Re: Bug report on iswalpha

[Rich Felker <dalias@libc.org>]

On Tue, Aug 05, 2014 at 01:35:27PM -0700, Alon Zakai wrote:
> I think we have encountered a bug in iswalpha, as shown by the following
> program:

At least an inconsistency with glibc. Not necessarily a bug.

> ====
> #include <locale.h>
> #include <stdio.h>
> #include <wctype.h>
> 
> int
> main(const int argc, const char * const * const argv)
> {
>   const char * const locale = (argc > 1 ? argv[1] : "C");
>   const char * const actual = setlocale(LC_ALL, locale);
>   if(actual == NULL) {
>     printf("%s locale not supported; skipped locale-dependent code\n",
>            locale);
>     return 0;
>   }
>   printf("locale set to %s: %s\n", locale, actual);
> 
>   const int result = iswalpha(0xf4); // ô
>   printf("iswalpha(\"\xc3\xb4\") = %d\n", result);
>   return 0;
> }
> ====
> 
> It returns 1 in the final printf, saying that that char is an walpha char,
> when I believe it is not. For comparison, glibc reports 0.
> 
> Tested on musl 1.0.3 (used in emscripten) and musl trunk on git, same
> result.

Expecting iswalpha(0xf4) to return 0 in the C locale is wron, since
0xf4 has not been established to be valid wchar_t value in the current
locale, and the behavior of iswalpha is _undefined_ unless the
argument is either WEOF or a valid wchar_t in the current locale.

As documented, musl's C locale contains all of Unicode, and
additionally classifies all Unicode characters into the C classes like
"alpha", etc. based on their Unicode identities. This behavior is
definitely conforming to the requirements of ISO C and likely (though
the specification is not entirely clear) conforming to the current
requirements of POSIX, but is expected to be forbidden in future
issues of POSIX.

This is actually a topic of current discussion and possible change
(depending on what happens in POSIX), but I don't think the behavior
of iswalpha is likely to change in any case. If the C locale in musl
is changed not to include all of Unicode, then iswalpha(0xf4) would
just be undefined behavior in the C locale, and there would be no
reason to make it check the locale and return false. If the above code
is part of a test, I think it's an invalid test. With a better idea of
what it's trying to test, I could possibly suggest a fix that avoids
the UB.

Rich