[ocaml-ctypes] Passing ocaml strings to C functions

[Jeremy Yallop]

Dear Thomas,

On 31 May 2018 at 09:41, Thomas Braibant <thomas.braibant at gmail.com> wrote:
> For some reason, I can't seem to find how to apply C.foo to an ocaml string.
>
> ```
> let foo_wrapped' (s : String.t) =
>   C.foo
>     (Ctypes.to_voidp (Ctypes.ocaml_string_start s)
>     (Unsigned.Size_t.of_int (String.length str))
> ;;
> ```

Right: the circumstances under which C parts of a program can read
directly from an OCaml string are deliberately very constrained.  Even
something like this is unsafe:

>     (Ctypes.to_voidp (Ctypes.ocaml_string_start s)

because allocating the pointer in 'to_voidp' could cause the GC to
run, changing the address of the string 's'.

> What's more surprising is the error message when trying to coerce the
> ptr char and ocaml strings together:
> ```
> Ctypes.(coerce ocaml_string (ptr char) (Ctypes.ocaml_string_start ""))
> ;;
> Exception: Coercion failure: char* is not coercible to char*.
> ```

This is indeed a surprising message, even to OCaml users, who might be
somewhat used to messages of the form "type t is not equal to type t".
Ctypes generally uses the underlying C type as the printed
representation of a 'typ' value, and in this case both 'ptr char' and
ocaml_string represent the same C type, albeit with different
constraints on the behaviour, and so they have the same printed
representation:

   # ptr char;;
   - : char ptr typ = char*
   # ocaml_string;;
   - : string ocaml typ = char*

> What I think is that I could rewrite the binding, to have something like:
> ```
> let foo' = foreign "fooC" (ocaml_string @-> size_t @-> returning int);;
> ```
> but it seems like unnecessary code duplication (and would make my
> actual use case harder to write).

One way to avoid the duplication is parameterizing the storage by the
'typ'.  David Sheets's ocaml-sodium library uses that approach in a
single set of bindings that work with both bigarrays and bytes:

    https://github.com/dsheets/ocaml-sodium/blob/c1ab5990/lib_gen/sodium_bindings.ml#L29-L32

Kind regards,

Jeremy