UMBC CS 201, Fall 06
UMBC CMSC 201
Fall '06

CSEE | 201 | 201 F'06 | lectures | news | help

Dangling Pointers

A dangling pointer is a pointer that points to memory that has already been free()d.

Dangling pointers are extremely dangerous, because you now have a pointer to memory that you have released back to the operating system. For the moment, that memory contains all of the information you had stored at that location. However, if any new space is dynamically allocated, it will likely be at the address you have just given up. Now your original pointer points to some new information that's been stored. Dangling pointers are especially brutal when working with linked lists.

To illustrate, here is a DestroyStack() function that does free all of the nodes that are in the stack, but fails to set top to NULL when the stack is empty. top is now a dangling pointer.

The Buggy Code

void DestroyStack(NODEPTR *topPtr) { NODEPTR prev, curr; prev = NULL; curr = *topPtr; while(curr != NULL) { prev = curr; curr = curr -> next; free(prev); printf("Just freed the node containing %d\n", prev -> data); } } Let's Push() some items onto a stack. We'll print it out to make sure the stack actually contains what we pushed. Then we'll call the buggy DestroyStack() function. top now points to a memory location that has been freed. If we call PrintStack() and pass it top, the original contents of the stack will print, because the garbage those memory locations contain is our old information. If we now create a new node, get a value for its data portion, and Push() it onto the stack, when we call PrintStack(), we find a disaster :

Output

The stack is empty Enter the value of data : 1 Pushing the value, 1 Enter the value of data : 2 Pushing the value, 2 Enter the value of data : 3 Pushing the value, 3 Enter the value of data : 4 Pushing the value, 4 The stack contains : 4 3 2 1 Destroying the Stack Just freed the node containing 4 Just freed the node containing 3 Just freed the node containing 2 Just freed the node containing 1 Finished destroying the Stack The stack contains : 4 3 2 1 Enter the value of data : 5 Pushing the value, 5 The stack contains : 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 (endless loop)


CSEE | 201 | 201 F'06 | lectures | news | help

Tuesday, 22-Aug-2006 07:14:08 EDT