ctypes and byte order

[Jean-Michel Pichavant]

Hi list,

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.

Thanks,

JM

here's a code sample (python 2.7):

--------------------
import ctypes, sys

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)] 

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)]


print sys.version
f = Foo(iarray(0,1,2,3,4))
print "uint32 array: ", f.bar

f_be = Foo_be_working(0,1,2,3,4)
print "bar0 and bar5: ", f_be.bar0, f_be.bar5

f_be = Foo_be(iarray(0,1,2,3,4)) # will raise an exception

--------------------


The output

2.7.3 (default, Mar 13 2014, 11:03:55) 
[GCC 4.7.2]
uint32 array:  <__main__.c_uint_Array_5 object at 0x1cf4560>
bar0 and bar4:  0 4


TypeError: incompatible types, c_uint_Array_5 instance instead of c_uint_be_Array_5 instance
     24 
---> 25 f_be = Foo_be(iarray(0,1,2,3,4))
     26 



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