calling c from python with ctypes: passing vectors

Question

I want to call a c function from python using ctypes. From the documentation I don\'t understand how to pass pointer to vectors. The function I want to call is:

Answer 1

Based on @Sven Marnach's answer:

#!/usr/bin/env python
import ctypes
import numpy as np
from numpy.ctypeslib import ndpointer

libf = ctypes.cdll.LoadLibrary('/path/to/lib.so')
libf.f.restype = ctypes.c_double
libf.f.argtypes = [ctypes.c_int, ndpointer(ctypes.c_double)]

def f(a):
    return libf.f(a.size, np.ascontiguousarray(a, np.float64))

if __name__=="__main__":
    # slice to create non-contiguous array
    a = np.arange(1, 7, dtype=np.float64)[::2]
    assert not a.flags['C_CONTIGUOUS']
    print(a)
    print(np.multiply.reduce(a))
    print(f(a))

Output

[ 1.  3.  5.]
15.0
15.0

Removing np.ascontiguousarray() call produces the wrong result (6.0 on my machine).