How does Python receive a string from a C function?

Question

C code:

#include "Python.h"
#include <windows.h>
__declspec(dllexport) PyObject* getTheString()
{
    auto p = Py_BuildValue("s","hello");
    char * s = PyString_AsString(p);
    MessageBoxA(NULL,s,"s",0);
    return p;
}

Python code:

import ctypes
import sys
sys.path.append('./')
dll = ctypes.CDLL('pythonCall_test.dll')
print type(dll.getTheString())

Result:

<type 'int'>

How can I get pytype(str)'hello' from C code? Or is there any Pythonic way to translate this pytype(int) to pytype(str)?

It looks like no matter what I change，the returned Pyobject* is a pytype(int) no else

Just thinking out loud: in the c code you are returning a PyObject pointer, which is just an address actually. And the python side is receiving this as an integer (since it is just an address) — Selçuk Cihan
– Selçuk Cihan, Commented Dec 28, 2015 at 8:46

Assem · Accepted Answer · 2015-12-28 10:46:31Z

2

By default functions are assumed to return the C int type. Other return types can be specified by setting the restype attribute of the function object. (ref)

Define the type returned by your function like that:

>>> from ctypes import c_char_p
>>> dll.getTheString.restype = c_char_p # c_char_p is a pointer to a string
>>> print type(dll.getTheString())

answered Dec 28, 2015 at 10:46

Assem

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1 Comment

rookie Over a year ago

can i return a Python type to python code? i wanna python code receive python value from C code.but thank u for the answer

PM 2Ring · Accepted Answer · 2016-01-01 08:01:52Z

0

int is the default return type, to specify another type you need to set the function object's restype attribute. See Return types in the ctype docs for details.

edited Jan 1, 2016 at 8:01

answered Dec 28, 2015 at 9:06

PM 2Ring

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