Re: pragma import (C, , ); -- import a constant value from C library

[Simon Wright <simon@pushface.org>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Fri, 26 Oct 2012 15:45:29 -0700 (PDT), Enzo Guerra wrote:
>
>> 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
>
> Don't do this. Ada.Numerics already defines Euler's constant.
>
> And in general, you can define any constant this way in Ada. The precision
> is limited only by the source where you get its tabulated value. And it
> will work for all real types. E.g.
>
> gamma : constant :=
>    0.57721_56649_01532_86060_65120_90082_40243;
>
>    X : Long_Float := gamma;
>    Y : Float := gamma;
>    ...
>
> The compiler truncates the value when you use a narrower target like
> Long_Float.

Yes.

I worked on a project where the system engineers on the other side had
defined pi to be 3.14159 (not even 3.141592); so when we sent a Float
equal to Ada.Numerics.Pi they got a range check. We ended up agreeing
the bounds to be one mantissa bit outside the actual limit we intended.