c - 循环链表在列表开头添加节点时出现段错误 11

Question

最初创建程序时，我可以创建仅添加第一个初始节点的循环链表。

一旦我尝试在开头添加新节点，它就会给我一个 Segmentation Fault 11。

我什至尝试在 Stack Overflow 中搜索这个问题，但他们中的许多人只是取消引用了指针变量。

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

struct node
{
    int data;
    struct node *next;
};

struct node *start=NULL;
struct node *create_cll(struct node *);
struct node *display(struct node *);
struct node *add_beg(struct node *);
struct node *add_end(struct node *);
struct node *del_beg(struct node *);
struct node *del_end(struct node *);
struct node *del_aftgiven(struct node *);
struct node *del_entire(struct node *);

int main()
{
    int option;
    do
    {
        printf("\n\n****MAIN MENU****");
        printf("\n\nEnter an option from the list given below");
        printf("\n1. Create a linked list");
        printf("\n2. Display the linked list");
        printf("\n3. Add a node at the beginning of the list");
        printf("\n4. Add a node at the end of the list");
        printf("\n5. Delete a node from the beginning of the list");
        printf("\n6. Delete a node from the ending of the list");
        printf("\n7. Delete a node after a given node");
        printf("\n8. Delete the entire linked list");
        printf("\n");
        scanf("%d",&option);

        switch(option)
        {
            case 1: start = create_cll(start);
                    printf("\nCircular linked list created!");
                    break;

            case 2: start = display(start);
                    break;

            case 3: start = add_beg(start);
                    break;

        }
    }while(option!=4);
    return 0;
}

struct node *create_cll(struct node *start)
{
    struct node *new_node,*ptr;
    int num;
    printf("\nEnter -1 to stop adding new nodes");
    printf("\nEnter the data:");
    scanf("%d",&num);
    if(num!=-1)
    {
        new_node=(struct node *)malloc(sizeof(struct node));
        new_node->data=num;
        if(start!=NULL)
        {
            while(ptr->next!=start)
            {
                ptr=ptr->next;
            }
            ptr->next=new_node;
            new_node->next=start;
        }
        else
        {
            new_node->next=new_node;
            start=new_node;
        }
    }
    return start;
}

struct node *display(struct node *start)
{
    struct node *ptr;
    ptr=start;
    while(ptr->next!=start)
    {
        printf("\t %d",ptr->data);
        ptr=ptr->next;
    }
    printf("\t %d",ptr->data);
    return start;
}

struct node *add_beg(struct node *start)
{
    struct node *ptr,*new_node;
    int num;
    printf("\nEnter the data you want to enter at the beginning:");
    scanf("%d",&num);
    new_node=(struct node *)malloc(sizeof(struct node));
    new_node->data=num;
    if(ptr->next!=start)
    {
        ptr=ptr->next;
    }
    ptr->next=new_node;
    new_node->next=start;
    start=new_node;
    printf("Node succesfully added:");
    return start;
}

Answer 1

ptr 在

while(ptr->next!=start)

未赋值，使用->会报错。尝试将 ptr = start 放在它之前的行中。这也将发生在 add_beg() 中。