Question

There is c struct definition

struct Matrix {
    int width;
    int height;
    int** data;
};

How to use python’s ctypes.Struct to define it?

class Matrix( ctypes.Structure ):
    _fields_ = [
        ( "width", ctypes.c_int ),
        ( "height", ctypes.c_int ),
        ( "data", ??? ),
    ]

I don’t know how to define the “data” ‘s type.
Anyone here help?

Answer 1

The most literal translation of int** would be:

ctypes.POINTER(ctypes.POINTER(ctypes.c_int))

That is, it is a pointer to a pointer to an int.

A potentially more idiomatic way is a 2D array:

ctypes.c_int * width * height

, but to do that you need to know how many elements are in the array – the size is part of the type in ctypes. If, as it seems, you don’t know that size in advance (and it can change per instance of the struct), a pointer to a pointer is the best option.

To instantiate this takes a little less indirection – by the time you want to create the array, by definition you will know how big it needs to be. So, you can just do:

m.data = (ctypes.c_int * width * height)()

Which will give you a height-length array of width-length arrays of c_int. That is, it does (broadly) the same thing as the C++ code:

m.data = new int[width][height];

Or, if you prefer:

m.data = (int**) malloc(sizeof(int) * width * height)