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C wrappers for Ocaml functions
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Date: -- (:)
From: Dmitry Bely <dmitry.bely@g...>
Subject: Re: [Caml-list] C wrappers for Ocaml functions
On Tue, Apr 14, 2009 at 3:51 PM, Tim Leek <trleek@gmail.com> wrote:
> Hello.  Not sure if this should have gone to beginners or the regular
> list.  Also not sure if its been answered before.  If so, please
> redirect me!  I did a bunch of searching both on google and in the lists
> and couldn't find the answers I needed.  So here goes.
>
> I have written a blob of code in Ocaml that I like very much.  I'd love
> to keep it in ocaml and not have to worry about things like memory leaks
> and so on.  However, much of what I do is in C and can't be in Ocaml.
> So I am investigating packing my nice Ocaml code into a library and
> writing C bindings so that I can talk to it.
>
> Virtually all of the tutorials out there and documentation cover how to
> create Ocaml bindings to a C library.  I don't want to do that.  The few
> examples I have found that are relevant are toy ones.  How to write C
> bindings to an Ocaml "fib" function, e.g.  In particular, I have found
> none that cover how to obtain pointers to OCaml function return values
> that are not strings or ints, how to store them in C-land, and how to pass
> them back to Ocaml as parameters.
>
> Let's take as a concrete example the following: creating C bindings for
> a simple hash table mapping string keys to integer values.  If I can
> generate bindings for this that work I should be able to do so for the
> library I really care about.
>
> Here's what I put together for the hash table slightly-less-than-toy
> example.  It doesn't even compile.  [Oddly, with very similar incantations
> my own code does compiles but then segfaults inside one of the Ocaml fns.]
> Any help much appreciated!
>
> -Tim
>
>
> 1. The implementation.
>
> % cat ht.ml
>
> type ht = (string,int) Hashtbl.t
>
> let create () : ht = Hashtbl.create 100
>
> let add (table:ht) key valu = Hashtbl.add table key valu
>
> let mem (table:ht) key = Hashtbl.mem table key
>
> let remove (table:ht) key = if (Hashtbl.mem table key) then
> Hashtbl.remove table key
>
> let _ = Callback.register "create" create
>
> let _ = Callback.register "add" add
>
> let _ = Callback.register "mem" mem
>
> let _ = Callback.register "remove" remove
>
> 2. The wrappers
>
> %  cat ht_wrap.c
> #include <caml/mlvalues.h>
> #include <caml/callback.h>
> #include <caml/alloc.h>
>
> void *ht_create (void) {
>  static value *create_closure = NULL;
>  if (create_closure == NULL)
>    create_closure = caml_named_value("create");
>  return ((void *) (caml_callback(*create_
> closure, Val_unit)));
> }
>
> void ht_add (void *ht, char *key, int val) {
>  static value *add_closure = NULL;
>  if (add_closure == NULL)
>    add_closure = caml_named_value("add");
>  caml_callback3(*add_closure, (value) ht, caml_copy_string(key),
> Val_int(val));
> }
>
> void ht_mem (void *ht, char *key) {
>  static value *mem_closure = NULL;
>  if (mem_closure == NULL)
>    mem_closure = caml_named_value("mem");
>  caml_callback2(*mem_closure, (value) ht, caml_copy_string(key));
> }
>
> void ht_remove (void *ht, char *key) {
>  static value *remove_closure = NULL;
>  if (remove_closure == NULL)
>    remove_closure = caml_named_value("remove");
>  caml_callback2(*remove_closure, (value) ht, caml_copy_string(key));
> }
>
>
> 3. Header file for those wrappers
>
> % cat ht.h
>
> void *ht_create (void);
> void ht_add (void *ht, char *key, int val);
> void ht_mem (void *ht, char *key);
> void ht_remove (void *ht, char *key);
>
> 4. The test program, with main() function.
>
> % cat ht_test.c
> #include <stdio.h>
> #include "ht.h"
>
> int main (int argc, char **argv) {
>  void  *ht;
>
>  caml_startup(argv);
>
>  ht = ht_create();
>  ht_add(ht, "foo", 1);
>  ht_add(ht, "foo", 1);
>  ht_add(ht, "bar", 1);
>  ht_remove(ht, "foo");
> }
>
>
> 5. And this is how I am attempting to compile it.  Note that error I'm
> getting here is in link.  If I add "-lm" it gets less noisy but still is
> mad at me. Why am I having to add math library?  Again, help!!
>
> % ocamlopt -output-obj -o ht.o ht.ml

Wrong! You cannot specify ht.o as an output as ht.o is an intermediate
result of ht.ml compilation. Activate -verbose flag and you'll see.
Replace it with something like

ocamlopt -output-obj -o ht_out.o ht.ml

> % ocamlopt -c ht_wrap.c
> % cp  /usr/lib/ocaml/3.10.0/libasmrun.a ./ht.a
> % ar r ht.a ht_wrap.o ht.o
> % gcc -o htt ht_test.c ht.a  -lcurses
> ht.a(floats.o): In function `caml_ceil_float':
> (.text+0x193): undefined reference to `ceil'
> ht.a(floats.o): In function `caml_atan2_float':
> (.text+0x1ae): undefined reference to `atan2'
(...)

Why simply not to use

ocamlopt -c ht_wrap.c
gcc -o htt ht_test.c ht_out.o ht_wrap.o -L${OCAMLLIB} -lasmrun -lcurses -lm

?

- Dmitry Bely