Re: Threads docs updated in CVS

From: williamkempf_at_[hidden]
Date: 2001-08-12 08:17:55

--- In boost_at_y..., John Max Skaller <skaller_at_m...> wrote:
> 	So please comment. It is my understanding that
> events are 
> 
> 	a) more general
> 	b) harder to use

Condition variables *MUST* be used in conjunction with a mutex which 
is locked before the wait, unlocked during the wait, and locked again 
after the wait in a thread safe manner.  Condition variables use an 
interface that makes this hard to do wrong (with Boost.Threads it 
becomes impossible).  Events *MUST NOT* be used in conjunction with a 
mutex, and if you attempt to you wind up with race conditions.  
Nothing in their interface can prevent this misuse, and unfortunately 
most programmers don't understand this danger.  So events are 
dangerous in comparison to condition variables.  Further, there are 
very few designs in which an event is the optimal synchronization 
object to use, and even in those rare cases a condition variable can 
easily be used instead, while the converse isn't true.

That said, those rare cases where an event is optimal are valid...

> than more specific synchronisation techniques. Would you agree?
> And, more for Bill: is there anything preventing an
> event synchronisation subsystem being added to the library?

No, event's can be trivially added to Boost.Threads, using the native 
event objects in Win32 and condition variables in POSIX.  However, 
because of the reasons I gave above, this is not an extension that 
I'll be making before submission.  If people find a legitimate need 
for them later we can discuss the pros and cons more carefully and 
add events if we then deem them to be needed.

Bill Kempf