OpenMP error message

Question

I got the following output with error message while executing the program that uses openmp in VS2008 C++

thread number: 0

thread number: 1

Fatal User Error 1002: 'for' loop executed with inconsistent parameters between threads

My program does the following:

    omp_set_dynamic(0);
    omp_set_num_threads(2);

    int i = 0;
    int start_pos   = 0;
    int end_pos     = 0;

#pragma omp parallel default(none) private(i, start_pos, end_pos)
    {
        int nThreadNum = omp_get_thread_num();
        printf("thread number: %d\n", nThreadNum);
        start_pos   = 0;
        end_pos     = (number_of_model_points / 2 + 1);
        if (nThreadNum != 0) {
            start_pos   = (number_of_model_points / 2) + 1;
            end_pos     = (number_of_model_points);
        }

#pragma omp for
        for(i = start_pos; i < end_pos; i++) {
                      ...some code here...
        }
}

What is the problem here? Please correct me if I made a mistake.

FFox · Accepted Answer · 2010-08-07 19:01:50Z

1

So why there is an additional pragma for the for loop? You already did fork your processes. If you really want to fork again (don't do that, please!) then "i" needs to be set private again. However I think you just would want to remove the "omp for" pragma.

answered Aug 7, 2010 at 19:01

FFox

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1 Comment

Jonathan Dursi Over a year ago

This is the right answer for the wrong reason. You normally do want to do a #pragma omp for inside the #pragma omp parallel to take advantage of work sharing - the omp for will split up the loop for you, otherwise you'd have all the threads (not processes) doing the same loop. But, here you have already done the work of manually breaking up the loop yourself; sometimes that's advantageous, although I don't see the benefit here. So you do want all the threads to do the same for loop, with each thread doing it's own region. So you don't want the omp for.

ramino · Accepted Answer · 2019-10-17 18:54:58Z

0

Jonathan Dursi is right, so in summary, for loop bounding parameters can not be private. In your example, for(i = start_pos; i < end_pos; i++), start_pos and end_pos are private per omp thread, hence consistency is not guaranteed and you get error 1002. Otherwise you can definitely have #pragma omp for inside #pragma omp parallel.

answered Oct 17, 2019 at 18:54

ramino

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