使用链表执行队列时出现“分段故障（核心已转储）”

Question

I wrote a program for implementation of queue using linked list..

#include<stdio.h>
#include<stdlib.h>
struct node
{
 int data;
 struct node *next;
};
struct queue
 {
  struct node *front;
  struct node *rear;
 };
struct queue *q;
void create_queue(struct queue *);
struct queue * insert(struct queue *,int);
struct queue * delete(struct queue *);
struct queue * display(struct queue *);
int peek(struct queue *);
int main()
{        
 printf("a");
 int value,option,t=0;
 create_queue(q);
 while(t==0)
 {
    printf("\n1.insert\n2.delete\n3.peek\n4.display\n");
    scanf("%d",&option);
    switch(option)
    {
        case 1:

                printf("enter the number to be inserted");
                scanf("%d",&value);
                q=insert(q,value);
                break;
        case 2:
                q=delete(q);
                break;
        case 3:
                value=peek(q);
                printf("the value pointed by front is %d",value);
                break;
        case 4:
                q=display(q);
                break;
        default:
                printf("invalid option");
    }
    printf("\n '0' to run again else '1' \n");
    scanf("%d",&t);
 }
 return 0;
}
void create_queue(struct queue *q)
 {
    q->rear=NULL;
    q->front=NULL;
 }
struct queue * insert(struct queue *q,int value)
 {
    struct node *ptr;
    ptr=(struct node *)malloc(sizeof(struct node *));
    ptr->data=value;
    if(q->front==NULL)
    {
        q->front=ptr;
        q->rear=ptr;
        q->front->next=q->rear->next=NULL;
    }
    else
    { 
        q->rear->next=ptr;
        q->rear=ptr;
        q->rear->next=NULL;
    }
   return q;
 }
struct queue * delete(struct queue *q)
 {
   struct node *ptr;
   ptr=q->front;
   if(q->front==NULL)
   printf("\n underflow");
   else
   {
    q->front=q->front->next;
    printf("\n the value being deleted is %d",ptr->data);
    free(ptr);
   }
   return q;
  }
 struct queue * display(struct queue *q)
 {
   struct node *ptr;
   ptr=q->front;
   if(ptr==NULL)
   printf("\n queue is empty");
   else
   {
    printf("\n");
    while(ptr!=q->rear)
    {
        printf("%d \t",ptr->data);
        ptr=ptr->next;
    }
    printf("%d \t",ptr->data);
  }
  return q;
 }
int peek(struct queue *q)
 {
  return (q->front->data);
 }

While execution: The terminal shows "Segmentation fault (core dumped)" and the execution of program stops. why this has happened? what modifications must be done in the code for avoiding this?

Answer 1

In addition to the problem pointed out by @FredK in his answer , you are not creating the struct queue properly.

Since you have defined q to be in global scope, it is initialized to NULL . And then you use it as an argument in create_queue before its value has been set to a valid pointer. In create_queue , you access the pointer as if it points to a valid object. Accessing members of a NULL pointer cause undefined behavior. In your case, that manifests as segmentation fault.

Change create_queue to:

struct queue * create_queue()
{
   struct queue *q = malloc(sizeof(*q));
   q->rear=NULL;
   q->front=NULL;
   return q;
}

Remove the global variable q and replace it with a local variable in main .

int main()
{        
   struct queue *q;
   printf("a");
   int value,option,t=0;
   q = create_queue();

   ...

}

Answer 2

ptr=(struct node *)malloc(sizeof(struct node *)); 

This is incorrect. You have allocated space for a pointer to a node rather than for a node. To avoid such confusion, always use

 ptr = malloc( sizeof(*ptr) );

Don't cast the result of malloc - it will hide the error that you will encounter if you forget to

#include <stdlib.h>