C wrappers for Ocaml functions

[Tim Leek <trleek@gmail.com>]

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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
% 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'
ht.a(floats.o): In function `caml_atan_float':
(.text+0x1c5): undefined reference to `atan'
ht.a(floats.o): In function `caml_acos_float':
(.text+0x1dc): undefined reference to `acos'
ht.a(floats.o): In function `caml_asin_float':
(.text+0x1f3): undefined reference to `asin'
ht.a(floats.o): In function `caml_tanh_float':
(.text+0x20a): undefined reference to `tanh'
ht.a(floats.o): In function `caml_tan_float':
(.text+0x221): undefined reference to `tan'
ht.a(floats.o): In function `caml_cosh_float':
(.text+0x238): undefined reference to `cosh'
ht.a(floats.o): In function `caml_cos_float':
(.text+0x24f): undefined reference to `cos'
ht.a(floats.o): In function `caml_sinh_float':
(.text+0x266): undefined reference to `sinh'
ht.a(floats.o): In function `caml_sin_float':
(.text+0x27d): undefined reference to `sin'
ht.a(floats.o): In function `caml_power_float':
(.text+0x298): undefined reference to `pow'
ht.a(floats.o): In function `caml_sqrt_float':
(.text+0x2bf): undefined reference to `sqrt'
ht.a(floats.o): In function `caml_log10_float':
(.text+0x400): undefined reference to `log10'
ht.a(floats.o): In function `caml_log_float':
(.text+0x417): undefined reference to `log'
ht.a(floats.o): In function `caml_fmod_float':
(.text+0x551): undefined reference to `fmod'
ht.a(floats.o): In function `caml_floor_float':
(.text+0x568): undefined reference to `floor'
ht.a(floats.o): In function `caml_exp_float':
(.text+0x57f): undefined reference to `exp'
ht.a(ht.o): In function `caml_program':
(.text+0x46): undefined reference to `camlHt__entry'
ht.a(ht.o): In function `caml_globals':
(.data+0x2e0): undefined reference to `camlHt'
ht.a(ht.o): In function `caml_data_segments':
(.data+0x410): undefined reference to `camlHt__data_begin'
ht.a(ht.o): In function `caml_data_segments':
(.data+0x418): undefined reference to `camlHt__data_end'
ht.a(ht.o): In function `caml_code_segments':
(.data+0x488): undefined reference to `camlHt__code_begin'
ht.a(ht.o): In function `caml_code_segments':
(.data+0x490): undefined reference to `camlHt__code_end'
ht.a(ht.o): In function `caml_frametable':
(.data+0x4d8): undefined reference to `camlHt__frametable'
collect2: ld returned 1 exit status



-- 
When  I see an adult on a bicycle, I do not despair for the future of the
human race. - H. G.Wells

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