Python, ctypes, multi-Dimensional Array

Question

I have structure in Python code and in C code. I fill these fields

("bones_pos_vect",((c_float*4)*30)),
("bones_rot_quat",((c_float*4)*30))

in python code with the right values, but when I request them in C code, I get only 0.0 from all array cells. Why do I lose the values? All other fields of my structures work fine.

class SceneObject(Structure):
    _fields_ = [("x_coord", c_float),
                ("y_coord", c_float),
                ("z_coord", c_float),
                ("x_angle", c_float),
                ("y_angle", c_float),
                ("z_angle", c_float),
                ("indexes_count", c_int),
                ("vertices_buffer", c_uint),
                ("indexes_buffer", c_uint),
                ("texture_buffer", c_uint),
                ("bones_pos_vect",((c_float*4)*30)),
                ("bones_rot_quat",((c_float*4)*30))]

typedef struct
{
    float x_coord;
    float y_coord;
    float z_coord;
    float x_angle;
    float y_angle;
    float z_angle;
    int indexes_count;
    unsigned int vertices_buffer;
    unsigned int indexes_buffer;
    unsigned int texture_buffer;
    float bones_pos_vect[30][4];
    float bones_rot_quat[30][4];    
} SceneObject;

Solution

Here's an example of how you can use a multidimensional array with Python and ctypes.

I wrote the following C code, and used gcc in MinGW to compile this to slib.dll:

#include <stdio.h>

typedef struct TestStruct {
    int     a;
    float   array[30][4];
} TestStruct;

extern void print_struct(TestStruct *ts) {
    int i,j;
    for (j = 0; j < 30; ++j) {
        for (i = 0; i < 4; ++i) {
            printf("%g ", ts->array[j][i]);
        }
        printf("\n");
    }
}

Note that the struct contains a 'two-dimensional' array.

I then wrote the following Python script:

from ctypes import *

class TestStruct(Structure):
    _fields_ = [("a", c_int),
                ("array", (c_float * 4) * 30)]

slib = CDLL("slib.dll")
slib.print_struct.argtypes = [POINTER(TestStruct)]
slib.print_struct.restype = None

t = TestStruct()

for i in range(30):
    for j in range(4):
        t.array[i][j] = i + 0.1*j

slib.print_struct(byref(t))

When I ran the Python script, it called the C function, which printed out the contents of the multidimensional array:

C:\>slib.py
0.1 0.2 0.3 0.4
1.1 1.2 1.3 1.4
2.1 2.2 2.3 2.4
3.1 3.2 3.3 3.4
4.1 4.2 4.3 4.4
5.1 5.2 5.3 5.4
... rest of output omitted

I've used Python 2, whereas the tags on your question indicate that you're using Python 3. However, I don't believe this should make a difference.

Comments

[Sebastian Hoffmann]

Multidimensional arrays are not mapped one to one in memory in c. Thus float[30][4] is actually an array (size=30) of float pointers (pointing to the beginning of the float array). The (c_float*4)*30) might be really a array of arrays (first 4 floats then the second 4 floats etc.) though. You should test that out.

[shestakoffvs]

So, I must send simply (c_float*(4*30))?

[Sebastian Hoffmann]

no i think it should be POINTER(c_float)*30; use [i] to access the single values then. e.g pbase = bones_pos_vect[17]; p = pbase[3] #third float in array; value = p.contents

[shestakoffvs]

No it's not working, i get only 0.0. Also, I tryed use (c_float*(4*30) and have same result((( i want to cry, cause it's not workin

[shestakoffvs]

and i want to send values to C code

[Sebastian Hoffmann]

if you iterate over the array after you filled it does it prints the correct values?

[shestakoffvs]

yes but only in C code, in python all good

[Mark Tolonen]

@Paranaix, float[30][4] is not an array of float pointers. @shestakoffvs has the correct definition.

[Mark Tolonen]

@shestakoffvs You must have a mistake in either your Python code calling C or in the C code displaying the array. Your definition is correct.

[Mark Tolonen]

Ah, beat me to it. I wrote basically the same code with the OP's structure and definition.

[shestakoffvs]

Oh guys, i'm so stupid))) And big thx for example code, with it i'm understand what wrong with my)))) insted of assign to every varing in structure field array i just assigned it whole new array: scene_object.bones_pos_vects = import_content["bones_pos_vects"] now i do this so: for n in range(len(import_content["bones_pos_vects"])): for u in range(3): scene_object.bones_pos_vects[n][u] = import_content["bones_pos_vects"][n][u] scene_object.bones_rot_quats[n][u] = import_content["bones_rot_quats"][n][u] Now all work very well))) big thx)))