[c++-pthreads] cancellation points report failure

[Alexander Terekhov]

Nathan Myers wrote:
[...]
>  C library and system calls never throw.
>  In the event of cancellation, cancellation point functions report 
>    failure via their normal means (e.g. return -1, or NULL), and set 
>    errno if appropriate.  Blocked calls fail immediately.

This will essentially break a lot of existing C/C++ code that is 
cancel-safe and doesn't known how to handle ECANCELED "error" 
(instead of expected behavior -- exception/unwinding).

>  In a catch block handling a cancellation exception, functions
>    identified as cancellation points work normally.

They work normally because "When a cancellation request is acted 
upon, or when a thread calls pthread_exit(), the thread first 
disables cancellation by setting its cancelability state to 
PTHREAD_CANCEL_DISABLE and its  cancelability type to 
PTHREAD_CANCEL_DEFERRED. " And, in plain-C POSIX, "The behavior 
is undefined if a cancellation cleanup handler or thread-
specific data destructor routine changes the cancelability 
state to PTHREAD_CANCEL_ENABLE" (source: TC2).

>  When a cancellation exception is swallowed by a catch block, 
>    subsequently-called cancellation point functions report failure, 
>    as before, until another cancellation exception is thrown.

That's sticky cancel. Not fun if you're using cancel for canceling 
*jobs* and really want to reuse worker threads (your simply catch 
and finalize cancel exception, enable cancel again, and continue 
with next job).

regards,
alexander.