On Wednesday 30 May 2007 13:00, Andrew Haley wrote:
> Mihai Don?u writes:
> > typedef struct _type2_t {
> > union {
> > uint32_t field1:31;
> > uint32_t field2:16;
> > };
> > uint32_t field3:1;
> > } __PACKED type2_t;
>
> I've read this three times now and I still don't understand your
> question. You declare a packed struct, but you don't want it to be
> packed? So don't declare it packed, then. But that can't be what you
> really meant.
>
> Andrew.
>
Ok, I'm sorry if I wasn't clear.
Let me begin with the problem that I'm facing: I have an array of dword-s
where each dword has the following "bit-fields":
* ordinal_or_name (bit 31, i.e. 0x80000000 )
* ordinal (bits 0-15, i.e. 0x0000ffff)
* name_rva (bits 0-30, i.e. 0x7fffffff)
(this is actually an entry in the ImportLookupTable of the .idata section of
a PE image (Windows .dll)).
If 'ordinal_or_name' is 0 then 'name_rva' contains a valid address (RVA). If
'ordinal_or_name' is 1 then 'ordinal' contains a valid number (import ordinal).
Instead of doing some "ugly" macros I wanted to do a dword-sized structure to
help me read the bit-fields more easily:
typedef struct _type2_t {
union {
uint32_t field1:31; /* bits 0-30 */
uint32_t field2:16; /* bits 0-15 */
};
uint32_t field3:1; /* bit 31 (or so I want) */
} __PACKED type2_t;
My problem is that gcc does not make the union *exactly* 31 bits, but rounds it
to a dword and as such, 'field3' maps to the first bit in the 2nd dword of a quad
(while I wanted it to map to the last bit in the 1st dword).
Btw, I'm talking x86(_x64) only here.
I hope this clears things up :)
--
Mihai Donțu
