why doesn't the following cause a coredump in Linux?

genie (genie@ucflink.com)
Sat, 5 Jul 1997 22:21:49 -0400 (EDT)

The following program will not cause a coredump even though it looks like
I am assigning a value to a illegal memory reference. Why is this? Am I
allocating more memory than I need in my dim2d or am I allocating it the
wrong way? I am not sure why the main() references won't cause a core
dump, since it looks like I'm poking past the array's boundaries (at least
in the static case, one would think so). Can someone give me a hint on
memory allocation and the layout of a statis/dynamic array in memory under
Linux(or UNIX even, perhaps)? Thank you dearly.

M. Naskovski

--
#include <stdio.h>
#include <stdlib.h>


double **dim2d(int n, int m) /* n = # of rows, m = # of columns */
{
        int i;
        double **narr;
        narr=(double **)calloc(n, sizeof(double *)); 
	if (narr == NULL) return NULL;
        for (i=0; i < n; i++) {
                narr[i]=(double *)calloc(m, sizeof(double));
                if (narr[i] == NULL) return NULL;
        }
        return narr;
}


main()
{
	double **a=dim2d(1,1);
	double a2[1][1];
	a[0][0]=255; 
	a[0][5]=256;


	a2[0][0]=255;
	a2[0][5]=256;
}