openmp OpenMP reductions Approximation of PI using reductions based on #pragma omp critical
Example
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 omp critical
{
area += (4.0 / (1.0 + x*x));
}
}
double 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.
