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

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

On Fri, Jan 8, 2016 at 3:19 PM, Malcolm Matalka <mmatalka@gmail.com> wrote:
> 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.

In the stub generation case, you will not have symbols that represent
the macros. Instead, C code will be generated which generates OCaml
code which contains the macro constants.

In the dlsym static linking case, you would build regular cm(x)a files
and a .a file and install them all with ocamlfind. Your users don't
have to know about C libraries or have them installed globally on the
system.

David

>>
>> 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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