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Message-ID: <20150417165238.GA6817@brightrain.aerifal.cx> Date: Fri, 17 Apr 2015 12:52:38 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Explicit casts in ctype.h suppress compiler warnings On Fri, Apr 17, 2015 at 06:49:54PM +0200, Jens Gustedt wrote: > Am Freitag, den 17.04.2015, 13:59 +0300 schrieb Alexander Monakov: > > For the following erroneous source code: > > > > #include <ctype.h> > > int f(char *c) > > { > > return isdigit(c) || isspace(c); > > } > > > > GCC warns only for passing a pointer to isspace; isdigit is implemented as a > > macro that casts its argument to unsigned, and the warning is suppresed > > because the origin of the cast is in a system header. Since isspace is > > implemented with a static inline helper function, there is a warning. With > > glibc headers, no warning is issued in either case for a similar reason. > > 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. Rich
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