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<CENTER><B><FONT SIZE=+2>ACE Tutorial 017</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Using the ACE_Barrier synch object</FONT></B></CENTER>
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The Barrier implementation is quite simple. The threads() mutator
took a couple of tries to get right. In particular, be sure you know
when to apply the _wait paramter and when not to! In fact, the
requirement that only the "owning" thread can change the thread count
is rather limiting. A more appropriate solution would allow any
thread to safely change the count but that would require more complex
locking that is just a bit more than what I wanted to present here.
<HR>
<PRE>
<font color=red>// page04.html,v 1.10 2000/03/19 20:09:32 jcej Exp</font>
<font color=blue>#include</font> "<font color=green>Barrier_i.h</font>"
<font color=red>/* Initialize the threads_ count to zero and the barrier_ pointer to a
safe value. At the same time, we remember the thread that created
us so that we can allow it to change the thread count.
*/</font>
<font color=#008888>Barrier::Barrier</font>(void)
: threads_(0)
,barrier_(0)
,new_barrier_(0)
{
owner_ = <font color=#008888>ACE_OS::thr_self</font>();
}
<font color=red>/* Ensure that barrier_ get's deleted so that we don't have a memory leak.
*/</font>
<font color=#008888>Barrier::~Barrier</font>(void)
{
delete barrier_;
}
void <font color=#008888>Barrier::owner</font>( ACE_thread_t _owner )
{
owner_ = _owner;
}
<font color=red>// Report on the number of threads.</font>
u_int <font color=#008888>Barrier::threads</font>(void)
{
return threads_.value();
}
<font color=red>/* Allow the owning thread to (re)set the number of threads.
make_barrier() is called because it will wait() if we were already
configured. Typical usage would be for the worker threads to
wait() while the primary (eg -- owner) thread adjusts the thread
count.
For instance:
In the worker threads:
if( myBarrier.threads() != current_thread_count )
myBarrier.wait();
In the primary thread:
if( myBarrier.threads() != current_thread_count )
myBarrier.threads( current_thread_count, 1 );
*/</font>
int <font color=#008888>Barrier::threads</font>( u_int _threads, int _wait )
{
if( ! <font color=#008888>ACE_OS::thr_equal</font>(ACE_OS::thr_self(), owner_) )
{
return -1;
}
threads_ = _threads;
return make_barrier(_wait);
}
<font color=red>/* Wait for all threads to synch if the thread count is valid. Note
that barrier_ will be 0 if the threads() mutator has not been
invoked.
*/</font>
int <font color=#008888>Barrier::wait</font>(void)
{
if( ! barrier_ )
{
return -1;
}
<font color=red>// If the threads() mutator has been used, new_barrier_ will</font>
<font color=red>// point to a new ACE_Barrier instance. We'll use a</font>
<font color=red>// traditional double-check here to move that new object into</font>
<font color=red>// place and cleanup the old one.</font>
if( new_barrier_ )
{
<font color=red>// mutex so that only one thread can do this part.</font>
ACE_Guard<ACE_Mutex> mutex(barrier_mutex_);
<font color=red>// We only want the first thread to plug in the new barrier...</font>
if( new_barrier_ )
{
<font color=red>// out with the old and in with the new.</font>
delete barrier_;
barrier_ = new_barrier_;
new_barrier_ = 0;
}
}
return barrier_->wait();
}
<font color=red>/* Wait for all threads to synch. As each thread passes wait(), it
will decrement our thread counter. (That is why we had to make
threads_ an atomic op.) When the last thread decrements the
counter it will also delete the ACE_Barrier & free up a little
memory.
*/</font>
int <font color=#008888>Barrier::done</font>(void)
{
if( this->wait() == -1 )
{
return -1;
}
--threads_;
if( ! threads_.value() )
{
delete barrier_;
barrier_ = 0;
}
return 0;
}
<font color=red>/* This will build the actual barrier. I broke this code out of the
threads() mutator in case it might be useful elsewhere.
If a barrier already exists, we will wait for all threads before
creating a new one. This trait is what allows the threads mutator
to be used as shown above.
*/</font>
int <font color=#008888>Barrier::make_barrier</font>( int _wait )
{
<font color=red>// Ensure we have a valid thread count.</font>
if( ! threads_.value() )
{
return -1;
}
<font color=red>// If a barrier already exists, we'll arrange for it to be</font>
<font color=red>// replaced through the wait() method above.</font>
if( barrier_ )
{
<font color=red>// Create the new barrier that wait() will install for us.</font>
ACE_NEW_RETURN(new_barrier_,ACE_Barrier(threads_.value()),-1);
<font color=red>// Wait for our siblings to synch before continuing</font>
if( _wait )
{
barrier_->wait();
}
}
else
{
<font color=red>// Create the initial barrier.</font>
ACE_NEW_RETURN(barrier_,ACE_Barrier(threads_.value()),-1);
}
return 0;
}
<font color=blue>#if defined</font> (<font color=purple>ACE_HAS_EXPLICIT_TEMPLATE_INSTANTIATION</font>)
template class ACE_Atomic_Op <ACE_Mutex, u_int>;
<font color=blue>#elif defined</font> (<font color=purple>ACE_HAS_TEMPLATE_INSTANTIATION_PRAGMA</font>)
<font color=blue>#pragma</font> <font color=purple>instantiate</font> ACE_Atomic_Op <ACE_Mutex, u_int>
<font color=blue>#endif</font> <font color=red>/* ACE_HAS_EXPLICIT_TEMPLATE_INSTANTIATION */</font>
</PRE>
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