(1) sem.c is a program usiung a single process.
    run in by typing in Ch 
       sem.c
(2) semabinit.c, sema.c and semab.c are 
    from http://docs.linux.cz/programming/c/unix_examples/semab.html
    run in by typing in Ch 
       semabinit.c
       sema.c
       semb.c
