How does the following omp parallel if works?-CodePudding

When I enter i = 1 it outputs:

c[0] = 7
c[0] = 7
c[0] = 7
c[0] = 7
c[0] = 7

but when i enter i = 0 it outputs:

c[1] = 9
c[2] = 11
c[3] = 13
c[4] = 15
c[0] = 7

shouldn't the output of i = 0 be the output of i = 1 and vice versa?

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

int main(){
    int a[5] = {1, 2, 3, 4, 5} ;
    int b[5] = {6, 7, 8, 9, 10} ;
    int c[5];
    int tid;
    int i;
    scanf("%d", &i);
    #pragma omp parallel if(i)
    {
        #pragma omp parallel num_threads(5)
            {
                tid = omp_get_thread_num();
                c[tid] = a[tid]   b[tid];
                printf("c[%d] = %d\n", tid, c[tid]);    
            }
    
    }
    
}```

CodePudding user response：

The behaviour you see is correct (assuming that the variable tid has been declared private as otherwise you have a race condition here).

Indeed, the first #pragama omp parallel if(i) will create a parallel region with an implementation defined number of threads if and only if i isn't 0. But then, if the parallel region has been created, then the following #pragma omp parallel num_threads(5) will be ignored as by default, nested parallelism is disabled.

And if i was 0 and the first parallel region wasn't created (no more nested parallelism here), then the second one will indeed be created and 5 threads will be spawned.

In summary, i non null prevents the inner parallel region to be created, while i null allows it.