...
extern float ** Distance;
...

float ** Distance;

#include "main.h"

...

function przyklad () {
... 
Distance = (float **) calloc( numOfLines, sizeof(float *) );
for ( i = 0; i < numOfLines; i++ )
    Distance[i] = (float *) calloc( numOfLines, sizeof(float) );
...
}
...

#include <stdio.h>

int** tab = NULL;

#include <stdio.h>
#include <stdlib.h>
#include "Unit1.cpp"

extern int** tab;

void przyklad(int rows, int columns);

int main(void)
{
   int rows = 6;
   int columns = 4;

   przyklad(rows, columns);

   for (int i = 0, counter = 0; i < rows; i++) {
      
      for (int j = 0; j < columns; j++) {
         
         tab[i][j] = counter++; // umieszczenie kolejnych liczb w komorkach tab
         printf("[%d,%d] = %2d ", i, j, tab[i][j]); // wyswietlenie zawartosci tab
      }
      printf("\n");
   }
   
   // zwolnienie pamieci
   for (int i = 0; i < rows; i++) {
      free(tab[i]);
   }
   free(tab);
   tab = NULL;

   return 0;
}

void przyklad(int rows, int columns)
{
   tab = (int**) calloc(rows, sizeof(int*));
   for (int i = 0; i < rows; i++) {
      tab[i] = (int*) calloc(columns, sizeof(int));
   }
}