openmp OpenMP reductions Approximation of PI using reductions based on #pragma atomic


double area;
double h = 1.0 / n;
#pragma omp parallel for shared(n, h, area) 
for (i = 1; i <= n; i++)
  double x = h * (i - 0.5);
  #pragma atomic
  area += (4.0 / (1.0 + x*x));
pi = h * area;

In this example, each threads execute a subset of the iteration count and they accumulate atomically into the shared variable area, which ensures that there are no lost updates. We can use the #pragma atomic in here because the given operation (+=) can be done atomically, which simplifies the readability compared to the usage of the #pragma omp critical.