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

[Malcolm Matalka <mmatalka@gmail.com>]

David Sheets <sheets@alum.mit.edu> writes:

> 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.

Will these approaches require that I have the C library installed to
compile against any binary using my library or will the symbols be part
of the ocaml library?  In my current version I have a libfoo.a that gets
created in the project and then linked against the library.

>
> 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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