Re: Interfacing Ada with C

[Simon Wright <simon@pushface.org>]

Ada novice <posts@gmx.us> writes:

> Many thanks for all your inputs. I'll give you an example. Say we want
> to compute the eigenvalues of this 3 X 3 matrix. The code in C (using
> the IMSL library) is as follows:
[...]
> What would the best way to interface this with Ada? The elements of my
> matrix will be formed in Ada, then the matrix will be passed to C to
> calculate the eigenvalues and then the latter passed back to Ada. The
> size of my matrix will be fixed say 3 x 3. As a novice in Ada, I would
> like to learn good programming practice and the best way is to use
> codes and learn from experts here in Ada.

I've encoded a general complex eigenvalues function, interfacing to the
LAPACK procedure zgeev, starting from a demo at
http://www.physics.oregonstate.edu/~rubin/nacphy/lapack/codes/eigen-c.html
& copying bits and pieces from within the GNAT implementation of
Ada.Numerics.Generic_Complex_Arrays.

Find it at http://public.me.com/simon.j.wright (folder numerics).

NB1 LAPACK is a Fortran code, hence the need to transpose the matrix.

NB2 the compilation gives warnings .. 

   $ gnatmake test_zgeev.adb
   gcc -c test_zgeev.adb
   test_zgeev.adb:9:06: warning: "Interfaces.Fortran.BLAS" is an internal GNAT unit
   test_zgeev.adb:9:06: warning: use of this unit is non-portable and version-dependent
   test_zgeev.adb:10:06: warning: "Interfaces.Fortran.LAPACK" is an internal GNAT unit
   test_zgeev.adb:10:06: warning: use of this unit is non-portable and version-dependent
   gnatbind -x test_zgeev.ali
   gnatlink test_zgeev.ali
   $

However, (a) one could always write ones own versions, (b) the results
are the same with GNAT GPL 2010 and GCC 4.5.0, (c) the output looks good
(from your inputs) ..

   $ ./test_zgeev 
    2.00000000000000E+00  4.00000000000000E+00
    2.00000000000000E+00 -4.00000000000000E+00
    9.99999999999996E-01  2.07319734774360E-16

I get the strong impression that to know what the arguments of the BLAS
& LAPACK subprograms are you have to buy the book or read the code!


This all worked without any need for additional link-time arguments on
Mac OS X, but YMMV on Windows. If not, I *think* that if you with
Ada.Numerics.Long_Complex_Arrays (no need to use anything from it) that
would be enough to bring in the necessary libraries.