Question

I attempting to call into existing C code from Python. The C code defines a struct B that contains an struct array of As. The C code also defines a function that puts values into the structure when called. I can access the array member variable, but it is not an list (or something that supports indexing). Instead, I am getting an object that is a proxy for B*.

I found this question, but it doesn’t look like it was completely resolved. I’m also not sure how to make an instance of the Python class B to replace the PyString_FromString().

Below is the code needed to demonstrate my issue and how to execute it:

example.h

typedef struct A_s
{
  unsigned int thing;
  unsigned int other;
} A_t;

typedef struct B_s
{
  unsigned int size;
  A_t items[16];
} B_t;

unsigned int foo(B_t* b);

example.c

#include "example.h"

unsigned int
foo(B_t* b)
{
  b->size = 1;
  b->items[0].thing = 1;
  b->items[0].other = 2;
}

example.i

%module example
%{
#define SWIG_FILE_WITH_INIT
#include "example.h"
%}

%include "example.h"

setup.py

from distutils.core import setup, Extension

module1 = Extension('example', sources=['example.c', 'example.i'])
setup(name='Example', version='0.1', ext_modules=[module1])

script.py – This uses library and demonstrates the failure.

import example
b = example.B_t()
example.foo(b)
print b.size
print b.items
for i in xrange(b.size):
    print b.items[i]

How to run everything:

python setup.py build_ext --inplace
mv example.so _example.so
python script.py

Answer 1

you could write some C helper functions like:

int get_thing(B_t *this_b_t, int idx);
int get_other(B_t *this_b_t, int idx);
void set_thing(B_t *this_b_t, int idx, int val);
void set_other(B_t *this_b_t, int idx, int val);

wrap these and you can then use the pointer that you get from example.B_t() to access values from within your data-structure arrangement, e.g.

>>> b = example.B_t()
>>> a_thing = example.get_thing(b, 0)
>>> example.set_thing(b,0,999)

Hopefully its obvious what the implementation of these C functions should be – if not I could provide an example…

granted this isn’t as seamless as being able to access C arrays as python lists – you might be able to use typemaps to achieve this but I can’t remember the exact syntax you would need in this instance – you’d have to do a bit of RTFM on the SWIG documentation

also possible duplicate here: nested structure array access in python using SWIG