How can I pass a pointer to array of pointers to structs from python

Question

I am using ctypes in Python and I need to pass a pointer to an array of pointers to structs to some C function. This is struct:

typedef struct {
    float x;
    float y;
    float z;
    float radius;
} Sphere;

And I have function with the following prototype:

void render(Sphere** spheres);

In Python I declared a class for the Sphere struct and I need to set argtypes to the render function:

lib_render = ctypes.cdll.LoadLibrary('librender.so')

class Sphere(ctypes.Structure):
    _fields_ = [('x', ctypes.c_float),
                ('y', ctypes.c_float),
                ('z', ctypes.c_float),
                ('radius', ctypes.c_float)]

render = lib_render.render
render.argtypes = [<cannot find out what needs to be here>]

spheres = numpy.array([Sphere(1, 2.8, 3, 0.5),
                       Sphere(4.2, 2, 1, 3.2)])
render(spheres)

How to pass that array correctly?

I have not tested this, but I'd assume it would be ctypes.POINTER(ctypes.POINTER(Sphere)) — jacob
– jacob, Commented Nov 3, 2017 at 0:56
I tried that, but doesn't work. It can't convert numpy array to LP_LP_Sphere automatically and if I'm passing to the function spheres.ctypes.data_as(POINTER(POINTER(Sphere))) the function receives unknown garbage, all different for each time. — Dimansel
– Dimansel, Commented Nov 3, 2017 at 1:01
Yes, once I used it for passing pointer to an array of integers or some other C types this way: np.ctypeslib.ndpointer(dtype = np.int32, ndim = 1, flags = 'C_CONTIGUOUS') But I had no idea how to apply this for array of pointers to structs. — Dimansel
– Dimansel, Commented Nov 3, 2017 at 7:35

Mark Tolonen · Accepted Answer · 2024-11-07 18:57:54Z

I don't use numpy much, but the following works without it. I am assuming if you are passing a pointer to pointers that the pointer list must be null-terminated.

import ctypes as ct

class Sphere(ct.Structure):
    _fields_ = (('x', ct.c_float),
                ('y', ct.c_float),
                ('z', ct.c_float),
                ('radius', ct.c_float))

dll = ct.CDLL('./test')
dll.render.argtypes = ct.POINTER(ct.POINTER(Sphere)),
dll.render.restype = None

# Create a couple of objects
a = Sphere(1, 2, 3, 4)
b = Sphere(5, 6, 7, 8)

# build a list of pointers, null-terminated.
c = (ct.POINTER(Sphere) * 3)(ct.pointer(a), ct.pointer(b), None)
dll.render(c)

Test DLL:

#include <stdio.h>

typedef struct Sphere {
    float x;
    float y;
    float z;
    float radius;
} Sphere;

__declspec(dllexport) void render(Sphere** spheres) {
    for( ; *spheres; ++spheres)
        printf("%f %f %f %f\n", (*spheres)->x, (*spheres)->y,
                                (*spheres)->z, (*spheres)->radius);
}

Output:

1.000000 2.000000 3.000000 4.000000
5.000000 6.000000 7.000000 8.000000

With numpy, using void render(Sphere* spheres, size_t len), this works. Maybe someone more familiar with numpy can comment if Sphere** can be supported.

import ctypes as ct
import numpy as np

class Sphere(ct.Structure):
    _fields_ = (('x', ct.c_float),
                ('y', ct.c_float),
                ('z', ct.c_float),
                ('radius', ct.c_float))

dll = ct.CDLL('./test')
dll.render.argtypes = ct.POINTER(Sphere), ct.c_size_t
dll.render.restype = None

a = Sphere(1, 2, 3, 4)
b = Sphere(5, 6, 7, 8)
# c = (Sphere * 2)(a, b)
# dll.render(c, len(c))
d = np.array([a, b])
dll.render(d.ctypes.data_as(ct.POINTER(Sphere)), len(d))

Nice approach. I didn't know you can just multiply pointer by a number and get array of pointers. I think I even don't need a numpy array.

Collectives™ on Stack Overflow

How can I pass a pointer to array of pointers to structs from python

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related