Ctypes returns wrong Result

Question

fI try to wrap ac function using ctypes, for example:

#include<stdio.h>

typedef struct {
    double x;
    double y;
}Number;

double add_numbers(Number *n){
    double x;
    x = n->x+n->y;
    printf("%e \n", x);
    return x;
}

I compile the c file with the option

gcc -shared -fPIC -o test.so test.c 

to a shared library.

The Python code looks like this:

from ctypes import * 

class Number(Structure):
    _fields_=[("x", c_double),
              ("y", c_double)]

def main():
    lib = cdll.LoadLibrary('./test.so')
    n = Number(10,20)
    print n.x, n.y
    lib.add_numbers.argtypes = [POINTER(Number)]
    lib.add_numbers.restypes = [c_double]

    print lib.add_numbers(n)

if __name__=="__main__":
    main()

The printf statement in the add_numbers function returns the expected value of 3.0e+1, but the return value of the lib.add_numbers function is always zero. I don't see the error, any Idea?

Answer 1

Change this:

lib.add_numbers.restypes = [c_double]

to this:

lib.add_numbers.restype = c_double

Note that it is restype , not restypes .