On 04/04/2017 06:56 AM, Florian Weimer wrote:
For the usual GNU/Linux targets, does GCC support unaligned loads
through pointers to packed structs? That is, something like that:
struct __attribute__ ((may_alias, packed, aligned (1))) header
{
uint16_t id;
uint16_t flags;
uint16_t qdcount;
uint16_t ancount;
uint16_t nscount;
uint16_t adcount;
};
uint16_t
get_question_count (const void *buf)
{
const struct header *header = buf;
return ntohs (header->qdcount);
}
And this should work even if buf is not properly aligned.
There are target-independent tests in the test suite that exercise
this support so I'd say the answer is yes.
For example gcc.c-torture/execute/strct-pack-1.c (there are others
as well).
Martin
I know there are problems if I take the address of a member in struct
header and use that pointer, but I hope that direct use of the member is
okay.
This would allow us to simplify some code in glibc and avoid defensive
copies to a properly aligned struct.
Thanks,
Florian
