C usage of malloc on struct

Question

I'm trying to use malloc on a struct called image. The function is:

void image_init(struct image* img, int w, int h) {
    img = malloc(sizeof(struct image));
    (*img).w = w;
    (*img).h = h;
}

And the function to free the image again is:

void image_destroy(struct image* img) {
    free(img);
}

But whenever I try to free the malloc-ed data I'm getting the error that the adress I'm trying to free was not malloc-ed before.

The struct of image is:

struct image {
    int w,h;
    int* data;
}

I call the functions with:

struct image img;
image_init(&img,100,100);
image_destroy(&img);

Answer 1

First here

void image_init(struct image* img, int w, int h) {
    img = malloc(sizeof(struct image));
    (*img).w = w;
    (*img).h = h;
}

img is a copy of the original pointer passed to it, hence the code on first line inside function has no effect, since it makes the copy point somewhere - not the original object.

This:

struct image img;
image_init(&img,100,100);
image_destroy(&img);

doesn't make sense either (assuming you expected img to point somewhere after call to init). img isn't a pointer, how you expect it to point somewhere?

---

One way you could solve this could be

struct image *img = image_init(100,100);

where

struct image* image_init(int w, int h) {
    img = malloc(sizeof(struct image));
    (*img).data = NULL;
    (*img).w = w;
    (*img).h = h;
    return img;
}

Don't forget to call free on the pointer returned by above function - you will need to free data pointer separately too, in case you allocate it too.

---

NOTE: My best guess (if you also can't change prototype) is you want something like this:

void image_init(struct image* img, int w, int h) {
    img->data = malloc(sizeof(int) * w * h);
    img->w = w;
    img->h = h;
}

destroy:

void image_destroy(struct image* img) {
    free(img->data);
    free(img);
}

In main

struct image* img = malloc(sizeof(struct image));
image_init(img, 100, 100);
image_destroy(img);

PS. Or if you want the usage of those functions remain as you had in main go with the answer of Johnny Mopp.