Poll System Call Timeout

poll() waits indefinitely although timeout is specified

Your code is unconditionally calling recv() even when there is no data to read. In fact, you are completely ignoring the fds.revents field altogether if poll() does not return an error/timeout.

Your loop should look more like this:

struct pollfd fds;
fds.fd = sock;
fds.events = POLLIN;

do {
  r = poll(&fds, 1, TIMEOUT*1000);
  if (r == -1) {
    if (errno == EINTR) continue;
    perror("poll() failed");
    break;
  }
  else if (r == 0) {
    printf("timeout expired");
    break;
  }
  else if (fds.revents & POLLIN) {
    r = recv(...);
    if (r < 0) {
      perror("recv() failed");
      break;
    }
    else if (r == 0) {
      printf("socket disconnected\n");
      break;
    }
    else {
      // process data as needed...
    }
  }
  else if (fds.revents & (POLLERR | POLLNVAL)) {
    printf("socket error\n");
    break;
  }
}
while (1);

close(sock);

What does poll() do with a timeout of 0?

It will return immediately:

If timeout is greater than zero, it specifies a maximum interval (in milliseconds) to wait for any file descriptor to become ready. If timeout is zero, then poll() will return without blocking. If the value of timeout is -1, the poll blocks indefinitely.

, as of Mac OS X 10.5;

Maximum interval to wait for the poll to complete, in milliseconds. If this value is 0, poll() will return immediately. If this value is INFTIM (-1), poll() will block indefinitely until a condition is found.

, as of OpenBSD 3.8

Poll system call - is this infiniband communication?

The fact that poll returns with a timeout only means that the process is waiting for some communication event to occur. This openmpi faq page lists some ways to debug.

What does poll(NULL, 0, timeout) mean?

It is basically a longhand way of writing a sleep() of timeout duration.

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