c - C中的排序链表

Question

根据一些论坛和教程，我编写了一个排序链表的代码。该代码导致核心转储。我相信错误发生在第 50 行之后，我试图在末尾插入节点。你能帮我解决这个问题吗？我不确定我做错了什么。谢谢。

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

struct node
{
    char val;
    struct node* next;
};

struct node *start = (struct node*) NULL;

// definition of creating only first node
struct node* create(char* data)
{
    struct node* temp;
    temp = (struct node*)malloc(sizeof(struct node));
        if (start == NULL)
        {
            temp->val = data;
            temp->next = NULL;
            start = temp;
        }
    printf("List created\n");
}

struct node* insert(char* data)
{
    int i, pos;
    struct node* tempnode, *ptr;
    ptr = start;

    while(ptr != NULL)
    {
        if (data <= ptr->val)
        {
            printf("!!!Inserting before!!!\n");
            tempnode = (struct node*)malloc(sizeof(struct node));
            tempnode->val = ptr->val;
            tempnode->next=ptr->next;
            ptr->val = data;
            ptr->next=tempnode;
            break;
        }
        else if (data > ptr->val)
        {
            printf("!!!Going next!!!\n");
            ptr = ptr->next;
        }
        if (ptr == NULL) //insert behind the node
        {
            tempnode = (struct node*)malloc(sizeof(struct node));
            tempnode->val = data;
            tempnode->next = NULL;
            ptr->next = tempnode;
            printf("!!!Placed at the end!!!\n");
            break;
        }
    }
}

void display()
{
    struct node* ptr; // ptr is pointer
    ptr = start;
    while (ptr != NULL)
    {
        printf("%s->", ptr->val);
        ptr = ptr->next;
    }
    printf("End of list\n");
}

int main()
{
    char* data;
    int i = 0;

    create(0);
    FILE* file = fopen("words.txt", "r");

    while (i < 10)
    {
        fscanf(file, "%s", &data);
        printf ("data is %s\n", &data);
        insert(data);
        i++;
    }
    display();

}

Answer 1

在这里，在您的insert 函数的底部，您已保证ptr 为NULL，但您还执行了ptr->next = ... 。这等同于 (*ptr).next = ...，它取消引用 NULL 指针!

if (ptr == NULL) //insert behind the node
    {
        tempnode = (struct node*)malloc(sizeof(struct node));
        tempnode->val = data;
        tempnode->next = NULL;
        ptr->next = tempnode;
        printf("!!!Placed at the end!!!\n");
        break;
    }