Nicolas Pinault <nicolasp@[...].com>  writes:

>  Hello,
> 
>  I need to access data structures which are in big-endian format. I
>  would like to be able to declare and use Structures which manage this
>  automaticaly.
>  A keyword could say the structure contains big-endian data. When
>  accessing this structure fields, byte ordering would be done
>  automaticaly.

This is not the first time this request comes up, although personally I
never had a need for it.

First, there are some 'Struct' modules that I know of which are based on
the standard Python struct module, and so can handle different byte
orders.  (Bob Ippolito has something called ptypes, iirc, and Just van
Rossum has sstruct, but I'm less sure on the name of the latter).
Neither of them is compatible with ctypes, afaik.

Second, I would probably accept a patch for ctypes if someone provides
one, although the C code can get hairy because it must also support bit
fields.

Third, if speed isn't too important, this can be done with a custom
metaclass.  I've experimented a bit, and wrote an only slightly tested
module that allows to define Structure types in network byte order - it
as attached to this email.  There are some limitations: it only supports
char/bytem, short, and long, it uses the socket module functions
ntohl/htonl and htons/ntohs to do the byte swapping, doesn't support bit
fields. Let me explain how it works:

The NetworkByteOrdered_Structure metaclass creates the new structure
type, then loops over the defined _fields_. It replaces the descriptors
that ctypes has created for the field by new descriptors that do the
byte swapping when assigning to a field and when retrieving a field's
value.

As usual, I would appreciate feedback ;-)

Thomas