Re: setlocale: Silence a -Wdiscarded-qualifiers warning on Windows.

Hi Bruno,

Bruno Haible <[email protected]> writes:

> Collin Funk wrote:
>> On Windows (x86_64-w64-mingw32), I see the following warning:
>> 
>>     $ gnulib-tool --create-testdir --dir testdir1
>>     $ cd testdir1
>>     $ ./configure CFLAGS='-Wdiscarded-qualifiers'
>>     $ cd glllib
>>     $ make setlocale.o
>>     depbase=`echo setlocale.o | sed 's|[^/]*$|.deps/&|;s|\.o$||'`;\
>>     gcc -DHAVE_CONFIG_H -DEXEEXT=\".exe\" -DEXEEXT=\".exe\" -DNO_XMALLOC -DEXEEXT=\".exe\" -I. -I..  -DGNULIB_STRICT_CHECKING=1 -I/mingw64/include  -Wdiscarded-qualifiers -MT setlocale.o -MD -MP -MF $depbase.Tpo -c -o setlocale.o setlocale.c &&\
>>     mv -f $depbase.Tpo $depbase.Po
>>     setlocale.c: In function 'setlocale_single':
>>     setlocale.c:889:12: warning: return discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
>>       889 |     return setlocale_messages (locale);
>>           |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>> 
>> The attached patch fixes it by casting away the const.
>
> I don't like this because, although in line with the previous code, it
> adds one more cast.

I agree with your dislike of additional casts. I figured it was okay in
this case, given the similar one in setlocale_mtsafe when the result of
setlocale_null is returned. Anyway, glad I asked before committing.

>> Another
>> reasonable solution is changing the definition of setlocale_messages to
>> return a plain "char *".
>
> Returning a 'char *' instead of 'const char *'? This goes in the backward
> direction.
>
> Look: ISO C has 'const char *' since 1990. It also has 'setlocale' since 1990.
> POSIX <https://pubs.opengroup.org/onlinepubs/9799919799/functions/setlocale.html>
> says:
>   "The application shall not modify the string returned."
> Which means that the intended return type is 'const char *'. The return type
> 'char *' is a historical accident. (Probably the ANSI C89 committee simply
> lacked the courage to add 'const'.)
>
> But we can add the intended return type in Gnulib. Done as follows. This
> patch removes two casts, instead of adding a cast.

I considered suggesting this as well for the same reason you mention,
i.e., the standard saying the returned string cannot be modified. But I
mistakenly assumed you would not want to diverge from the standardized
prototype.

Perhaps it is worth bringing this up with the ISO C people. I have CC'd
Alex in case he decides he wants more work to do. I vaguely remember
POSIX being willing to change the return type of gettext to be "const
char *" for similar reasons to this getlocale case. I can hunt down
those links again if they will be useful for a proposal.

Collin