C++ program crashes while checking if a pointer is NULL using if condition

Question

I was trying a C program on queue as a linked list. Whenever I try to execute, it crashes whenever it encounters if condition comparing a pointer (q->front in this case) with NULL. Please check the following code:

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

struct que
{
    struct node *front
    struct node *rear;
    
    que() { front = rear = NULL; }
};
struct que *pq;

/* prototypes */
void Displace(struct que *q);
int Empty(struct que *q);
void Insert(struct que *q,int x);
void Delete(struct que *q);

int main()
{
    int cho;
    while(1)
    {
        printf("Enter 1 to insert in a queue\n");
        printf("Enter 2 to delete in a queue\n");
        printf("Enter 3 to display the queue\n");
        scanf("%d", &cho);

        if (cho == 1)
        {
            int x;
            printf("Enter the info to be added\n");
            scanf("%d",&x);
            Insert(pq, x);
        }
        else if (cho == 2)
            Delete(pq);
        else if (cho == 3)
            Displace(pq);
    }
    return 0;
}

int Empty(struct que *q) 
{
    return ((q-> front == NULL) ? 1 : 0); //Error
}
void Insert(struct que *q, int a)
{
    node *p;
    p = new node;
    p->info = a;
    p->next = NULL;
    
    if ((q->r) == NULL)   //Error. I get crash and this statement is never executed.
        (q->f) = p;
    else (q->r)->next = p;
    (q->r) = p;
    
    printf("Node added\n");
}
void Delete(struct que *q)
{
    node *p = NULL;
    if (Empty(q))
    {
        printf("Empty queue.Insert some elements\n");
        return;
    }
    p = q->front;
    q->front = p->next;
    delete p;
    printf("Node deleted\n");
}
void Displace(struct que *q)
{
    if (Empty(q))
    {
        printf("Empty queue.Insert some elements\n");
        return;
    }
    node *i = NULL;
    for (i = q->front; i != NULL; i = i->next)
        printf("%d\n", i->info);
}

I suspect there's something wrong with the statement if ((q->rear) == NULL). Executing the program results in a crash " has stopped working". I have also tried replacing it with if (!q->rear) , but without much success. I am unable to find a problem in my code.Please help me..Thanks

Can you maybe use real variable names? There's no need to have one letter variable names and 3 letter function names — AustinWBryan
– AustinWBryan, Commented Feb 20, 2023 at 17:36

AustinWBryan · Accepted Answer · 2023-02-20 17:42:27Z

4

You never initialize pq, so q->rear in the following has undefined behaviour:

if ((q->rear) == NULL)   // Error. I get crash and this statement is never executed.

One way to fix this is by turning

struct que *pq;

into

struct que pq;

and then passing &pq into Insert() et al.

edited Feb 20, 2023 at 17:42

AustinWBryan

3,3373 gold badges28 silver badges44 bronze badges

answered Jan 11, 2015 at 17:13

NPE

503k114 gold badges970 silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Vaibhav Jain Over a year ago

Thank you for answer.But what should i initialize pq with it. Sorry, if u find this question stupid. I'm a beginner :).

Vaibhav Jain Over a year ago

The updated answer helped me..Thanks. But just to clarify concept, why can't I use the former (struct que *pq;)?

NPE Over a year ago

@VaibhavJain: You could, but you'd need to allocate memory for it, which is more complicated than the solution I proposed in the answer.

Collectives™ on Stack Overflow

C++ program crashes while checking if a pointer is NULL using if condition

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related