On 12 February 2017 at 00:55, David Guillen Fandos <david@xxxxxxxxxxx> wrote:
> Hello!
>
> I have the following function
> std::tuple<uint32_t, uint8_t> getRawIdx(uint16_t tidx) {
> return std::make_tuple(localidx.entries[tidx].indx_ptr,
> localidx.entries[tidx].indx_size);
> }
>
> Where s is a struct like
>
> typedef struct __attribute__((packed)) {
> uint32_t indx_ptr;
> uint8_t indx_size;
> } _i_idx_entry ;
>
> The error I get is:
>
> cannot bind packed field
> ‘((DBIndex*)this)->DBIndex::localidx.DBIndex::_i_idx::entries[((int)tidx)].DBIndex::_i_idx_entry::indx_ptr’
> to ‘unsigned int&’
>
> If extend the tuple to include a 3rd field and I return some other thing
> it just works:
>
> return std::make_tuple(localidx.entries[tidx].indx_ptr,
> localidx.entries[tidx].indx_size,
> tidx, prefb);
>
> I think gcc is trying to do some smart optimization or something and
> packed does not work because of tuple alignments not being packed.
>
> Any ideas?
> Thanks!
> David
>
This is the wrong mailing list for questions about possible bugs or
for help using GCC, please read https://gcc.gnu.org/lists.html and use
the list where this is on-topic (gcc-help). I've redirected this reply
that list.
This has nothing to do with std::tuple (the alignment of your struct
has no effect on the alignment of std::tuple<uint32_t, uin8_t>,
they're completely different types).
You get the same error without std::tuple:
struct __attribute__((packed)) S {
int indx_ptr;
char indx_size;
};
S s;
struct T {
T(int&, char&) { }
};
T getRawIdx() {
return T(s.indx_ptr, s.indx_size);
}
It's related to binding references to unaligned variables, because
int& is a reference to an int with normal alignment, but the member of
your struct has different alignment to a normal int.
I think this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36566
