Multi tool use

Transferring Pointers from Python to dll and get modified values back

I am struggeling with the following problem, which is, I guess, not that complicated for people with more advanced C skills.

My intention is to access C++ funtions stored in an DLL from Python.
I managed to get this running for simple functions as ADD and MULT.

As the funtions I'd like to access (later) will return multiple float values I need tu use pointers.

I'd like to do the following basic steps:

-Create a variable in python,

-Transfer the pointer to the dll (ctypes --> c_pointer(...)),

-Update the value the pointer is referring to;

-go back to python and use the modified value.

My C++ funtion I used for dll creation is:

#include <stdio.h>
#include <wchar.h>
#include <iostream>
#include <vector>
#include <math.h>
#include <fstream>
using namespace::std;

extern "C" {

__declspec(dllexport)
int Kappel(double a, double *f){
*f = 25.5;

return 0;
}
} // extern "C"


My python code to call the function in the dll looks like:

import os
import os.path
import ctypes
from ctypes import pointer,c_long,c_int,c_double
lib_path=os.path.dirname(os.path.abspath(__file__))+os.path.sep+'test.dll'
lib = ctypes.CDLL(lib_path)

lib.Kappel.resttype = pointer(c_double())

p = 1.5
Cp = c_double(1.5)
Cp_ = pointer(c_double(p))

def test3(wert, zeiger):
func = lib.Kappel
Res = func(wert, zeiger)
return Res

#Output after modification
print('Returnvalue: ', test3(Cp,Cp_))
print(p)
print(Cp)
print(Cp_)


Thanks for any help
Erik

2 Answers
2

Solved the issue minutes ago.
It was as simple as supposed...

Does not work:

p = 1.5
Cp = c_double(p)
Cp_ = pointer(c_double(p))


Works:

p = 1.5
Cp = c_double(p)
Cp_ = pointer(Cp)


ctypes has byref, which is the preferred way to do what you want:

ctypes

byref

# Given: int Kappel(double a, double *f)
lib.Kappel.argtypes = c_double,POINTER(c_double)
lib.Kappel.restype = c_int

out = c_double() # Create instance of a C type to pass by reference.
lib.Kappel(1.5,byref(out))
print(out.value) # Retrieve the value.


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