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
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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