Simplify sendmmsg code

[Florian Weimer]

On 05/09/2017 10:58 PM, Adhemerval Zanella wrote:

> So if I understood correctly, the idea is to *remove* sendmmsg on resolv code then?
> Because now with __ASSUME_SENDMMSG being used as default for Linux this code will
> be used on all architectures.

Yes, once we have two sockets, we need to call sendmsg for each one 
separately.  But that's an *algorithmic* change, it's not simply about 
replacing one call to sendmmsg with two to sendmsg.  But that would be 
fine as an intermediate step as well if you want to simplify things in 
this way.

> In any case, my understanding for sendmmsg is it is a GNU extension (similar to
> pwritev), so the idea is to have a fallback usable code on sysdeps/{posix,unix}
> to we do not need to actually tests its usability in the rest of glibc code.

As far as I understand things, both sendmmsg and pwritev cannot be 
emulated in userspace because the fallback implementation would not have 
the atomicity guarantees provided by the kernel implementation.  IMHO, 
the generic versions should just return ENOSYS (or not linked in at all, 
so that configure scripts can note their absence).

Thanks,
Florian