C qsort compare function issue

I'm attempting to sort what would normally be a multi-dimensional array (CIDR block in 0, CIDR mask in 1) as a single-dimensional array of structures.  Using qsort, telling it the data size is the size of the structure, and a hopefully-clever compare function to compare only the CIDR blocks using structure offsets.

The compilation fails in the compare function with the error:

sorttest.c: In function 'ib_cmpfunc':
sorttest.c:22:27: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) > 0) return 1;
                           ^
sorttest.c:22:62: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) > 0) return 1;
                                                              ^
sorttest.c:23:27: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) < 0) return -1;
                           ^
sorttest.c:23:62: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) < 0) return -1;

Here's the code used to generate the test program.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#define BLACKLIST_SIZE 512

typedef struct {
    /* IPV4 address as 32-bit integer */
  unsigned long ib_blackaddr;
    /* CIDR mask as 32-bit integer */
  unsigned long ib_blackmask;
}  ib_arrayelement;

  /* Number of entries in the blacklist table */
int ib_blacklist_size;

  /* IPv4 address - CIDR mask blacklist */
ib_arrayelement ib_blackarray[BLACKLIST_SIZE];

static int ib_cmpfunc (const void *a, const void *b) {
  if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) > 0) return 1;
  if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) < 0) return -1;
  return 0;
}

void main (int argc, char **argv) {

  qsort(ib_blackarray, ib_blacklist_size,
        sizeof(ib_arrayelement), ib_cmpfunc);

  return;
}

The qsort compare function is too deep in pointers for me to follow what the problem is; I don't normally do anything this obscure.  Can someone point me at a solution?

Side note:  This is c on a Debian linux host.  Not C++, nor Windows.