Python ctype Structure/Union Issue

So I am trying to do a bitmap. The data comes from a device as a 32bit number and each bit means something. So I was playing around with ctype Structures and Unions. Trying to do the classic int in and it maps to the individual bits.

from ctypes import *

class Bits(Structure):
    _fields_ = [("Bit_0", c_bool),
                ("Bit_1", c_bool),
                ("Bit_2", c_bool),
                ("Bit_3", c_bool),
                ("Bit_4", c_bool),
                ("Bit_5", c_bool),
                ("Bit_6", c_bool),
                ("Bit_7", c_bool),
                ("Bit_8", c_bool),
                ("Bit_9", c_bool),
                ("Bit_10", c_bool),
                ("Bit_11", c_bool),
                ("Bit_12", c_bool),
                ("Bit_13", c_bool),
                ("Bit_14", c_bool),
                ("Bit_15", c_bool),
                ("Bit_16", c_bool),
                ("Bit_17", c_bool),
                ("Bit_18", c_bool),
                ("Bit_19", c_bool),
                ("Bit_20", c_bool),
                ("Bit_21", c_bool),
                ("Bit_22", c_bool),
                ("Bit_23", c_bool),
                ("Bit_24", c_bool),
                ("Bit_25", c_bool),
                ("Bit_26", c_bool),
                ("Bit_27", c_bool),
                ("Bit_28", c_bool),
                ("Bit_29", c_bool),
                ("Bit_30", c_bool),
                ("Bit_31", c_bool),
                ]
    def return_dic(self):
        dic = {}
        for field in self._fields_:
            dic[field[0]] = getattr(self, field[0])
        return(dic)

class Int(Structure):
    _fields_ = [("Value", c_int32)]

class union(Union):
    _fields_ = [("Descriptor", Int),("Bits", Bits)]
    
test = union(Int(0b01111111111111111111111111111111))
print("{0:b}".format(test.Descriptor.Value))
dic = test.Bits.return_dic()
for x in dic.keys():
    print("{0} : {1:b}".format(x,dic[x]))

and this output. This first line is the 32bit value and the rest are individual bits.

1111111111111111111111111111111
Bit_0 : 1
Bit_1 : 1
Bit_2 : 1
Bit_3 : 1
Bit_4 : 0
Bit_5 : 0
Bit_6 : 0
Bit_7 : 0
Bit_8 : 0
Bit_9 : 0
Bit_10 : 0
Bit_11 : 0
Bit_12 : 0
Bit_13 : 0
Bit_14 : 0
Bit_15 : 0
Bit_16 : 0
Bit_17 : 0
Bit_18 : 0
Bit_19 : 0
Bit_20 : 0
Bit_21 : 0
Bit_22 : 0
Bit_23 : 0
Bit_24 : 0
Bit_25 : 0
Bit_26 : 0
Bit_27 : 0
Bit_28 : 0
Bit_29 : 0
Bit_30 : 0
Bit_31 : 0

Why are the bits 4 – 31 not being set? and bit 0 – 3 are behaving very weird too

Answer

c_bool is one byte in size and prints 0 for zero and 1 for non-zero. You can see that the size in bytes for the structures are incorrect:

>>> sizeof(Bits)
32
>>> sizeof(Int)
4
>>> sizeof(union)
32

Since Value is a 4-byte integer and all four bytes are non-zero the first four byte-sized c_bool fields in the union print 1.

To specify bit fields correctly, specify a 3rd parameter with the number of bits to use. See Bit fields in structures and unions in the ctypes documentation.

from ctypes import *

class Bits(Structure):
    _fields_ = [('Bit_0', c_uint32, 1),  # Use 1 bit of an unsigned 32-bit integer.
                ('Bit_1', c_uint32, 1),  # etc.
                ('Bit_2', c_uint32, 1),
                ('Bit_3', c_uint32, 1),
                ('Bit_4', c_uint32, 1),
                ('Bit_5', c_uint32, 1),
                ('Bit_6', c_uint32, 1),
                ('Bit_7', c_uint32, 1),
                ('Bit_8', c_uint32, 1),
                ('Bit_9', c_uint32, 1),
                ('Bit_10', c_uint32, 1),
                ('Bit_11', c_uint32, 1),
                ('Bit_12', c_uint32, 1),
                ('Bit_13', c_uint32, 1),
                ('Bit_14', c_uint32, 1),
                ('Bit_15', c_uint32, 1),
                ('Bit_16', c_uint32, 1),
                ('Bit_17', c_uint32, 1),
                ('Bit_18', c_uint32, 1),
                ('Bit_19', c_uint32, 1),
                ('Bit_20', c_uint32, 1),
                ('Bit_21', c_uint32, 1),
                ('Bit_22', c_uint32, 1),
                ('Bit_23', c_uint32, 1),
                ('Bit_24', c_uint32, 1),
                ('Bit_25', c_uint32, 1),
                ('Bit_26', c_uint32, 1),
                ('Bit_27', c_uint32, 1),
                ('Bit_28', c_uint32, 1),
                ('Bit_29', c_uint32, 1),
                ('Bit_30', c_uint32, 1),
                ('Bit_31', c_uint32, 1)]

    def return_dic(self):
        dic = {}
        for field in self._fields_:
            dic[field[0]] = getattr(self, field[0])
        return(dic)

class Int(Structure):
    _fields_ = [('Value', c_uint32)]

class union(Union):
    _fields_ = [('Descriptor', Int),('Bits', Bits)]
        
test = union(Int(0x87654321))
print('{0:b}'.format(test.Descriptor.Value))
dic = test.Bits.return_dic()
for x in dic.keys():
    print('{0} : {1:b}'.format(x,dic[x]))

Output:

10000111011001010100001100100001
Bit_0 : 1
Bit_1 : 0
Bit_2 : 0
Bit_3 : 0
Bit_4 : 0
Bit_5 : 1
Bit_6 : 0
Bit_7 : 0
Bit_8 : 1
Bit_9 : 1
Bit_10 : 0
Bit_11 : 0
Bit_12 : 0
Bit_13 : 0
Bit_14 : 1
Bit_15 : 0
Bit_16 : 1
Bit_17 : 0
Bit_18 : 1
Bit_19 : 0
Bit_20 : 0
Bit_21 : 1
Bit_22 : 1
Bit_23 : 0
Bit_24 : 1
Bit_25 : 1
Bit_26 : 1
Bit_27 : 0
Bit_28 : 0
Bit_29 : 0
Bit_30 : 0
Bit_31 : 1