Linked Implementation of Queues in C

Question

I have written a function Serve to get the front node in the queue and remove it. It is as follows:

void Serve(QueueNode **p,Queue *q)
{
    if(QueueEmpty(q))
        Error("Queue is empty!");
    else
    {
        if(q->front==q->rear)
        {
            *p=q->front;
            q->front=q->rear=NULL;
        }
        else{
            *p=(q->front);
            q->front=q->front->next;
            (*p)->next=NULL;
        }
    }
}

I call it from the main function as follows:

int main() {
    QueueNode **p=malloc(sizeof(QueueNode));
    Queue *q=CreateQueue();
    Append('a',q);
    Append('b',q);
    Append('c',q);
    Append('d',q);
    Serve(p,q);
}

While declaring the pointer to pointer QueueNode, if I set it to NULL or leave it uninitialized the program gave me NULL pointer dereference or accessing wrong memory location. I can understand why. But, here, when I create the QueueNode using malloc in this line:

QueueNode **p=malloc(sizeof(QueueNode));

the space allocated is getting wasted when I reassign it to q->front in the Serve function. How can I save this space. Also, I just need a pointer that points to q->front. Thanks !

Please create a minimal reproducible example. The malloc() probably isn't needed at all, but no way to be sure without having a self-contained example. — user2371524
– user2371524, Commented Aug 28, 2017 at 16:19
@BLUEPIXY That leaks a QueueNode. There is no need to malloc a node. It should be QueueNode* p = NULL. — Banex
– Banex, Commented Aug 28, 2017 at 16:19
QueueNode **p=malloc(sizeof(QueueNode)); this is wrong, no matter what. It should be sizeof(QueueNode *) or better just sizeof *p. But then, as I already said, I don't see why you're allocating memory here at all. — user2371524
– user2371524, Commented Aug 28, 2017 at 16:26

vadim_hr · Accepted Answer · 2017-08-29 07:38:51Z

1

you don't need allocate memory for p. it's already allocated in function Append().

int main() {
    QueueNode *p=NULL; 
    Queue *q=CreateQueue(); 
    Append('a',q); 
    Append('b',q); 
    Append('c',q); 
    Append('d',q); 
    Serve(&p,q); 
}

edited Aug 29, 2017 at 7:38

answered Aug 28, 2017 at 18:05

vadim_hr

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