结构体中的指针大小

Question

So, I have this structure used to implement a circular buffer of another structure ( plane )

typedef struct queue
{
    struct plane *q;        
    int size, capacity, front, rear;  
}queue;

After this I declare the structure

queue *q;

and later, to inicialize the buffer, I use this method

int queue_init(int size){

    q = (queue*) malloc(sizeof(queue));

    q->q=malloc(sizeof(struct plane) * size); 

    q->size = 0;
    q->capacity = size;
    q->front = 0;
    q->rear = 0;

    return 0;
}

which is supposed to be the one that inicializes all the variables I need to use the buffer and the buffer itself with q->q=malloc(sizeof(struct plane) * size); . The problem is the size of the buffer is always 8 bytes, where it should be as it says, size times the size of plane , which is actually 16. The thing is how should I initialize the variables so I can get the buffer the size I want. I can't change the return value or the parameters of the function as limitations. Thank you beforehand!

Answer 1

If you're trying to get the size of the array by doing sizeof(q->q) , this won't work as you expect.

Since the q field of struct queue is a pointer and not an array, sizeof(q->q) actually gives you the size of the pointer which on your machine appears to be 8 bytes in size.

The language doesn't keep track of how much memory was allocated in a call to malloc , so you need to keep track of that yourself. The good thing is that you're already doing that.

You correctly allocated room for an array of struct place of size size by using malloc(sizeof(struct plane) * size) , and you stored size in q->capacity . So you know how big the array is.