Rename struct ucontext tag (bug 21457)

[Joseph Myers]

On Tue, 13 Jun 2017, Florian Weimer wrote:

> > In this case, the type does need to have some tag rather than just a
> > typedef name, because it includes a pointer to itself.  This patch
> > uses struct ucontext_t as the new tag, so the type is mangled as
> > ucontext_t (the POSIX *_t reservation applies in all namespaces, not
> > just the namespace of ordinary identifiers).  Another reserved name
> > such as struct __ucontext could of course be used.
> 
> How widely known is the reserved nature of _t names?  Maybe the __
> prefix would communicate better that this is an internal name?  Although
> I assume we will never change it again, so applications referencing the
> new name should not face any future problems.

Since this is a (C++) ABI, we don't want to change it unnecessarily, so I 
don't see a reason for subsequent changes.  We just need to pick the name 
we want to use (and then people with patches such as the RISC-V port that 
add sys/ucontext.h files will need to follow accordingly).

-- 
Joseph S. Myers
joseph@codesourcery.com