Python ctype结构/联合问题

Question

所以我正在尝试制作位图。数据以32位数字的形式来自设备，每一位都有意义。所以我在玩ctype结构和工会。尝试在中执行经典的int，它会映射到各个位。

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]))

以及该输出。第一行是32位的值，其余的是单独的位。

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

为什么没有设置位4-31？和位0-3的行为也很奇怪

Answer 1

c_bool是一个字节大小，打印0表示零，打印1表示非零。您可以看到结构的字节大小不正确：

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

由于Value是4字节整数并且所有四个字节都是非零的，所以联合打印1中的前四个字节大小的c_bool字段。

要正确指定位字段，请指定具有要使用的位数的第三个参数。请参阅ctypes文档中结构和并集中的位字段。

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]))

输出：

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