c - 打印双向链表时发生访问冲突

Question

我需要一些帮助来弄清楚为什么我会遇到此访问冲突。这是家庭作业，我已经全部写好了，但在打印列表时最终遇到了访问冲突。我正在尝试向前和向后打印列表。我怀疑问题出在反向函数上。这是代码，感谢您的帮助。

List.h

typedef int Titem;

// Interface of list
typedef struct node *Tpointer;
typedef struct node 
{
    Titem item;
    Tpointer next, previous;
} Tnode;

typedef struct
{
Tpointer first;
Tpointer last;
}Tdbl;


void initialize_dbl (Tdbl *list);
void insert_to_dbl_front (Tdbl *list, Titem data);
void insert_to_dbl_back (Tdbl *list, Titem data);
void print_dbl (Tdbl const list);
void print_dbl_reverse (Tdbl const list);

List.c

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

#define TYPE_INT 0
#define TYPE_FLOAT 1



// Implementation of list (only obj is need in appl)
void initialize_dbl (Tdbl *list) 
{
   list->first = NULL;
   list->last = NULL;
}


void insert_to_dbl_front (Tdbl *list, Titem data)
{
    Tpointer newnode;

    if(list->first == NULL)
    {
       newnode = (Tpointer) malloc(sizeof(Tnode));
       newnode->item = data;
       newnode->next = NULL;
       newnode->previous = NULL;
       list->first = newnode;
    }
    else
    {
        newnode = (Tpointer) malloc(sizeof(Tnode));
        newnode->item = data;
        newnode->next = list->first;
        newnode->previous = NULL;
        list->first = newnode;

    }
}

void insert_to_dbl_back (Tdbl *list, Titem data)
{
    Tpointer newnode;

    newnode = (Tpointer) malloc(sizeof(Tnode));
    newnode -> item = data;
    if (list->first == NULL)
        list->first = newnode;       //first node
    else
        list->last->next = newnode;  //not first node
    list->last = newnode;
    list->last->next = NULL;
}

void print_dbl (Tdbl const list) 
{
    Tpointer what;

    printf("\nList forward:");
    what = list.first;
    while (what != NULL) {
        printf("%d ", what->item);
        what = what->next;
    }

}


void print_dbl_reverse (Tdbl const list)
{
    Tpointer last = list.last;
    Tpointer temp = NULL;

    printf("\nList reversed: ");
    if(last == NULL)
    {
        printf("");
    }
    else
    {   
        while(last != NULL)
        {
            temp = last->next;  
            last->next = last->previous;
            last->previous = temp;
            last = temp;

        }

        printf("\nList reverse:");

        while (last != NULL) 
        {
            printf("%d ", last->item);
            last = last->next;
        }
    }
}

main.c

#include "list.h"
#include <stdio.h>


int main(void) {
Tdbl dbl;
initialize_dbl(&dbl);
print_dbl(dbl);
print_dbl_reverse(dbl);
insert_to_dbl_back(&dbl, 10);
print_dbl(dbl);
print_dbl_reverse(dbl);
insert_to_dbl_front(&dbl, 20);
print_dbl(dbl);
print_dbl_reverse(dbl);
insert_to_dbl_back(&dbl, 30);
print_dbl(dbl);
print_dbl_reverse(dbl);
insert_to_dbl_front(&dbl, 40);
print_dbl(dbl);
print_dbl_reverse(dbl);
insert_to_dbl_back(&dbl, 50);
print_dbl(dbl);
print_dbl_reverse(dbl);


fflush(stdin); getchar();
}

我查看了大约 10 个不同的链接列表示例并搜索了论坛 来回答我的问题。我尝试过的反转列表的每个示例似乎都没有执行任何操作或最终出现此访问冲突错误。哦，是的，主文件或头文件中的任何内容都不能更改。

Answer 1

在

void insert_to_dbl_front (Tdbl *list, Titem data)
{
    Tpointer newnode;

    if(list->first == NULL)
    {
       newnode = (Tpointer) malloc(sizeof(Tnode));
       newnode->item = data;
       newnode->next = NULL;
       newnode->previous = NULL;
       list->first = newnode;
    }

您没有设置list->last，因此仍然设置为NULL。但是，如果 list->first 不是 NULL，

else
{
    newnode = (Tpointer) malloc(sizeof(Tnode));
    newnode->item = data;
    newnode->next = list->first;
    newnode->previous = NULL;
    list->first = newnode;

}

您从未将当前第一个节点的previous指针设置为新节点，因此您实际上并没有双向链表。

当你插入后面时，

void insert_to_dbl_back (Tdbl *list, Titem data)
{
    Tpointer newnode;

    newnode = (Tpointer) malloc(sizeof(Tnode));
    newnode -> item = data;
    if (list->first == NULL)
        list->first = newnode;       //first node
    else
        list->last->next = newnode;  //not first node
    list->last = newnode;
    list->last->next = NULL;
}

您从未设置新节点的previous指针。所以你仍然只有一个单链表。仅此一点不会导致访问冲突，但在这里您从未将newnode->previous设置为任何内容，因此它包含该内存位置处发生的任何位。

然后在print_dbl_reverse中交换一些previous和next指针并得到

while (last != NULL) 
{
    printf("%d ", last->item);
    last = last->next;
}

在某个时刻将 last 设置为未初始化的非 NULL 指针，这会导致访问冲突。