The answer by Chinmay Kanchi is excellent but I wanted an example of a function which passes and returns a variables/arrays to a C++ code. I though I'd include it here in case it is useful to others.
Passing and returning an integer
The C++ code for a function which takes an integer and adds one to the returned value,
extern "C" int add_one(int i)
{
return i+1;
}
Saved as file test.cpp, note the required extern "C" (this can be removed for C code).
This is compiled using g++, with arguments similar to Chinmay Kanchi answer,
g++ -shared -o testlib.so -fPIC test.cpp
The Python code uses load_library from the numpy.ctypeslib assuming the path to the shared library in the same directory as the Python script,
import numpy.ctypeslib as ctl
import ctypes
libname = 'testlib.so'
libdir = './'
lib=ctl.load_library(libname, libdir)
py_add_one = lib.add_one
py_add_one.argtypes = [ctypes.c_int]
value = 5
results = py_add_one(value)
print(results)
This prints 6 as expected.
Passing and printing an array
You can also pass arrays as follows, for a C code to print the element of an array,
extern "C" void print_array(double* array, int N)
{
for (int i=0; i<N; i++)
cout << i << " " << array[i] << endl;
}
which is compiled as before and the imported in the same way. The extra Python code to use this function would then be,
import numpy as np
py_print_array = lib.print_array
py_print_array.argtypes = [ctl.ndpointer(np.float64,
flags='aligned, c_contiguous'),
ctypes.c_int]
A = np.array([1.4,2.6,3.0], dtype=np.float64)
py_print_array(A, 3)
where we specify the array, the first argument to print_array, as a pointer to a Numpy array of aligned, c_contiguous 64 bit floats and the second argument as an integer which tells the C code the number of elements in the Numpy array. This then printed by the C code as follows,
1.4
2.6
3.0