_NONBLOCK) == -1) {
syslog(LOG_USER|LOG_ERR, "set fd blocking failed: %s", strerror(errno));
close(fd);
return -1;
}

Answer 1

Somewhere (I did not bookmark that page) in another post I found that the connect() only establishes a TCP connection. It only means that on the other end, there is a working TCP stack, but it does not mean, the server has actually accept()-ed!
The example there was connect() is like calling a support center, and the automatic voice tells you, you are in a queue, but you still cannot communicate. accept() is the actual operator taking your call.
My solution for the same problem will be to have the client wait for the server to actually send something, before moving on with other client-stuff. I can put this in a select-timeout loop.
listen() has a parameter, how many connections can be put in a backlog before starting to drop client connection attempts.

Answer 2

You can use select() or poll() after EINPROGRESS, as described in the connect man page. If you get EAGAIN or EWOULDBLOCK, the Unix domain socket has run out of backlog entries, the queue length specified by the server with the listen() call. The connect() failed.
Note that a connecting client can be able to write to Unix domain sockets until the system buffer is full, before the server even accepted the call. That works for each backlog buffer. Failures occur afterwards.
A failed connect() might need a new socket before retrying. select() might return 0 also if the connection was refused, such as if the server didn't listen(). That depends on system and libray. At any rate, after an error of EAGAIN, it is necessary to retry. For example:
int rtc, so_error, max_retry = 5;
socklen_t len = sizeof so_error;
while ((rtc = connect(fd, (struct sockaddr *)&address, sizeof address)) != 0
&& errno == EAGAIN && --max_retry >= 0) {
sleep(1);
// new socket?
}
if (rtc < 0 && errno != EINPROGRESS) {
syslog(LOG_USER|LOG_ERR, "connect returned %d: %s", rtc, strerror(errno));
close(fd);
return -1;
}
if (rtc < 0)
{
fd_set set, wset, eset;
struct timeval timeout;
timeout.tv_sec = 10;
timeout.tv_usec = 0;
FD_ZERO(&set);
FD_SET(fd, &set);
wset = set;
eset = set;
if(select(fd + 1, &set, &wset, &eset, &timeout) <= 0) {
close(fd);
return -1;
}
// [...]
}