#C++

Copy this into the file hello_threads.cpp.

#include <iostream>

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

int main(int argc, const char **argv)
{
    std::cout << "I am the main thread.\n";

    #pragma omp parallel
    {
        int nthreads = omp_get_num_threads();
        int thread_id = omp_get_thread_num();

        std::cout << "Hello. I am thread " << thread_id
                  << " out of a team of " << nthreads 
                  << std::endl;
    }

    std::cout << "Here I am, back to the main thread.\n";

    return 0;
}

This example uses two OpenMP functions;

• omp_get_num_threads() : Returns the number of threads in the OpenMP thread team.
• omp_get_thread_num() : Returns the identifying number of the thread in the team.

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.

Note also that because the nthreads and thread_id variables exist only in the scope of the omp parallel section, each thread will have its own private copy of these variables.

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

• g++ : g++ -fopenmp hello_threads.cpp -o hello_threads
• icpc : icpc -openmp hello_threads.cpp -o hello_threads -cxxlib-icc
• pgCC : pgCC -mp hello_threads.cpp -o hello_threads

This will produce the executable, hello_threads.

Now return to the previous page.