UNIX for Advanced & Expert Users

 #define MASK(f)     (1 << (f))
           #define NTTYS 4

                int tty[NTTYS];
                int ttymask[NTTYS];
                int readmask = 0;
                int readfds;
                int nfound, i;
                struct timeval timeout;

                   /* First open each terminal for reading and put the
                    * file descriptors into array tty[NTTYS].  The code
                    * for opening the terminals is not shown here.
                    */

                for (i=0; i < NTTYS; i++) {
                   ttymask[i] = MASK(tty[i]);
                   readmask |= ttymask[i];
                }

                timeout.tv_sec  = 5;
                timeout.tv_usec = 0;
                readfds = readmask;

                /* select on NTTYS+3 file descriptors if stdin, stdout
                 * and stderr are also open
                 */
                if ((nfound = select (NTTYS+3, &readfds, 0, 0, &timeout)) == -1)
                   perror ("select failed");
                else if (nfound == 0)
                   printf ("select timed out \n");
                else for (i=0; i < NTTYS; i++)
                   if (ttymask[i] & readfds)
                      /* Read from tty[i].  The code for reading
                       * is not shown here.
                       */
                else printf ("tty[%d] is not ready for reading \n",i);