Thread: doubly linked list problem

Hi, I am experiencing a runtime error with a function of my program that tries to output a doubly linked list backwards. I am not sure where the problem originates, but the debug shows a runtime error at the Retreat() function. I suspect I might not have pointers correctly set up in my Insert() function.

The code below is a global function in main().

Code:

void
PrintBackwards( const List L )
{
    Position P = Tail(L);
	 
    if( IsEmpty( L ) )
        printf( "Empty list\n" );
    else
    {
        do
        {
            P = Retreat( P );
            printf( "%d ", Retrieve( P ) );
        } while( !IsFirst( P, L ) );
        printf( "\n" );
    }
}

The next code is the structure itself and the functions used in PrintBackwards() function. They are in a separate functions file (list.c).

Code:

struct Node
{
   ElementType Element;
   Position    Next;
   Position    Prev;
};

// Inserts an integer after Position P
void Insert( ElementType X, Position P )
{
   Position NewCell;

   NewCell = (Position) malloc( sizeof( struct Node) ); // create NewCell
   if( NewCell == NULL )
     FatalError( "Out of space!!!" );

   NewCell->Element = X; // assign a value to the NewCell

   /* Assigning and Reassigning the Pointers After Adding a NewCell */			
				
   NewCell->Prev = P; // assign the NewCell's Prev pointer to the left adjacent node
   NewCell->Next = P->Next; // reassign the NewCell's Next pointer to the right adjacent node

   P->Next = NewCell;       // reassign left adjacent next ptr to NewCell
				
// condition needed since a NULL cannot point back to anything
  if (NewCell->Next != NULL) {
					
  NewCell->Next->Prev = NewCell; 
  // assign right adjacent previous ptr to NewCell
  // NewCell->Next->Next may point to other numbers in the list
														
   }
}

Position Tail( List L )
{
   while (L != NULL) { // traverse to the end of the list
    L = L->Next;
				}
    return L;
}

// Return true if L is empty
int IsEmpty( List L ) {
   return L->Next == NULL;
}

Position Retreat( Position P ){
   return P->Prev;
}

ElementType Retrieve( Position P ) {
   return P->Element;
}

// Return true if P is the first position in list L
int IsFirst( Position P, List L ) {
   return P->Prev == NULL;
}

I also have a PrintForwards() function, which works fine. You can download the entire code if the above info. does not help.

1) list.h is the header file for function
2) list.c has all the functions and the structure itself
3) lab4.c has the main() function including PrintForwards() function

Thank you very much for your help in advance.

Yev