ctypes struct containing arrays and manipulating-CodePudding

I'm trying to use ctypes in my python code test.pyto pass the array to test.c and do some calculation and bring the result to python code.

Here is 'test.c'

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "test.h"

void test(int N, complex *v){
    int k;
    for(k=0; k<N; k  ){
        v[k].Re = v[k].Re   1;
        v[k].Im = v[k].Im   1;    
        printf("%d %f %f\n", k, v[k].Re, v[k].Im);
    }
}

v is the structed array defined in test.h

typedef struct{float Re; float Im;} complex;

From Python I can do something like this

import numpy as np
from ctypes import cdll
from ctypes import *

class complex(Structure):
    _fields_ = (('Re', POINTER(c_float)),
                ('Im', POINTER(c_float)))

    def __init__(self, I, Q, lenIQ):
        array_type = c_float * lenIQ
        self.Re = array_type(*I)
        self.Im = array_type(*Q)

    def __repr__(self):
        return f'Complex({self.Re}, {self.Im})'

cgmap = cdll.LoadLibrary('./test.dll')
test = cgmap.test
test.argtypes = (c_int, complex)
I = [2,2,3,4]
Q = [5,6,7,8]
lenIQ = len(I)

myarray = complex(I, Q, lenIQ)
test(c_int(lenIQ), myarray)
print(myarray)
print(myarray[0])

But when I execute test.py, then :

0 4284182633119744.000000 1.000000
1 4585659272527872.000000 1.000000
2 3.983704 1.000000
3 3.983746 1.000000
Complex(<__main__.LP_c_float object at 0x0000029959825A48>, <__main__.LP_c_float object at 0x0000029959825A48>)
Traceback (most recent call last):
  File "test.py", line 28, in <module>
    print(myarray[0])
TypeError: 'complex' object does not support indexing

Is there anyway to solve without modifying test.c? Any help would be appreciated ;)

CodePudding user response：

you are mixing up between creating a struct, creating an array of struct, and creating a pointer to an array of struct, those are 3 different operations, i have commented the code for clarity.

import numpy as np
from ctypes import cdll
from ctypes import *

class complex(Structure):  # struct with two floats
    _fields_ = (('Re', c_float),
                ('Im', c_float))

    def __init__(self, I_val, Q_val):
        self.Re = I_val
        self.Im = Q_val

    def __repr__(self):
        return f'Complex({self.Re}, {self.Im})'

I = [2,2,3,4]
Q = [5,6,7,8]
lenIQ = len(I)
my_array_type = complex*lenIQ  # create the type of array (complex[])
my_array = my_array_type()  # create the array (complex my_array[4])

for i in range(len(I)):
    my_array[i] = complex(I[i],Q[i])  # fill each individual object in the array

# next line is equivalent to "complex* pointer_to_array = (complex*)(my_array);"
pointer_to_array = cast(my_array, POINTER(complex))  # create the pointer to the first element of the array (complex*)

print(my_array)  # prints type of my_array
print(my_array[0])  # equivalent to c's my_array[0]
print(pointer_to_array[0])  # equivalent to c's pointer_to_array[0]

<__main__.complex_Array_4 object at 0x000002195A9B8C40>
Complex(2.0, 5.0)
Complex(2.0, 5.0)

now for calling

cgmap = cdll.LoadLibrary('./test.dll')
test = cgmap.test
test.argtypes = (c_int, POINTER(complex))
test.restype  = None
test(c_int(lenIQ), pointer_to_array)