Question

Let’s say I have the following structures:

typedef struct first {
  int a;
  int b;
  char *c;
} *first_t;

typedef struct second {
  char *a;
  first_t b;
} *second_t;

The user does something like this:

first_t first = first_new();
second_t second = second_new();
second->b = first;

Where first_new and second_new allocate memory.

I want to write a free function for the second_t structure.
I think I would have to let the user the responsability to free his first_t structure. Is it correct ? or do I have to write something like:

void second_free(second_t second) {
  free(second->a);
  first_free(second->b);
  free(second);
}

Answer 1

It depends. What are the semantics of your code?

If the semantics of second->b = first are that you are transferring ownership, then second_free should indeed call first_free. (Although in this case, it may be appropriate to have a second_create_first function, to keep things encapsulated.)

If you are not transferring ownership, then calling first_free should be the responsibility of the client code.

Whatever semantics you choose, you should make things very clear in your documentation.