Polyglot

Mixed Language Programming: Mix up a quick and useful cocktail today!

Introduction

One of the ey underling themes in this series of pragrmatic programming workshops is getting things done, with a minimum of fuss and wasted effort, and this workshop on mixed language programming is no exception.

When we sit down to a keyboard, we do so with a particular goal in mind. We have a task, idea or experiment and the computer is our workbench. The languages we write programes with, along with the text editors, compilers, debuggers etc. are our tools. Now some tools are better than others (e.g. Subversion is an improvment upon CVS). Some we just may have a preference for (e.g. emacs vs. vi for editing files). None are perfect, however, and all have their pros and cons.

What's all this got to do with mixed language programming? you ask. Well, imagine a scenario:

You sit down to your workbench. You have your goal and you ask yourself whether somebody else has written some code which will do at least part of what you want. Perhaps they've bundled it up into a nice open-source library that you can use? That way you'll save truck-loads of time. A couple of web searches later, and bingo! You've found an ideal library for the job. There's only one snag. You like programming in Fortran, and the library is written in C. Scuppered! Well, perhaps not..

What are your options? You could use something not so suitable because it happens to be written in Fortran. Hmm, that doesn't sound so good. You could translate the library from C to Fortran. Hmm, that sounds like tedious and time-consuming work. Plus you'd need to understand the C, and perhaps a direct translation can't be made anyhow? This is looking like a dead-end too. It would be far better to leave the library as it is and to call the routines from your favoured language. Is that possible? Sure it is! Read on and find out how..

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. In the first two examples we'll look at calling C code from Fortran and then calling Fortran from C. To get the code for examples, 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

The cfortran Header File Project

• [1]

Calling C from Python using SWIG

NB You will need swig installed on your machine for this example. swig is instaled on dylan

Currently as per:

• [2]

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