gcc/libgomp/testsuite/libgomp.c/appendix-a/a.18.1.c

/* { dg-do run } */

#include <omp.h>
#include <stdio.h>

extern void abort (void);

#define NUMBER_OF_THREADS 4

int synch[NUMBER_OF_THREADS];
int work[NUMBER_OF_THREADS];
int result[NUMBER_OF_THREADS];
int
fn1 (int i)
{
  return i * 2;
}

int
fn2 (int a, int b)
{
  return a + b;
}

int
main ()
{
  int i, iam, neighbor;
  omp_set_num_threads (NUMBER_OF_THREADS);
#pragma omp parallel private(iam,neighbor) shared(work,synch)
  {
    iam = omp_get_thread_num ();
    synch[iam] = 0;
#pragma omp barrier
    /*Do computation into my portion of work array */
    work[iam] = fn1 (iam);
    /* Announce that I am done with my work. The first flush
     * ensures that my work is made visible before synch.
     * The second flush ensures that synch is made visible.
     */
#pragma omp flush(work,synch)
    synch[iam] = 1;
#pragma omp flush(synch)
    /* Wait for neighbor. The first flush ensures that synch is read
     * from memory, rather than from the temporary view of memory.
     * The second flush ensures that work is read from memory, and
     * is done so after the while loop exits.
     */
    neighbor = (iam > 0 ? iam : omp_get_num_threads ()) - 1;
    while (synch[neighbor] == 0)
      {
#pragma omp flush(synch)
      }
#pragma omp flush(work,synch)
    /* Read neighbor's values of work array */
    result[iam] = fn2 (work[neighbor], work[iam]);
  }
  /* output result here */
  for (i = 0; i < NUMBER_OF_THREADS; i++)
    {
      neighbor = (i > 0 ? i : NUMBER_OF_THREADS) - 1;
      if (result[i] != i * 2 + neighbor * 2)
	abort ();
    }

  return 0;
}