On Tuesday, 15 May 2012 16:23:14 Armin Rigo wrote: > Hi all, Hi, > Fijal and me would like to raise interest among various groups of > people about building a better ctypes replacement for Python.
I have a suggestion, and it (possibly) involves using ctypes...
> Opinions? Interests? This mail is deliberately low on details about
> how we think we can do it. Instead I'm seeking general reactions for
> now and would like to move this soon to its own project, independent
> of PyPy.
First, I think when one interfaces with a C(++) library a limited
understanding of C(++) is required, at least the minimum of data types,
structures and (to an extent) memory management (think pointers and who owns
which structures).
Using that understanding one could then use tool X to create a pythonic
wrapper around the underlying C CPI and expose a "pure" python API to the rest
of the world.
For me, the ideal would be (using a trivial [bad] example):
<module name="sysctl.py>
import ctypes
# See sysctl(3) for details (from FreeBSD)
# Note 1
from c.sys import sysctl as _sysctl #include <sys/sysctl.h>
__all__ = ["get", "set"]
def get(name):
# Assume all values are c_int (for now)
cint = ctypes.c_int()
# Note 2
_sysctl.sysctl(name, len(name), cint, ctypes.sizeof(cint), None, 0)
return cint.value
def set(name, value):
# Assume all values are c_int (for now)
# Note 3
_sysctl.sysctl(name, len(name), None, 0, value,
ctypes.sizeof(ctypes.c_int))
</module>
Notes:
1) The module "c" reads the C header file <sys/sysctl.h> and generates (or
loads a cached result, possible bundled with the program) glue that provides a
low level interface to the C API described in that header file.
2) See (*) for the description of _sysctl.sysctl. The wrapper for sysctl(3)
knows that the third parameter is needed by reference (and that we want the
stored value), the 5th parameter None translates to NULL.
3) See (*), the wrapper knows the 5th parameter is needed by reference (and
that we don't want the result).
I think the trick here is automating the tedious description of the C API in
ctypes (note, the above doesn't need to be done in ctypes, but can be thought
of a pythonic interface to low-level C). AFAIK LLVM provides python binding
for the language and (with clang) could possibly provide all the information
required to parse the C header files, then python can be used to generate the
interface provided by LLVM (this, to my understanding, is starting to sound
like rpython...).
I think it is quite possible that the interface modules for the C API could be
pre-generated (in rpython, cython, etc and compiled) and distributed with the
python codes (similarly to how C/Python mixed code works).
Which, all the above, is very similar to the Lua FFI library, just on a
different scale.
Regards,
David
P.S. Would it not be possible for the pypy jit to inline c macros, given the
right information ;-)
(*) int sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
const void *newp, size_t newlen);
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