struct foo {
	void *data;
};

struct foo *foo_new (void) {
	struct foo *self = (struct foo *)malloc(sizeof(*self));

	self->data = NULL;

	return self;
}

void foo_set (struct foo *self, void *data, size_t l_data) {
	free(self->data);
	self->data = (void *)malloc(l_data);
	memcpy(self->data, data, l_data);
}

void foo_free (struct foo *self) {
	free(self->data);
	free(self);
}

struct bar {
	void *always_allocated;
};

struct bar *bar_new (void) {
	struct bar *self = (struct bar *)malloc(sizeof(*self));

	self->always_allocated = (void *)malloc(0);

	return self;
}

void bar_free (struct bar *self) {
	free(self->always_allocated);
	free(self);
}

int main () {
	struct foo *f = foo_new();

	/* This would loose track of the original pointer and allocate new memory.
	 * foo_free wouldn't call bar_free, but the "generic" free function.
	 *
	 * How would I correctly store my struct bar inside my struct foo without having memory leaks?
	**/
	foo_set(f, bar_new());

	foo_free(f);
}