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).

Answer 2

You can call it like this:

#!python
from ctypes import *

#!python
from ctypes import *

# double f(int n, double* x)
f = CDLL('/path/to/lib.so').f
f.argtypes = [c_int, POINTER(c_double)]
f.restype = c_double

if __name__ == '__main__':
    array = (c_double * 5)(1, 2, 3, 4, 5)
    r = f(len(array), array)
    print(r)

If you have numpy array, you can use numpy.array.ctypes.data_as:

#!python
from ctypes import *
import numpy

# double f(int n, double* x)
f = CDLL('/path/to/lib.so').f
f.argtypes = [c_int, POINTER(c_double)]
f.restype = c_double

if __name__ == '__main__':
    array = numpy.array([1, 2, 3, 4, 5])
    r = f(array.size, array.astype(numpy.double).ctypes.data_as(POINTER(c_double)))
    print(r)

or:

#!python
from ctypes import *
import numpy

# double f(int n, double* x)
f = CDLL('/path/to/lib.so').f
f.argtypes = [c_int, POINTER(c_double)]
f.restype = c_double

if __name__ == '__main__':
    array = numpy.double([1, 2, 3, 4, 5])
    r = f(array.size, array.ctypes.data_as(POINTER(c_double)))
    print(r)

Answer 3

apparently I needed to specify the flag contigous to make it work

http://scipy-lectures.github.io/advanced/interfacing_with_c/interfacing_with_c.html

this is an extract from my code:

array_1d_double = numpy.ctypeslib.ndpointer(dtype=numpy.double, ndim=1, 
                                            flags='CONTIGUOUS')
libc.f.argtypes = [c_int, array_1d_double]
libc.f(n, x)