雙向鏈表釋放內存valgrind錯誤

Question

嗨，我正在嘗試釋放分配給雙鏈表的內存，但是當我用valgrind檢查它時，free_all函數出現了一些錯誤（我認為），但是我不知道如何避免它。

我認為在free_all函數中，我使用了temp和node指針錯誤，或者我需要先分配它，然后再使用它們，但是當我嘗試這種方法時，valgrind仍然給了我一些錯誤。

#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);
}

Answer 1

您的free_all至少有兩個錯誤：while條件引用head-> prev，但是在第一次迭代中您釋放了head（在free之后使用）。 退出循環后，盡管在第一次迭代中釋放了頭部，您仍然可以釋放頭部。 free_all（）適用於單個元素的情況。

Answer 2

修改后，valgrind中沒有錯誤或內存泄漏

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);
}

Answer 3

這篇文章是由mevets撰寫的，是對我的解決方案的編輯，但我認為最好也將其包含在線程中：

mevets：

我傾向於這樣的事情：

void Unlink(ITEM **head, ITEM *t) {
        if (t->next == t) {
                /* remove head */
                *head = NULL;
        } else {
                t->prev->next = t->next;
                t->next->prev = t->prev;
                if (t == *head) {
                        *head = t->next;
                }
        }
}

/*
   remove and return the element after head
*/
ITEM *Pop(ITEM **head) {
        ITEM *node;
        if ((node = *head) != NULL) {
                node = node->next;
                Unlink(head, node);
        }
        return node;
}


void free_all (ITEM *head) {
        ITEM *node;
        while ((node = Pop(&head)) != NULL) {
            free(node);
        }
}

這將列表維護（取消鏈接）與訂購（Pop）和內存管理（free_all）分開。 這給您留下了更多的邊界，您可以在其中對列表進行斷言，例如在取消鏈接之前和之后，列表應該是有效的並且可以檢查。 此外，如果可以同時訪問列表，則可以將Pop（）放在方括號內，以最大程度地減少沖突。

緩存分配器本身就是一件事，但是合同的一個典型部分是您釋放處於已知狀態的節點，這樣當重新分配節點時，它可以跳過它們的構造。