c - 我的程序给出了 Segmentation Fault 错误

Question

我正在用 Dev C++ 编写一个单链表的 C 程序其代码如下:

#include<stdio.h>
typedef struct node
{
     int data;
     struct node *next;
}node;
node *insert(node*,int,int);
node *create(int);
node *del(node*,int);
int print(node*);
int main()
{
    node *head;
    int n,ch,n1,x,key,k;
    head = NULL;
    printf("\n Number of nodes:");
    scanf("%d",&n);
    while(1)
    {
    printf("\nYour choices are:");
    printf("\n1) Create");
    printf("\n2) Print");
    printf("\n3) Insert");
    printf("\n4) Delete");
    printf("\n5) Reverse\n");        
    printf("Enter your choice:");
    scanf("%d",&ch);
    switch(ch)
    {
     case 1:        
     head = create(n);
     break;

     case 2:
     n1 = print(head);
     break;

     case 3:
     printf("Enter the element to be inserted:");
     scanf("%d",&x);
     printf("Enter the position where it is to be inserted:");
     scanf("%d",&key);    
     head = insert(head,x,key);
     break;

     case 4:
     printf("Enter the element to be deleted:");
     scanf("%d",&k);
     head = del(head,k);
     break;

     case 5:
     break;

     default:
     exit(0);
     break;
    }   
   }  
   return(0);
}
node *create(int n)
{
     node *head,*p;
     int i;
     printf("Enter %d data fields",n);
     head = (node*)malloc(sizeof(node));
     head->next = NULL;
     scanf("%d",&(head->data));
     p=head;

     for(i=1;i<n;i++)
     {
          p->next = (node*)malloc(sizeof(node));
          p=p->next;
          scanf("%d",&(p->data));
          p->next=NULL;
     }
     printf("Linked list created.");
     return(head);
}
int print(node *p)
{

     while(p!=NULL)
     {
          printf("%d-->",p->data);
          p=p->next;
     }
     printf("NULL");
     return(0);
}
node *insert(node *head,int x,int key)
{
     node *p,*q;
     p = (node*)malloc(sizeof(node));
     p->data = x;
     if(key==-1)
     {
         p->next = head;
         head=p;
     }
     else
     {
         q = head;
         while(key != q ->data && q!=NULL)
            q=q->next;
         if(q!=NULL)
         {
            p->next = q->next;
            q->next = p;
         }
     }
     printf("Element Inserted.");
     return(head);    
}    
node *del(node *head,int x)
{
     node *p,*q;
     if(x == head->data)
     {
          p = head;
          head = head->next;
          free(p);
     }
     else
     {
         while(x != (p->next)->data && p->next != NULL)
            p=p->next;
         if(p->next != NULL)
         {
            q = p->next;
            p->next = (p->next)->next;
            free(q);
         }
     }
     return(head);  
}    

一切顺利，但是当我尝试删除一个节点时，控制台崩溃了，当我调试它时它显示“访问冲突错误:段错误”我的程序出了什么问题。

Answer 1

指针 p 在 del(..) 的 else block 之前没有被初始化。

当 p 是空指针时，您不能访问 p->next。