#Directives (Pragmas)

So what was going on in the last example?

A standard program works by executing one line of code at a time, starting from the main function (or “program” function in Fortran) and working down line by line. This single thread of execution is known as the main thread. All programs have a single main thread, and in most programs, this is the only thread of execution, hence why the program can only do one thing at a time.

The hello_openmp program also has a single main thread of execution. However, this main thread is split into a team of threads within the OpenMP parallel section. Each parallel thread in the team executes all of the code in the parallel section, hence each thread executes the line of code that prints Hello OpenMP!.

We can see this more clearly by getting each thread to identify itself. Please copy the code from one of these examples to create the executable hello_threads.

Try running this executable using different values of OMP_NUM_THREADS.

The OpenMP parallel section is specified by using compiler directives. These directives (also called compiler pragmas) are instructions to the compiler to tell it how to create the team of threads, and to help tell the compiler how to assign threads to tasks. These OpenMP directives are only followed if the program is compiled with OpenMP support. If the program is compiled without OpenMP support, then they are ignored.

There are several OpenMP directives. This course will cover the basic usage of just a selection;

Pragmas are added to the code using either;

#pragma omp name_of_directive

in C or C++, or using;

C$OMP name_of_directive

at the start of the line in fixed-format Fortran, or using;

!$OMP name_of_directive

anywhere in the line using free-format Fortran.

Compare with MPI

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