#Fortran

Copy this into the file hello_threads.F (make sure you use a capital .F).

      program main

#ifdef _OPENMP
      use omp_lib
#endif

      implicit none

      integer thread_id
      integer num_threads

      print *,"I am the main thread."

C$OMP PARALLEL PRIVATE(thread_id, num_threads)

#ifdef _OPENMP
      thread_id = omp_get_thread_num()
      num_threads = omp_get_num_threads()
#else
      thread_id = 0
      num_threads = 1
#endif

      print *,"Hello. I am thread ",thread_id," out of a team of ",
     .        num_threads
C$OMP END PARALLEL

      print *,"Here I am, back to the main thread."

      end

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 use the omp_lib Fortran library. To ensure portability (if OpenMP is not supported) we hide this header file behind an #ifdef _OPENMP guard, and manually set the values of num_threads and thread_id if _OPENMP is not set.

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);

• gfortran : gfortran -fopenmp hello_threads.F -o hello_threads
• ifort : ifort -openmp hello_threads.F -o hello_threads -cxxlib-icc
• pgf77 : pgf77 -mp hello_threads.F -o hello_threads

This will produce the executable, hello_threads.

Now return to the previous page.