Question

I compile this bit of code on Snow Leopard and linux and I get different results. On Snow leopard, the first call of omp_get_max_threads returns 2, which is my number of cores, while the second returns 1. On linux, both calls return 4, which is my number of cores. I think Linux has the correct behavior, am I right? Are both correct and I just have a misunderstanding of this function?

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

int main() {
  printf(" In a serial region; max threads are : %d\n", omp_get_max_threads());
#pragma omp parallel
{
  #pragma omp master
  printf(" In a parallel region; max threads are : %d\n", omp_get_max_threads());
}
}

Mac output:

 In a serial region; max threads are : 2
 In a parallel region; max threads are : 1

Linux output:

 In a serial region; max threads are : 4
 In a parallel region; max threads are : 4

Answer 1

this call is well specified in the openmp spec. linux has the correct behavior here.
with that being said, you are in a master region which is effectively serial and fhe the main thread, so the num threads call is explainable. if you arent tied to pure c I would encourage you to look at the c++ tbb library and particularly the ppl subset, you will find more generality and composability like for nested parallelism. I’m on myphone so I apologize for typos here.