Re: Redesigned thread class

From: williamkempf_at_[hidden]
Date: 2001-08-23 20:31:27

--- In boost_at_y..., "Peter Dimov" <pdimov_at_m...> wrote:
> Another question:
> 
> How do I create a 'joinable' thread object representing the main 
thread?

You can't.  And my first thought was "trying to join the main thread 
sounds like a major design flaw".  That aside, once I thought this 
one through (which is why I didn't respond sooner) I realize that the 
real problem you have here is with thread adoption semantics.

This is a similar problem to what I encountered when trying to design 
semantics that allowed multiple "joins".  POSIX simply doesn't 
support this.  The best you can do is allow for multiple "waits".  
What's the difference?  A "join" gaurantees that the thread has fully 
completed, including the running of cleanup handlers and the 
recycling of the "thread id", while a "wait" only gaurantees that the 
thread is done running it's thread entry procedure.  I honestly don't 
think you can get POSIX (or the proposed Boost.Threads semantics) to 
allow for multiple "joins" given the need to adopt threads.  You 
simply can't tell if the thread has already been joined, nor can you 
insure that the creator won't attempt to do a "native join".

That said, supporting multiple "waits" is trivial for both POSIX and 
the proposed Boost.Threads semantics.  You simply create a monitor 
(mutex and condition variable) stored in TLS and ref-counted.  The 
TLS cleanup routine is used to notify all threads waiting on the 
monitor.

This will _usually_ be enough.  The only thread that absolutely has 
to "join" instead of "wait" is the "last" thread, in order to insure 
that all cleanup occurs before the process ends.

The bigger issue for you with the proposed Boost.Threads semantics is 
that the thread object created through the default constructor is 
valid only within that thread.  I can lift that restriction... I 
added it only to simplify the Win32 implementation so that I didn't 
have to call DuplicateHandle().  In other words it was an 
optimization.  I expected someone to ask about it, but no one did :).

Bill Kempf