semaphore
    n 1: an apparatus for visual signaling with lights or
         mechanically moving arms
    v 1: send signals by or as if by semaphore
    2: convey by semaphore, of information

Semaphore \Sem"a*phore\, n. [Gr. sh^ma a sign + fe`rein to bear:
   cf. F. s['e]maphore.]
   A signal telegraph; an apparatus for giving signals by the
   disposition of lanterns, flags, oscillating arms, etc.
   [1913 Webster] Semaphoric

semaphore

    The classic method for
   restricting access to shared resources (e.g. storage) in a
   multi-processing environment.  They were invented by
   Dijkstra and first used in T.H.E operating system.

   A semaphore is a protected variable (or abstract data
   type) which can only be accessed using the following
   operations:

   	P(s)
   	Semaphore s;
   	
   	  while (s == 0) ;	/* wait until s>0 */
   	  s = s-1;
   	

   	V(s)
   	Semaphore s;
   	
   	  s = s+1;
   	

   	Init(s, v)
   	Semaphore s;
   	Int v;
   	
   	  s = v;
   	

   P and V stand for Dutch "Proberen", to test, and "Verhogen",
   to increment.  The value of a semaphore is the number of units
   of the resource which are free (if there is only one resource
   a "binary semaphore" with values 0 or 1 is used).  The P
   operation busy-waits (or maybe sleeps) until a resource is
   available whereupon it immediately claims one.  V is the
   inverse, it simply makes a resource available again after the
   process has finished using it.  Init is only used to
   initialise the semaphore before any requests are made.  The P
   and V operations must be indivisible, i.e. no other process
   can access the semaphore during the their execution.

   To avoid busy-waiting, a semaphore may have an associated
   queue of processes (usually a FIFO).  If a process does a
   P on a semaphore which is zero the process is added to the
   semaphore's queue.  When another process increments the
   semaphore by doing a V and there are tasks on the queue, one
   is taken off and resumed.

   (1995-02-01)