[Boost-users] Condition Variable and optimization

Subject: [Boost-users] Condition Variable and optimization
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2011-12-15 13:29:10

The example in the documentation for Condition Variables illustrates the implication that 
these are Mesa-style (non-blocking signals) with this example:

boost::condition_variable  cond;
boost::mutex  mut;
bool  data_ready;

void  process_data();

void  wait_for_data_to_process()
{
     boost::unique_lock<boost::mutex>  lock(mut);
     while(!data_ready)
     {
         cond.wait(lock);
     }
     process_data();
}

This needs to declare data_ready as mutable, so the compiler doesn't reduce the while loop 
to a no-op and assume the value doesn't change, or just hold it in a register so it 
doesn't see the change made by another thread.

More generally, how do you write it in this case?

     while ( !collection.empty() )  cond.wait(lock);

where the condition is a function of an existing class?  The compiler might inline that 
function and give the same issue.

I think a form of wait that takes a predicate ought to be introduced, that prevents this 
particular issue and ensures correct use.

—John