Question

I’m trying to write a server that handles multiple clients at a time. Apparently I’m supposed to use select. I’ve found numerous tutorials but none of them handle writing.

I get how I’m supposed to read data in, but what’s the.. appropriate way to handle writing back to the clients in my ‘loop’?

Thanks in advance.

Answer 1

If you have data that you want to send to a particular socket, then you need select() to return when there is space available in that socket’s output-buffer. The way to tell select() to do that is similar to using select() just for read — except that also you do the FD_SET on the second fd_set object.

int socks[10] = {... some client sockets...}

while(1)
{
   fd_set readSet, writeSet;

   FD_ZERO(&readSet);
   FD_ZERO(&writeSet);

   int maxSock = -1;
   for (int i=0; i<10; i++)
   {
      FD_SET(socks[i], &readSet);
      if (socks[i] > maxSock) maxSock = socks[i];

      if (IHaveDataToSendToThisSocket(i))  // implement this function as appropriate to your program
      {
         FD_SET(socks[i], &writeSet);
         if (socks[i] > maxSock) maxSock = socks[i];
      }
   }

   int ret = select(maxSock+1, &readSet, &writeSet, NULL, NULL);
   if (ret < 0)
   {
      perror("select() failed");
      break;
   }

   // Do I/O for sockets that are ready
   for (int i=0; i<10; i++)
   {
      if (FD_ISSET(socks[i], &readSet))
      {
         // there is data to read on this socket, so call recv() on it
      }

      if (FD_ISSET(socks[i], &writeSet))
      {
         // this socket has space available to write data to, so call send() on it
      }
   }
}