在选择时阻止时将新FD添加到fd_Set

Question

I have a question regarding adding new socket file descriptors to an FDSET. Lets say we've already connected to a socket s1:

fd_set readfds;

//s1 = socket(...);
//connect(s1, ...)...

FD_ZERO(&readfds);
FD_SET(s1, &readfds);

and we are waiting for data to come down the socket, by calling select in a thread:

socket_reader_thread() {

   for (;;)
   {
      int rv = select(n, &readfds, NULL, NULL, &tv);

      if (rv == -1) {
         perror("select"); // error occurred in select()
      }
      else if (rv == 0) {
         printf("Timeout occurred!  No data after 10.5 seconds.\n");
      }
      else {
         // one the descriptors have data
         .....
      }
   }

}

If I now wanted to add another socket (or may be two more socket etc) to the readfds set, given that select is blocking, how should I proceed? how can I interrupt select

Is the trick to add a zero timeout and use select like poll?

Answer 1

You need to use the "pipe-trick".

This is where an additional socket or pipe is created add it to the fd_set.

Then to interrupt a running or pending select, send a 1 byte message to it via another thread.

The select will then return and if the special pipe FD is one of the ones that are ready in the set, that means you need to say look at a list or something "do work" - like add any new FDs to the fd_set before returning to the select call.

Answer 2

You can interrupt select by sending (and catching) a signal to your process, for example using raise . select will return in this case with -1 and errno set to EINTR . You can then change the events you want to wait for and call select again.

Is the trick to add a zero timeout and use select like poll?

One can simply use a timeout of 0 in which case it will just do a non-blocking check if any of the events got triggered, ie polling. But this should only be done in a few cases since busy polling instead of a blocking wait uses lots of resources of machine. And I would even consider the interrupting of a blocking select a questionable design, although probably not as bad as busy polling.