On Jun 18, 6:29 pm, Nick Craig-Wood <n...@craig-wood.com> wrote:
> Karthik <karthik301...@gmail.com> wrote:
> > Hello Everybody,
>
> > I'm trying to create a packed structure in ctypes (with one 64-bit
> > element that is bitfielded to 48 bits),
> > unsuccessfully:
>
> > ===================================
> > from ctypes import *
>
> > class foo (Structure):
> > _pack_ = 1
> > _fields_ = [
> > ("bar", c_ulonglong, 48),
> > ]
>
> > print("sizeof(foo) = %d" % sizeof(foo))
> > ===================================
> > I'm expecting that this should print 6 - however, on my box, it prints
> > 8.
>
> > The following piece of C code, when compiled and run, prints 6, which
> > is correct.
> > ===================================
> > struct foo {
> > unsigned long long bar: 48;
> > };
>
> > printf("sizeof(foo) = %d", sizeof(foo));
> > ===================================
>
> > So... what am I doing wrong?
>
> I compiled and ran the above with gcc on my linux box - it prints 8
> unless I add __attribute__((__packed__)) to the struct.
>
> I'm not sure that using bitfields like that is a portable at all
> between compilers let alone architectures.
>
> I'd probably do
>
> from ctypes import *
>
> class foo (Structure):
> _pack_ = 1
> _fields_ = [
> ("bar0", c_uint32),
> ("bar1", c_uint16),
> ]
> def set_bar(self, bar):
> self.bar0 = bar & 0xFFFFFFFF
> self.bar1 = (bar >> 32) & 0xFFFF
> def get_bar(self):
> return (self.bar1 << 32) + self.bar0
> bar = property(get_bar, set_bar)
>
> print "sizeof(foo) = %d" % sizeof(foo)
> f = foo()
> print f.bar
> f.bar = 123456789012345
> print f.bar
>
> Which prints
>
> sizeof(foo) = 6
> 0
> 123456789012345
>
> --
> Nick Craig-Wood <n...@craig-wood.com> --http://www.craig-wood.com/nick
Oops, sorry about the missing __attribute__((packed)) - that was an
error of
omission.
Thank you, Nick :)
I'm looking at some C code that's using bitfields in structs, and I'm
writing
python code that "imports" these structs and messes around with them -
unfortunately,
I'm not at liberty to change the C code to not use bitfields.
Your solution works fine, so that's what I'm going to use.
Thanks,
Karthik.
--
http://mail.python.org/mailman/listinfo/python-list
