I successfully compiled a shared library for use with
CTypes and linked it to an external library (Gnu
Scientific Library) on Mac OS X 10.4. I hope this
helps Mac people and anyone else who wants to use
CTypes to access their own C extensions and use other
C libraries in the process. I want to thank several
people on this list who gave me many helpful
suggestions and asked me good questions. I also want
to thank the several people who kept nudging me to try
CTypes even though I was reluctant. It is much easier
than programming an extension all in C.
Below are 4 files that enable building of a C shared
library in Mac OS X (10.4) that can be used with
CTypes to call a function from the Gnu Scientific
Library (a Bessel function program gsl_sf_bessel_J0).
You can see that the idea is pretty simple. The code
requires that you have ctypes (in site-packages) and
GSL (dynlib version in /usr/local/lib) or your desired
C library installed. I suspect on other platforms
what will be different will be the make file. I do
not know enough to provide Linux or Windows versions.
I'm sorry.
Note: This works best if the libraries are shared
(e.g. the GSL library to use is the dynlib version).
That way only the code that's needed is loaded when
the C functions are called from python.
Comments welcome. Of course, I am responsible for any
and all mistakes. So, I make no guarantees or
warrenties. These are examples and should not be used
where loss of property, life, or other dangers exist.
#==== Source code 'bess.c' ======================
#include <stdio.h>
#include "bess.h"
#include <gsl/gsl_sf_bessel.h> /* Must include the
header to define the function for compiler */
/* ---- test fcns ------------------------- */
#ifdef __cplusplus
extern "C" {
#endif
double J0_bess (double x) {
/* Call the GSL Bessel function order 0 of the first
kind */
double y = gsl_sf_bessel_J0 (x);
/* Print the value right here */
printf ("J0(%g) = %.18e\n", x, y);
return y;
}
#ifdef __cplusplus
}
#endif
#==== Header file 'bess.h' =====================
/* ---- Prototypes -------------------- */
#ifdef __cplusplus
extern "C" {
#endif
double J0_bess(double x);
#ifdef __cplusplus
}
#endif
#==== Make file 'bess.make' ===================
# ---- Link to existing library in this directory
------------
bess.so: bess.o bess.mak
gcc -bundle -flat_namespace -undefined suppress -o
bess.so bess.o -lgsl
# ---- gcc C compile ------------------
bess.o: bess.c bess.h bess.mak
gcc -c bess.c -o bess.o
#==== Python file 'bess.py' =======================
#!/usr/local/bin/pythonw
import numpy as N
import ctypes as C
# Put the name of your library in place of 'bess.so'
and the path to
# it in place of the path below in load_library
_bess = N.ctypeslib.load_library('bess.so',
'/Users/loupecora/Code_py/test_folder/ctypes_tests/test3ctypes/simplelink-GSL/')
_bess.J0_bess.restype = C.c_double
_bess.J0_bess.argtypes = [C.c_double]
def fcn_J0(x):
return _bess.J0_bess(x)
x = 0.2
y = fcn_J0(x)
print "x, y: %e %.18e" % (x, y)
#==== Typical output ===============
# The first line is printed from the shared library
function J0_bess
# The second line is from the python code that called
the shared lib. function
J0(0.2) = 9.900249722395765284e-01
x, y: 2.000000e-01 9.900249722395765284e-01
-- Lou Pecora, my views are my own.
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