Free-ing a doubly linked list - hangs

Question

I am trying to free a doubly linked list. This is the function I am using.

static void clean_list(particles * plist)
{

  particles  *n_list = plist;
  particles *temp_nlist;

  while(n_list){
    temp_nlist = n_list->next;

    free(n_list);
    n_list = temp_nlist;

  }
 }

When I try to call this function in my program, the program hangs without returning from this function. Few things to note: plist is a doubly linked list with a prev, a next, a pointer to a struct which in turn has int and double as data and a linked list itself as member data. Do you think that since plist has pointers to other data, it hangs? In that case I have even tying freeing the pointer, and running the same clean_list on the linked list which a member of plist. I tried searching around for a solution and I didn't find any. Please help.

This is probably caused by the creation of the list (most likely next never becomes NULL). — cnicutar
There is no inherent reason that should cause a problem. There is probably a problem with the list itself. Maybe it is circular. You should print out some information about the nodes you are freeing to get a better understanding of what is going on. — Vaughn Cato
If you want any help solving this you will have to show the code that creates the list. — David Heffernan

ObscureRobot ObscureRobot · Accepted Answer · 2011-10-17T00:19:57

I agree with the other commenters that this question needs more information about the initial conditions of your problem before it can be properly answered. However, a scenario that causes your code to fail comes to mind immediately.

Suppose that plist is a circularly linked list with three elements: A <-> B <-> C <-> A, and plist points to A.

When your code runs, it will: deallocate A, advance to B, deallocate B, advance to C, deallocate C and then advance to the freed memory that was A. Now your code blows up. (or runs forever)

Since it is a doubly linked list, you should use your previous links to null out your next links before deallocating. And for good measure, null out your prev links as well.

temp_nlist = n_list->next;
temp_nlist->prev = NULL;
if (n_list->prev != NULL) n_list->prev->next = NULL;

free(n_list);
n_list = temp_nlist;

Free-ing a doubly linked list - hangs

2 Answers