OpenMP-Examples/synchronization/sources/atomic.3.c

/*
* @@name:	atomic.3
* @@type:	C
* @@operation:	compile
* @@expect:	success
* @@version:	omp_3.1
*/
int fetch_and_add(int *p)
{
   /* Atomically read the value of *p and then increment it. The
      previous value is returned. This can be used to implement a
      simple lock as shown below.
    */
   int old;
#pragma omp atomic capture
   { old = *p; (*p)++; }
   return old;
}

/*
 * Use fetch_and_add to implement a lock
 */
struct locktype {
   int ticketnumber;
   int turn;
};
void do_locked_work(struct locktype *lock)
{
   int atomic_read(const int *p);
   void work();

   // Obtain the lock
   int myturn = fetch_and_add(&lock->ticketnumber);
   while (atomic_read(&lock->turn) != myturn)
      ;
   // Do some work. The flush is needed to ensure visibility of
   // variables not involved in atomic directives

   #pragma omp flush
   work();
   #pragma omp flush
   // Release the lock
   fetch_and_add(&lock->turn);
}