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From: Lei Zhang <zhanglei.april@gmail.com>
Date: Wed, 22 Jun 2016 22:02:45 +0800
Message-ID: <CAOYuCc3ZvVF9EnNTr_sCdj1zqaZ6cFYHumuNfzHF6Qda35UL2w@mail.gmail.com>
Subject: Re: [gentoo-musl] [GSoC] _GNU_SOURCE in C++
To: Luca Barbato <lu_zero@gentoo.org>
Cc: gentoo-musl@lists.gentoo.org
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2016-06-22 21:54 GMT+08:00 Lei Zhang <zhanglei.april@gmail.com>:
> 2016-06-21 12:54 GMT+08:00 Lei Zhang <zhanglei.april@gmail.com>:
>> 2016-06-17 20:44 GMT+08:00 Lei Zhang <zhanglei.april@gmail.com>:
>>> After putting about eight "#define _GNU_SOURCE"s in various *.cpp
>>> files, it seems to work now :)
>>
>> I was wrong; it doesn't work... I still can't figure out a good solution :(
>>
>> Take glibc's strtoll_l for example: it's hidden by _GNU_SOURCE in
>> stdlib.h and used exclusively by libc++'s header <locale>; but
>> defining _GNU_SOURCE in <locale> could be too late, since stdlib.h
>> might be included prior to <locale>.
>>
>> Though glibc's stdlib.h is only explicitly included in <cstdlib>,
>> defining _GNU_SOURCE in <cstdlib> doesn't work either. It turns out
>> strtoll_l (and others) is not directly protected by _GNU_SOURCE, but
>> by __USE_GNU, which is in turn defined in features.h according to
>> _GNU_SOURCE. That means "#define _GNU_SOURCE" must appear before the
>> inclusion of features.h, and it's not very clear where "#undef
>> _GNU_SOURCE" should be put.
>>
>> Any suggestions?
>
> So far the only feasible solution I can think of is to wrap every
> inclusion of C headers in libc++ with a pair of "#define _GNU_SOURCE"
> and "#undef _GNU_SOURCE", thus making sure _GNU_SOURCE is defined
> before any implicitly inclusion of features.h. It is cumbersome, but
> works. I haven't really figured out if this solution is 100%
> error-proof.
As an after-thought, I don't this justifies as a *real* solution, as
it still exposes nearly all the hidden symbols to user program; it
just enables us to compile LLVM against musl...
Lei