Re: [PATCH rdma-core 2/5] util: Fix check_snprintf to use __rc__ for local

[Leon Romanovsky <leon-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

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On Fri, Sep 01, 2017 at 11:46:02AM +0200, Nicolas Morey-Chaisemartin wrote:
>
>
> Le 01/09/2017 à 11:35, Leon Romanovsky a écrit :
> > On Fri, Sep 01, 2017 at 09:00:01AM +0200, Nicolas Morey-Chaisemartin wrote:
> >>
> >> Le 01/09/2017 à 06:56, Leon Romanovsky a écrit :
> >>> On Thu, Aug 31, 2017 at 02:50:56PM -0600, Jason Gunthorpe wrote:
> >>>> To avoid clashing with user variables.
> >>>>
> >>>> Signed-off-by: Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>
> >>>> ---
> >>>>  util/util.h | 4 ++--
> >>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/util/util.h b/util/util.h
> >>>> index cbf0deca2dde7b..30d32ee6b51e24 100644
> >>>> --- a/util/util.h
> >>>> +++ b/util/util.h
> >>>> @@ -8,8 +8,8 @@
> >>>>   * error */
> >>>>  #define check_snprintf(buf, len, fmt, ...)                                     \
> >>>>  	({                                                                     \
> >>>> -		int rc = snprintf(buf, len, fmt, ##__VA_ARGS__);               \
> >>>> -		(rc < len && rc >= 0);                                         \
> >>>> +		int __rc__ = snprintf(buf, len, fmt, ##__VA_ARGS__);           \
> >>> It can't clash, because it is declared in the scope {..} of that define and
> >>> it is local to check_snprintf macro.
> >> It does if any of the va-args is called rc.
> >> This is "valid" C although not sure what printfs does print here (probably random stack value)
> >> {
> >>  int i = printf("i = %d\n", i);
> >> }
> > So how does this rename solve the issue?
> > Now, we can clash with __rc__ variable.
> >
> > Thanks
>
> There is no safe way to handle that. It can be detected with -Wshadow though.
> But rc is quite a common name (39 definitions in the code base), __rc__ much less (unused... yet)

Or commit message should reflect that it is not fix, but attempt to
reduce name collision, or the patch should provide real fix. For
example, create inline function and convert all 5 callers to use it
without ##__VA_ARGS__.

>
>
> If you want some real fun, try writing macros with local variables that can be called recursively ;)
>
> Nicolas
>
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