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Date: Thu, 26 Apr 2012 09:58:51 +0200
From: Xavier ALLAMIGEON <xavier.allamigeon@inria.fr>
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Subject: [Caml-list] static C library of wrappers: compilation issues

Dear caml-list,

I'd like to build a static C library implementing an interface to an 
OCaml library, but I get some compilation errors. Here's an example of 
the problem.

1) The ml_code.* files contain a hello_world function which I'd like to 
provide in C.

<ml_code.ml>
let ml_hello_world () =
   print_endline "Hello world!"

let _ =
   Callback.register "ml_hello_world" ml_hello_world

<ml_code.mli>
val ml_hello_world: unit -> unit

2) On the C part, I created .c/.h files calling the ml function with 
camlback:
<c_code.c>
#include <caml/callback.h>
#include <caml/memory.h>

void init(void) {
   char* dummy = '\0';
   caml_main(&dummy);
}

void c_hello_world(void) {
   CAMLparam0();
   static value *closure_ml_hello_world = NULL;
   if (closure_ml_hello_world == NULL) {
     closure_ml_hello_world = caml_named_value("ml_hello_world");
   }
   caml_callback(*closure_ml_hello_world, Val_unit);
   CAMLreturn0;
}

<c_code.h>
void init(void);
void c_hello_world(void);

3) Finally, I created a test file in C:
<test.c>
#include "c_code.h"

int main(int argc, char **argv) {
   init();
   c_hello_world();
}

Here's the way I'm compiling everything. It builds a libhello_world.a 
static library from ml_code.obj.o and c_code.o.
ocamlc -c ml_code.mli
ocamlopt -c ml_code.ml
ocamlopt -output-obj ml_code.cmx -o ml_code.obj.o
gcc -c c_code.c -I"`ocamlc -where`"
ar rcs libhello_world.a ml_code.obj.o c_code.o
gcc -o test -L. -L"`ocamlc -where`" test.c -lhello_world -lasmrun -lm -ldl

The compilation error happens at the last line, where I get the 
following messages:
/home/allamigeon/Applications/godi/lib/ocaml/std-lib/libasmrun.a(startup.o): 
In function `caml_main':
startup.c:(.text+0x239): undefined reference to `caml_data_segments'
startup.c:(.text+0x243): undefined reference to `caml_data_segments'
startup.c:(.text+0x249): undefined reference to `caml_data_segments'
startup.c:(.text+0x297): undefined reference to `caml_code_segments'
startup.c:(.text+0x2a5): undefined reference to `caml_code_segments'
startup.c:(.text+0x2b3): undefined reference to `caml_code_segments'
startup.c:(.text+0x2c9): undefined reference to `caml_code_segments'
startup.c:(.text+0x2ce): undefined reference to `caml_code_segments'
/home/allamigeon/Applications/godi/lib/ocaml/std-lib/libasmrun.a(roots.o): 
In function `caml_init_frame_descriptors':
roots.c:(.text+0x22b): undefined reference to `caml_frametable'
roots.c:(.text+0x23a): undefined reference to `caml_frametable'
/home/allamigeon/Applications/godi/lib/ocaml/std-lib/libasmrun.a(roots.o): 
In function `caml_do_roots':
roots.c:(.text+0x3a6): undefined reference to `caml_globals'
roots.c:(.text+0x3b2): undefined reference to `caml_globals'
/home/allamigeon/Applications/godi/lib/ocaml/std-lib/libasmrun.a(roots.o): 
In function `caml_oldify_local_roots':
roots.c:(.text+0x505): undefined reference to `caml_globals'
roots.c:(.text+0x511): undefined reference to `caml_globals'
/home/allamigeon/Applications/godi/lib/ocaml/std-lib/libasmrun.a(amd64.o): 
In function `caml_start_program':
(.text+0x251): undefined reference to `caml_program'
...

Which is really weird is that the problem happens only with Linux, and 
not with Mac OS. Of course, if I replace the last line by:
gcc -o test -L"`ocamlc -where`" test.c ml_code.obj.o c_code.o -lasmrun 
-lm -ldl
then it works like a charm:
./test
"Hello world!"

Any idea?

Thanks in advance for your help!

Xavier