Memory allocated in caml_*_of_value leaks

[goodpaul6]

Once again, great work here :) Definitely going to be using this for some of my projects.

As it stands, caml_eleven_out_of_ten_majestic_af leaks the Doggo allocated in caml_Doggo_of_value:

https://github.com/ocaml-sys/ocaml-bindgen/blob/main/examples/caml_doggo.c

Will brainstorm some ways to deal with this effectively in the comments below.

[goodpaul6]

The following modifications to the Doggo stubs allow it to work:

void caml_Doggo_of_value(value caml_x, Doggo* dest) {
  dest->many = Int_val(Field(caml_x, 0));
  dest->wow = Int_val(Field(caml_x, 1));
}

// ...

void caml_eleven_out_of_ten_majestic_af(value caml_pupper) {
  CAMLparam1(caml_pupper);

  Doggo pupper = { 0 };
  caml_Doggo_of_value(caml_pupper, &pupper);
  eleven_out_of_ten_majestic_af(pupper);
  CAMLreturn0;
}

[leostera]

I like this! it should make it easier to reason about the memory allocations when writing the wrapper as well as allow for these of_value/to_value functions to be a little more flexible regarding where they are used.

[goodpaul6]

The above should work for all non-opaque structs (including nested ones) but it won't work if there's indirection (e.g. the Doggo struct contains a pointer to another Doggo):

struct Doggo {
  struct Doggo* fren;
  int wow;
  char many;
};

Would that be represented as a fren: doggo option at the OCaml level?

[omnisci3nce]

I like your approach of allocating on the stack also for its performance improvement too!

I'm currently trying to write some code that can handle the string type as a parameter to a function call from OCaml. This is surprisingly tricky because the only information we get is the parsed C header file, so we can only intuit that we must pass a char* but not that we can pass an ocaml string and use String_val macro. For example if I used an an int* instead its obvious we cant magically assume some type conversion like in the char* -> string case.

I think relates to this issue because it signifies we will need to introduce some additional code into bindgen that is used at runtime so that the user can configure how they want the bindings to be called and types to be marshalled.