Python3 ctypes 复制指向 ctypes.structure 的指针

问题描述

Linux 内核的 ioctl 将结构体放入结构体中。我想弄清楚如何用 ctypes 做同样的事情。

我从 ioctl 收到了 ParentRq。我想要一种将 Message.data 中的数据复制到另一个结构（例如 EventID）中的通用方法。

from ctypes import Structure,c_uint8,c_uint16,POINTER

class EventID(Structure):
    _pack_ = 1
    _fields_ = [
        ('recent_addition_timestamp',c_uint8 * 4),('last_recid',c_uint8 * 2),('last_sw_processed_id',('last_processed_id',]
    _eventid_sampledata = [0x88,0x29,0xfd,0x60,0x3c,0x02,0xff,0x02]

class Message(Structure):
    _pack_ = 1
    _fields_ = [
        ('data_len',c_uint16),('data',POINTER(c_uint8))
]

class ParentRq(Structure):
    _pack_ = 1
    _fields_ = [
        ('msg',Message)
    ]

p = ParentRq()
p.msg.data_len = len(EventID._eventid_sampledata)
p.msg.data = (c_uint8 * p.msg.data_len)(*EventID._eventid_sampledata)

# Proof that items got copied into the pointer correctly.
items = []
for i in range(p.msg.data_len):
    items.append(hex(p.msg.data[i]))
# items -> ['0x88','0x29','0xfd','0x60','0x3c','0x2','0xff','0x2']

# copy p.msg.data into EventID
# How do I copy p.msg.data into EventID?
eid = EventID()

解决方法

列表[Python.Docs]: ctypes - A foreign function library for Python。

有多种方法可以实现，一种是使用 memmove：

>>> import ctypes as ct
>>>
>>>
>>> class EventID(ct.Structure):
...     _pack_ = 1
...     _fields_ = [
...         ("recent_addition_timestamp",ct.c_uint8 * 4),...         ("last_recid",ct.c_uint8 * 2),...         ("last_sw_processed_id",...         ("last_processed_id",...     ]
...
>>>
>>> l = [0x88,0x29,0xfd,0x60,0x3c,0x02,0xff,0x02]
>>> size = len(l)
>>> arr = (ct.c_ubyte * size)(*l)
>>> ptr = ct.cast(arr,ct.POINTER(ct.c_ubyte))  # The equivalent of Message.data (ParentRq.msg.data) instance
[hex(ptr[i]) for i in range(size)]
>>> ['0x88','0x29','0xfd','0x60','0x3c','0x2','0xff','0x2']
>>>
>>> eid = EventID()  # Initialize members to default values (0)
>>>
>>> for f,_ in EventID._fields_:
...     print(f,[hex(i) for i in getattr(eid,f)[:]])
...
recent_addition_timestamp ['0x0','0x0','0x0']
last_recid ['0x0','0x0']
last_sw_processed_id ['0x0','0x0']
last_processed_id ['0x0','0x0']
>>>
>>> ct.memmove(ct.addressof(eid),ptr,size)
2361449631368
>>>
>>> for f,f)[:]])
...
recent_addition_timestamp ['0x88','0x60']
last_recid ['0x3c','0x2']
last_sw_processed_id ['0xff','0xff']
last_processed_id ['0x3c','0x2']