Long-overdue Update — ‘f2cy’

Much progress over the past few weeks, I’m glad to say.  Here are the main threads that are out there.

First, the 10000 meter view:  For my GSoC, I’m developing, under the guidance of Dag Sverre Seljebotn, (1) a python package that wraps fortran so it can be easily called from a cython program; (2) improving cython’s buffer support to allow the passing of buffers/arrays to external code (C, fortran, or whatever can be linked in) or cdef functions.

The fortran-side:

Dag Sverre and I discussed a bit about the naming of the fortran wrapper project — right now it’s under the working title “f2cy,” in the long tradition of “<some-language/protocol/format>2<other-language/protocol/format>” converter names.  There are at least 85 similarly-named executables on my ubuntu system (apropos 2 | awk ‘{ print $1 }’ | grep ‘[a-zA-Z]2[a-zA-Z]’ | wc -l).  Since the wrapper and the code it generates has very little direct dependence on python/cython, and since it may be useful for those who want to automate wrapping fortran inside a C program, the name will likely change.  I’m thinking of something along the lines of ‘fwrap’ — Dag suggested ‘fbridge’ to emphasize the connectivity aspect.

f2cy, which is using the fortran parser ‘fparser’ from the latest version of f2py (http://code.google.com/p/f2py/) is able to parse pretty much everything required for the midterm review.  The rest of f2cy is still rapidly developing.  The package lives inside the Tools directory of the cython distribution package. To take a look (it’s still in pre-alpha stage) checkout the mercurial repo here: http://hg.cython.org/gsoc-kurt/.

After fparser generates the parse tree, f2cy makes a few passes on the AST (using the Visitor pattern); one pass adds type ‘annotations’ to the dummy arguments of the subprograms to be wrapped, the next pass generates the autoconfig fortran source file, the next one generates the fortran wrapper.  Only scalar-valued arguments are supported for now, but by August f2cy will support arrays (assumed-shape, assumed-size and explicitly shaped) and passing derived types (structs) end-to-end from cython/C.  How is this done?  Thanks to the iso_c_binding intrinsic module within the Fortran 2003 standard, a Fortran program can be portably linked to C code without too much pain — certainly less pain than was required before.  This module allows f2cy to easily wrap subprograms that take scalars, arrays and derived types — in the distant future, f2cy may support passing C callbacks.  There still is an issue of resolving the kind-type-parameters for the dummy arguments, but that is what the autoconfig file is for.

Our goal in the project is to generate fortran wrappers that are completely portable, in the same way that Cython itself is portable.  A number of projects will generate an extension module with Cython and ship it without a Cython dependence, so the source has to work with any C compiler.  In the same way, f2py will generate wrappers that do not have any compiler-specific knowledge (including the particular values for the kind-type-parameters for the various types).  This isn’t purely academic, since some compilers define integer type KTPs consecutively (1,2,3,…) while for others (gfortran, xlf) the KTPs correspond to the byte-size (1,4,8,10,…).  This makes for some complications and makes the autoconfig file necessary.  More on the autoconfig in a future post.

I’ll address the cython-side in a short while.

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