Hi,

I've run into a problem where omp_in_parallel() is returning 0 where I 
expect it to return 1. The minimal working example is:

#include <omp.h>

#include <iostream>
#include <vector>

const std::size_t problem_size = 10000;

int main(void) {
   std::vector<bool> parallel_checks(problem_size, false);
#pragma omp parallel for
   for (std::size_t i = 0; i < problem_size; i++) {
     parallel_checks[i] = omp_in_parallel();
   }
   for (std::size_t i = 0; i < problem_size; i++) {
     if (!parallel_checks[i]) {
       std::cout << "failed at i = " << i << "\n";
     }
   }
   return 0;
}

where parallel_checks[i] should be true for all i. This fails only 
inconsistently so I've attached a slightly longer version that runs this 
check thousands of times. On my personal computer (4 cores, g++ 9.3.0), 
it fails about 10-30 times out of 100,000 attempts (failure meaning that 
any element in parallel_checks[] is false). On a supercomputing node I 
have access to (64 cores, g++ 11.2.0), it fails much more frequently 
(300-900 attempts out of 1000). I've tested this across optimization 
settings and while the numbers vary a bit, it is always a problem even 
without optimizations on.

Am I misunderstanding what omp_in_parallel() is supposed to provide? 
Does it not guarantee a true value at any point inside a parallel section?

Thanks for your help.

Best,
- Matthias