python - 在Python中转换结构

Question

我正在使用 ctypes 从外部数据库读取一些数据。

这个数据是写在struct中的。问题是，收到的数据可能有不同的结果。为了更好地理解:我创建了两个结构:

class BEAM(Structure):
    _fields_ = [
        ('NR', c_ulong),
        ("NODE1", c_ulong),
        ("NODE2", c_ulong),
        ("NP", c_ulong),
        ("DL", c_float),
        ("foo", c_ulong),
        ("foobar", c_ulong),
        ("bar", c_ulong),
        ("barfoo", c_ulong)
    ]

class DUMMY(Structure):
    _fields_ = [
        ('ID', c_ulong),
        ("NODE1", c_ulong),
        ("NODE2", c_ulong),
        ("NP", c_ulong),
        ("DL", c_ulong),
        ("foo", c_ulong),
        ("foobar", c_ulong),
        ("bar", c_ulong),
        ("barfoo", c_ulong)
    ]

这个结构之间的区别在于“DL”中的 u_long 类型...在 DUMMY 中它是 u_long，在 BEAM 中它是 u_float。

读取数据库后，我进入 DL = 1056964624，但在 float 中应该是 0.5

我的问题是如何将 DUMMY 转换到 BEAM 中。

我已经尝试过了BEAMRecord = cast(Record, POINTER(BEAMRecord)) 但有一个错误称为类型错误:必须是 ctypes 类型

这是我的代码:

'''
Structure for DataLength
'''
class Len(Structure):
    _fields_ = [
        ('buffer', c_int)
    ]


SLNRecord = element.SLN()
BEAMRecord = element.BEAM()

Record = element.DUMMY()
RecLen = Len()

sofistik = cdll.LoadLibrary("cdb_w30_x64.dll")

py_sof_cdb_init = sofistik.sof_cdb_init
py_sof_cdb_close = sofistik.sof_cdb_close
py_sof_cdb_get = sofistik.sof_cdb_get

py_sof_cdb_get.restype = c_int

Index = py_sof_cdb_init("system.cdb", 99)

pos = c_int(0)

while True:
    RecLen.buffer = sizeof(Record)

    ie = py_sof_cdb_get(Index, 100, 0, byref(Record), byref(RecLen), pos)

    pos.value += 1

    if ie > 1:
        break

    if Record.ID > 0:
        BEAMRecord = cast(Record, POINTER(BEAMRecord))
        print BEAMRecord

py_sof_cdb_close(0)

exit()

感谢您的帮助

<小时/>

解决方案:

通过阅读this thread我修改了@临时先生的问题

我创建了一个 BEAMRecordPointer = POINTER(element.BEAM) 和 BEAMRecord = cast(Record, POINTER(BEAMRecord)) 我重写为 BAR = cast( byref(Record), BEAMRecordPointer).contents所以解决方案如下所示

if Record.ID > 0:
        BAR = cast(byref(Record), BEAMRecordPointer).contents
        print BAR

我做错了吗？

<小时/>

更新1

@eryksun 对cast() 函数有非常好的截图。谢谢。

Answer 1

您可以将结构加载到Union中，然后按照您需要的方式访问它:

from ctypes import *

class BEAM(Structure):
    _fields_ = [('NR', c_ulong),
                ("NODE1", c_ulong),
                ("NODE2", c_ulong),
                ("NP", c_ulong),
                ("DL", c_float),
                ("foo", c_ulong),
                ("foobar", c_ulong),
                ("bar", c_ulong),
                ("barfoo", c_ulong)]

class DUMMY(Structure):
    _fields_ = [('ID', c_ulong),
                ("NODE1", c_ulong),
                ("NODE2", c_ulong),
                ("NP", c_ulong),
                ("DL", c_ulong),
                ("foo", c_ulong),
                ("foobar", c_ulong),
                ("bar", c_ulong),
                ("barfoo", c_ulong)]

class Both(Union):
    _fields_ = [('Beam',BEAM),('Dummy',DUMMY)]

x = Both()
x.Dummy.DL = 1056964624
print(x.Beam.DL)

输出:

0.5000009536743164

或者更简单地说:

from ctypes import *

class DL(Union):
    _fields_ = [('DUMMY',c_ulong),('BEAM',c_float)]

class Hybrid(Structure):
    _fields_ = [('NR', c_ulong),
                ("NODE1", c_ulong),
                ("NODE2", c_ulong),
                ("NP", c_ulong),
                ("DL", DL),
                ("foo", c_ulong),
                ("foobar", c_ulong),
                ("bar", c_ulong),
                ("barfoo", c_ulong)]

x = Hybrid()
x.DL.DUMMY = 1056964624
print(x.DL.BEAM)

(相同的输出)