[Boost-users] thread: condition::notify() does not require a lock - why?

From: Pete Chapman (p.chapman_at_[hidden])
Date: 2005-10-25 03:46:45

condition::wait requires that a lock is acquired prior to calling it,
but condition::notify does not.
The Java threading API (which in my limited experience seems similar to
boost::thread) requires that a lock is acquired prior to calling
Object.notify().

Is this potentially a problem? For example:

scoped_lock myLock(myMutex);
while (!some_variable)
{
   // Another thread now sets some_variable = true, and
   // calls myCondition.notify()
   // then...

   // will this wait indefinitely?
   myCondition.wait(myLock);
}

Clearly if the other thread in this example were forced to obtain a lock
on myMutex before calling myCondition.notify(), there would be no
danger. In practice this is what I do anyway, but the boost:thread API
doesn't force me to. Why not? Is there a potential problem if I don't?

Many thanks...