[cfe-dev] Clang++ always defines _GNU_SOURCE

[Joerg Sonnenberger via cfe-dev]

On Tue, Jun 14, 2016 at 08:28:33PM +0800, Lei Zhang wrote:
> 2016-06-14 20:22 GMT+08:00 Lei Zhang <zhanglei.april at gmail.com>:
> > 2016-06-14 20:12 GMT+08:00 Joerg Sonnenberger via cfe-dev
> > <cfe-dev at lists.llvm.org>:
> >> Compared to breaking lots of applications that expect the GNU symbols by
> >> default?
> >
> > That's a valid point. OTOH, should we encourage applications to use
> > GNU symbols with out explicitly defining _GNU_SOURCE? Then what's the
> > point of such a macro?
> 
> BTW, _GNU_SOURCE is *not* unconditionally defined by gcc/clang when
> compiling C code on Linux.

You can't change the visibility macros after the first header has been
included. C language mode doesn't ship with a C library set that
requires many of the non-standard / non-ISO functions. That's completely
different from C++, where many typical STL includes include more than
the functions supported by ISO C (and the default namespace with glibc).
That's why it is pushed by default.

Joerg