Question

I’m new to the world of parallel programming and openmp, so this may be a futile question, but I can’t really come up with good answer to what I’m experiencing, so I hope someone will be able to shed some light on the matter.

What I am trying to achieve is to have a private copy of a dinamically allocated matrix (of integers) for every thread that will handle the following parallel section, but as soon as the flow of execution enters said region the reference to the supposedly private matrix holds a null value.

Is there any limitation of this directive I’m not aware of? Everything seems to work just fine with monodimensional dynamic arrays.

A snippet of the code is the following one…

#define n 10000
int **matrix;
#pragma omp threadprivate(matrix)

int main()
{

    matrix = (int**) calloc(n, sizeof(int*));
    for(i=0;i<n;i++) matrix[i] = (int*) calloc(n, sizeof(int));
    AdjacencyMatrix(n, matrix);

    ...

   /* Explicitly turn off dynamic threads */
   omp_set_dynamic(0);
   #pragma omp parallel
   {    
      // From now on, matrix is NULL... 
      executor_p(matrix, n);
   }
   ....

Answer 1

Look at the OpenMP documentation regarding what happens with the threadprivate clause:

On first entry to a parallel region, data in THREADPRIVATE variables and common blocks should be assumed undefined, unless a COPYIN clause is specified in the PARALLEL directive

There’s no guarantee of what value is going to be stored in the matrix variable in the parallel region.