From: David Allsopp <dra-news@metastack.com>
To: Edgar A <edgar.factorial@gmail.com>,
"caml-list@inria.fr" <caml-list@inria.fr>
Subject: RE: [Caml-list] Getting some C values over to the OCaml side correctly
Date: Sun, 25 Sep 2016 08:40:38 +0000 [thread overview]
Message-ID: <E51C5B015DBD1348A1D85763337FB6D901351FA074@Remus.metastack.local> (raw)
In-Reply-To: <090F68D4-7CA4-4091-BC5D-5F2C9BFB2AEC@gmail.com>
Edgar A wrote:
> Greetings,
>
> I have some C values that I want to get over to the OCaml side using the
> FFI.
>
> Specifically I have `uint32_t`, `intptr_t` and `int32_t` and I’m not sure
> how to get it over to the OCaml side correctly.
>
> Would the uint32_t be expressed as a Int32.t? Do I really need to put an
> integer as block just to get it to the OCaml side, because of the 31 bit
> integers on the OCaml side right?
Yes, you do, unless you can guarantee that you don't need bit 31 ever. The Unix library, for example, uses a 31/63-bit int for Unix FDs (https://github.com/ocaml/ocaml/blob/trunk/otherlibs/unix/open.c) on the realistic assumption that bit 31 will never be set.
You should use a nativeint (still boxed) for an intptr_t, as that will transparently work for 32/64-bit variants.
> Been looking for examples but can’t find any, example code greatly
> appreciated, the C side and the ml side please.
There's quite a lot in the manual for this (http://caml.inria.fr/pub/docs/manual-ocaml/intfc.html) - Int32_val and Nativeint_val are the two functions for converting an OCaml value to a C type and caml_copy_int32 and caml_copy_nativeint are the equivalents for converting the C type back to an OCaml value.
Putting caml_copy_nativeint into a GitHub search reveals, for example, https://github.com/yallop/ocaml-unix-type-representations/blob/master/lib/unix_type_representation_stubs.c (you can find many more examples by searching for caml_copy_nativeint or caml_copy_int32)
David
next prev parent reply other threads:[~2016-09-25 8:40 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-25 8:02 Edgar A
2016-09-25 8:40 ` David Allsopp [this message]
2016-09-25 8:43 ` Edgar A
2016-09-25 12:28 ` David Allsopp
2016-10-06 9:19 ` Goswin von Brederlow
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