OpenMP stackvariables

Question

I am currently learning about OpenMP. As default variables declared outside of the parallel region are public, where as variables inside of a parallel region are private. Also stack variables from inside the parallel regions are private.

double A[10];
int index[10];
#pragma omp parallel
{
 work(index);
}
printf(%d\n”,index[0]);

But why is "index" on the above example public for each thread? Shouldn't it be private, since its put on the stack, and stack variables are private?

Thanks in advance

Why would you think that "stack variables are private"? Do you mean "shared" if you write "public"? Are you, perhaps, suffering from the same misunderstandings as discussed in this question? — Zulan
Well I am learning for my exam, and on the slides there is a line "But not everything is shared - Stack variables in C functions called from parallel regions are PRIVATE". Sorry, yes I mean shared when I say public — User20139023

Zulan Zulan · Accepted Answer · 2019-01-22T12:23:47

The statement

Stack variables in C functions called from parallel regions are private

is true, but you need to differentiate in your case. First,

int index[10];
#pragma omp parallel
{
    // index is a shared variable here
    work(index);
}

But when it comes to the function you call, imagine:

void work(int* passed_index)
{
    ...
}

passed_index - the pointer - is in fact a private variable within work. You can change the pointer, and no other thread will notice.

But the data pointed to by *passed_index is still shared.

OpenMP stackvariables

1 Answers