Date: Mon, 17 Feb 2014 19:04:31 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: Julio Merino <jmmv@outlook.com> Cc: Garrett Cooper <yanegomi@gmail.com>, freebsd-testing@FreeBSD.org, David Chisnall <theraven@FreeBSD.org> Subject: Re: Compile error with gcc Message-ID: <EB8398A1-4FF4-4DA4-9B8B-FF4D27A7E9A6@FreeBSD.org> In-Reply-To: <BLU0-SMTP3338CC4EB901473B7FC4A23C0990@phx.gbl> References: <695E42A3-2009-4DD7-B10E-BF8465C89D39@gmail.com> <A8EF2DCC-5F11-4405-88D1-05A193AB7BAF@gmail.com> <D10C2EBB-36EC-4292-A944-4356EA5657F2@FreeBSD.org> <BLU0-SMTP284F6428835416F4D9E3727C09E0@phx.gbl> <849648F5-7834-45DD-8BDF-6385BF4F82DB@FreeBSD.org> <BLU0-SMTP35FABDADF1BB4D66F59E02C09E0@phx.gbl> <2281FB5D-2BF0-4763-AC24-67EC3864D39B@gmail.com> <BLU0-SMTP3338CC4EB901473B7FC4A23C0990@phx.gbl>
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[-- Attachment #1 --] On 17 Feb 2014, at 14:55, Julio Merino <jmmv@outlook.com> wrote: ... > I'm not sure it's going to work. I think Dimitry mentioned that our libstdc++ should already provide std::vsnprintf, but due to the flags it's built with, it doesn't. I'm not sure if, due to this, the version check above would do the right thing. Our libstdc++ config.h is configured that way, because if you run its configure script, it notices that a few long double maths functions are missing. Then it disables *all* C99 support in libstdc++, including all the stuff we do have, like vsnprintf(). I'm not sure what would be broken if we overrode its automatic detection, and forced libstdc++ to expose C99 functions. Maybe some other programs would then complain about missing math functions... >> etc, and if so, what c++ standard was vsnprintf incorporated into? It looks like <some-date> should be 201103L (c++11) (http://en.cppreference.com/w/cpp/io/c/vfprintf ). If so, then the configure.ac tests should instead set the appropriate -std variable (or setting), then test for c++’s existence. > > The point of tests in configure.ac scripts is to _not_ do the above style of checks. If you are going to use those, then you don't need configure because you can stick those into the code: configure should be checking what the compiler actually does, not what it claims to support. This way the checks are future-proof and are going to work even for compilers you have not tried yet. > > Note that the check in configure.ac is working just fine and that's not the problem. The problem is in FreeBSD, where we have a single bconfig.h for the two compilers and the two compilers behave differently in at least one of the detected settings. It's not the compilers that behave differently, it is the C++ standard library. If you compile with clang, but use -stdlib=libstdc++, you will encounter the same issue, e.g. std::vsnprintf() will not be available. > So... we could generate two bconfig.h files, one for each compiler, and use them accordingly... or we could try to eliminate the divergence altogether. To do the latter, I think it's enough to remove the conditional and leave the code doing: > > namespace std { > using ::vsnprintf; > } > > in all cases. It works in clang but I don't know what the standard has to say in this regard! You would simply be using the non-std version of vsnprintf(). I don't think this should cause any trouble. Having said all this, I guess it would be simpler to just patch up our libstdc++ to expose the functions we do have for C99, and not care about what its configure script outputs. -Dimitry [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.22 (Darwin) iEYEARECAAYFAlMCTzsACgkQsF6jCi4glqMlogCgy7NanLOdqiErh9XUYNeXtFd4 x/cAoIr9KGAXJ9Ig000iVjKUeozbIqtF =bwVo -----END PGP SIGNATURE-----help
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