Calling global variable of C in Python

Question

I want to access the global variable of c in python. I can already use/access the functions of c in python using ctypes after making a .so file. Here's the C code :

#include <stdio.h>

int globalVar = 2;

void add_int(int num1,int num2){
    globalVar = num1+num2;
    fprintf(globalVar, "%d\n", );
}

float add_float(float num1,float num2){
    return num1+num2;
}

and the Python Code:

from ctypes import *

adder = CDLL('/home/akshay/Desktop/ffmpeg/linking/addr.so')

adder.add_int(4,5)

var = c_int.in_dll(adder,"globalVar")
print "Sum of 4 and 5 = " + str(adder.add_int(4,5))

a = c_float(5.5)
b = c_float(4.1)

add_float = adder.add_float
add_float.restype = c_float
print "Sum of 5.5 and 4.1 = ", str(add_float(a, b))

[SO]: How to create a Minimal, Complete, and Verifiable example (mcve). — CristiFati
– CristiFati, Commented Jun 20, 2018 at 14:55
See Accessing values exported from dlls from the python documentation. — Neitsa
– Neitsa, Commented Jun 20, 2018 at 16:20

Mark Tolonen · Accepted Answer · 2018-06-20 18:37:49Z

1

Given this one-line source for a Windows DLL:

__declspec(dllexport) int test = 5;

You access global variables by using the .in_dll() method of a ctypes type, which takes a ctypes DLL reference and the name of the global variable:

>>> from ctypes import *
>>> dll = CDLL('test')   # test.dll of the above source.
>>> c_int.in_dll(dll,'test')
c_long(5)

answered Jun 20, 2018 at 18:37

Mark Tolonen

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5 Comments

Akshay Kumar Meena Over a year ago

I want to use it in ubuntu. So how to implement this for .so files

Mark Tolonen Over a year ago

@Akshay Same Python code, I just had a Windows system to test with. Just export a global variable in a .so file.

Akshay Kumar Meena Over a year ago

It is showing me the error of undefined symbol. ValueError: /home/akshay/Desktop/ffmpeg/linking/addr.so: undefined symbol: globalVar

Mark Tolonen Over a year ago

You may need something like __attribute__ ((visibility ("default"))) in Linux to do the equivalent of Windows' __declspec(dllexport) to export the definition from the shared library. See gcc.gnu.org/wiki/Visibility. I don't have a Linux account handy to work it out.

Mark Tolonen Over a year ago

@Akshay extern int globalVar; may also work. My Linux is rusty.

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