Copy this into the file hello_threads.c.

#include <stdio.h>

#ifdef _OPENMP
    #include <omp.h>
    #define omp_get_num_threads() 0
    #define omp_get_thread_num() 0

int main(int argc, char **argv)
    int nthreads, thread_id;

    printf("I am the main thread.\n");

    #pragma omp parallel private(nthreads, thread_id)
        nthreads = omp_get_num_threads();
        thread_id = omp_get_thread_num();

        printf("Hello. I am thread %d out of a team of %d\n", 
                 thread_id, nthreads);

    printf("Here I am, back to the main thread.\n");

    return 0;

This example uses two OpenMP functions;

Note that using these functions requires you to include the omp.h header file. To ensure portability (if OpenMP is not supported) we hide this header file behind an #ifdef _OPENMP guard, and add stubs for the two OpenMP functions to set them to 0.

This example uses a slightly modified omp parallel line. In this case, private(nthreads, thread_id) is added to specify that each thread should have its own copy of the nthreads and thread_id variables.

You can compile this program using one of these commands (choose one for the compiler you wish to use);

This will produce the executable, hello_threads.

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