Question

Unix/C question here.

I have multiple sockets that I am trying to poll for periodic data. I don’t want select to wait indefinitely so I have a timeout in place and I’m running in a loop. I have found that once a socket is ready to read, it is always ready to read. As in, I cannot have select go to sleep when there is no data to be read from any of the sockets.

for (i = 0; i < n_connections; i++) {
  FD_SET( sockfd[i], &master );
  if (sockfd[i] > fdmax) 
    fdmax = sockfd[i];
  }

for(;;) {
  int nready = 0;
  timeout.tv_sec  = 1;
  timeout.tv_usec = 0;
  read_fds = master;
  if ( (nready = select(fdmax+1, &read_fds, NULL, NULL, NULL)) == -1 ) {
    fprintf( stderr, "Select Error\n" );
    return FAILURE;
  }
  printf( "Number of ready descriptors: %d\n", nready );

  for (i = 0; i <= fdmax; i++) {
    if (FD_ISSET(i, &read_fds)) {
      if (( nbytes = recv(i, buf, sizeof(buf), 0)) <= 0 ) {
        if (nbytes == 0) {
          //connection closed
          printf("Socket %d hung up\n", i );
        }
        else {
          fprintf( stderr, "Recv Error %d\n", nbytes);
        }
      }
    else {
      printf( "Data Received on %d: %s\n", i, buf );
    }
  }
} // end file descriptor loop

It seems that after my first read, the 1 second timeout no longer applies and the socket is always “ready to read”, even if there are 0 bytes available. How can I get select to sleep until data comes in (for the one second, or by switching the final argument to NULL, indefinitely waiting for data to come in on the socket?)

Output:

Number of Ready Descriptors: 2
Data Received on 4: GreetingsChap
Data Received on 5: HiMatengsChap
Loop...
Number of Ready Descriptors: 2
Socket 4 hung up
Socket 5 hung up
Loop...
Number of Ready Descriptors: 2
Socket 4 hung up
Socket 5 hung up
Loop...

Thank you,

Note: Code updated for clarity
Updated based on @yvesBraumes suggestions – still doesn’t work.

Answer 1

If you detect that a connection is closed, remove the socket from the fd set, otherwise select is going to report them (Socket 4 hung up).. select is not edge triggered, if you don’t handle the event, it’s going to report it again.