translating arrays from c to python ctypes

Question

I have the below arrays on C how can i interpert them to ctypes datatypes inside structre

struct a {

BYTE    a[30];
CHAR    b[256];
};

should i interpert a fixed array as the datatype * the size i want like the below and if yes how can i call this structure as a parameter to fun that takes instance from this structure

class a(structure) :

_fields_ = [ ("a",c_bytes*30 ),
                 ("b",c_char*256 ),]

Answer 1

This should work:

from ctypes import Structure, c_bytes, c_char

class A(Structure):
    _fields_ = [("a", c_bytes*30), ("b", c_char*256)]

Then you can simply access the fields of the structure using the dot operator:

>>> my_a = A()
>>> my_a.a[4] = 127
>>> my_a.a[4]
127
>>> my_a.b = "test string"
>>> my_a.b
'test string'
>>> my_a.b[2]
's'

You can also pass the structure directly to an arbitrary Python function:

def my_func(a):
    print "a[0] + a[1] = %d" % (a.a[0] + a.a[1], )
    print "Length of b = %d" % len(a.b)

>>> my_a = A()
>>> my_a.a[0:2] = 19, 23
>>> my_a.b = "test"
>>> my_func(my_a)
a[0] + a[1] = 42
Length of b = 4

Answer 2

You're on the right track. You're probably just missing the byref() function. Assuming the function you want to call is named *print_struct*, do the following:

from ctypes import *

class MyStruct(Structure):
    _fields_ = [('a',c_byte*30), ('b',c_char*256)]

s = MyStruct() # Allocates a new instance of the structure from Python

s.a[5] = 10 # Use as normal

d = CDLL('yourdll.so')
d.print_struct( byref(s) )  # byref() passes a pointer rather than passing by copy