[c++-pthreads] Re: cancellation points report failure

[Alexander Terekhov]

David Abrahams wrote:
[...]
> Of course it's not safe.  That's my point, sort of: if you use
> synchronous cancellation, you have to give up on any *guarantee* that
> the thread will be cancelled, so we shouldn't be considering measures
> that take heroic steps to try to ensure it.  If you want to guarantee
> that cancellation happens, you have to do something unsafe.

Note that the use of POSIX asynchronous cancellation (I mean the 
presence of async-cancel{-safe} regions on the execution path) does 
NOT guarantee thread termination (cancel request delivery) at all. 
Conforming implementations are free to ignore it completely, so to
speak. I wish the standard would define pthread_testcancel() "in
terms" of an empty async-cancel region (and it would also provide 
async-cancel-safety for pthread_testcancel() itself):

void pthread_testcancel() { /* mandatory shall occur semantics */
  int oldtype; 
  pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, &oldtype);
  pthread_setcanceltype(oldtype, &oldtype);
}

regards,
alexander.