[Bug c/53769] [C11]: Macros __STDC_NO_THREADS__ / __STDC_NO_ATOMIC__ missing.

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53769

--- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-06-26 17:13:52 UTC ---
On Tue, 26 Jun 2012, vincent-gcc at vinc17 dot net wrote:

> OK, this makes clear that for portability, programs should do configure tests
> instead of relying on the values of standard macros. I think this should be

Exactly.  Cf. the issues GCC had with system headers on FreeBSD checking 
__cplusplus to enable C++11 features (where GCC generally has more C++11 
support than Clang, but not the particular feature used by those headers; 
bug 51705).

Eventually there will be features implemented in all compilers people are 
likely to care about and those features can be used without worrying about 
compilers from during the transitional period.  But during the 
transitional period while major features of a new standard such as C++11 
or C11 are being implemented by compilers, you need to test for the 
features you want to use.  And it's entirely plausible that an 
implementation will have fully functional _Thread_local without C11 
threads.h (thread-local storage for pthreads or another form of threads), 
so test for what you actually want to use.