Saving errno around signal handlers

[Roland McGrath]

> > So the standard is not going to be changed to require that signal handling
> > save and restore errno implicitly.  (It would have been a highly suspect
> > change.)  It's being clarified to say that the implementation is free to do
> > so.  But applications cannot assume any such thing, so any such future
> > application remains as broken as such applications are today.  I see no
> > rationale even slightly compelling for making libc do this magic.  In fact,
> > it would clearly do harm to people developing applications on glibc-based
> > systems but intending to write POSIX-conformant code.
> 
> In that it would mask potential errors in the signal handling code?

Hide real bugs that many manifest on any POSIX-compliant system, yes.