Accessing the c pointer to structure in python

Question

Is it possible to cast an int to a class type ?

I have the following code in C:

#include "Python.h"

#define PYTHON_FILENAME "modelparam"

void getmodelparam(long pModelParam) ;

typedef struct {
 int seconds;
 int nanoseconds;
} someTime;


int main ()
{
someTime *pSome ;
long a ;
printf ("Testing the python interfaces\n") ;

pSome = (someTime *) calloc(1, sizeof(someTime)) ;

pSome->seconds = 10 ;
pSome->nanoseconds = 20 ;

a = (long) pSome ;
printf ("a is %d, pSome is %d\n", a, pSome) ;

getmodelparam(a) ;  

printf ("After the python call values are : %d, %d\n",pSome->seconds, pSome->nanoseconds) ; 

return 0 ;

}
void getmodelparam(long pModelParam)
{
    PyObject    *pName ;
PyObject    *pModule ; 
PyObject    *pDict ;
PyObject    *pFunc ;
int         iSize = 0 ;
char        pcFunctionName[] = "modifymodelparam" ;

double      dTemp1, dTemp2 ; 

/* Initialize the python interpreter */
Py_Initialize() ;

/* Get Python code/module */
pName = PyUnicode_FromString(PYTHON_FILENAME);
if (NULL != pName)
{
    /* Import the module equivalent to doing 'import calresidual' in python */
    pModule = PyImport_Import(pName);
    Py_DECREF(pName) ;
    if (NULL != pModule)
    {
        /* Get the function and check if its callable function */   
        pFunc = PyObject_GetAttrString(pModule, pcFunctionName);
        if (pFunc && PyCallable_Check(pFunc))
        {
            /* Build the input arguments */
            PyObject *pResult = PyObject_CallFunction(pFunc,"i", pModelParam) ;
        }
        else
        {
            printf ("Some error with the function\n") ;
        }
    }
    else
    {
        printf ("Couldnt load the module %s\n", PYTHON_FILENAME) ;
    }
}
else
{
    printf ("Couldnt convert the name of the module to python name\n") ;
}
/* Release the resources. */
Py_DECREF(pModule) ;
Py_DECREF(pFunc) ;
Py_DECREF(pName) ;

/*Release the interpreter */
Py_Finalize() ;
}

And in Python code:

import ctypes

class someTime(ctypes.Structure):
     _fields_ = [("seconds", ctypes.c_uint),
                 ("nanoseconds", ctypes.c_uint)]  

def modifymodelparam(m):
# Not sure how to access internal elements using m ?? 
    # How to typecast m ?? 
    n = someTime(m)
print ('Seconds', n.seconds)

How can typecast the address passed from C to a class type in Python so that I can access those class parameters or indirectly saying accessing the structure parameters?

score 1 · Accepted Answer · answered Mar 24 '14 at 07:04

Well there are some mistakes in the above code.

Need to pass the pointer address as an unsigned long/int to the python module. So

void getmodelparam(long pModelParam)

//has to become

void getmodelparam(unsigned long pModelParam)

//and 

PyObject *pResult = PyObject_CallFunction(pFunc,"i", pModelParam) ;

// has to become 

PyObject *pResult = PyObject_CallFunction(pFunc,"k", pModelParam) ;

and then in the python file:

def modifymodelparam(m):
    n = ctypes.cast(m, ctypes.POINTER(someTime))
    print (n.contents.seconds)
    print (n.contents.nanoseconds)

score 0 · Answer 2 · answered Mar 23 '14 at 07:07

0

To typecast, cast function can be used

n = ctypes.cast(m, someTime)

[Google]http://docs.python.org/2/library/ctypes.html

answered Mar 23 '14 at 07:07

user1118142

43
1
5

I have already tried this one. But I am not able to get the contents by doing n.contents or n[0] as listed in the same document. – Sagar Masuti Mar 23 '14 at 08:02

Accessing the c pointer to structure in python

2 Answers2