How to define a object or struct as threadprivate in OpenMP?

Question

I don\'t know how to make a struct or object as threadprivate, what I\'m doing generates a error:

    struct point2d{
        int x;
        int y;
        p         


        

Answer 1

What you do in your code is completely correct. Quoting the OpenMP standard (emphasis mine):

A threadprivate variable with class type must have:

• an accessible, unambiguous default constructor in case of default initialization without a given initializer;
• an accessible, unambiguous constructor accepting the given argument in case of direct initialization;
• an accessible, unambiguous copy constructor in case of copy initialization with an explicit initializer.

The one in bold seems exactly your case.

The behavior you encounter seems a missing feature or a bug in the compiler. Strangely enough, even GCC seems to have problem with that, while Intel is claimed to work fine.

Answer 2

In C++ a struct with methods is a Class where the default is public. It's not plain-old-data (POD). MSVC seems to imply that it can handle threadprivate objects (i.e. non-POD) but I can't seem to get it to work. I did get it working in GCC like this:

extern point2d myPoint;
#pragma omp threadprivate(myPoint)
point2d myPoint;

But there is a work around which will work with MSVC (as well as GCC and ICC). You can use threadprivate pointers.

The purpuse of threadprivate is to have private version of an object/type for each thread and have the values persistent between parallel regions. You can do that by delcaring a pointer to point2d, making that threadprivate, and then allocating memory for the private pointer for each thread in a parallel region. Make sure you delete the allocated memory at your last parallel call.

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

struct point2d {
    int x;
    int y;
    point2d(){
        x = 0;
        y = 0;
    }
    //copy constructor
    point2d(point2d& p){
        x = p.x;
        y = p.y;
    }
};      

static point2d *myPoint;
#pragma omp threadprivate(myPoint)

int main() {

    #pragma omp parallel 
    {
        myPoint = new point2d();
        myPoint->x = omp_get_thread_num();
        myPoint->y = omp_get_thread_num()*10;
        #pragma omp critical
        {
            printf("thread %d myPoint->x %d myPoint->y %d\n", omp_get_thread_num(),myPoint->x, myPoint->y);
        }
    }   
    #pragma omp parallel
    {
        #pragma omp critical
        {
            printf("thread %d myPoint->x %d myPoint->y %d\n", omp_get_thread_num(),myPoint->x, myPoint->y);
        }
        delete myPoint;
    }
}