Question

I am trying to implement a Queue in C. Coming from Java and other managed languages, I am really struggling with memory management. Here is the enqueue() function:

int enqueue(Queue q, int value) {

    Node newNode = malloc(sizeof(Node));
    /*newNode->value = value;

    if (q->size == 0)
        q->head = newNode;
    else
        q->head->next = &newNode;

    q->size++;*/
}

I am getting this error :

malloc.c:3096: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.

FWIW, here’s the rest of the code (is this even right?):

typedef struct NodeStruct *Node;
struct NodeStruct {
    Node* prev;
    Node* next;
    int value;
};

typedef struct QueueStruct *Queue;
struct QueueStruct {
    Node* head;
    Node* tail;
    int size;
    int capacity;
};

Queue newQueue(int size) {
    Queue q = malloc(sizeof(Queue));

    q->capacity = size;
    q->size = 0;
    q->head = NULL;
    q->tail = NULL;

    return q;
}

void printQueue(Queue q) {
    printf("Queue of size %d, capacity %d", q->size, q->capacity);
}    

int main() {
    Queue myQ = newQueue(10);

    // this seems to work
    printQueue(myQ);
    // epic fail
    enqueue(myQ, 5);

    return 0;
}

Why is this happening?

Answer 1

The following line is probably giving you grief:

Node newNode = malloc(sizeof(Node));

Node is a pointer type, so you’re only allocating enough space to hold a pointer, not an entire NodeStruct. I think what you want to do is:

Node newNode = malloc(sizeof(*newNode));

or

Node newNode = malloc(sizeof(NodeStruct));

The same issue exists for Queue, you’re only allocated space to hold a pointer, not a QueueStruct. Something else that I only just noticed, is that in your NodeStruct and QueueStruct, you are using the type Node*, which is actually NodeStruct **, which is probably not what you want, since Node is already a pointer.