Marcin
Fri Oct 27 03:36:10 CDT 2006
Ulrich Eckhardt napisaÅ?(a):
> Marcin Gabryszewski wrote:
>> typedef union
>> {
>> unsigned int n32bits;
>> struct
>> {
>> char p1 : 1;
>> char p2 : 1;
>> char p3 : 1;
>> unsigned int p4 : 29;
>> }s;
>> }u;
>>
>> u u1;
>> u1.s.p1 = 0;
>> u1.s.p2 = 1;
>> u1.s.p3 = 1;
>> u1.s.p4 = 1;
>>
>> what value should have u1.n32bits ??
>
> 1. You are not allowed to write to one field of a union and read from the
> other, according to the C standard. It can be made to work with most
> compilers though if some care is taken.
> 2. Bitfields are only packed up to the size of their base type. Therefore,
> you had better use the same for all if you want them packed together.
> 3. Assuming that 'unsigned int' is 32 bit is just stupid as there is no
> guarantee for that. Use an explicitly sized integer like uint32_t (C99) or
> UINT32 (win32).
> 4. The order in those bitfields is even dependant on the endianess!
> 5. assert(sizeof u == sizeof u.n32bits);
> 6. [C++ only] No need to create an anonymous union and then use typedef to
> assign a name to it.
>
> Uli
>
ok, I know that, this code is a SAMPLE
which shows
gcc can align this union to 1 bit, vc not
of course a can solve this problem easily (all fields in struct are 32
bits).
anyway,
thx
--
Marcin Gabryszewski
G DATA Software
www.gdata.pl
address:<FirstName><dot><Surname><at><gdata><dot><pl>