[CMake] Extension proposal: CheckFortranFuinctionExists.cmake

[Bernd Schubert]

On Wednesday 25 June 2008, Alan W. Irwin wrote:
> On 2008-06-25 15:15+0200 Hauke Heibel wrote:
> > Hi there,
> >
> > I started diving into the world of CMake and spend the last two days in
> > trying to convert a project of mine to be working with CMake.
> >
> > We are using BLAS in the project, so obviously I gave FIND_PACKAGE(BLAS)
> > a shot. In the beginning it was not really working, but I found a
> > workaround which I would like to discuss - maybe it will be useful for
> > others as well.
> >
> > So here is the scenario. I am given a machine having only access to
> > static pre-compiled libraries of BLAS (in my case atlas.lib, cblas.lib
> > and f77blas.lib). It would be great if the FIND_PACKAGE script would
> > allow me to use these libraries even when I do not have a Fortran
> > compiler at hand. Why should I need it anyways - the libs are
> > pre-compiled.
> >
> > So in the first step I managed to get FindBLAS.cmake running that far,
> > that it finds the libs but as said before, it was failing due to the
> > missing Fortran compiler. To resolve this final issue I did the
> > following.
> >
> > Since Fortran compiled libraries can be used from C/C++, why not simply
> > create a test based on a .cpp file. I gave it a try and it seems to work:
>
> I think that assumption needs lots of qualifiers.  Did your run-time
> actually give correct numerical results when linking from a C main
> programme to f77blas.lib? Interfacing C/C++ and Fortran is a tricky
> business. There is a project called cfortran that tries to do it for all
> the different Unix variants, but it is pretty complicated because of
> various naming conventions on each brand of Unix. Because of those
> complications, systems often have both C and Fortran versions of BLAS
> installed.  For example, it appears your system has cblas.lib which is the
> library I think you want to link to from C/C++ code.

No, it is really no problem to link fortran libraries and to call their 
functions. It is onkly a bit nasty sometimes to write the proto types of the 
blas/lapack functions, since nobody seems to have created a complete set of 
these, afaik. For real number crunching one also uses the intel/amd or goto 
libraries and there is always only one library of these, for both C and 
Fortran (afaik, these optimized libraries are also not written in fortran 
anymore, they just have this stupid interface to be compatible with fortran.
The other issue with fortran libraries is to figure out, how these have been 
compiled (additional underscores, etc).

On the other hand, cblas even has a slightly different interface than nativ 
blas, so without changing the existing code it is not suitable for linking.

Cheers,
Bernd