Re: [boost] Re: first sight

From: Lee Brown (lee_at_[hidden])
Date: 2002-01-14 19:22:02

Thank you all, this "thread" ("Hey! I made that funny" a la Foghorn Leghorn) 
has been very insightful to me.

These are things as I see them. I know nothing of Windows.
I am quite conversant with Posix threads and thread issues in general.

1) Thread cancelation comes in three flavors: immediate, deferred, and 		
ignored. Immediate means one thread cancels another and the other immediately
exits.  Deferred means the other doesn't exit until it hits a cancelation 
point. Ignored means cancelation is not possible(honored). Boost is ignored 
mode capable only.

2) Immediate cancelation is not possible without a thread being able to put 
itself into defer or ignore state and back again.

3) Cancelation is not possible without a resource cleanup stack.

4) If cancelation is in effect, resources should be aquired and released in 
the same scope and in reverse order.(FYI: I believe reverse order is the only 
important fact,  same scope merely ensures this. This is the bizarre 
implementation of pthead_cancel_push() and pthread_cancel_pop())

5) Thread cancellation is NOT an exception. Period. It is an event to
be handled.  In fact I believe Xavier Leroy implemented thread_cancelation 
through a signal handler.  Thread cancelation was merely a special case of 
sending a signal to a thread. 

6) Actually, these mailings have reinforced my opinion that cancelation would 
be quite safe and easy to implement in C++ (at least in UNIX), with simple 
rules to keep everything cancel safe. I think policy based programming would 
really help here as Andrei shows it helps concurrent in general. Of course 
its hard to know until it is done because multithreaded stuff can be subtle.

i.e.

thread_routine()
{
  cancel_safe_lock l(mutex_);

   smart_ptr<Object> o =      
CreatePolicy<Object,cancel_safe_thread_policy>create();
}	

Then again maybe I'm an idiot.

lee