C - accessing dynamic array within a struct

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?

Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. Questions without a clear problem statement are not useful to other readers. See: How to create a minimal reproducible example. — AndyG
– AndyG, Commented Dec 1, 2017 at 15:30
I have added cluster initialization and resizing functions to question. — ten_druhy
– ten_druhy, Commented Dec 1, 2017 at 15:31
You are passing c by value to init_cluster(). So whatever you are passing as an argument remains uninitialized. You could change to: struct cluster_t * init_cluster(int cap) {....;return c;}. — 001
– 001, Commented Dec 1, 2017 at 15:33

001 · Accepted Answer · 2017-12-01 15:41:48Z

3

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);

answered Dec 1, 2017 at 15:41

001

13.6k5 gold badges38 silver badges72 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

ten_druhy Over a year ago

Removing c = malloc(sizeof(struct cluster_t)); from init_cluster() works as I wanted. Thanks a lot.

Collectives™ on Stack Overflow

C - accessing dynamic array within a struct

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related