Open MP: private structure with shared element?

Question

Is the following possible?

EDIT: Minimal working example:

#include <omp.h>
#include <stdio.h>

struct foo
{
        int a;
        int b;
};

int main(int argc, char ** argv)
{
  struct foo outerFoo;
  int * p = &outerFoo.a;

  outerFoo.a = 1;

  printf("outerFoo @ %p, a=%d\n", &outerFoo, outerFoo.a);

#pragma omp parallel private(outerFoo), shared(p)
  {
    printf("Thread %d: outerFoo @ %p, a=%d, p = %p,p->:%d\n",
           omp_get_thread_num(),
           &outerFoo, outerFoo.a,p,*p);

#pragma omp critical // i know i can use atomic here
    {
        *p=*p+1;
    }
  }

  printf("a:%d\n",outerFoo.a);
  return 0;
}

Answer 1

What you have written is certainly possible. Whether it does what you want is impossible for us to tell, since you haven't told us what that is!

I strongly suspect that it doesn't, since it will generate new, uninitialized instances of somestruct in each thread, and a shared pointer to the outer scope instance.

Consider this code which is effectively the same as yours, but compileable

#include <omp.h>
#include <cstdio>

class foo
{
public:
  int a;
  int b;
  foo() : a(0), b(1) {}
};

int main(int argc, char ** argv)
{
  foo outerFoo;
  int * p = &outerFoo.a;

  outerFoo.a = 27;

  printf("outerFoo @ %p, a=%d, b=%d\n", &outerFoo, outerFoo.a, outerFoo.b);
#pragma omp parallel private(outerFoo), shared(p)
  {
    printf("Thread %d: outerFoo @ %p, a=%d, b=%d, p = %p\n",
           omp_get_thread_num(),
           &outerFoo, outerFoo.a, outerFoo.b,p);
  }

  return 0;
}

It prints something like this

OMP_NUM_THREADS=4 ./a.out
outerFoo @ 0x7fffffffd080, a=27, b=1
Thread 0: outerFoo @ 0x7fffffffcb80, a=0, b=1, p = 0x7fffffffd080
Thread 3: outerFoo @ 0x2aaaacf4fa80, a=0, b=1, p = 0x7fffffffd080
Thread 2: outerFoo @ 0x2aaaacb4da80, a=0, b=1, p = 0x7fffffffd080
Thread 1: outerFoo @ 0x2aaaac74ba00, a=0, b=1, p = 0x7fffffffd080

So you can see that happening. There is one outer instance of the class and four inner ones. All of them have all of the struct fields.