C - accessing dynamic array within a struct - c

Question

I'm trying to create cluster with dynamic array objects.
Struct definitions are following:
struct obj_t {
int id;
float x;
float y;
};
struct cluster_t {
int size;
int capacity;
struct obj_t *obj;
};
Function for adding object to cluster is:
void append_cluster(struct cluster_t *c, struct obj_t obj)
{
if(c->capacity < (c->size + 1))
{
c = resize_cluster(c, c->size + 1);
}
if(c == NULL)
return;
c->obj[c->size] = obj; //at this point program crashes.
c->size++;
}
EDIT: Here is resize_cluster() function:
struct cluster_t *resize_cluster(struct cluster_t *c, int new_cap)
{
if (c->capacity >= new_cap)
return c;
size_t size = sizeof(struct obj_t) * new_cap;
void *arr = realloc(c->obj, size);
if (arr == NULL)
return NULL;
c->obj = (struct obj_t*)arr;
c->capacity = new_cap;
return c;
}
EDIT 2: Here is cluster initialization:
void init_cluster(struct cluster_t *c, int cap)
{
c = malloc(sizeof(struct cluster_t));
c->size = 0;
c->capacity = cap;
c->obj = (struct obj_t*)malloc(cap * sizeof(struct obj_t));
}
I can't figure out why program crashes when I try to add the object to the array in cluster. Is accessing array this way wrong? If so, how should I access it?

Answer 1

The issue is the call to init_cluster(). The c parameter is passed-by-value, so whatever you are sending remains unmodified:
struct cluster_t * c;
init_cluster(c, 1);
// c is uninitialized!
One fix would be to pass a pointer to an object:
struct cluster_t c;
init_cluster(&c, 1);
Then remove c = malloc(sizeof(struct cluster_t)); from init_cluster();
Or, you could create an alloc_cluster function:
struct cluster_t * alloc_cluster(int cap)
{
c = malloc(sizeof(struct cluster_t));
c->size = 0;
c->capacity = cap;
c->obj = malloc(cap * sizeof(struct obj_t));
return c;
}
And call it like:
struct cluster_t *c = init_cluster(1);