Re: [musl] [PATCH v2] prevent from redefining __STDC_UTF_{16,32}__ macros

[Jens Gustedt <jens.gustedt@inria.fr>]

Hi

Am 26. Februar 2024 04:47:22 MEZ schrieb Gabriel Ravier <gabravier@gmail.com>:
> On 2/26/24 02:17, Rich Felker wrote:
> > On Thu, Jan 25, 2024 at 03:14:40PM +0100, Jules Maselbas wrote:
> >> Undefine any previous __STDC_UTF_{16,32}__ macros before defining
> >> them to prenvent any warnings of redefining macros.
> >> 
> >> ---
> >> v2:
> >>   - changed `#if !define(...)` guard to `#undef`, as suggested by Rich Felker
> >> 
> >> I encountered this "issue" trying to compile a program with the -isystem
> >> option to override toolchain/installed musl headers with one from source.
> >> 
> >>   include/stdc-predef.h | 3 +++
> >>   1 file changed, 3 insertions(+)
> >> 
> >> diff --git a/include/stdc-predef.h b/include/stdc-predef.h
> >> index af1a2799..5ccd884d 100644
> >> --- a/include/stdc-predef.h
> >> +++ b/include/stdc-predef.h
> >> @@ -7,7 +7,10 @@
> >>   #define __STDC_IEC_559__ 1
> >>   #endif
> >>   +#undef __STDC_UTF_16__
> >>   #define __STDC_UTF_16__ 1
> >> +
> >> +#undef __STDC_UTF_32__
> >>   #define __STDC_UTF_32__ 1
> >>     #endif
> >> -- 
> >> 2.43.0
> > I merged this, but now gcc warns about undefining them if system
> > header warnings aren't suppressed. I'm not sure what the justification
> > is for that... *sigh*
> > 
> > Rich
> 
> It appears as though since 2001 GCC makes it so that any redefinition or undefinition of any macro with a name starting with "__STDC_" (with hard-coded exceptions for __STDC_FORMAT_MACROS, __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS) always results in a warning (from reading the code this doesn't seem to be tied to any -W switch, so it's simply entirely impossible to work around)
> 

The way to go would be to #ifdef them, not to use #undef. 
These are really a language feature and not library, so anything that this should do is repair work for older compilers.  The compiler's setting, if any, should prime over the C library. 

With C23 these two macros become mandatory.

Jens

-- 
Jens Gustedt - INRIA & ICube, Strasbourg, France