/* libadd.c */
#include <stdio.h>

extern int var;

int add(int a, int b) {
    return (a + b + var);
}

gcc -c -Wall -Werror -fpic libadd.c
gcc -shared -o libadd.so libadd.o

/* test.c */
#include <stdio.h>
#include "libadd.h"

int var = 1;

int main(void) {
    printf("%d", add(1,2));
    return 0;
}

gcc -L/path/to/libadd -Wall -o test test.c -ladd

# test.py
from ctypes import *

#what to do to define var?

sharedlib = CDLL("sharedlib/libadd.so", mode = RTLD_GLOBAL)
print( sharedlib.add(1,2) )

OSError: sharedlib/libadd.so: undefined symbol: var