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<META NAME="Author" CONTENT="James CE Johnson">
<META NAME="Description" CONTENT="A first step towards using ACE productively">
<TITLE>ACE Tutorial 007</TITLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 007</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Creating a thread-pool server</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
<P>Finally, <A HREF="thread_pool.cpp">thread_pool.cpp</A>
where we have the Thread_Pool object implementation.
<P>
<HR WIDTH="100%">
<P><FONT FACE="Arial,Helvetica">// $Id: thread_pool.cpp,v 1.1 1998/08/30
23:47:15 schmidt Exp $</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">#include "thread_pool.h"</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> We need this header so that
we can invoke handle_input() on the objects we dequeue.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">#include "ace/Event_Handler.h"</FONT>
<BR><FONT FACE="Arial,Helvetica"></FONT> <FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> All we do here is initialize
our active thread counter.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">Thread_Pool::Thread_Pool(void)</FONT>
<BR><FONT FACE="Arial,Helvetica"> : active_threads_(0)</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> Our open() method is a thin
disguise around the ACE_Task<> activate() method. By</FONT>
<BR><FONT FACE="Arial,Helvetica"> hiding activate() in this
way, the users of Thread_Pool don't have to worry about</FONT>
<BR><FONT FACE="Arial,Helvetica"> the thread configuration
flags.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">int Thread_Pool::open( int _pool_size
)</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica"> return this->activate(THR_NEW_LWP|THR_DETACHED,_pool_size);</FONT>
<BR><FONT FACE="Arial,Helvetica">}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> Closing the thread pool can
be a tricky exercise. I've decided to take an easy approach</FONT>
<BR><FONT FACE="Arial,Helvetica"> and simply enqueue a secret
message for each thread we have active.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">int Thread_Pool::close(void)</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Find out how many threads are currently active</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
int counter = active_threads_.value();</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
For each one of the active threads, enqueue a "null" event handler.
Below, we'll</FONT>
<BR><FONT FACE="Arial,Helvetica">
teach our svc() method that "null" means "shutdown".</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
while( counter-- )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
this->enqueue( 0 );</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
As each svc() method exits, it will decrement the active thread counter.
We just wait</FONT>
<BR><FONT FACE="Arial,Helvetica">
here for it to reach zero. Since we don't know how long it will take,
we sleep for</FONT>
<BR><FONT FACE="Arial,Helvetica">
a quarter-second or so between tries.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
while( active_threads_.value() )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_OS::sleep( ACE_Time_Value(0.25) );</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
return(0);</FONT>
<BR><FONT FACE="Arial,Helvetica">}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> When an object wants to do
work in the pool, it should call the enqueue() method.</FONT>
<BR><FONT FACE="Arial,Helvetica"> We introduce the ACE_Message_Block
here but, unfortunately, we seriously missuse it.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">int Thread_Pool::enqueue( ACE_Event_Handler
* _handler )</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
An ACE_Message_Block is a chunk of data. You put them into an ACE_Message_Queue.</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Task<> has an ACE_Message_Queue built in. In fact, the parameter
to ACE_Task<></FONT>
<BR><FONT FACE="Arial,Helvetica">
is passed directly to ACE_Message_Queue. If you look back at our
header file you'll</FONT>
<BR><FONT FACE="Arial,Helvetica">
see that we used ACE_MT_SYNCH as the parameter indicating that we want
MultiThread</FONT>
<BR><FONT FACE="Arial,Helvetica">
Synch safety. This allows us to safely put ACE_Message_Block objects
into the</FONT>
<BR><FONT FACE="Arial,Helvetica">
message queue in one thread and take them out in another.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
An ACE_Message_Block wants to have char* data. We don't have that.
