OpenMP: a Standard for Directive Based Parallel Programming

• Although standardization and support for the threaded APIs has a considerable progress, their use is still restricted to system programmers as opposed to application programmers.
• One of the reasons for this is that APIs such as Pthreads are considered to be low-level primitives.
• A large class of applications can be efficiently supported by higher level constructs (or directives)
• Which rid the programmer of the mechanics of manipulating threads.
• Such directive-based languages have standardization efforts succeeded in the form of OpenMP.
• OpenMP is an API that can be used with FORTRAN, C, and C++ for programming shared address space machines.

• Standard API for defining multi-threaded shared-memory programs.
• Allow a programmer to separate a program into serial regions and parallel regions, rather than concurrently-executing threads.
• NOT parallelize automatically and NOT guarantee speedup.
• General structure:

      1 #include <omp.h>
      2 main ()  {
      3 int var1, var2, var3;
      4 Serial code 
      5 Beginning of parallel section. Fork a team of threads
      6 Specify variable scoping 
      7 #pragma omp parallel private(var1, var2) shared(var3)
      8   {
      9   Parallel section executed by all threads 
     10   All threads join master thread and disband 
     11   }  
     12 Resume serial code 
     13 }