calling a function inside a multithreaded loop

Question

I have a question regarding multithreaded loops in OpenMP. The private-clause declares variables in its list to be private to each thread. So far so good. But what happens when I call a function inside a multithreaded loop? See this minimal example:

#include <iostream>
#include <vector>
#include "omp.h"

using namespace std;



int second(int num)
{
    int ret2 = 2*num;
    return ret2;
}

int first(int num)
{
    int ret1 = num;
    return second(ret1);
}


int main()
{
    int i;
    #pragma omp parallel
    {
        vector<int> test_vec;
        #pragma omp for
        for(i=0; i<100; i++)
        {
            test_vec.push_back(first(omp_get_thread_num()));
        }
        #pragma omp critical
        cout << test_vec[0] << endl;
    }
    return 0;
}

Will each thread get its own version of the functions first and second such that the threads can call them independently of each other? Or will the threads have to "queue" in order not to call them at the same time?

Whatever happens, I would expect the variables ret1 and ret2 to be private to each thread

Nate Hekman · Accepted Answer · 2013-04-12 20:17:21Z

1

ret2 and ret1 are declared on the stack, and each thread will have its own stack, therefore there will be no interference if first or second get called by multiple threads simultaneously.

Am I understanding your question correctly?

answered Apr 12, 2013 at 20:17

Nate Hekman

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1 Comment

BillyJean Over a year ago

Yes, you understood it correctly. I was worried that I had to include a clause such that interference would be avoided. But if each thread has its own private stack, where variables are declared, then there will be no interference to begin with, so all OK

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