Re: [Caml-list] ctypes with -thread: inconsistent assumptions over implementation Foreign

[Jeremy Yallop <yallop@gmail.com>]

On 19 November 2013 22:57, Mike McClurg <mike.mcclurg@gmail.com> wrote:
> $ ocamlfind ocamlopt -verbose -package ctypes,ctypes.foreign -linkpkg
> -thread -o test test.ml
[...]
> File "test.ml", line 1:
> Error: Files test.cmx
>        and /Users/mike/.opam/system/lib/ctypes/ctypes-foreign-threaded.cmxa
>        make inconsistent assumptions over implementation Foreign
[...]
> Am I doing something wrong here? Or is something wrong with the way ctypes
> is built and packaged?

It's a packaging problem.  Ctypes uses a findlib feature[1] that
selects the library to link according to whether threads are enabled,
as you can see in the META file:

    archive(native, mt) = "ctypes-foreign-threaded.cmxa"
    [...]
    archive(native) = "ctypes-foreign-unthreaded.cmxa"
    (from [2])

This shows up in your command-line as expected: when you don't pass
-thread to ocamlfind it picks the -unthreaded version of the
ctypes-foreign library

   + ocamlopt.opt [...]
/Users/mike/.opam/system/lib/ctypes/ctypes-foreign-unthreaded.cmxa
test.ml

and when you pass -thread you get the -threaded version

   + ocamlopt.opt [...]
/Users/mike/.opam/system/lib/ctypes/ctypes-foreign-threaded.cmxa
test.ml

So far, so good.  However, although there are two cmxa implementations
installed for ctypes-foreign there's only one set of cmx files, which
happen to match the -unthreaded version of the library.  Compiling
with the -thread option picks the -threaded library but the
-unthreaded cmx, leading to the mismatch that you're seeing.  If you
simply delete the offending cmx file (as a diagnostic step, not a
long-term solution!) you should see the problem disappear:

   rm /Users/mike/.opam/system/lib/ctypes/foreign.cmx

[1] http://projects.camlcity.org/projects/dl/findlib-1.4/doc/guide-html/x261.html
[2] https://github.com/ocamllabs/ocaml-ctypes/blob/ocaml-ctypes-0.2.1/META#L39-L43