[ovs-dev] [poll-logging 2/2] poll-loop: Make wakeup logging more portable and easier to understand.

[Ben Pfaff]

On Fri, May 13, 2011 at 02:14:33PM -0700, Ethan Jackson wrote:
> > + ? ?if (!getaddr(fd, (struct sockaddr *) &ss, &len)) {
> > + ? ? ? ?if (ss.ss_family == AF_INET) {
> > + ? ? ? ? ? ?struct sockaddr_in sin;
> > +
> > + ? ? ? ? ? ?memcpy(&sin, &ss, sizeof sin);
> > + ? ? ? ? ? ?ds_put_format(string, IP_FMT":%"PRIu16,
> > + ? ? ? ? ? ? ? ? ? ? ? ? ?IP_ARGS(&sin.sin_addr.s_addr), ntohs(sin.sin_port));
> 
> I be misunderstanding some basic principle of C here, but why do you
> need to memcpy?  Can't you simply do struct sockaddr_in *sin = (struct
> sockaddr_in *)&ss ?

In practice, probably you could.

In theory, this might violate C's aliasing rules, which (in outline)
say that an object declared of one that may only be accessed either as
that type (or a compatible type) or as a character type.  GCC these
days warns about potential or actual violations, so I prefer to avoid
them.

memcpy() always (conceptually) accesses its arguments as arrays of
character type.

In practice again, glibc defines sockaddr and friends as a weird GCC
extension that avoids aliasing violations and warnings for them.  But
other OSes don't necessarily do that.

In summary, it's easier to memcpy() on slow paths where it doesn't
matter anyway.  Makes life easier, in my opinion.