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Dynamic libs w/ Ocaml + C code under Mac OS X
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Date: 2008-02-05 (04:57)
From: Alain Frisch <alain@f...>
Subject: Re: [Caml-list] Dynamic libs w/ Ocaml + C code under Mac OS X

Joel Stanley wrote:
> The short question is: can I currently generate a shared library 
> containing natively-compiled ocaml code together with arbitrary C code 
> under Mac OS X?

I'll reply assuming you want to build a plugin that can be loaded with 
Dynlink, not a stand-alone shared library that includes the OCaml 
runtime. Otherwise let me know.

> To wit, should I be doing something other than:
> $ gcc -c wrapper.c
> $ ocamlopt -dlcode -c
> ...
> $ ocamlopt -shared -o test simple.cmx wrapper.o
> or perhaps just ocamlopt -shared -o test wrapper.c?

Both should work under Mac OS X < 10.5 on Intel. PowerPC versions are 
not supported and for 10.5, there are some issues (see below). What are 
your OS version and cpu?

> The immediate problem is that this seems to be passing the wrong 
> arguments to ld:
> + ld -bundle -flat_namespace -undefined suppress -read_only_relocs 
> suppress -o 'test'  -I'/Users/jstanley/hbin/lib/ocaml' 
> '-L/Users/jstanley/hbin/lib/ocaml' 'test.startup.o' 'simple.o' 'wrapper.o'
> (I'd expect to see gcc -dynamiclib (instead of bundle) here if Mac OS X 
> is supported, and without the -I switch).

I'm not very familiar with the linker of Mac OS X. In particular, I 
don't really know the differences between bundles and shared libraries.
OCaml used to produce bundles (not shared libs) for stub libraries 
(dll*.so), and I think I just decided to use the same approach.

What's wrong with the above linker invocation? Do the errors you mention 
appear with it, or only if you change the command-line to "gcc 
-dynamiclib ..."?

> which lead me to believe that simple.o is not correctly being generated 
> with position independence.

Indeed. The code generators don't produce PIC code. The Mac OS X linker 
is supposed to be able to build bundle with position-dependent code, 
using the -read_only_relocs suppress flag. This flag has been removed 
from the new linker in Leopard. Apple provides a ld_classic with the old 
behavior, but unfortunately, some libraries (like X11) seems to require 
the new linker. I think we have no good solution for Mac OS X 10.5 on 
Intel currently. Any help is welcome, of course.

-- Alain