Deleting Linked LIst in C

Question

I just stuck with the problem couple of hours, trying to find where my code breaks. I know how to delete linked list but something doesn't work.

First it is a very simple struct with a dataype of int and 2 struct *next and *prev.

struct _list_{ 
    struct _list_ *next;
    struct _list_ *prev;
    float distance;
}

Now i am making a push_front function and it works great. I get the result that i am looking for. But now i am making pop_front function and something is missing.

The function should return the distance and then remove that list from the linked list but i can't make it do it.

here is the code that i wrote

int pop_front(list** header)
{
  float number = (*header)->data;
  list *head = *header;
  list *remove = head;

  // This should check if the pointer is pointing at the first element
  while (head->prev != NULL) {
    head = head->prev;
  }

  if (head) {
    head = head->next;
    free(remove);
    remove = head;
    remove->prev = NULL;
    //if i remove the code below then i get this error 
    //*** Error in `./double_ended_queue.out': double free or 
    //corruption (fasttop): 0x0000000001d5a050 ***
    //Pop up: 3 pointer: 3 Aborted (core dumped)
    *header = *remove;
    //And with this code i get a Segmentation fault (core dumped
    return number;
  }
  return 0;
}

Any help would be great, thank you.

P.S. checked all the linked list question here and none helped.

Either this is C and then you can use a variable named delete (although I would not recommend it), or it is not and then you should not. Either way, using both language tags is wrong. — Jongware
this looks wonky. *remove = head; free(remove); remove=head. aren't you freeing what head points at, and then immediately using head again? — Marc B

Bodo Thiesen Bodo Thiesen · Accepted Answer · 2016-10-11T21:27:53

Where are you all getting this homework from? The API sux. Here, someone else did have almost the same homework (link pointing to my answer, which has many issues left): Pointer Dequeue - pointer training

Anyways:

Do you want to return int or float? The element data is type float, your variable "number" too, but your function returns int.

int pop_front(list** header)
{
  float number = (*header)->data;

so, here you get the value of the element you're trying to remove, but then ...

  list *head = *header;
  list *remove = head;

  // This should check if the pointer is pointing at the first element
  while (head->prev != NULL) {
    head = head->prev;
  }

... you actually SEARCH for the element to remove.

Obviously, you have to do it the other way round:

int pop_front(list** header)
{
    list * head = *header;
    while (head->prev) head = head->prev;

now, you should check, weather you need to adjust the *header pointer (and do it right away):

    if (*header == head) {
        *header = head->next;
    }

the only thing to do now is to remove the object from the list, get it's value and free it's memory before return.

    head->next->prev = NULL;
    float retval = head->data;
    free(head);
    return retval;
}

As exercise left to you: Make sure, that an empty list doesn't crash ;)

/edit: This will also crash for removal of the last element, so you have two exercises left ;)

Deleting Linked LIst in C

1 Answers