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Date: Tue, 3 Jan 2017 18:17:28 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: libstdc++ namespace pollution
On Tue, Jan 03, 2017 at 10:52:19PM +0000, Justin Cormack wrote:
> On 3 January 2017 at 21:35, Rich Felker <dalias@...c.org> wrote:
> > On Tue, Jan 03, 2017 at 09:16:29PM +0000, Justin Cormack wrote:
> >> On 3 January 2017 at 18:29, Rich Felker <dalias@...c.org> wrote:
> >> > On Tue, Jan 03, 2017 at 05:44:47PM +0000, Justin Cormack wrote:
> >> >> I have been trying to build a C++ program recently, and came across
> >> >> the issue that
> >> >>
> >> >> 1. libstdc++ always defines _GNU_SOURCE see
> >> >> https://gcc.gnu.org/onlinedocs/libstdc++/faq.html#faq.predefined
> >> >> 2. Musl defines pretty much everything once _GNU_SOURCE is defined
> >> >
> >> > So does glibc. :)
> >>
> >> This particular issue only happens with Musl, it includes more...
> >
> > That might be a bug/unwanted behavior on musl's side then. Could you
> > help me check? I'd be happy to remove namespace-polluting cruft that's
> > not actually needed to meet what applications can reasonably expect
> > from _GNU_SOURCE.
> >
>
> This minimal test case compiles with c++ on Debian but not on Alpine:
>
> #include <signal.h>
>
> class ArgumentParser_x64 {
> enum Register {
> REG_A,
> REG_B,
> REG_C,
> REG_D,
> REG_SI,
> REG_DI,
> REG_BP,
> REG_SP,
> REG_8,
> REG_9,
> REG_10,
> REG_11,
> REG_12,
> REG_13,
> REG_14,
> REG_15,
> REG_RIP,
> };
> };
>
> main() {}
I see. It's a bit of luck that it happens to work on glibc, I think --
they define the REG_* identifiers as enum constants and then #define
them to themselves in order to satisfy programs which are checking for
their presence with #ifdef. So while the above code has macros
clashing with the identifier names it wants to use, they end up being
benign because they're defined to themselves.
In general I don't do this (the enum approach) in musl because (1) I
don't like enums, and (2) it breaks things that want to use the macros
in preprocessor #if conditionals. However for macros like this that
aren't specified by any standard and which are fundamentally namespace
pollution, it seems like a better approach, so I'm not opposed to
switching. We should probably do the same on all affected archs if we
do.
Rich
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