Re: [Xen-devel] [PATCH v11 1/9] xen: use __UINTPTR_TYPE__ for uintptr_t

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 6 Mar 2019, Jan Beulich wrote:
> >>> On 05.03.19 at 23:38, <sstabellini@xxxxxxxxxx> wrote:
> > Use __UINTPTR_TYPE__ to define uintptr_t. A later patch will make use of
> > __PTRDIFF_TYPE__ to define ptrdiff_t.
> > 
> > Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> > Reviewed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> 
> As before - I object to this change without the description supplying
> both a reason (which would better also explain why the current way
> of defining uintptr_t is detrimental) and a discussion why it is okay for
> us to use __UINTPTR_TYPE__, despite (at least) gcc making this
> available only under certain conditions (i.e. it would need to be
> confirmed that whatever the conditions they're always met for us).

Is the following good enough:

Use __UINTPTR_TYPE__ to define uintptr_t for consistency with ptrdiff_t.
__UINTPTR_TYPE__ is safe to use as it is a common predefined macro of
GNU C; it is defined to the correct underlying type as per GCC
documentation[1].

[1] https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html

Also, it is not a good idea to use __mode__(__pointer__) to define
uintptr_t, because it relies on an obscurely defined GCC feature whose
semantics might be taken to imply that the things are really pointers.


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