Implementation of Queues in Linked Lists - c

Question

I am given these structure declarations in order to implement a queue collection that uses a circular linked list.
typedef struct intnode {
int value;
struct intnode *next;
} intnode_t;
typedef struct {
intnode_t *rear; // Points to the node at the tail of the
// queue's linked list
int size; // The # of nodes in the queue's linked list
} intqueue_t;
intnode_t *intnode_construct(int value, intnode_t *next)
{
intnode_t *p = malloc(sizeof(intnode_t));
assert (p != NULL);
p->value = value;
p->next = next;
return p;
}
/* Return a pointer to a new, empty queue.
* Terminate (via assert) if memory for the queue cannot be allocated.
*/
intqueue_t *intqueue_construct(void)
{
intqueue_t *queue = malloc(sizeof(intqueue_t));
assert(queue != NULL);
queue->rear = NULL;
queue->size = 0;
return queue;
}
I'm trying to create a function that will enqueue at a specified value (append it to the rear of the queue), and I need to consider the two cases in which the queue is empty and when the queue has one or more elements. This is the code I have so far:
void intqueue_enqueue(intqueue_t *queue, int value)
{
intnode_t *p = intnode_construct(value, NULL);
if(queue->rear->next == NULL) {
//the queue is empty
queue->rear->next =p;
} else {
//the queue is not empty
queue->rear=p;
}
queue->rear=p;
queue->size++;
}
This code gives me a runtime error so I'm not sure whats wrong. In the code, I'm assuming queue->rear->next is the front, however I think this is where the problem might be. All help is greatly appreciated. Thanks!

Answer 1

Your problem occurs on this line:
if(queue->rear->next == NULL) {
The first time you call the function, queue->rear is NULL. Thus when you try to dereference it to get queue->rear->next you get the runtime error.
To fix this code, update intqueue_enqueue to just check if queue->size==0, and if so then you need to initialize it by setting queue->rear=p and p->next=p. Then update the else clause so that it inserts the element between the two existing elements. Hint: you'll need to store queue->rear->next in p.
Edit
To address your comment, here's how to graphically think about a list with three elements:
<element1: next==element2> <element2: next==element3> <element3: next==element1>
And queue->rear points to element3. So, to insert a fourth element, you need to make it so that queue->rear points to element4 and element4->rear needs to point to element1. Remember that the location of element is stored in rear->next.