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Wrapping C function with pointer arguments using SWIG

I'm trying to use SWIG to wrap an existing C library for use in Python. I'm running swig 2.0.10 on Windows XP with Python 2.7.4. The problem I'm encountering is that I'm unable to call a wrapped C function that has a pointer to an int as an argument which is where the function result is to be stored. I've distilled the problem into the follow example code:

The C function in convert.c:

#include <stdio.h>
#include "convert.h"
#include <stdlib.h>

int convert(char *s, int *i)
{
   *i = atoi(s);
   return 0;
}

The header file in convert.h

#ifndef _convert_h_
#define _convert_h_

int convert(char *, int *);

#endif

The swig interface file in convert.i

/* File : convert.i */
%module convert
%{
#include "convert.h"
%}

%include "convert.h"

All of this is being built into a .pyd file using Visual C++ 2010. When the build is complete, I'm left with two files: convert.py and _convert.pyd in the build directory. I open a command window in this directory and start python session and enter the following:

Python 2.7.4 (default, Apr  6 2013, 19:54:46) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import *
>>> import convert
>>> dir(convert)
['__builtins__', '__doc__', '__file__', '__name__', '__package__', '_convert', '_newclass', '_object', '_swig_getattr', '_swig_property', '_swig_repr', '_swig_setattr', '_swig_setattr_nondynamic', 'convert']
>>> i = c_int()
>>> i
c_long(0)
>>> convert.convert('1234', byref(i))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: in method 'convert', argument 2 of type 'int *'

Why is my pointer object being rejected? What should I do to make this work?

Solution

  • SWIG and ctypes are different libraries, so you can't pass ctypes objects directly to SWIG-wrapped functions.

    In SWIG, the %apply command can apply typemaps to common parameter types to configure them as INPUT, INOUT, or OUTPUT parameters. Try the following:

    %module convert
%{
#include "convert.h"
%}

%apply int *OUTPUT {int*};
%include "convert.h"

    Python will no longer require the parameter on input, and will change the output of the function to be a tuple of the return value and any INOUT or OUTPUT parameters:

    >>> import convert
>>> convert.convert('123')
[0, 123]

    Note that parameters beyond the POD (plain old data) types usually require writing your own typemaps. Consult the SWIG Documentation for more details.