Miscellaneous
C | Fortran-90
parallel is the clause that tells OpenMP to create a parallel region, in which threads are spawned. The number of threads spawned in parallel regions is specified by the environment variable OMP_NUM_THREADS. It can be overriden during program execution via the OpenMP routine omp_set_num_threads.
#pragma omp parallel [clause[[,]clause]...] <new-line>
<structured-block>The clause to appear on the parallel construct, which is one of the following:
#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
/**
* @brief Illustrates the OpenMP parallel region creation.
* @details Two parallel regions are created:
* - 1st one: using the number of threads indicated by the OMP_NUM_THREADS environment variable
* - 2nd step: using the number of threads overriden by omp_set_num_threads
**/
int main(int argc, char* argv[])
{
#pragma omp parallel
{
#pragma omp single
{
printf("From the environment variable, parallel regions contain %d threads.\n", omp_get_num_threads());
}
}
omp_set_num_threads(7);
#pragma omp parallel
{
#pragma omp single
{
printf("After overwriting, parallel regions contain %d threads.\n", omp_get_num_threads());
}
}
return EXIT_SUCCESS;
}