How to create an `omp parallel for` with synchronization (`barrier`) of all threads in the middle with OpenMP

Question

I have two functions, do_step_one(i) and do_step_two(i), for i from 0 to N-1.

Currently, I have this (sequential) code:

for(unsigned int i=0; i<N; i++) {
     do_step_one(i);
}

for(unsigned int i=0; i<N; i++) {
     do_step_two(i);
}

Each call of do_step_one() and do_step2() can be done in any order and in parallel, but any do_step_two() needs the end of all the do_step_one() to start (it use do_step_one() results).

I tried the following :

#omp parallel for
for(unsigned int i=0; i<N; i++) {
    do_step_one(i);

#omp barrier

    do_step_two(i);
}

But gcc complains

convolve_slices.c:21: warning: barrier region may not be closely nested inside of work-sharing, critical, ordered, master or explicit task region.

What do I misunderstand? How to solve that issue?

Answer 1

Just a side note, if you want to make sure the threads are not recreated, separate the declaration of parallel and declaration of for:

#pragma omp parallel
{
  #pragma omp for
  for(unsigned int i=0; i<N; i++){
    do_step_one(i);
  }
  //implicit barrier here
  #pragma omp for
  for(unsigned int i=0; i<N; i++){
    do_step_two(i);
  }
}