Using std::cout in a parallel for loop using OpenMP

Question

I want to parallelize a for-loop in C++ using OpenMP. In that loop I want to print some results:

#pragma omp parallel for
for (int i=0; i<1000000; i++) {
    if (test.successful() == true) {
        std::cout << test.id() << " " << test.result() << std::endl;
    }
}

The result I get without using OpenMP:

3 23923.23
1 32329.32
2 23239.45

However I get the following using a parallel for-loop:

314924.4244.5
434.
4343.43123

How can I avoid such an output?

score 0 · Answer 1 · edited Aug 27 '19 at 11:36

0

The reason is the printing is not defined as an atomic operation, thus context switch occurs while printing to the standard output is still ongoing.

Solution: You must make the for loop body an atomic operation, using #pragma omp atomic

#pragma omp parallel for
for (int i=0; i<1000000; i++) {
    if (test.successful() == true) {
        #pragma omp atomic
        std::cout << test.id() << " " << test.result() << std::endl;
    }
}

edited Aug 27 '19 at 11:36

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answered Aug 27 '19 at 11:31

nivpeled

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What does `atomic` mean? – Gilfoyle Aug 27 '19 at 11:34
I get the following error: `error: invalid operator for ‘#pragma omp atomic’ before ‘<<’ token`. – Gilfoyle Aug 27 '19 at 11:37
@Samuel please put the if body inside block brackets: `{ std::cout << test.id() << " " << test.result() << std::endl; } ` – nivpeled Aug 27 '19 at 12:19
2

I don't think `#pragma omp atomic` does work here, you need `#pragma omp critical` – Mike van Dyke Aug 27 '19 at 12:32
1

As the [duplicate](https://stackoverflow.com/questions/4106992/parallelize-output-using-openmp) I pointed out says, use `#pragma omp critical`. – Ted Lyngmo Aug 27 '19 at 12:43

Using std::cout in a parallel for loop using OpenMP

1 Answers1