Target support for packed/aligned (1) structs

Florian Weimer
Tue Apr 4 12:56:00 GMT 2017

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;

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.

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 

This would allow us to simplify some code in glibc and avoid defensive 
copies to a properly aligned struct.


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