casting to Arrays in Python ctypes

Question

I'm trying to convert a 16 byte blob of data returned by socket.inet_pton into a ctypes array of unsigned bytes. My data structure looks like this:

class in6_addr(ctypes.Structure):
    _fields_ = (("Byte", ctypes.c_ubyte * 16),)

And the blob is just:

data = socket.inet_pton(socket.AF_INET6, "2001::3")

However, these attempts get errors:

sin6 = in6_addr()

# TypeError: expected c_ubyte_Array_16 instance, got str
sin6.Byte = data
# TypeError: cast() argument 2 must be a pointer type, not c_ubyte_Array_16
sin6.Byte = ctypes.cast(data, ctypes.c_ubyte * 16)
# TypeError: incompatible types, LP_c_ubyte instance instead of c_ubyte_Array_16 instance
sin6.Byte = ctypes.cast(data, ctypes.POINTER(ctypes.c_ubyte))

All of the code: http://codepad.org/2cjyVXBA

Any ideas what type I need to cast to?

Scott Griffiths · Accepted Answer · 2010-07-20 09:51:45Z

3

I might be completely wrong here (and it does seem a bit complex) but this works for me:

sin6.Byte = (ctypes.c_ubyte*16)(*list(bytearray(data)))

I had to convert the data into a list of integers and unpack them for the constructor. There must be an easier way!

answered Jul 20, 2010 at 9:51

Scott Griffiths

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Mark Tolonen Over a year ago

Can be more simple: sin6.Byte = (ctypes.c_ubyte * 16)(*data)

Alex · Accepted Answer · 2023-08-25 13:02:38Z

2

Arguably easier:

sin6.Byte = cast(data, ctypes.POINTER(ctypes.c_ubyte * 16)).contents

or:

sin6.Byte = (ctypes.c_ubyte * 16)(*[x for x in data])

Using bytes stream:

import io
io.BytesIO(data).readinto(sin6.Byte)

And since the considered structure contains the single field, the field name can be omited:

sin6 = (ctypes.c_ubyte * 16)(*data)

sin6 = cast(data, ctypes.POINTER(ctypes.c_ubyte * 16)).contents

io.BytesIO(data).readinto(sin6)

edited Aug 25, 2023 at 13:02

answered Mar 24, 2020 at 16:08

Alex

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Mark Tolonen · Accepted Answer · 2023-05-25 21:44:39Z

0

Easiest way:

import ctypes
import socket

class in6_addr(ctypes.Structure):
    _fields_ = (("Byte", ctypes.c_ubyte * 16),)

data = socket.inet_pton(socket.AF_INET6, "2001::3")

sin6 = in6_addr()
sin6.Byte[:] = data   # copy the data into a complete slice of the array

A couple of helper methods make the class easier to use as well:

import ctypes
import socket

class in6_addr(ctypes.Structure):
    _fields_ = ("Byte", ctypes.c_ubyte * 16),

    def __init__(self, data):
        self.Byte[:] = data

    def __repr__(self):
        return f'in6_addr(Bytes=bytes({list(self.Byte)}))'

data = socket.inet_pton(socket.AF_INET6, "2001::3")
sin6 = in6_addr(data)
print(sin6)

Output:

in6_addr(Bytes=bytes([32, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3]))

edited May 25, 2023 at 21:44

answered May 25, 2023 at 21:36

Mark Tolonen

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