Fwd: Re: Major issues with g++ 6 and gnulib

[Orion Poplawski]

Some comments from the Fedora devel list.


-------- Forwarded Message --------
Subject: Re: Major issues with g++ 6 and gnulib
Date: Sat, 20 Feb 2016 01:40:43 +0000
From: Jonathan Wakely <address@hidden>
Reply-To: Development discussions related to Fedora 
<address@hidden>
To: Development discussions related to Fedora 
<address@hidden>

On 19/02/16 16:33 -0700, Orion Poplawski wrote:
> I seem to be able to work around it at the moment by explicitly including
> <math.h> before <cmath>,

That might actually be a reasonable fix.

There is no guarantee whatsoever that <cmath> includes <math.h>, so it
is not portable and not guaranteed to work when Octave assumes that
including <cmath> will indirectly include gnulib's <math.h>.

If Octave wants to include gnulib's <math.h> then it should be
including it itself, not relying on some other header doing so
indirectly.

Furthermore, when GCC's <cmath> wants to include <math.h> it *really*
does want libc's <math.h>, which is what GCC was configured against,
and what it expects to find again when you compile. If some other
<math.h> turns up it could break all sorts of things.

Gnulib does seem to be going to some lengths to co-operate with the
real <math.h> (using #include_next itself, and putting everything in a
namespace when requested to) but it's entirely right that <cmath> gets
to decide what it wants to include, Octave should not be interfering
with the implementation's headers.

So for Octave to include gnulib's <math.h> either it should have a
different name, such as <gl_math.h>, or it should include it as
<libgnu/math.h>, or just ensure it is included directly, and not as a
side effect of including <cmath>.
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