We could</FONT>
<BR><FONT FACE="Arial,Helvetica">
cast our ACE_Event_Handler* directly to a char* but I wanted to be more
explicit.</FONT>
<BR><FONT FACE="Arial,Helvetica">
Since casting pointers around is a dangerous thing, I've gone out of my
way here</FONT>
<BR><FONT FACE="Arial,Helvetica">
to be very clear about what we're doing.</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
First: Cast the handler pointer to a void pointer. You can't
do any useful work</FONT>
<BR><FONT FACE="Arial,Helvetica">
on a void pointer, so this is a clear message that we're making the</FONT>
<BR><FONT FACE="Arial,Helvetica">
pointer unusable.</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
Next: Cast the void pointer to a char pointer that the ACE_Message_Block
will accept.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
void * v_data = (void*)_handler;</FONT>
<BR><FONT FACE="Arial,Helvetica">
char * c_data = (char*)v_data;</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Construct a new ACE_Message_Block. For efficiency, you might want
to preallocate a</FONT>
<BR><FONT FACE="Arial,Helvetica">
stack of these and reuse them. For simplicity, I'll just create what
I need as I need it.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Message_Block * mb = new ACE_Message_Block( c_data );</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Our putq() method is a wrapper around one of the enqueue methods of the
ACE_Message_Queue</FONT>
<BR><FONT FACE="Arial,Helvetica">
that we own. Like all good methods, it returns -1 if it fails for
some reason.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
if( this->putq(mb) == -1 )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Another trait of the ACE_Message_Block objects is that they are reference
counted.</FONT>
<BR><FONT FACE="Arial,Helvetica">
Since they're designed to be passed around between various objects in several
threads</FONT>
<BR><FONT FACE="Arial,Helvetica">
we can't just delete them whenever we feel like it. The release()
method is similar</FONT>
<BR><FONT FACE="Arial,Helvetica">
to the destroy() method we've used elsewhere. It watches the reference
count and will</FONT>
<BR><FONT FACE="Arial,Helvetica">
delete the object when possible.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
mb->release();</FONT>
<BR><FONT FACE="Arial,Helvetica">
return(-1);</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
return(0);</FONT>
<BR><FONT FACE="Arial,Helvetica">}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> The "guard" concept is very
powerful and used throughout multi-threaded applications.</FONT>
<BR><FONT FACE="Arial,Helvetica"> A guard normally does some
operation on an object at construction and the "opposite"</FONT>
<BR><FONT FACE="Arial,Helvetica"> operation at destruction.
For instance, when you guard a mutex (lock) object, the guard</FONT>
<BR><FONT FACE="Arial,Helvetica"> will acquire the lock on
construction and release it on destruction. In this way, your</FONT>
<BR><FONT FACE="Arial,Helvetica"> method can simply let the
guard go out of scope and know that the lock is released.</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica"> Guards aren't only useful
for locks however. In this application I've created two guard</FONT>
<BR><FONT FACE="Arial,Helvetica"> objects for quite a different
purpose.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> The Counter_Guard is constructed
with a reference to the thread pool's active thread</FONT>
<BR><FONT FACE="Arial,Helvetica"> counter. The guard
increments the counter when it is created and decrements it at</FONT>
<BR><FONT FACE="Arial,Helvetica"> destruction. By creating
one of these in svc(), I know that the counter will be decremented</FONT>
<BR><FONT FACE="Arial,Helvetica"> no matter how or where svc()
returns.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">class Counter_Guard</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">public:</FONT>
<BR><FONT FACE="Arial,Helvetica">
Counter_Guard( Thread_Pool::counter_t & _counter )</FONT>
<BR><FONT FACE="Arial,Helvetica">
: counter_(_counter)</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
++counter_;</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
~Counter_Guard(void)</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
--counter_;</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">protected:</FONT>
<BR><FONT FACE="Arial,Helvetica">
Thread_Pool::counter_t & counter_;</FONT>
<BR><FONT FACE="Arial,Helvetica">};</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> My Message_Block_Guard is
also a little non-traditional. It doesn't do anything in the</FONT>
<BR><FONT FACE="Arial,Helvetica"> constructor but it's destructor
ensures that the message block's release() method is called.</FONT>
<BR><FONT FACE="Arial,Helvetica"> This is a cheap way to prevent
a memory leak if I need an additional exit point in svc().</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">class Message_Block_Guard</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">public:</FONT>
<BR><FONT FACE="Arial,Helvetica">
Message_Block_Guard( ACE_Message_Block * & _mb )</FONT>
<BR><FONT FACE="Arial,Helvetica">
: mb_(_mb)</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
~Message_Block_Guard( void )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
mb_->release();</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">protected:</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Message_Block * & mb_;</FONT>
<BR><FONT FACE="Arial,Helvetica">};</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> Now we come to the svc()
method. As I said, this is being executed in each thread of the</FONT>
<BR><FONT FACE="Arial,Helvetica"> Thread_Pool. Here,
we pull messages off of our built-in ACE_Message_Queue and cause them</FONT>
<BR><FONT FACE="Arial,Helvetica"> to do work.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">int Thread_Pool::svc(void)</FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
The getq() method takes a reference to a pointer. So... we need a
pointer to give it</FONT>
<BR><FONT FACE="Arial,Helvetica">
a reference to.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Message_Block * mb;</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Create the guard for our active thread counter object. No matter
where we choose to</FONT>
<BR><FONT FACE="Arial,Helvetica">
return() from svc(), we no know that the counter will be decremented.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
Counter_Guard counter_guard(active_threads_);</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Get messages from the queue until we have a failure. There's no real
good reason</FONT>
<BR><FONT FACE="Arial,Helvetica">
for failure so if it happens, we leave immediately.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
while( this->getq(mb) != -1 )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
A successful getq() will cause "mb" to point to a valid refernce-counted</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Message_Block. We use our guard object here so that we're sure
to call</FONT>
<BR><FONT FACE="Arial,Helvetica">
the release() method of that message block and reduce it's reference count.</FONT>
<BR><FONT FACE="Arial,Helvetica">
Once the count reaches zero, it will be deleted.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
Message_Block_Guard message_block_guard(mb);</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
As noted before, the ACE_Message_Block stores it's data as a char*.
