#Hello OpenMP!

The first stage is to write a small OpenMP program. Choose from C, C++ or Fortran to write the program hello_openmp. Once you have written and compiled hello_openmp return to this page.

Now that you have the compiled the hello_openmp executable, the next step is to run it. The way to run the executable is the same regardless of the programming language used to produce it. The first step is to set the number of threads of execution that you want to run in parallel. This is achieved by setting the environmental variable OMP_NUM_THREADS to the desired number of threads, e.g. in the bash shell;

export OMP_NUM_THREADS=4

or in the tcsh shell;

setenv OMP_NUM_THREADS 4

Now that you have set the number of threads, run the executable by typing;

./hello_openmp

You should see the following output.

Hello OpenMP!
Hello OpenMP!
Hello OpenMP!
Hello OpenMP!

The line Hello OpenMP! is output four times, as the program split into four threads, and each thread printed Hello OpenMP!. Play with the number of threads (by changing OMP_NUM_THREADS) and see how that affects the output.

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Compare with MPI

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