FAQ: how to vary the byte offset of a field of a ctypes.Structure
Larry Bates
larry.bates at websafe.com
Thu May 31 14:20:38 EDT 2007
p.lavarre at ieee.org wrote:
> How do I vary the byte offset of a field of a ctypes.Structure?
>
> How do I "use the dynamic nature of Python, and (re-)define the data
> type after the required size is already known, on a case by case
> basis"?
>
> \\\
>
> For example, suppose sometimes I receive the value '\x03hi' + \x04bye'
> for the struct:
>
> class Struct34(ctypes.Structure):
> _pack_ = 1
> _fields_ = [('first', 3 * ctypes.c_ubyte),
> ('second', 4 * ctypes.c_ubyte)]
>
> but then sometimes instead I receive the value '\x05left' + \x06right'
> for the struct:
>
> class Struct56(ctypes.Structure):
> _pack_ = 1
> _fields_ = [('first', 5 * ctypes.c_ubyte),
> ('second', 6 * ctypes.c_ubyte)]
>
> Thus in general I receive (0xFF ** 2) possible combinations of field
> lengths.
>
> ///
>
> How do I declare all those hugely many simply regular combinations as
> one CTypes.structure?
>
> I also need to do series of 3 or 4 or 5 strings, not just 2 strings.
> But always the byte offsets of the subsequent fields vary when the
> byte sizes of the preceding fields vary. The byte size of the
> enclosing packed struct varies as the length of the packed bytes it
> contains.
>
> The errors I get as I try techniques that don't work include:
>
> AttributeError: '_fields_' must be a sequence of pairs
> AttributeError: _fields_ is final
> ValueError: Memory cannot be resized because this object doesn't own
> it
> TypeError: incompatible types, c_ubyte_Array_2 instance instead of
> c_ubyte_Array_1 instance
>
> How do I change the offset of a field of a ctypes.Structure?
>
> Is the answer to contribute to the next version of CTypes? Or is this
> feature already supported somehow?
>
> Curiously yours, thank in advance,
>
How about something like:
class fooStruct(ctypes.Structure):
_pack_ = 1
_fields_=[]
def __init__(self, fields):
self._fields_=fields
ctypes.Structure.__init__(self)
a=fooStruct([('first', 3*ctypes.c_ubyte),
('second', 4*ctypes.c_ubyte)])
print a._fields_
-Larry
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