Passing multiple parameters and allocating strings in C using Swig/Python

Question

I am using SWIG to wrap the following C interface to access it from Python:

void some_method(char **output, int paramA, const char *paramB, int paramC);
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Answer 1

%cstring_output_allocate does work with multiple parameters. It declares "If you see char **output as a parameter to any method, hide the parameter and return it as an additional output.

Here's an example. I declare two methods: one that returns a value and one that doesn't. Note in the output how the output parameter is not passed, but it's result is returned as an additional return value.

Example

x.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

void some_method(char **output, int paramA, const char *paramB, int paramC)
{
    *output = malloc(paramA);
    sprintf_s(*output,paramA,"%s_%d",paramB,paramC);
}

int some_method2(char **output, int paramA, const char *paramB, int paramC)
{
    *output = malloc(paramA);
    sprintf_s(*output,paramA,"%s_%d",paramB,paramC);
    return strlen(*output);
}

x.h

void some_method(char **output, int paramA, const char *paramB, int paramC);
int some_method2(char **output, int paramA, const char *paramB, int paramC);

x.i

%module x

%begin %{
#pragma warning(disable:4100 4127 4211 4706)
%}

%{
#include "x.h"
%}

%include<cstring.i>
%cstring_output_allocate(char **output, free(*$1));
%include "x.h"

makefile

_x.pyd: x.c x_wrap.c x.h
    cl /LD /W4 /MD /Ic:\python27\include x.c x_wrap.c -link /LIBPATH:c:\python27\libs

x_wrap.c: x.i x.h
    swig -python x.i

Output

Python 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import x
>>> x.some_method(100,'blah',123)
'blah_123'
>>> x.some_method2(100,'blah',123)
[8, 'blah_123']

Answer 2

Not a direct answer, but what you're working on sounds sufficiently low-level that you may want to consider forgoing SWIG and using the CPython api directly. SWIG is great, but it adds another dependency and loads of generated code.