Freeing array of structs inside struct

I have two structs

struct obj_t {
    int id;
    float x;
    float y;
};

struct cluster_t {
    int size;
    int capacity;
    struct obj_t *obj;
};

As you can see, there is pointer to first obj_t inside cluster_t

What I want to do is to free every obj_t from array inside cluster_t

Do I have to write it with for loop like this?

void clear_cluster(struct cluster_t *c)
{
    for(int i = 0; i<c->size;i++)
    {
        free(&c->obj[i]);
    }
    free(c->obj);
}

Or is it ok to free the memory just like this ?

void clear_cluster(struct cluster_t *c)
{
    free(c->obj);
}

Solution

There should be one free() for every malloc() you have, and executed in the opposite order from which it was allocated.

The field obj of cluster_t is a pointer to an array of object_t. This is probably allocated with one malloc() when initializing your cluster_t (something like c->obj = malloc(c->capacity*sizeof(*c->obj))), so it only needs to be freed with one call to free(). You would then want to free the cluster_t allocation itself (assuming it too was dynamically allocated):

free(c->obj);
free(c);

There would be a difference, however, if each object_t itself had a dynamic allocation within it. (In your example, object_t does not.) In that case, you would have needed to iterate through the array and malloc() an allocation when you created the array, and therefore do the reverse and free() each at the end.