Re: [Caml-list] Including a C library statically in an Ocaml library

[David Sheets <sheets@alum.mit.edu>]

On Fri, Jan 8, 2016 at 2:14 PM, Malcolm Matalka <mmatalka@gmail.com> wrote:
> The core problem I am having is a C library I want to bind has a number
> of macros which I need the value of.  Here is how I am trying to solve
> it, but perhaps there is a better way:
>
> I have a small C library which gets compiled to libfoo.a which provides
> functions that return the macro values, like:
>
> int macro1() { return MACRO1; }
>
> I then have an ocaml library, called ofoo, that uses Ctypes to bind to macro1:
>
> let macro1 = foreign "macro" (void @-> returning int)
>
> Where I am having issues is this small library, I'd prefer it to not
> have to be installed on the system but just compiled into the Ocaml
> library so that a user just has to link against that library.  Right
> now, none of the symbols (macro1) are being included in the library, I'm
> guessing because the linker sees no direct use of them.  And I'm not
> even sure if I can get it included in the ocaml library.  I'm also not
> able to get the libfoo symbols linked into a final executable, I'm
> guessing for similar reasons.
>
> What are my options here?

I'd recommend using ctypes 0.4+ stub generation support which can bind
macro values and detect struct layout at an early compilation stage.
You can see it in use to do this in my ocaml-unix-errno library
<https://github.com/dsheets/ocaml-unix-errno>. One benefit with this
approach is greater static checking and c <-> ocaml type safety.

If you continue using dynamic bindings, two lonker flags may be of use to you:

--no-as-needed : on recent Ubuntu distributions, gcc automatically
includes the --as-needed flag which drops symbols that are not
referenced. Unfortunately, clang does not understand this flag so you
need to have a conditional in your build system to detect the compiler
in use if you want a cross-platform library.

-E : The Exports local symbols which are statically linked into a
binary into the dynamic symbol table so that they can be found with
dlsym.

You can use these with gcc like -Wl,-E for example.

Hope this helps,

David Sheets


> If I've missed any useful information, let me know.  I haven't interoped
> much with C directory in Ocaml so I'm not sure what information is
> important.
>
> /Malcolm
>
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