Polyglot

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

Introduction

One of the key underling themes in this series of pragmatic 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

Making a Fortran wrapped C parcel

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

cd examples/example1

To compile the example, type:

make

and to run it, type:

call_c.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 call_c.f90. In this simple program, we have a character string that we pass to the C function--called cfunc--which in turn modifies the string and we can print the result. For those familiar with Fotran90, the main program is trivial enough. So, let's turn to the C code in func.c. Again this is a simple function and the syntax will be familiar to those who know some C. An interesting detail, however, is the name of the function. Note that we have called it cfunc_, i.e. with a trailing underscore. Why on earth have we done that? Well, we are anticipating the 'decoration' or name mangling that the compiler will apply.

We can look inside the object files using a utility called nm. First let's look inside call_c.o. On my machine I get:

> nm call_c.o
00000000 T MAIN__
         U _gfortran_set_std
         U _gfortran_st_write
         U _gfortran_st_write_done
         U _gfortran_transfer_character
         U cfunc_

These are the symbols created by the compiler in the object file. Now let's look inside func.o:

>  nm func.o
00000000 T cfunc_

Note the two cfunc_s? They match! If we had not pre-decorated the name of the function in func.c, we would not have got a match and we would have got a link-time error. However, we were smart and got ourselves a working executable--hurrah!

A word of caution, however. Different compilers 'mangle' subroutine and function names differently, i.e. dependening upon the mix of compilers, we can't always rely on a single trailing underscore decoration. You will need to use nm to determine the exact decoration your Fortran compiler expects and to design your code so that any changes are easily accomodated. One way to do this is to include a define preprocessor macro in you C code, such as:

#define CFUNC cfunc_
...
void CFUNC(int* string_size, char* string)

In that way, a change of decoration can be easily, and consistently made across the whole file/library.

Turning the Parcel Inside-Out

OK let's turn our mind to working the other way 'round and call a Fortran subroutine from a C program:

cd ../example2

We can compile and run in a similar way:

make
./call_fort.exe

If you look inside the two key files, sub.f90 and call_fort.c, you can see that we've still kept the program fairly simple, but we've introduced a few more variable types (integers, floating-point numbers and arrays) and syntactic refinements (use of preprocessor macros and 'const' and 'intent(in)' declarations).

The decoration and name-mangling will be familiar to you from the last example. Note the use of the define macro in call_fort.c:

#define FORT_SUB fort_sub_

The code comments in the last example touched upon the distinction between the pass-by-value default behaviour of C and the pass-by-reference adopted by Fortran. Thus, in order to dovetail the two languages, we must prevail upon our C code to use pass-by-reference. We can see this in the function prototype in call_fort.c:

extern void FORT_SUB(const int*, const int*, float[]);

Here we are declaring that we will use pointers-to variables, i.e. addresseses or references, as the arguments to the routine, denoted by the *s. The routine, of course, is written in Fortran, and so is expecting these references. By default, C would have taken a copy of the value of a variable and passed that. This is where the term pass-by-value comes from. (Note that a side effect of pass-by-value is that any changes are only local to a routine and so do not have an effect on the value of the variable used in the calling routine.) Note the the const modifiers match with the intent(in) attributes in sub.f90. This is an example of defensive programming. Normally, when we pass references, we run the risk that called routine could modify the values of the variables in a lasting way. This may not be what you would like, and so it would be unsafe. However, by using const and intent(in), we are saying from both sides that some values are not to be changed and so we protect ourselves.

In general with mixed langauge programming, we need to be careful about mathing the variable types we pass between the languages. For example, C typically uses 4 bytes to store a float. However, Fortran may use 4 or 8 bytes to store a real, depending upon the type of processor in the machine (32 ior 64 bit). We need to be careful to ensure a match, otherwise we could have all sorts errors in store realated to truncation or uninitialsed memory space. Thus in sub.f90, we have limited our variables to 4 bytes:

  integer(kind=4),intent(in)                     :: arg_in
  integer(kind=4),intent(in)                     :: array_size
  real(kind=4),dimension(array_size),intent(out) :: arg_out

An intersting point of difference to note between Fortran and C is that arrays are indexed differently. Firstly, Fortran gives the first element of an array the index 1, by default. C gives it an index of 0. Compare the two loops in the source code files. First for the calling C program:

  /* initialise array to ones */
  for(ii=0;ii<MAXSIZE;++ii) {
    from_fort[ii] = 1.0;
  }

and second for the Fortran subroutine:

  ! just set output to be equal to input
  do ii=1,array_size
     arg_out(ii) = real(arg_in)
  end do

Things diverge further for multi-dimensional arrays. This is because arrays in C are 'row-major' and arrays in Fortran are 'column major' in their ordering. Thus for a defualt 2-dimensional array, the cell (3,7) in C would map to (8,4) in Fortran. Oh the joys of mixed languages! Thankfully, since program units are typically self-contained and we would pass arrays in their entirity (actually just the memory address of the first cell), this difference in behaviour does not cause many headaches.

Fortran 2003 Improves Matters

cd ../example3

make
./call_c.exe

Exchanging Fortran derived types for C structures

Mapping user-derived types from Fortran to structures in C and vice versa used to be a somewhat fraught affair. Matters have been significantly improved with the advent of Fortran2003. However, we will see that full support is still not available and limits remain upon what you can achieve.

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

cd ../exampleX

make

./test.py

Currently as per:

• [2]

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