[Ffmpeg-devel] [PATCH] Machine endian bytestream functions

[Trent Piepho]

On Tue, 24 Apr 2007, Ramiro Ribeiro Polla wrote:
> M?ns Rullg?rd wrote:
> >>> I'm thinking the unaligned16/32/64 macros from bitstream.h should be
> >>> renamed and moved here instead.
> >>>
> >>>
> >> I'll take a look.
> >>
> >
> > This lets the compiler choose whatever access method is appropriate,
> > and saves us the trouble of maintaining a fragile list
>
> Is there a way for us to get that information off the compiler? (assume
> only gcc atm)
> I thought maybe compile a simple read with __attribute__((packed)) and
> another without the attribute. Then compare object files and assume that
> if they're equal, unaligned access are ok.
> Is that too ugly/random/unreliable?

It won't work.  Pretty much all architectures align fields in structures to
some kind of boundary.  When packed, the structures will be different, even
on those architectures where unaligned accesses are possible.