linking the same module more than once

linking the same module more than once

[Jake Donham]

Hi,

Consider the following code:

  a.ml: let a () = raise (Unix.Unix_error (Unix.E2BIG, "", ""))
  b.ml: try A.a () with Unix.Unix_error (_,_,_) -> ()

compiled with

  ocamlc -o test unix.cma a.ml unix.cma b.ml

Run it:

  > ./test
  Fatal error: exception Unix.Unix_error(0, "", "")

With exceptions raised from C code this can even happen within the same module:

  a.ml: let a () = try Unix.mkdir "/tmp" 0o777 with Unix.Unix_error
(_,_,_) -> ()
  b.ml: A.a ()

There is no trouble linking a module more than once, but different
parts of the code can wind up linked to different instances of the
module, leading to the confusing behavior that exception handling
seems not to work (and generally that things you think are equal are
not). For exceptions raised from C code, since
Callback.register_exception is called twice, the C code has a
different instance from the ML code.

Ocamlopt catches multiple definitions; seems like ocamlc should also.
But I see bugs

  http://caml.inria.fr/mantis/view.php?id=1522
  http://caml.inria.fr/mantis/view.php?id=1657

suggesting that this is expected behavior. Is there some use for it?

(I ran into this trying to use the ocaml-ssl module, which for some
reason builds unix.cma into ssl.cma.)

Jake

Re: [Caml-list] linking the same module more than once

[Stéphane Glondu]

Jake Donham wrote:
> [...]
> There is no trouble linking a module more than once, but different
> parts of the code can wind up linked to different instances of the
> module, leading to the confusing behavior that exception handling
> seems not to work (and generally that things you think are equal are
> not). [...]

This is a problem I faced myself. My advice is to make sure some module
is not linked twice in an executable. For this, I build a .cma with the
same objects as the executable (basically, add -a to the final linking
stage), and run ocamlobjinfo on the .cma.

Sometimes, it is not up to you. See for example:

  http://caml.inria.fr/mantis/view.php?id=4568

> [...]
> (I ran into this trying to use the ocaml-ssl module, which for some
> reason builds unix.cma into ssl.cma.)

I've also noticed this and submitted a patch upstream for this.


Cheers,

-- 
Stéphane Glondu

Re: [Caml-list] linking the same module more than once

[Alain Frisch]

Jake Donham wrote:
> There is no trouble linking a module more than once, but different
> parts of the code can wind up linked to different instances of the
> module, leading to the confusing behavior that exception handling
> seems not to work (and generally that things you think are equal are
> not).

You should really avoid linking two modules with the same name. This can 
lead to all kind of errors and the compiler will not always catch you if 
they have the same signature. You can even break type safety by linking 
two modules with the same name and signature but with different 
implementations:

- without Dynlink: http://caml.inria.fr/mantis/view.php?id=4231
(quite simple with ocaml; more difficult with ocamlopt, but still 
possible with -pack)

- with Dynlink: http://caml.inria.fr/mantis/view.php?id=4229



-- Alain

Re: [Caml-list] linking the same module more than once

[Jake Donham]

On Thu, Aug 7, 2008 at 11:13 PM, Alain Frisch <alain@frisch.fr> wrote:
> You should really avoid linking two modules with the same name.

Yes, I agree vehemently :). I would like the bytecode linker to help
me out by (at least) warning me when I do it by accident.

Jake