Atomic construct

Apart from using critical construct to synchronize accessing the same variable, atomic is another choice. Check the following code:

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

int main(void)
{    
    int sum = 0;

    #pragma omp parallel for
    for (int index = 1; index <= 10; index++)
    {
        #pragma omp atomic
        sum += index;
    }

    printf("Sum is %d\n", sum);
    return 0;
} 

Build and run it on my 24-core machine:

# gcc -fopenmp parallel.c
# ./a.out
Sum is 55
# ./a.out
Sum is 55
# ./a.out
Sum is 55
# ./a.out
Sum is 55

The atomic ensures updating sum variable is an atomic operation, so the program always calculate the correct result. Besides +, atomic construct also supports other operators, such as: -, *, /, etc. Similarly, -=, *=, /= are covered too.