Re: Calling __cxa_thread_atexit_impl directly, from C code

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 30/08/22 04:37, Florian Weimer wrote:
> * Adhemerval Zanella Netto:
> 
>> It would mean that libgcc_s would need to build and use the fallback
>> implementation in the case of failure, which is suboptimal (although not
>> sure it would be an improvement over abort on failure).
> 
> The fallback implementation also has to allocate memory.
> 
> The alternative would be to throw std::bad_alloc.

Yeah, but the suboptimal is not solely for the allocation memory part,
but also for the missing synchronization and ordering.  But I also think
moving the failing handling to caller it still better than the hard hammer
or aborting the process (even though I agree it won't improve that much).

> 
>> But I also think for compat reasons we can't really change 
>> __cxa_thread_atexit_impl, since C++ constructors will be the ones responsible
>> to call __cxa_thread_atexit and afaik it assumes it can not fail (meaning
>> that any failure will be ignored).
> 
> Yes, there is the general problem that for registering an object for
> destruction, as a matter of principle, you need to try to allocate the
> data structure in the registry first, and if that is successful, create
> the object.  Otherwise you may end up with an object and no way to
> register its destructor.  Perhaps you should just call the destructor at
> this point and throw std::bad_alloc.
> 
> I guess we should go with the static destructor counting approach
> instead. 8-/

Why strategy more specially do you mean the counting approach?  I just reread
the 'Counting static __cxa_atexit calls' thread and tend to agree with you
that having the number of required unique __cxa_atexit calls seems slight
better than a failable .init_array mode.