How can I pass an array to shared library(.dll) written in c using python

Question

the function code of test.dll file:

double __cdecl add(int len,double array[]){}

(and I have tested it in vc)

python code:

import ctypes
from ctypes import *

N=...
arr=(c_double*N)()
...
...
dll=CDLL("test.dll")
sum=dll.add(c_int(N),byref(arr))

print sum

but the python code doesn't work, and the "sum"is always equal to N .(i.g. when N=10 ,it print"sum=10") what's wrong with it?

Answer 1

Passing an array in ctypes mirrors passing an array in C, i.e. you do not need to pass a reference to it as the array is already a reference to its first element.

from ctypes import *

N = ...
arr=(c_double*N)()
dll=CDLL("test.dll")
sum=dll.add(c_int(N),arr)

print sum

An example of this can be seen in the ctypes documentation under the callbacks section when discussing interfacing with qsort.

Edit: As per David Heffernan's comment, you also require a dll.add.restype = c_double otherwise ctypes will assume the returned type is c_int.