[cfe-dev] Clang++ always defines _GNU_SOURCE

[Lei Zhang via cfe-dev]

2016-06-14 21:40 GMT+08:00 Joerg Sonnenberger via cfe-dev
<cfe-dev at lists.llvm.org>:
> 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.

Perhaps I'm not seeing the whole picture, but it looks to me that
using finer-grained visibility macros like _BSD_SOURCE,
_POSIX_C_SOURCE, etc does *not* violates your points. We could still
expose those non-standard functions without blindly defining
_GNU_SOURCE, don't we?


Lei