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Fortran O my Fortran

June 7, 2014

Ever wondered why research scholars, PhD students and nowadays interns start hating Fortran?

I experienced my share today. And it was amazing. I was in mixed emotions. So fortran comes in to the picture because the Fast Multipole Method (FMM) library is a Fortran library. You can find the FMM in the last century’s top 10 algorithms here. In our N-body simulation problem, we aim to replicate a biological system consisting of a cell membrane and a number of molecules/particles moving within it. The problem has to be scaled to a large number of particles, about a million!! That is how it would be close to a real system.

So I have a C code and my goal is to check whether the Fast Multipole approximation works for this case. How to call a fortran subroutine from a C code?

From what I knew, I needed an object file (.o extension) and linking the correct object file to the fortran code would let me use it. Let me begin by explaining how you call a fortran subroutine from a C code. Step 1 is to create a header file that declares the subroutine you wish to call and what kind of variables you would need.

Fortran reminder number 1 – You must pass 1-dimensional arrays. If you ever happen to pass a 2-dimensional arrays, don’t be scared by the junk values it outputs, because C may not allocate the pointers in contiguous memory. The fortran compiler isn’t smart enough and it fails to locate them. So the simplest and an important note! Avoid 2-dimensional arrays, make use of 1-dimensional arrays. Get used to indexing. But remember!

Fortran reminder number 2 – Fortran interprets 2-dimensional arrays (matrices) by the column major rule. So it’d be better to know this before you start coding in C and using a fortran library.

You create a header file and include it in your C code.

extern void your_fortran_subroutine_(argument list);

and save it as “your_header_file.h”.

Include this .h file in your C code. While compiling your code, just link your object file for the fortran code. Something similar to this:

$ gcc /your_object_file_directory/object_file.o -lgfortran 

But just the way I came to know, its always better to create a library out of all your object files, so that you can link the entire library at once.

$ ar -cvq libctest.a ctest1.o ctest2.o

Be sure that you add an underscore character in your fortran subroutine, this is the standard way of calling fortran from C.

But even if the entire code looks flawless, there may be an error popping out which says “can’t find ‘your_fortran_subroutine_()'”. You may wonder if the compiler isn’t able to locate the fortran object file, or you are missing out on including some necessary files. Finally, after an hour of intense debugging, I came to know that fortran is CASE-INSENSITIVE. Yes people, when fortran was made, it was all in Caps. But slowly, people preferred writing in lower-case and so in order to incorporate this feature without making undesirable tweaks, fortran was made case-insensitive.

And another point to be noted… Fortran interprets matrices by COLUMN MAJOR rule and NOT row major. So what you pass to a fortran subroutine is the transpose of the regular matrix.

Remember guys…. a friend in need, will never ever be fortran indeed 🙂

But as we know, some of the most useful libraries, for example LAPACK, BLAS (linear algebra libraries) are Fortran libraries.

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