fields ordering in structure RTL instructions.

I'm messing with the followig structure, compiling with GNU GCC on an
ARM architecture.

typedef union _waltdisney{
struct{
uint8_t goofy : 5;
uint8_t pluto : 1;
uint8_t donald : 2;
} __packed family;
uint8_t val;
} __packed waltdisney_t;

It looks like the first fields of "family" packed structure are mapped
into the less significant bits of the waltdisney_t type value. So for
instance, the following assignment

waltdisney_t wd;
wd.val = 0x0F;

is interpreted as assigning 0 to donald, 0 to pluto and 15 to goofy.Is
this always true? Can I get to know or even force the behavior for the
field ordering?

Thanks in advance.
RM

Riccardo Manfrin wrote:
> I'm messing with the followig structure, compiling with GNU GCC on an
> ARM architecture.
>
> typedef union _waltdisney{
> struct{
> uint8_t goofy : 5;
> uint8_t pluto : 1;
> uint8_t donald : 2;
> } __packed family;
> uint8_t val;
> } __packed waltdisney_t;
>
> It looks like the first fields of "family" packed structure are mapped
> into the less significant bits of the waltdisney_t type value. So for
> instance, the following assignment
>
> waltdisney_t wd;
> wd.val = 0x0F;
>
> is interpreted as assigning 0 to donald, 0 to pluto and 15 to goofy.Is
> this always true? Can I get to know or even force the behavior for the
> field ordering?
>
> Thanks in advance.
> RM


There is even no guarantee of byte ordering
in ARM cores. There is a signal between the
core and the surrounding chip selecting the
byte ordering. It depends on the whim of the
chip designer how the signal is connected.

You will invite problems later on if you
trust a probed packing ordering.

The way of setting up a packed structure is
an implementation-defined part in the C standard.

A better way would be to define bit masks
and pick the bits by hand, if you're trying
to match hardware-defined bit fields.

What are you attempting to achieve?

--

Tauno Voipio
tauno voipio (at) iki fi

On Mon, 2011-04-04, Riccardo Manfrin wrote:
> I'm messing with the followig structure, compiling with GNU GCC on an
> ARM architecture.
>
> typedef union _waltdisney{
> struct{
> uint8_t goofy : 5;
> uint8_t pluto : 1;
> uint8_t donald : 2;
> } __packed family;
> uint8_t val;
> } __packed waltdisney_t;

Which version of gcc is this? My documentation for gcc 4.4 doesn't
mention "__packed", except perhaps in combination with __attribute__.
And it doesn't compile:

salix:~% gcc -c /tmp/q.c
/tmp/q.c:8: error: expected ':', ',', ';', '}' or '__attribute__' before 'family'
/tmp/q.c:10: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'waltdisney_t'
/tmp/q.c:12: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'foo'

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .

> A better way would be to define bit masks
> and pick the bits by hand, if you're trying
> to match hardware-defined bit fields.
>
> What are you attempting to achieve?

bit masking is definitely the cleaner way, but I'd like to have a
simple yet flexible way to frame the bits, just like the reported data
structure shows.
I found some workaround which seem to work quite well and whose I'll
be posting of soon (as soon as I have time to)..
Thanks in advance

> Which version of gcc is this? My documentation for gcc 4.4 doesn't
> mention "__packed", except perhaps in combination with __attribute__.
> And it doesn't compile:

Sorry, __packed is my define for __attibute__ (( packed ))