Polyglot

Language Cocktails: Mix up something tasty in no time!

Introduction

All languages have their pros and cons. None is perfect.

Imagine that you've found a very useful third-party library written in C. Alas, you're most comfortable writing Fortran. It would take you a very long time to re-write the C code in Fortran and would be tedious to boot. Worst of all, you'd have to understand the C to make the translation and lo, we've come full circle. Far better then to leave the library as it is and to call the routines from your favoured language. In this workshop, we'll look at ways in which we can mix languages and in the process create a useful end product which plays to the strengths of it's components and gets you to where you want to be, without any laborious re-writes.

To get the examples for the practicals, log into your preferred Linux machine and cut and paste the following into your terminal.

svn export http://source.ggy.bris.ac.uk/subversion-open/polyglot/trunk ./polyglot

Wrapping up some Fortran with C

In the first example we'll call a Fortran subroutine from C program:

cd examples/example1

To compile the example, type:

make

and to run it, type:

call_fort.exe

Tada! We've mixed our languages into a single executable and it works! Cool. Very cool. OK, so much for the magic, let's take a look inside the files. Open up sub.f90.

Things to get right:

• use 'nm' or a similar program to investigate the name mangling used by your Fortran compiler, so that you can match the name in your C program.
• matching the sizes of variables
• pass by reference, pass by value
• fortran indexes arrays from 1, by defualt; C from 0.
• The rows/columns are reversed for 2-dimensional arrays in Fortran to C. Arrays in C are 'row-major' and arrays in Fortran are 'column major'.

Turning the Parcel Inside-Out

Now let's work the other way 'round and call some C from a Fortran program:

cd ../example2

Calling some C++ from Python

Here we move from 'mixing', to more of an API arrangement.