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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

User20139023
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  • 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](https://stackoverflow.com/q/48135794/620382)? – Zulan Jan 22 '19 at 12:14
  • 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 Jan 22 '19 at 12:16

1 Answers1

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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.

Zulan
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  • Aaah know I understand. So I could change the pointer of passed_index, but when I try to, lets say do passed_index[0] = 1; This would have an effect on the other threads, right? Also when I define a variable inside of work() it gets private I guess – User20139023 Jan 22 '19 at 12:27
  • You can stil safely work on different *elements* of the array (e.g. thread `i` on `passed_index[i]`). And yes, variables defined within `work` are private, but of course you can't just `int* foo = passed_index` - that would still be a pointer to shared data. – Zulan Jan 22 '19 at 12:58