[patch] libstdc++/69945 Add __gnu_cxx::__freeres hook

[Richard Biener]

On Wed, Feb 24, 2016 at 7:35 PM, Jonathan Wakely <jwakely@redhat.com> wrote:
> This adds a new function to libsupc++ which will free the memory still
> in use by the pool used for allocating exceptions when malloc fails.
>
> This is similar to glibc's __libc_freeres, which valgrind (and other
> tools?) use to tell glibc to deallocate everything before exiting.
>
> I initially called it __gnu_cxx::__free_eh_pool() but I figured we
> might have other memory in use at some later date, and we wouldn't
> want valgrind to have to start calling a second function, nor make a
> function called __free_eh_pool() actually free other things.
>
> I intentionally *didn't* lock the pool's mutex before freeing it,
> because this should never be called in a context where multiple
> threads are still active and accessing the pool.
>
> Thoughts?
>
> Is the new test a good idea, or will exposing it like that in the
> testsuite just give users the idea that they can/should be doing the
> same themselves?

The goal is to lookup the function via dlsym and only call it before
glibcs freeres (which will probably make free() in-operatable?)

>
> [ Aside: should the actual pool be created with placement-new to
> [ ensure it doesn't ever get destroyed?
> [
> [   alignas(pool) unsigned char poolbuf[sizeof(pool)];
> [   pool& emergency_pool = *::new(poolbuf) pool;

Hmm, for pedantic correctness yes I guess.  But it hasn't any destructor,
it's global (so no CLOBBER / memory re-use issue) and so the destruction
won't do anything.  Of course it might simply save a global dtor (does it
even register one?  I don't see one).

There is still the missing feature of allowing to size the emergency
pool using an environment variable (properly securely implemented, of course).

Richard.

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