From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7408 Path: news.gmane.org!not-for-mail From: William Ahern Newsgroups: gmane.linux.lib.musl.general Subject: Re: Explicit casts in ctype.h suppress compiler warnings Date: Fri, 17 Apr 2015 12:33:20 -0700 Message-ID: <20150417193320.GA11349@wilbur.25thandClement.com> References: <1429289394.7038.3.camel@inria.fr> <20150417165238.GA6817@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1429299216 30343 80.91.229.3 (17 Apr 2015 19:33:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Apr 2015 19:33:36 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7421-gllmg-musl=m.gmane.org@lists.openwall.com Fri Apr 17 21:33:36 2015 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1YjC18-0006qu-TV for gllmg-musl@m.gmane.org; Fri, 17 Apr 2015 21:33:35 +0200 Original-Received: (qmail 5728 invoked by uid 550); 17 Apr 2015 19:33:33 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 5710 invoked from network); 17 Apr 2015 19:33:33 -0000 Content-Disposition: inline In-Reply-To: <20150417165238.GA6817@brightrain.aerifal.cx> User-Agent: Mutt/1.4.2.3i Xref: news.gmane.org gmane.linux.lib.musl.general:7408 Archived-At: On Fri, Apr 17, 2015 at 12:52:38PM -0400, Rich Felker wrote: > On Fri, Apr 17, 2015 at 06:49:54PM +0200, Jens Gustedt wrote: > > I generally think that casts are a bad idea, anyhow, and should only > > be used where it must be done, that is basically for pointer to > > integer conversion (and back). Code like this > > > > #define isdigit(a) (((unsigned)(a)-'0') < 10) > > > > can easily be replaced by > > > > #define isdigit(a) (((unsigned const){a}-'0') < 10) > > > > to change the explicit conversion to an implicit one in the > > initializer of the compound literal. Then, any compiler would have to > > diagnose if "a" would be a pointer. > > In another place (math.h) I removed this type of compound literal > usage because it was incompatible with C++, but the macros are > suppressed in C++ anyway. Still they might break -pedantic with > -std=c89. I do like this approach best in principle if it works > though, because the rules for when an error occurs are basically the > same as the rules for a real function. I think C++11 brace initialization int { 42 } works nearly identically to C99 compound literal definition (int){ 42 } The big difference is that the lifetime of C++ temporaries have expression scope, whereas compound literals have block scope. But that's irrelevant where the values will be copied and no pointer is derived.* A simple macro could be used to select the syntax. However, I don't think something like `unsigned char { 42 }' will work. The type name needs to be a single identifier, so a typedef would be needed: u_char { 42 }. * Unfortunately, by default g++ accepts C99-style compound literals, but for whatever reason gives them expression-scoped lifetimes as-if they were C++ temporaries. This gave me and some other people grief when when using pointers to compound literals, either explicitly or implicitly through array-to-pointer decay). See Bug #53220. Everything seemed to work when compiled as C++, unless you were fortunate enough to get a crash near the offending code. More recent versions of g++ now at least warn when a pointer is derived from a compound literal. I think clang++ has the same behavior.