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A.15 Determining the Number of Threads Used

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Consider the following incorrect example (for Section 3.1.2 on page 37):

np = omp_get_num_threads(); // misplaced 
#pragma omp parallel for schedule(static)
    for (i=0; i<np; i++)
        work(i);

The omp_get_num_threads() call returns 1 in the serial section of the code, so np will always be equal to 1 in the preceding example. To determine the number of threads that will be deployed for the parallel region, the call should be inside the parallel region.

The following example shows how to rewrite this program without including a query for the number of threads:

#pragma omp parallel private(i)
{
    i = omp_get_thread_num();
    work(i);
}