python ctypes array of structs

Question

I have the followng code and it ends up to segmentation fault.

import ctypes
from random import randint
class STRUCT_2(ctypes.Structure):
 #_pack_=2
 _fields_ = [('field_1', ctypes.c_short),
 ('field_2', ctypes.c_short),
 ('field_3', ctypes.c_short)]
class STRUCT_1(ctypes.Structure):
 #_pack_=2
 _fields_ = [('elements', ctypes.c_short),
 ('STRUCT_ARRAY', ctypes.POINTER(STRUCT_2))]
 def __init__(self,num_of_structs):
 elems = (ctypes.POINTER(STRUCT_2) * num_of_structs)()
 self.STRUCT_ARRAY = ctypes.cast(elems,ctypes.POINTER(STRUCT_2))
 self.elements = num_of_structs
 for num in range(0,num_of_structs):
 self.STRUCT_ARRAY[num].field_1 = 1
 self.STRUCT_ARRAY[num].field_2 = 2
 self.STRUCT_ARRAY[num].field_3 = 3
for num in range(0,100):
 test = STRUCT_1(num)
 print "%i done" % num

output: 5 done Segmentation fault

But if field_3 is not present in struct_2 then it seems to work correctly. If I add one short field (field_4) more it ends to segmentation fault...

So what i'am doing incorrectly or what i have missed here??

Is there someother way to define array size?

• mishik: Why not just do self.STRUCT_ARRAY.append({"field_1": 1, "field_2": 2, "field_3": 3}) ?

Answer 1

STRUCT_ARRAY in your code should be a pointer to a pointer array, not a pointer to an element in the array, as you are appending STRUCT_2 pointers to the array.

In C, STRUCT_ARRAY can be defined as *STRUCT_2[length], aka an array of STRUCT_2 pointers

import ctypes
from random import randint
class STRUCT_2(ctypes.Structure):
 #_pack_=2
 _fields_ = [('field_1', ctypes.c_short),
 ('field_2', ctypes.c_short),
 ('field_3', ctypes.c_short),
 ('field_4', ctypes.c_short),
 ('field_5', ctypes.c_short),
 ('field_6', ctypes.c_short),
 ('field_7', ctypes.c_short),
 ('field_8', ctypes.c_short)]
class STRUCT_1(ctypes.Structure):
 #_pack_=2
 _fields_ = [('elements', ctypes.c_short),
 #an array of pointers
 ('STRUCT_ARRAY', ctypes.POINTER(ctypes.POINTER(STRUCT_2)))]
 def __init__(self,num_of_structs):
 elems = (ctypes.POINTER(STRUCT_2) * num_of_structs)()
 self.STRUCT_ARRAY = ctypes.cast(elems,ctypes.POINTER(ctypes.POINTER(STRUCT_2)))
 self.elements = num_of_structs
 for num in range(0,num_of_structs):
 self.STRUCT_ARRAY[num].field_1 = 1
 self.STRUCT_ARRAY[num].field_2 = 2
 self.STRUCT_ARRAY[num].field_3 = 3
for num in range(0,100):
 test = STRUCT_1(num)
 print("%i done" % num)

If instead what you want is an array of structs, use the following code:

import ctypes
from random import randint
class STRUCT_2(ctypes.Structure):
 #_pack_=2
 _fields_ = [('field_1', ctypes.c_short),
 ('field_2', ctypes.c_short),
 ('field_3', ctypes.c_short),
 ('field_4', ctypes.c_short),
 ('field_5', ctypes.c_short),
 ('field_6', ctypes.c_short),
 ('field_7', ctypes.c_short),
 ('field_8', ctypes.c_short)]
class STRUCT_1(ctypes.Structure):
 #_pack_=2
 _fields_ = [('elements', ctypes.c_short),
 #an array of structs
 ('STRUCT_ARRAY', ctypes.POINTER(STRUCT_2))]
 def __init__(self,num_of_structs):
 elems = (STRUCT_2 * num_of_structs)()
 self.STRUCT_ARRAY = ctypes.cast(elems,ctypes.POINTER(STRUCT_2))
 self.elements = num_of_structs
 for num in range(0,num_of_structs):
 self.STRUCT_ARRAY[num].field_1 = 1
 self.STRUCT_ARRAY[num].field_2 = 2
 self.STRUCT_ARRAY[num].field_3 = 3
 self.STRUCT_ARRAY[num].field_4 = 4
for num in range(0,100):
 test = STRUCT_1(num)
 print("%i done" % num)

• Anurag Singh: If I want to pass self.STRUCT_ARRAY in above code through udp.Can you tell me how can I do this?One more problem is exist in above code if i am trying to print the field_1 ,field_2 ,field_3 variables values , it is showing me error like this AttributeError: 'LP_STRUCT_2' object has no attribute 'field_1'