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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: pragma import (C, , ); -- import a constant value from C library
Date: Sat, 27 Oct 2012 00:06:06 +0100
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Date: 2012-10-27T00:06:06+01:00
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Enzo Guerra <enzoguerra1@gmail.com> writes:

> trying to import a constant value from C library (libgsl)
> but having no luck
>
>  M_EULER : Long_Float;
>  -- M_EULER, Euler's constant, \gamma
> pragma Import (C, M_EULER, "M_EULER");
>
> get error undefined reference to `M_EULER'
>
> would appreciate any help

I wonder if the C M_EULER is in fact a macro? If so, it doesn't exist in
the library as a symbol in its own right.

You might try defining it as a variable in C and importing that:

   /* m_euler_definition.c */
   #include <libgsl.h>           /* wherever M_EULER's defined */ 
   const double m_euler_for_gnat = M_EULER;

   -- m_euler.ads
   M_EULER : constant Long_Float;  -- or Interfaces.C.double
   pragma Import (C, "M_EULER", "m_euler_for_gnat");

and then when you build you'll need to include m_euler_definition.o.