PGI Fortran Reference manual - The Portland Group

OpenMP Directives for Fortran‣ An active parallel region is a parallel region that is executed by a team consisting of morethan one thread.The definition of an active parallel region changed between OpenMP 2.5 and OpenMP 3.0. InOpenMP 2.5, the definition was a parallel region whose IF clause evaluates to true. To examinethe significance of this change, look at the following example:program testlogical omp_in_parallel!$omp parallelprint *, omp_in_parallel()!$omp end parallelstopendSuppose we run this program with OMP_NUM_THREADS set to one. In OpenMP 2.5, thisprogram yields T while in OpenMP 3.0, the program yields F. In OpenMP 3.0, execution is notoccurring by more than one thread. Therefore, change in this definition may mean previousprograms require modification.TaskA specific instance of executable code and its data environment, generated when a threadencounters a task construct or a parallel construct.8.1.3. OpenMP ExampleLook at the following simple OpenMP example involving loops.OpenMP Loop ExamplePROGRAM MAININTEGER I, N, OMP_GET_THREAD_NUMREAL*8 V(1000), GSUM, LSUMGSUM = 0.0D0N = 1000DO I = 1, NV(I) = DBLE(I)ENDDO!$OMP PARALLEL PRIVATE(I,LSUM) SHARED(V,GSUM,N)LSUM = 0.0D0!$OMP DODO I = 1, NLSUM = LSUM + V(I)ENDDO!$OMP END DO!$OMP CRITICALprint *, "Thread ",OMP_GET_THREAD_NUM()," local sum: ",LSUMGSUM = GSUM + LSUM!$OMP END CRITICAL!$OMP END PARALLELPRINT *, "Global Sum: ",GSUMSTOPENDPGI Fortran Reference Guide 152