How does the SECTIONS directive in OpenMP distribute work?

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The code posted by the OP will never execute in parallel, because the parallel keyword does not appear. The fact that the OP got ids different from 0 shows that probably his code was embedded in a parallel directive. However, this is not clear from his post, and might confuse beginners.

The minimum sensible example is (for the first example posted by the OP):

#pragma omp parallel sections
{
    #pragma omp section
    { 
        printf ("id = %d, \n", omp_get_thread_num());
    }

    #pragma omp section
    { 
        printf ("id = %d, \n", omp_get_thread_num());
    }
}

On my machine, this prints

id = 0,
id = 1,

showing that the two sections are being executed by different threads.

It’s worth noting that however this code can not extract more parallelism than two threads: if it is executed with more threads, the other threads don’t have any work to do and will just sit down idle.

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