ctypes and array of structs

Question

I'd like to read an array of structs defined in a C library using ctypes and python.

The C struct is simply

struct particle {
  double x;
  double y;
}

I have a function that returns a pointer to an array of structs:

struct particle* getParticles();

In python I define

class Particle(Structure):
    _field_ = [("x", c_double),("y", c_double)]

Then I'm trying to parse the returned pointer from python, but seem to be doing something wrong:

getp = libparticles.getParticles
getp.restype = POINTER(Particle)
particles = getp()

particles is of type LP_Particle, which seems to make sense. But the values (e.g. particles[0].x) are garbage.

How are you initializing libparticles? It helps to post a fully functional example that illustrates the problem. My guess is the function uses "C" calling convention and you've initialized as "stdcall" or vice versa. — Mark Tolonen
– Mark Tolonen, Commented Sep 15, 2014 at 3:17

Mark Tolonen · Accepted Answer · 2014-09-15 03:27:20Z

3

Here's a working example for Windows DLL with default "C" calling convention. Without a working, complete example of your code and an example of the error you get it's difficult to tell where you went wrong. One observation is _fields_ was spelled _field_ in your code.

C source

struct particle { double x,y; };

__declspec(dllexport) struct particle* getParticles()
{
    static struct particle p[3] = {1.1,2.2,3.3,4.4,5.5,6.6};
    return p;
}

Python

from ctypes import *

class Particle(Structure):
    _fields_ = [("x", c_double),("y", c_double)]

getp = cdll.x.getParticles
getp.restype = POINTER(Particle)
particles = getp()
for i in range(3):
    print(particles[i].x,particles[i].y)

Output

1.1 2.2
3.3 4.4
5.5 6.6

answered Sep 15, 2014 at 3:27

Mark Tolonen

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2 Comments

hanno Over a year ago

You were right about the initialization being the problem.

Mark Tolonen Over a year ago

Of course, just another example of no working, complete example. It wasn't usable code.

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