Re: trying to call HDF5 from ada -- problems with constants

["Randy Brukardt" <randy@rrsoftware.com>]

"Christopher Broeg" <broeg@astro.uni-jena.de~> wrote in message
news:cgkii4$lga$1@fsuj29.rz.uni-jena.de...
...
> tx. I just realized that with gnat I can import the constants directly.
> I realized that the ada specifications in the RM do never explicitly
> mention functions or procedures with respect to pragma import. So I did:
>
>     H5T_STD_I32BE_g : Hid_t;
>     pragma Import(C,H5T_STD_I32BE_g , "H5T_STD_I32BE_g");
>
> and it works! All you have to do is call H5open manually, which is done
> automatically with the c preprocessor. Is this behaviour
> compiler-dependant, or ADA standard?

Pragma Import on objects is standard (presuming interface to C is supported
by the implementation).
Since this object is going to be a constant in the Ada code, it's
unfortunate that you can't declare it that way:

    H5T_STD_I32BE_g : constant Hid_t;

but that lack of an explicit initializer makes it illegal. (I think an
implementation could support this if it wanted to - pragma Import can be a
completion - but I don't know if any do.)

> I'm surprised, because there was this long thread on this very topic
> explicitly stating that ada cannot import c constants... But I guess it
> only referred to preprocessor constants. All "variing" constants are of
> course real c constants visible in the library.

I certainly thought you were talking about preprocessor constants. I've
never seen a library level constant in C, so I never thought that you might
be talking about that. I suspect that is true of most of the people here.

             Randy.