Python ctypes. Get 2-dim array

Question

I have this c++ code:

#define DLLEXPORT extern "C"

DLLEXPORT double **points(unsigned N, unsigned D)
{

double **POINTS = new double * [N];
for (unsigned i=0;i<=N-1;i++) POINTS[i] = new double [D];
for (unsigned j=0;j<=D-1;j++) POINTS[0][j] = 0;

// do some calculations, f.e:
// POINTS[i][j] = do_smth()
//


return POINTS
}

which I compile using this command:

g++ -shared gpoints.cc -o gpoints.so -fPIC

Now I want to call this function from python using ctypes. I tried smth like this:

mydll = ctypes.cdll.LoadLibrary('/.gpoints.so')
func = mydll.points
mytype = c.c_double * 3 * 10
func.restype = mytype
arr = func(10, 3)
print(arr)

When I try to run it I get:

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc
[1]    2794 abort      python3 test.py

So, how to properly call this function from Python?

P.S. I got this library "as is", so, it is desirable to use it without rewriting c++ code.

Mark Tolonen · Accepted Answer · 2017-10-30 17:28:44Z

1

Your restype is equivalent to double[10][3], but what it actually returns is double**:

#!python3.6
import ctypes as c
dll = c.CDLL('test')
dll.points.argtypes = c.c_int,c.c_int
dll.points.restype = c.POINTER(c.POINTER(c.c_double))
arr = dll.points(10,3)
for i in range(10):
    for j in range(3):
        print(f'({i},{j}) = {arr[i][j]}')

I used the following DLL code to test (Windows):

extern "C" __declspec(dllexport)
double **points(unsigned N, unsigned D)
{
    double **POINTS = new double * [N];
    for (unsigned i=0;i<=N-1;i++)
    {
        POINTS[i] = new double [D];
        for(unsigned j=0;j<=D-1;j++)
            POINTS[i][j] = i*N+j;
    }
    return POINTS;
}

Output:

(0,0) = 0.0
(0,1) = 1.0
(0,2) = 2.0
(1,0) = 10.0
(1,1) = 11.0
(1,2) = 12.0
(2,0) = 20.0
(2,1) = 21.0
   :

answered Oct 30, 2017 at 17:28

Mark Tolonen

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1 Comment

danielleontiev Over a year ago

It's awsome! Why manual does not contain such clear examples. I spent a lot of time on this problem. Thanks a lot

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