how can I use C struct pointers in python using swig?

Question

I have a pointer to a C structure and i need know how i can initialize the struct from python after generating the python library using swig.

I was able to compile the code and run swig without any errors , I was also able to import the library into python .

My aim is to learn how to use pointers with c/swig/python.

This is the c function i want to integrate into python :

#include <stdio.h>
#include "myswig.h"


myData* myhfun(myData *data){

    data->s = data->r1 + data->r2;
    printf("sum(%f+%f)=%f\n", data->r1, data->r2, data->s);
    return data;
}

This is the header file for the same:

//file myswig.h
struct myData{
  double r1;
  double r2;
  double s;
};

typedef struct myData myData;

myData* myhfun(myData *data);

My swig interface file looks like this

/* file: myswig.i */
%module myswig
%{
#include "myswig.h"
%}

/*my function */

myData* myhfun(myData *data);

Script to run swig and test:

swig -python -Isrc myswig.i 
gcc -Isrc -fPIC -I/usr/include/python3.5m -I/usr/include/x86_64-linux-gnu/python3.5m -lpython3.5m -c src/myswig.c myswig_wrap.c
gcc -shared -fPIC -o _myswig.so myswig.o myswig_wrap.o

python3 -c "import myswig"

Answer 1

I found one way to achieve this ,

first write a function which initializes a struct pointer and returns the pointer to the struct,

you call this function from the python side and store the pointer value in a variable ,

now you can pass this variable around as if it were a pointer :).

C functions

#include<stdlib.h>
#include "myswig.h"


    myDataHolder* initStructMem(){
      myDataHolder *myStruct;
      myStruct=malloc(sizeof(myStruct));
      return myStruct;
    }

    myDataHolder* setVal(int a ,int b, myDataHolder *data){
      data->r1=a;
      data->r2=b;
      return data;
    }

    int getSum(myDataHolder *data){
      data->s = data->r1 + data->r2;
      return 0;
    }

    void printVal(myDataHolder *data){
      printf("sum of %d and %d = %d",data->r1, data->r2, data->s);
    }

Python code

import myswig


x = myswig.initStructMem()
y = myswig.setVal(1,2,x)

myswig.getSum(y)
myswig.printVal(y)

Swig definition file

 /* file: myswig.i */
%module myswig
%{
#include "myswig.h"
%}

/*my func */

myDataHolder* initStructMem(void);

myDataHolder* setVal(int a ,int b, myDataHolder *data);
int getSum(myDataHolder *data);
void printVal(myDataHolder *data);