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

[Christopher Zimmermann <christopher@gmerlin.de>]

On Fri, 08 Jan 2016 14:14:22 +0000 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,

That's no surprise. Macros are evaluated at compile time. You won't see
their name (only their value) in the compiled object file.

> 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?
> 
> 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.

You could hardcode the value of the macros into your ocaml code like
this:

let macro1 = 4096;;

But I don't see anything wrong with your approach. If you have really
many macros and don't want to add that many functions you could create 
a static record of all the macro values in your C stub code like this 
(not tested):

value get_constants(value) {
  static value constants[5] = {
    (4 << 10), /* header for block of size 4 */
     Val_long(Macro1),
     Val_long(Macro2),
     NULL,
     Val_long(Macro4)
  };

  if (constants.3 == NULL) {
    constants.3 = caml_copy_string(Macro3);
  }

  return (constants + 1);
}


Christopher



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