Date: Wed, 24 May 2023 10:30:26 -0400 From: Rich Felker <dalias@...c.org> To: Jₑₙₛ Gustedt <jens.gustedt@...ia.fr> Cc: musl@...ts.openwall.com Subject: Re: [C23 divers headers 15/17] C23: add the unreachable macro On Wed, May 24, 2023 at 04:26:43PM +0200, Jₑₙₛ Gustedt wrote: > Rich, > > on Wed, 24 May 2023 10:21:05 -0400 you (Rich Felker <dalias@...c.org>) > wrote: > > > On Wed, Apr 19, 2023 at 08:38:04AM +0200, Jens Gustedt wrote: > > > --- > > > include/stddef.h | 2 ++ > > > 1 file changed, 2 insertions(+) > > > > > > diff --git a/include/stddef.h b/include/stddef.h > > > index f25b8639..09be5fb6 100644 > > > --- a/include/stddef.h > > > +++ b/include/stddef.h > > > @@ -24,4 +24,6 @@ > > > #define offsetof(type, member) ((size_t)( (char *)&(((type > > > *)0)->member) - (char *)0 )) #endif > > > > > > +#define unreachable() __builtin_unreachable() > > > + > > > #endif > > > -- > > > 2.34.1 > > > > This is a namespace violation unless protected by a C version check. > > That would have been my first take also. I changed these things after > you (I think it was you) telling me that generally interfaces that are > added in newer versions of C need not be protected for older > version. I probably misunderstood something. Can you please clarify > under which circumstances you want feature tests and when not? OK, I agree it's a little bit unclear, but here it's a case of a very minimalist header (stddef.h) defining a new, plain-english-word macro that's very likely to conflict with existing code. I'm not sure where exactly the cutoff point for whether it matters should be -- this is something community should probably discuss. But I think this is a really clear case where the namespace should not be stomped. Cases where it's almost surely OK, OTOH, are things like promoting common existing functions to standard -- existing code is almost surely already avoiding conflicts with these (e.g. the time.h stuff). Rich
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