OpenMP-Examples/sources/Example_atomic.3c.c

/*
* @@name:	atomic.3c
* @@type:	C
* @@compilable:	yes
* @@linkable:	no
* @@expect:	success
*/
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);
}