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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: ctypes and byte order
Date: Thu, 18 Jun 2015 12:33:08 +0200
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Jean-Michel Pichavant wrote:

> I'm currently writing python code that writes a small binary file to be
> used by another device which code is written in C. The python code runs on
> a little endian CPU, and unfortunately, the other device is using a big
> endian MIPS.
> 
> My problem is the following: I cannot make an array of n int work in big
> endian, I can workaround the problem by creating n intergers, which is
> nicely done thanks to list comprehension but that seems to me like a
> misuse of the ctypes modules.
> 
> ctypes is expecting a 'c_uint_be_Array_5' (note the _be_ for big endian),
> and I don't know how to construct such object.
> 
> I hope I am clear.

Yes, very clear, thanks for the effort, although the nitpicker in me has to 
mention that an off-by-one bug slipped into your code ;)

> class Foo_be_working(ctypes.BigEndianStructure):
>     """Working big endian version, looks more like a workaround."""
>     _fields_ = [('bar%i'%i, ctypes.c_uint32) for i in range(5)]
[...]
> f_be = Foo_be_working(0,1,2,3,4)
> print "bar0 and bar5: ", f_be.bar0, f_be.bar5

I'm not very familiar with ctypes, but a random walk through the source 
found the _endian module and the __ctypes_be__ attribute. So
> 
> iarray = ctypes.c_uint32*5  # array of 5 unsigned 32 bits
> 
> class Foo(ctypes.Structure):
>     """Native byte order, won't work on my big endian device"""
>     _fields_ = [('bar', iarray)]
> 
> class Foo_be(ctypes.BigEndianStructure):
>     """big endian version, but python code fails"""
>     _fields_ = [('bar', iarray)]

becomes

$ cat be2.py
import ctypes, sys

iarray_be = ctypes.c_uint32.__ctype_be__*5

class Foo_be(ctypes.BigEndianStructure):
        _fields_ = [('bar', iarray_be)] 

print sys.version
f_be = Foo_be((0,1,2,3,0x11223344))
print hex(f_be.bar[4])

$ python be2.py 
2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2]
0x11223344L

which might do what you want.