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First time using select(), maybe a basic question?

Answer 1

A:

You might read the introductory tutorial Look at blocking vs non-blocking connections

Jay 2010-06-10 16:09:14

I am sorry for the poor formatting. The first thing i've tried to do is to use send() and recv() instead of read() and write(), but still it doesn't quite work ok. Thanks for the replies anyways.

darkletter 2010-06-10 16:25:41

Answer 2

A:

Hmm, I think your logic is all wrong. It should look something more like this (warning, untested pseudo-code):

for (;;)
{
   // First, set up the fd_sets to specify the sockets we want to be notified about
   fd_set readSet;  FD_CLR(&readSet);
   fd_set writeSet; FD_CLR(&writeSet);
   int maxFD = -1;
   for (int i=0; i<num_consumers; i++)
   {
      if (consumer_sockets[i] > maxFD) maxFD = consumer_sockets[i];
      FD_SET(consumer_sockets[i], &readSet);
      if (consumer_has_data_he_wants_to_send[i]) FD_SET(consumer_sockets[i], &writeSet);
   }
   for (int i=0; i<num_producers; i++)
   {
      if (producer_sockets[i] > maxFD) maxFD = producer_sockets[i];
      FD_SET(producer_sockets[i], &readSet);
      if (producer_has_data_he_wants_to_send[i]) FD_SET(producer_sockets[i], &writeSet);
   }

   // Now we block in select() until something is ready to be handled on a socket
   int selResult = select(maxFD+1, &readSet, &writeSet, NULL, NULL);
   if (selResult < 0) {perror("select"); exit(10);}

   for (int i=0; i<num_consumers; i++)
   {
      if (FD_ISSET(consumer_sockets[i], &readSet)
      {
         // There is some incoming data ready to be read from consumer_socket[i], so recv() it now
         [...]
      }
      if (FD_ISSET(consumer_sockets[i], &writeSet)
      {
         // There is buffer space in consumer_socket[i] to hold more outgoing
         // data for consumer_socket[i], so send() it now
         [...]
      }
   }
   for (int i=0; i<num_producers; i++)
   {
      if (FD_ISSET(&producer_sockets[i], &readSet)
      {
         // There is some data ready to be read from producer_socket[i], so recv() it now
         [...]
      }
      if (FD_ISSET(producer_sockets[i], &writeSet)
      {
         // There is buffer space in producer_socket[i] to hold more outgoing
         // data for producer_socket[i], so send() it now
         [...]
      }
   }
}

Note that to really do this properly, you'd need to set all of your sockets to non-blocking I/O and be able to handle partial reads and writes (by storing the partial data into a local memory-buffer associated with that consumer/producer, until you have enough data to act on), otherwise you risk having a call to recv() or send() block, which would prevent the event loop from being able to service any of the other consumers or producers. Ideally the only place you should ever block in is select()... every other call should be non-blocking. But if you want to keep things simple to start out with, you may be able to get away with using blocking I/O for a while.

Jeremy Friesner 2010-06-10 16:29:15

Thanks a lot for your answer. It is kind of depressing to see how you work of some days has to be thrown into the bin, but it's good to know the truth. It's a shame because when executing the server it really seemed i was close to finishing it. Would your little example change much if i used read(), write(), and blocking sockets? This is college homework, and we've never used either non-blocking sockets or non-blocking I/O. This is all new to me, and the deadline is somehow near.

darkletter 2010-06-10 16:41:39

On Linux/Unix, anyway, recv()/read() and write()/send() are equivalent (at least when they are used on network sockets). As far as using blocking sockets/IO goes, you can try it, but it's tricky because if you ever try to read()/recv() on a socket that has no data ready to read, then read()/recv() will not return until more bytes arrive from the network... which could be a long time, or never, and in the meantime, all other socket I/O is blocked. (there's a similar issue with send()/write() blocking if the remote client refuses to read its incoming data, but it's not quite as big a problem).

Jeremy Friesner 2010-06-10 18:07:40

Answer 3

A:

Thanks for the insight Jeremy, tomorrow i will ask the professor whether it's compulsory or not to use blocking sockets (he never explained to us non-blocking sockets, so i suppose it will be compulsory), but if not, i will proceed to investigate the non-blocking way.

Thanks a lot

PS: Thanks for the formatting abelenky.

darkletter 2010-06-10 19:36:30

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First time using select(), maybe a basic question?

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