CODEWITHSUNDEEP
c
carl.hiass
carl.hiass

Reputation: 1774

Why does logical-or cause this `-Wint-conversion` warning?

bool EnQueue(Queue *pq, Item item) {
    
    // 1. create a new node
    Node *node;
    if (QueueIsFull(pq))
        return false;
    else {
        node = malloc(sizeof(Node));
        if (node == NULL) {
            fprintf(stderr, "Unable to allocate memory");
            exit(EXIT_FAILURE);
        }
    }
    
    // 2. Copy the item to the node, update next
    node->item = item;
    node->next = NULL;

    // 3. update the rear of the queue (and front if first object)
    pq->rear->next = node;
    pq->rear  = node;
    pq->front = pq->front || NULL; // ***********************

    // 4. update the queue size
    pq->items++;
    return true;
}

The starred line gives me the following warning:

q.c:49:15: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
    pq->front = pq->front || NULL;

If I remove the ||, and just set it to one or the other value, it works fine with no warning. Why does || give a warning?

Upvotes: 0

Views: 68

Answers (1)

dbush
dbush

Reputation: 224437

The logical OR operator || results in either 0 if both arguments are 0 (or NULL) or 1 otherwise. So what that statement is doing is setting pq->front to the integer value 0 or 1.

You say you want the result to be the first non-null value, similar to how or works in Python. If that's the case, the statement would essentially do nothing, as it would just assign pq->front back to itself if non-null or to NULL if it is NULL.

What you probably were looking for is:

if (!pq->front) pq->front = node;

Which would set the front of the queue to node if the queue is empty.

On a related note, this is a problem:

pq->rear->next = node;

if pq->rear is NULL. To fix both issues, change this:

pq->rear->next = node;
pq->rear  = node;
pq->front = pq->front || NULL;

to this:

if (!pq->front) {
    pq->front = node;
    pq->rear = node;
} else {
    pq->rear->next = node;
    pq->rear = node;
}

This of course assumes that if either pq->front or pq->rear is NULL then the other is also NULL.

Upvotes: 1

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