Queue using linked list in c

Question

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;
}

Where is the code "stopping"? Is there a segmentation fault? Is it just not running? Is there an error being displayed? — pstrjds
– pstrjds, Commented Oct 8, 2014 at 16:08

Jonathan Leffler · Accepted Answer · 2017-09-14 04:50:22Z

5

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.

edited Sep 14, 2017 at 4:50

Jonathan Leffler

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answered Oct 8, 2014 at 16:16

honk

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Comments

RockOnRockOut · Accepted Answer · 2014-10-08 16:31:25Z

1

You're passing pointer q of type struct queue * to the function. But you have not allocated memory for that pointer.

So you need to allocate memory to the pointer q and then pass to your functions. You need to allocate memory like this

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

then pass q to your functions.

edited Oct 8, 2014 at 16:31

RockOnRockOut

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answered Oct 8, 2014 at 16:20

rpk

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