C语言打印双链接列表时违反访问冲突

我需要一些帮助来弄清楚为什么我会收到这种访问违规。这是家庭作业，我都写好了，但最终在打印列表时遇到访问冲突。我正在尝试正向和反向打印列表。我怀疑问题出在反向功能上。这是代码，感谢您的帮助。

列表.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);

列表.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;
        }
    }
}

主.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();
}

我看了大约 10 个不同的链表示例，并搜索了论坛以回答我的问题。我尝试反转列表的每个示例似乎都无济于事，或者最终出现此访问违规错误。哦，是的，主文件或头文件中的任何内容都无法更改。

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;
}

从不设置新节点的previous指针。所以你仍然只有一个单链表。仅此一项不会导致访问冲突，但在这里您从未将newnode->previous设置为任何内容，因此它包含恰好位于该内存位置的任何位。

然后在print_dbl_reverse中交换一些previous和next指针并拥有

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

在某些时候，last设置为未初始化的非NULL指针，这会导致访问冲突。

你的代码中有很多错误。我写了一个简单的，所以你可以研究它，但把写反向功能留给你。玩得愉快！

#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 = (Tpointer) malloc(sizeof(Tnode));;
    newnode->item = data;
    newnode->previous = NULL;
    newnode->next = list->first;
    if(list->first != NULL) 
        list->first->previous = newnode;
    else 
        list->last = newnode;
    list->first = newnode;
}
void insert_to_dbl_back (Tdbl *list, Titem data)
{
    Tpointer newnode = (Tpointer) malloc(sizeof(Tnode));
    newnode->item = data;
    newnode->next = NULL;
    newnode->previous = list->last;
    if (list->first == NULL)
        list->first = newnode;
    else
        list->last->next = newnode;
    list->last = newnode;
}
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 = last;
    printf("nList reverse: ");
    while (last != NULL) 
    {
            last = last->previous;
    }
}

下面是调试此问题的一些建议。

1）修改源代码以打印调试语句。您甚至可以根据 #ifdef DEBUG_STATEMENTS 标志编译调试信息。

2）您可以使用调试器并在程序崩溃之前单步执行此操作和/或设置断点。如果程序不是守护程序/系统服务或驱动程序，调试器将轻松工作。

我实际上围绕实现而不是调试的其他建议，并且与预分配内存和在预分配内存中操作链表有关。但是，由于这比前两个建议更复杂，因此我将停止对该想法的概述。

祝你好运。

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