Pass arrays to shared libraries from python using C types

README.md

Pass arrays to shared libraries from python using C types

This example implements a simple shared library in C.

This library called libfifthy exposes a function called add_fifthy which takes an array of integers and modify it in place by adding 50 to all elements.

Then there's some code in python which builds an array and invokes the add_fifthy from the shared library and reads the altered array values.

Usage

Compile the shared lib with:

gcc -g -fPIC -Wall -Werror -Wextra -pedantic *.c -shared -o libfifthy.so

This will produce the file libfifthy.so

Now run the python script with:

python test.py

If all went OK, you should visualize something like the following output:

('Original array: ', [1, 2, 3, 4, 5])
('Array altered from shared lib: ', <__main__.c_int_Array_5 object at 0x10b8aa9e0>)
All values in altered array:
51
52
53
54
55

libfifthy.c

#include <stdio.h>

/**
 * Add 50 to all the elements in the array
 */
void add_fifthy (int arr[], int length) {
  for (int i=0; i < length; i++) {
    arr[i] = arr[i] + 50;
  }
}

test.py

import os
from ctypes import *

script_dir = os.path.abspath(os.path.dirname(__file__))
lib_path = os.path.join(script_dir, "libfifthy.so")

# import shared lib
libfifthy = cdll.LoadLibrary(lib_path)

# define argument types for the add_fifthy function from the shared lib
libfifthy.add_fifthy.argtypes = [POINTER(c_int), c_int]

# build python array
arr =  [1,2,3,4,5]

# allocates memory for an equivalent array in C and populates it with
# values from `arr`
arr_c = (c_int * 5)(*arr)

# Call the C function
libfifthy.add_fifthy(arr_c, c_int(5))

# original array unchanged
print('Original array: ', arr)

# this will print a reference like: <__main__.c_int_Array_5 object at 0x104e7d9e0>
print('Array altered from shared lib: ', arr_c)

# prints all the changed items in a_c
print('All values in altered array:')
for i in arr_c:
  print(i)