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Re: libbacktrace integration for _GLIBCXX_DEBUG mode

On 12/21/18 10:03 PM, Jonathan Wakely wrote:
On 21/12/18 22:47 +0200, Ville Voutilainen wrote:
On Fri, 21 Dec 2018 at 22:35, Jonathan Wakely <> wrote:
>    I also explcitely define BACKTRACE_SUPPORTED to 0 to make sure
>libstdc++ has no libbacktrace dependency after usual build.

I'm concerned about the requirement to link to libbacktrace
explicitly (which will break existing makefiles and build systems that
currently use debug mode in testing).

But see what Francois wrote, "I also explcitely define
BACKTRACE_SUPPORTED to 0 to make sure
libstdc++ has no libbacktrace dependency after usual build."

Yes, but if you happen to install libbacktrace headers, the behaviour
for users building their own code changes. I agree that if you install
those headers, it's probably for a reason, but it might be a different
reason to "so that libstdc++ prints better backtraces".

Also, some of the glibc team pointed out to me that running *any*
extra code after undefined behaviour has been detected is a potential
risk. The less that you do between detecting UB and calling abort(),
the better. Giving the users more information is helpful, but comes
with some additional risk.

Ditto. Having said those things, I think we need to figure out a good
way to provide this sensibly
as an opt-in. The backtrace support is bloody useful, and dovetails
into a possible Contracts-aware
implementation of our library, but I think we need to do some more
thought-work on this, thus I agree
that it's not stage3 material. I do think it's something that we need
to keep in mind, thanks
for working on it, Francois!

Yes, I agree that making it available via a more explicit opt-in would
be good. Maybe require users to define _GLIBCXX_DEBUG_BACKTRACE as well
as _GLIBCXX_DEBUG, or something like that.

Yes, I considered that if you had install libbacktrace you wanted to use it. But I agree that an explicit activation might be better.

I'll prepare that.


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