We pull that</FONT>
<BR><FONT FACE="Arial,Helvetica">
out here and later turn it into an ACE_Event_Handler*</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
char * c_data = mb->base();</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
We've chosen to use a "null" value as an indication to leave. If
the data we got</FONT>
<BR><FONT FACE="Arial,Helvetica">
from the queue is not null then we have some work to do.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
if( c_data )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Once again, we go to great lengths to emphasize the fact that we're casting
pointers</FONT>
<BR><FONT FACE="Arial,Helvetica">
around in rather impolite ways. We could have cast the char* directly
to an</FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Event_Handler* but then folks might think that's an OK thing to do.</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
(Note: The correct way to use an ACE_Message_Block is to write data
into it.</FONT>
<BR><FONT FACE="Arial,Helvetica">
What I should have done was create a message block big enough to hold an</FONT>
<BR><FONT FACE="Arial,Helvetica">
event handler pointer and then written the pointer value into the block.
When</FONT>
<BR><FONT FACE="Arial,Helvetica">
we got here, I would have to read that data back into a pointer.
While politically</FONT>
<BR><FONT FACE="Arial,Helvetica">
correct, it is also a lot of work. If you're careful you can get
away with casting</FONT>
<BR><FONT FACE="Arial,Helvetica">
pointers around.)</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
void * v_data = (void*)c_data;</FONT>
<BR><FONT FACE="Arial,Helvetica"> </FONT>
<BR><FONT FACE="Arial,Helvetica">
ACE_Event_Handler * handler = (ACE_Event_Handler*)v_data;</FONT>
<BR><FONT FACE="Arial,Helvetica"> </FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Now that we finally have an event handler pointer, invoke it's handle_input()
method.</FONT>
<BR><FONT FACE="Arial,Helvetica">
Since we don't know it's handle, we just give it a default. That's
OK because we</FONT>
<BR><FONT FACE="Arial,Helvetica">
know that we're not using the handle in the method anyway.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
if( handler->handle_input(ACE_INVALID_HANDLE) == -1 )</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Tell the handler that it's time to go home. The "normal" method for
shutting</FONT>
<BR><FONT FACE="Arial,Helvetica">
down a handler whose handler failed is to invoke handle_close().
This will</FONT>
<BR><FONT FACE="Arial,Helvetica">
take care of cleaning it up for us.</FONT>
<BR><FONT FACE="Arial,Helvetica">
Notice how we use the handler's get_handle() method to populate it's "handle"</FONT>
<BR><FONT FACE="Arial,Helvetica">
parameter. Convenient isn't it?</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
handler->handle_close(handler->get_handle(),0);</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Also notice that we don't exit the svc() method here! The first time
I did</FONT>
<BR><FONT FACE="Arial,Helvetica">
this, I was exiting. After a few clients disconnect you have an empty</FONT>
<BR><FONT FACE="Arial,Helvetica">
thread pool. Hard to do any more work after that...</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT>
<BR><FONT FACE="Arial,Helvetica">
else</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
If we get here, we were given a message block with "null" data. That
is our</FONT>
<BR><FONT FACE="Arial,Helvetica">
signal to leave, so we return(0) to leave gracefully.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
return(0);
// Ok, shutdown request</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
// message_block_guard goes out of scope here</FONT>
<BR><FONT FACE="Arial,Helvetica">
// and releases the message_block instance.</FONT>
<BR><FONT FACE="Arial,Helvetica">
}</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
return(0);</FONT>
<BR><FONT FACE="Arial,Helvetica">}</FONT>
<P>
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