On 23/04/20 06:32 +0200, Helmut Grohne wrote:

On Mon, Apr 20, 2020 at 10:12:37AM +0100, Jonathan Wakely wrote:
> Now you are probably going to say that "-isystem /usr/include" is a bad
> idea and that you shouldn't do that.


> I'm inclined to agree. This isn't a
> problem just yet. Debian wants to move /usr/include/stdlib.h to
> /usr/include/<multiarch>/stdlib.h. After that move, the problematic flag
> becomes "-isystem /usr/include/<multiarch>". Unfortunately, around 30
> Debian packages[1] do pass exactly that flag. Regardless whether doing
> so is a bad idea, I guess we will have to support that.

Or Debian should fix what they're going to break.

This is not quite precise. The offending -isystem
/usr/include/<multiarch> flag is already being passed. According to what
you write later, doing so is broken today. It just happens to work by
accident. So all we do is making the present breakage visible.

> I am proposing to replace those two #include_next with plain #include.
> That'll solve the problem described above, but it is not entirely
> obvious that doing so doesn't break something else.
> After switching those #include_next to #include,
> libstdc++-v3/include/c_global/cstdlib will continue to temporarily
> will #include <stdlib.h>. Now, it'll search all include directories. It
> may find libstdc++-v3/include/c_comaptibility/stdlib.h or the libc's
> version. We cannot tell which. If it finds the one from libstdc++-v3,
> the header will notice the _GLIBCXX_INCLUDE_NEXT_C_HEADERS macro and
> immediately #include_next <stdlib.h> skipping the rest of the header.
> That in turn will find the libc version. So in both cases, it ends up
> using the right one. Precisely what we wanted.

As Marc said, this doesn't work.

That is not very precise either. Marc said that it won't fix all cases.
In practice, it would make those work that don't #include <stdlib.h> but
use #include <cstdlib> instead.

Marc also indicated that using include_next for a header of a different
name is wrong. So this is a bug in libstdc++ regardless of whether it
breaks or unbreaks other pieces of software.

He said he doesn't like it, that doesn't mean it's a bug or actually
causes incorrect results.

Whereas using -isystem provably *does* break the implementation,
making it impossible for #include <stdlib.h> to meet the requirements
of the C++ standard. And your proposed patch doesn't prevent that.

If a program tries to include <stdlib.h> it needs to get the libstdc++
version, otherwise only the libc versions of certain functions are
defined. That means the additional C++ overloads such as ::abs(long)
and ::abs(long long) won't be defined. That is the reason why
libstdc++ provides its own <stdlib.h>.

And if you do -isystem /usr/include (or any other option that causes
libstdc++'s <stdlib.h> to be skipped) that doesn't work. Only
::abs(int) gets defined.

So -isystem /usr/include breaks code, with or without your patch.

It is very difficult to disagree with -isystem /usr/include or -isystem
/usr/include/<triplet> being broken and unsupported. Having you state it
that clearly does help with communicating to other upstreams. For this
reason, I've looked into the remaining cases. It turns out that there
aren't that many left. In particular chromium, opencv and vtk got fixed
in the mean time. Basically all remaining failures could be attributed
to qmake, which passes all directories below /usr/include (including
/usr/include and /usr/include/<triplet> if a .pc file mentions them)
using -isystem. I've sent a patch https://bugs.debian.org/958479 to make
qmake stop doing that.

I therefore agree with you that the patch I sent for libstdc++ is not
necessary to make packages build on Debian. Removing the offending
-isystem flags from the respective builds is a manageable option and has
already happened to a large extend.

Yes, I introduced the current <stdlib.h> and <math.h> wrappers years
ago in GCC 6, and so I'm surprised to see it coming up again now.
Several packages had problems and already fixed them.

We can conclude that the motivation for my patch is not a good one,
because it embraces broken behaviour. However, the use of include_next
remains a bug, because the name of the including and the name of the
included header differ, and it should be fixed on that ground.

Not liking something is not a bug.

You need to demonstrate an actual bug (e.g. failure to compile,
non-conformance to the C++ standard) that is not caused by user error
(like misuse of -isystem) to argue for fixing something.

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