The mutex is used to keep the "state" of what you are waiting to happen from being changed in the window of time between your test of it and your thread actually being placed on the wait queue of the condition primitive. Otherwise you could get: Thread A tests the "state" and gets a false result. Thread B changes the "state" to true and then wakes all the waiters on the queue. Thread A is added to the wait queue. Which would potentially leave Thread A waiting forever for something that has already occurred. On 8 Sep 2005, at 12:06 PM, Johan 't Hart wrote:
Thanks for your replies.
To me boost::condition looks very much like the old event variable, except that it works together with a mutex. I am now wondering why you should lock a mutex before waiting for the condition? Is there a rationale about this?
Johan
----- Original Message ----- From: "Oleg Smolsky" <oleg.smolsky@pacific-simulators.co.nz> Newsgroups: gmane.comp.lib.boost.user Sent: Thursday, September 08, 2005 6:24 AM Subject: Re: Thread events
Hello Johan,
Johan 't Hart wrote on 8/09/2005 at 11:04 a.m.:
I'm quite a newbie in the boost world, and I read the documentation about boost::threads. Inthere I read that events were not included in boost because they were too error-phrone. That made me wonder how to do multithreading without events, for example, how do you make a thread wait for something to do without events?
Well, I don't know if it's at all possible to implement a powerful (or flexible) multi-threaded project using just conditions.
I use sockets and select() instead of WaitForMultipleObjects(). IE select() is the cross-platform multiplexor that you can use for threading and network events. That's the "worker thread" side.
As for the "user code" side, you can implement a simple event that can be signalled and waited upon using conditions. IE it's common to have event.Wait() and event.Signal(), which are very very basic but cross-platform: - you can use boost conditions - or you can use WaitForSingleObject() on win32 and pthread condition on unix. AFAIK that's what boost does.
Best regards, Oleg.
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