Re: Can't create small bitfields

On Tue, 15 Apr 2014 22:54:45 -0400
eryksun <[email protected]> wrote:

> On Tue, Apr 15, 2014 at 8:01 PM, Rob Gaddi
> <[email protected]> wrote:
> > the smallest Structure I can get is 4 bytes.  Any ideas on what I'm
> > doing wrong?
> >
> > from __future__ import print_function
> > from ctypes import *
> >
> > class Struct(Structure):
> >     _pack_ = 1
> >
> > Struct._fields_ = [
> >         ('low', c_uint, 4),
> >         ('high', c_uint, 4),
> >     ]
> 
> You're using c_uint as the storage unit size. Instead use c_ubyte:
> 
>     from ctypes import *
> 
>     class Struct(Structure):
>         _fields_ = [('low', c_ubyte, 4),
>                     ('high', c_ubyte, 4)]
> 
>     x = Struct(low=7, high=15)
> 
> 
>     >>> sizeof(x)
>     1
>     >>> (x.high << 4) + x.low
>     247
>     >>> list(bytes(x))
>     [247]

Eryksun --

Thanks, that fixed the immediate problem.  Though it did help point out
the degree to which I really don't understand what makes Structure
bitfields work.  For instance, I would have expected that if I took the
above, and made each field a c_uint8 of size 6, that I'd get a two byte
structure with 'high' spanning the byte boundary.  But in order to get
that, I need to use c_uint16 as the underlying type instead; c_uint8s
give me two bits of padding between.

So, is the underlying type size supposed to match the size of the
overall structure you're trying to fill?  Is there any simple reference
on this, or is it just a matter of sitting down with a solid 4 hours to
burn to read the _ctypes code?

Thanks again,
Rob

-- 
Rob Gaddi, Highland Technology
18 Otis St.
San Francisco, CA 94103
415-551-1700

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