Re: [boost] [threads] two threads gain lock on the same mutex??

Subject: Re: [boost] [threads] two threads gain lock on the same mutex??
From: Ryan Gallagher (ryan.gallagher_at_[hidden])
Date: 2010-01-02 17:00:28

David M. Cotter <me <at> davecotter.com> writes:

> 
> i thought this was impossible.  but i've caught it in the 
> debugger *right now*!
> 
> on thread 28, i've got this code:
> 
> boost::mutex::scoped_lock	lock(i_mutex);
[snip]
> PC -->		if (!i_semID->timed_wait(lock, xt)) {

The timed_wait will not hold the lock for the duration of the wait.  
It takes the lock so it can release it for the duration of the wait 
and then re-acquire the lock upon the end of the wait before 
returning.  See how condition variables work as you're essentially
waiting on the condition of the time duration (xt):

http://tinyurl.com/ycn7rnl

HTH,

-Ryan