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

[Malcolm Matalka <mmatalka@gmail.com>]

Christopher Zimmermann <christopher@gmerlin.de> writes:

> 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.

This is why I'm creating a function that returns the macro value.

>
>> 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