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Debugging C and OCaml
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Date: 2010-03-02 (00:48)
From: Jianzhou Zhao <jianzhou@s...>
Subject: Re: [Caml-list] Debugging C and OCaml
On Mon, Mar 1, 2010 at 10:19 AM, Mark Shinwell <> wrote:
> On Mon, Mar 01, 2010 at 10:10:44AM -0500, Jianzhou Zhao wrote:
>> My main program is C++. It uses C functions to call
>> OCaml functions, and these OCaml functions also call
>> C functions and wrapped C++ functions sometimes.
>> I can debug from C++, but it stops when it meets
>> an OCaml binding. Does anyone know an efficient
>> solution about how to debug across different languages?
> Is this native code compilation?

I compiled OCaml code into *.o by 'ocamlc -custom -output-obj...',
and then linked it with *.o from C and C++ code.
I think this isnt a native compilation.

My main C++ program is eventually compiled into a shared library.
If I do 'ocamlopt', ld reports:
relocation R_X86_64_PC32 against symbol `caml_last_return_address' can
not be used when making a shared object; recompile with -fPIC

So I am doing ''ocamlc -custom' for the time being.

> If so, you should be able to use gdb to debug into the Caml bindings at the
> assembly level -- does that work?  Unfortunately at the moment there is no Caml
> compiler support for emitting the debug info required by gdb to debug at the
> language level.  That said, ocamlopt-compiled assembly code is fairly easy to
> read, and you should be able to get something resembling a backtrace using
> "where".

Now I can debug into OCaml assembly from C, and its stack frames.

#0  0x00007ffff6cad3f4 in caml_interprete ()
#1  0x00007ffff6caa54f in caml_callbackN_exn ()
#2  0x00007ffff6caa64a in caml_callback_exn ()
#3  0x00007ffff6caa658 in caml_callback ()

It seems that OCaml runtime is interpreting via 'caml_interprete' the
OCaml function which C
calls at runtime. If this is true, we cannot really debug that OCaml
function, but we can
see how it is interpreted. But I guess we can debug the real assembly if I do
native compilation.

> Mark