双链表释放内存valgrind错误

Hi I'm trying to free the memory that I was allocated in the doubly linked list but when I check it with valgrind I have some error in the free_all function ( I think ) but I don't know how to avoid it.

嗨我正在尝试释放我在双链表中分配的内存，但是当我用valgrind检查它时，我在free_all函数中有一些错误（我想），但我不知道如何避免它。

I think in the free_all function I'm using temp and node pointer wrong or I need to allocate it first and then use them, but when I've tried this method valgrind still gave me some error.

我认为在free_all函数中我使用temp和节点指针错误或者我需要先分配它然后再使用它们，但是当我尝试这种方法时，valgrind仍然给了我一些错误。

#include <stdio.h>
#include <stdlib.h>

/*
  to compile it:
  gcc -g -Wall -ggdb3  double_linkedlist2.c -o double_linkedlist
  to check for memory leak and error:
  valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --verbose --log-file=valgrind-out.txt ./double_linkedlist
*/
typedef struct listitem
{
  struct listitem *next;    // pointer to next item
  struct listitem *prev;    // pointer to previous item
  int data;                 // some data
} ITEM;


int main (void)
{
  // prototype functions
  void free_all (ITEM *lst_ptr);


  // Variables
  ITEM *p_temp, *head;

  head = malloc (sizeof (ITEM));  // head will keep first and last element in its pointers
  head -> next = head;            // the last element in the list (at first head -> next and head -> prev will point to the head)
  head -> prev = head;            // the first element in the list


     for (int i = 0; i < 3; i++)
       {
         p_temp = malloc (sizeof (ITEM));     // allocate some memory for the new list item
         p_temp -> data = i;                  // set the list item's data to the loop count so that we can see where it is in the list
         p_temp -> next = head -> next;          // this will insert at the FRONT of the list
         head -> next = p_temp;                  // and set the list head to the newly created list item
         p_temp -> prev = head;              // this will insert at the BACK of the list
         p_temp -> next -> prev = p_temp;       // and set the list 'tail' to the newly created item
       }

     // now let's see what we got going forward
     printf ("Going forward\n");
     p_temp = head -> next;

     while (p_temp != head)
       {
         printf ("forward list item: current is %p; next is %p; prev is %p; data is %d\n", p_temp, p_temp -> next, p_temp -> prev, p_temp -> data);
         p_temp = p_temp -> next;
       }

     // now let's see what we got going backward
     printf ("Going backwards\n");
     p_temp = head -> prev;

     while (p_temp != head)
       {
         printf ("backward list item; current is %p; next is %p; prev is %p; data is %d\n", p_temp, p_temp -> next, p_temp -> prev, p_temp -> data);
         p_temp = p_temp -> prev;
       }

     printf ("\n");
     free_all (head);

     return 0;
}

void free_all (ITEM *head)
{
  ITEM *temp, *node;

  node = head;

  while (node != head -> prev)
    {
      temp = node;
      printf ("freed list item: current is %p; next is %p; prev is %p; data is %d\n", temp, temp -> next, temp -> prev, temp -> data);
      node = node -> next;
      free (temp);
    }
  free (node);
  free (head);
}

2 个解决方案

#1

your free_all has at least two errors: the while condition references head->prev, but in the first iteration you free head (use after free). after you exit the loop, you free head despite having free'd it in the first iteration. free_all() does work for the single element case.

你的free_all至少有两个错误：while条件引用head-> prev，但是在第一次迭代中你释放头（免费后使用）。退出循环后，尽管在第一次迭代中已将其释放，但您仍然可以释放头部。 free_all（）确实适用于单个元素的情况。

#2

There was no error or memory leak in valgrind after this modification

修改后，valgrind中没有错误或内存泄漏

void free_all (ITEM *head)
{
  ITEM *temp, *node = NULL;

  node = head -> next;

  while (node != head -> prev)
    {
      temp = node;
      printf ("freed list item: current is %p; next is %p; prev is %p; data is %d\n", node, node -> next, node -> prev, node -> data);
      node = node -> next;
      free (temp);
    }

  node = head -> prev;
  printf ("freed list item: current is %p; next is %p; prev is %p; data is %d\n", node, node -> next, node -> prev, node -> data);
  free (head);
  free (node);
}