determining if a variable is defined or not

Hi,

using one simple list, located in the external sdram device, the program is not able to figure out if the pItemNext from the list is defined or not... Therefore I always get an data abort error.

LISTITEM* pItemNext = (LITEM*)0;

/* the first item in the list will be defined... */

/* second item in the list is not defined;
the value of pItemNext = 0xE59FF018 */
pItemNext = pItem->pNextItem;


/* while loop will be executed for this item which is
not defined*/
while(pItemNext != (LITEM *)0)
{
  /* -> data abort error */
  pItemNext->data -= 1;
  pItemNext = pItem->pNextItem;
}

How is it possible to get the while loop working? At the moment the while-loop will be executed for the pItemNext = 0xE59FF018... which is wrong.

best regards
Hannes