Hi,
> struct fields {
> long long unsigned f0:12;
> long long unsigned f1:52;
> } __attribute__((__packed__));
the C99 standard ISO/IEC 9899 forbids this type:
6.7.2.1 Structure and union specifiers
4 A bit-field shall have a type that is a qualified or unqualified version of
_Bool, signed int,
unsigned int, or some other implementation-defined type.
The C standard simply does not promote the bit-field value to any type larger
than int or
unsigned int.
GCC chooses to do the larger than int computations in an artificial 52-bit
type, but it is a
non-standard extension.
And if you compile your example with -Wall you'll see the problem:
gcc -m32 -O3 -Wall test.c
test.c: In function 'main':
test.c:17:21: warning: format '%llx' expects argument of type 'long long
unsigned int', but argument 2 has type 'int' [-Wformat=]
printf("x.f0=0x%llx\n", x.f0);
^
test.c:19:21: warning: format '%llx' expects argument of type 'long long
unsigned int', but argument 2 has type 'long long unsigned int:52' [-Wformat=]
printf("x.f1=0x%llx\n", x.f1);
^
so especially the first warning is no joke:
./a.out
x.f0=0x80497b400000fff
g0=0x1ffe expect 0x1ffe
x.f1=0xfffffffffffff
g1=0xffffffffffffe expect 0x1ffffffffffffe
OTOH that is perfectly OK for C++:
gcc -x c++ -m32 -O3 -Wall test.c
./a.out
x.f0=0xfff
g0=0x1ffe expect 0x1ffe
x.f1=0xfffffffffffff
g1=0x1ffffffffffffe expect 0x1ffffffffffffe
Regards
Bernd.
