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Date: Fri, 17 Apr 2015 12:52:38 -0400
From: Rich Felker <>
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.


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