[Caml-list] interfacing C with Ocaml (beginner questions)

[Caml-list] interfacing C with Ocaml (beginner questions)

[francois bereux]

Dear list members,

I have two (beginnner) questions about linking C and Ocaml.

Following the documentation, I built a very simple example (see code
below) that runs fine by doing :
ocamlc -c add.c
ocamlc -c math.mli
ocamlc -c prog.ml
ocamlc -custom -o myprog myprog.cmo add.o

If I run ./myprog, I get the expected result.

a) Is it possible (and if yes, how shall I do) to build a toplevel where
my add function defined in C would be available ?

b) I managed to have C and Ocaml communicate using the Val_int and
Int_val macros to convert value to the appropriate int type. But how
should I proceed if I want that add operates on float (in Caml) i.e. on
double in C ?
I found a Double_val macro, but no Val_double. What is the solution ?

Thank you in advance for any help,

François


------------- add.c -----------------
#include  <caml/mlvalues.h>
#include  <caml/memory.h>

value add(value x, value y)
{
   CAMLparam2(x,y);
   return Val_int(Int_val(x)+Int_val(y));
}

------------- math.mli -----------------
external add: int -> int -> int = "add";;

--------------myprog.ml---------------
open Math;;

let x=3 and y=86 in
   print_int (add x y);;



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Re: [Caml-list] interfacing C with Ocaml (beginner questions)

[Markus Mottl]

On Thu, 07 Jun 2001, francois bereux wrote:
> a) Is it possible (and if yes, how shall I do) to build a toplevel
> where my add function defined in C would be available ?

Yes, it is possible: just use the command "ocamlmktop" to link your
program, using all required files as arguments. This will produce
a toplevel.

If you find it cumbersome to remember how to link stuff for specific
targets, you can also use "OcamlMakefile", which automates all these
steps. Then you'd only have to write "make top" if you want to create
a toplevel out of your project. You can get it here:

  http://www.ai.univie.ac.at/~markus/home/ocaml_sources.html

> b) I managed to have C and Ocaml communicate using the Val_int and
> Int_val macros to convert value to the appropriate int type. But how
> should I proceed if I want that add operates on float (in Caml) i.e. on
> double in C ?
> I found a Double_val macro, but no Val_double. What is the solution ?

You have to use "copy_double" if you want to make an OCaml-float out
of a C-double. Note that this really allocates memory, it's not just a
conversion. See the chapter on interfacing to C in the manual for details:

  http://caml.inria.fr/ocaml/htmlman/manual030.html

Regards,
Markus Mottl

-- 
Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl
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Re: [Caml-list] interfacing C with Ocaml (beginner questions)

[Damien Doligez]

>From: francois bereux <francois.bereux@fr.thalesgroup.com>

>I found a Double_val macro, but no Val_double. What is the solution ?

You must use copy_double (which will do the allocation) because
floating-point values are boxed in O'Caml.


>value add(value x, value y)
>{
>   CAMLparam2(x,y);
>   return Val_int(Int_val(x)+Int_val(y));
>}

This is wrong.  You must use CAMLreturn.

-- Damien
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