Using SWIG with pointer to function in C struct

Question

I'm trying to write a SWIG wrapper for a C library that uses pointers to functions in its structs. I can't figure out how to handle structs that contain function pointers. A simplified example follows.

test.i:

/* test.i */

%module test
%{

typedef struct {
    int (*my_func)(int);
} test_struct;

int add1(int n) { return n+1; }

test_struct *init_test()
{
    test_struct *t = (test_struct*) malloc(sizeof(test_struct));
    t->my_func = add1;
}
%}

typedef struct {
    int (*my_func)(int);
} test_struct;

extern test_struct *init_test();

sample session:

Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) 
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import test
>>> t = test.init_test()
>>> t
<test.test_struct; proxy of <Swig Object of type 'test_struct *' at 0xa1cafd0> >
>>> t.my_func
<Swig Object of type 'int (*)(int)' at 0xb8009810>
>>> t.my_func(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'PySwigObject' object is not callable

Anyone know if it's possible to get t.my_func(1) to return 2?

Thanks!

Steve · Accepted Answer · 2009-10-19 00:31:42Z

I found an answer. If I declare the function pointer as a SWIG "member function", it seems to work as expected:

%module test
%{

typedef struct {
  int (*my_func)(int);
} test_struct;

int add1(int n) { return n+1; }

test_struct *init_test()
{
    test_struct *t = (test_struct*) malloc(sizeof(test_struct));
    t->my_func = add1;
    return t;
}

%}

typedef struct {
    int my_func(int);
} test_struct;

extern test_struct *init_test();

Session:

$ python
Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) 
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import test
>>> t = test.init_test()
>>> t.my_func(1)
2

I was hoping for something that wouldn't require writing any custom SWIG-specific code (I'd prefer to just "%include" my headers without modification), but this will do I guess.

sambowry · Accepted Answer · 2009-10-17 22:05:01Z

0

You forget to "return t;" in init_test():

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

typedef struct {
 int (*my_func)(int);
} test_struct;

int add1(int n) { return n+1; }

test_struct *init_test(){
  test_struct *t = (test_struct*) malloc(sizeof(test_struct));
  t->my_func = add1;
  return t;
}

int main(){
  test_struct *s=init_test();

  printf( "%i\n", s->my_func(1) );
}

answered Oct 17, 2009 at 22:05

sambowry

2,4761 gold badge16 silver badges13 bronze badges

1 Comment

Steve Over a year ago

You're right, thanks. Sorry it was an error in transcribing my question from the test code I was using.

ultimate-anti-reversing · Accepted Answer · 2023-12-17 22:17:04Z

0

There is a current work with already satisfacting result about to have C bindings from C++ code.

So the simplest way seems to

Create C++ code instead of C.
Clone this unmerged branch https://github.com/swig/swig/pull/2086 and to compile from source.
Create python bindings.

Then you could use the -c option in order to provide C bindings as well as python. Then your code will be avaible in C as well as in Python.

answered Dec 17, 2023 at 22:17

ultimate-anti-reversing

212 bronze badges

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