The remote login problem recast as two threads (see bottom of page 42) and each thread only takes a few lines and is easy to understand (each thread is sequential):

1. One thread just reads from the remote user and writes to the local application:

          void incoming(int r_in, int l_out) {
             int eof = 0;
             char buf[BSIZE];
             int size;
   
             while (!eof) {
                 size = read(r_in, buf, BSIZE);
                 if (size <= 0)
                   eof = 1;
                 if (write(l_out, buf, size) <= 0)
                   eof = 1;
             }
          }
          
   

2. The other thread just reads the response from the local application and writes back to the remote user:

          void outgoing(int l_in, int r_out) {
             int eof = 0;
             char buf[BSIZE];
             int size;
   
             while (!eof) {
               size = read(l_in, buf, BSIZE);
               if (size <= 0)
                 eof = 1;
               if (write(r_out, buf, size) <= 0)
                 eof = 1;
               }
          }