Question

I have two threads with a parallel for but this one does not break up into threads:

#pragma omp parallel sections
  {
#pragma omp section
    {
      for(int i=0;i<4;++i)
        printf("Loop A %d %d\n", omp_get_thread_num(),2);
    }
#pragma omp section
    {
      for(int i=0;i<4;++i)
        printf("Loop B %d %d\n", omp_get_thread_num(),3);
    }           
  }

Output:

Running…

Loop A 0 2
Loop A 0 2
Loop A 0 2
Loop A 0 2
Loop B 0 3
Loop B 0 3
Loop B 0 3
Loop B 0 3

Answer 1

This can happen for a number of reasons. I’ll list the possible ones I know:

1: Make sure you actually have a multi-core machine. If it’s a single core machine (no HT), it’ll only run with one thread.

2: If this is on Visual Studio, you need to enable OpenMP support. Just including the header is not enough:

Project -> Properties -> Configuration Properties -> C/C++ -> Language -> Open MP Support

change it to Yes (/openmp), and it should be enabled.

I ran your code with OpenMP setup properly and I get this:

Loop A 0 2
Loop B 2 3
Loop B 2 3
Loop B 2 3
Loop B 2 3
Loop A 0 2
Loop A 0 2
Loop A 0 2

So I think your code is correct.