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Date: Fri, 17 Apr 2015 18:49:54 +0200
From: Jens Gustedt <>
Subject: Re: Explicit casts in ctype.h suppress compiler warnings

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.


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