gmeh94 gmeh94 - 2 months ago 20
C Question

Queue using linked list in c

During compilation this code gives no error, but the code stops abruptly. According to me the problem is with the

createq
function where
q->front=q->rear=NULL
is declared. It does have to be initialised. Is anything wrong with that?

#include<stdio.h>
#include<malloc.h>
#include<stdlib.h>
struct node
{
struct node *next;
int data;
};

struct queue
{
struct node *front;
struct node *rear;
};

struct queue *q;

void createq(struct queue *);
struct queue *insert(struct queue *);
struct queue *delete_q(struct queue *);
struct queue *display(struct queue *);

int main()
{
int option;
printf("\tMAIN MENU\n");
printf("\n1. Create\n2. Display\n3. Insert\n4. Delete\n5. Exit\n");
while(option!=5)
{
printf("\nEnter a choice:");
scanf("%d",&option);
switch(option)
{
case 1:
createq(q);
break;

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

case 3:
q=insert(q);
break;

case 4:
q=delete_q(q);
break;
}
}
return 0;
}

void createq(struct queue *q)
{
q->rear=NULL;
q->front=NULL;
printf("q intialized");
}

struct queue *insert(struct queue *q)
{
struct node *newnode;
int val;
newnode=(struct node *)malloc(sizeof(struct node));
printf("Enter the value to be inserted:");
scanf("%d",&val);
newnode->data=val;
if(q->front==NULL)
{
q->front=newnode;
q->rear=newnode;
q->front->next=q->rear->next=NULL;
}
else
{
q->rear->next=newnode;
q->rear=newnode;
q->rear->next=NULL;
}
return q;
}

struct queue *delete_q(struct queue *q)
{
struct node *ptr;
if(q->front==NULL)
{
printf("Queue Empty\n");
}
else
{
ptr=q->front;
q->front=q->front->next;
printf("Element being deleted is %d\n",ptr->data);
free(ptr);
}
return q;
}

struct queue *display(struct queue *q)
{
struct node *ptr;
ptr=q->front;
if(q->front==NULL)
printf("Queue Empty!!\n");
else
{
while(ptr!=q->rear)
{
printf("%d\t",ptr->data);
ptr=ptr->next;
}
printf("%d\t",ptr->data);
printf("\n");
}
return q;
}

Answer

You declare a pointer to a queue structure in the following way:

struct queue *q;

Note that you do not allocate memory for the structure here. Next, in your main() function you call:

createq(q);

Then you access rear and front via q in the createq() function:

q->rear=NULL;
q->front=NULL;

This way you access memory that you did not allocate. You should put something like the following at the beginning of your main() function:

q = (struct queue *)malloc(sizeof(struct queue));

And do not forget to put free(q) at the end of your main() function to prevent a memory leak.