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<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<META NAME="Author" CONTENT="James CE Johnson">
<TITLE>ACE Tutorial 012</TITLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 012</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Passing classes through ACE_Message_Queue</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
<P>
The Task is the only object we've not been through yet. I'll go ahead
and show both the header and cpp on this one page since the header
isn't very large.
<P>
<HR WIDTH="100%">
<PRE>
/*
This is our basic thread-pool Task. We have a choice of pool size
on the open() and the usual svc() and close() methods.
A new addition is the ACE_Barrier object. This will allow the
synchronization of our svc() methods so that they all start at the
"same" time. The normal case may allow one thread to start working
earlier than others. There's no real harm in it but you can get
better "work by thread" statistics if they start out together.
*/
class Task : public ACE_Task < ACE_MT_SYNCH >
{
public:
typedef ACE_Task < ACE_MT_SYNCH > inherited;
Task (void);
~Task (void);
int open (int threads = 1);
int svc (void);
int close (u_long flags = 0);
protected:
ACE_Barrier * barrier_;
};
</PRE>
<HR WIDTH="50%">
<PRE>
/*
Boring default constructor. Be sure our barrier_ is initialized in
case we get destructed before opened.
*/
Task::Task (void)
: barrier_ (0)
{
ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task ctor 0x%x\n", (void *) this));
}
/*
You'll see in the svc() method that when we get a shutdown request,
we always putq() it back into our message queue. The last thread in
the pool will do this also and result in there always being one
shutdown request left in the queue when we get here. Just to be
polite, we'll go ahead and get that message and release it.
We also delete the barrier_ object we used to synch the svc()
methods.
*/
Task::~Task (void)
{
ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task dtor 0x%x\n", (void *) this));
ACE_Message_Block *message;
this->getq (message);
message->release ();
delete barrier_;
}
/*
The ACE_Barrier needs to know how many threads it will be working
for. For that reason, we have to put off it's construction until we
get here. We then pass the thread count through to our base class'
activate().
*/
int Task::open (int threads)
{
barrier_ = new ACE_Barrier (threads);
return this->activate (THR_NEW_LWP, threads);
}
/*
We don't really do anything here but I wanted to provide a message
in the output.
*/
int Task::close (u_long flags)
{
ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task close 0x%x\n", (void *) this));
return inherited::close (flags);
}
/*
Now the svc() method where everything interesting happens.
*/
int Task::svc (void)
{
/*
All of the threads will block here until the last thread
arrives. They will all then be free to begin doing work.
*/
this->barrier_->wait ();
ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task 0x%x starts in thread %u\n", (void *) this, ACE_Thread::self ()));
// Where we getq() the message
ACE_Message_Block *message;
// What we really put into the queue is a Message_Block, so we'll
// cast the 'message' to 'message_block' after getting it. I'm
// going through some extra steps here just to be explicit
Message_Block * message_block;
// The baseclass of the work object we put into the queue. Notice
// that we can use this and not bother with the Work object at all.
Unit_Of_Work * unit_of_work;
while (1)
{
// Get the message...
if (this->getq (message) == -1)
{
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "getq"), -1);
}
// Is it a shutdown request?
if (message->msg_type () == ACE_Message_Block::MB_HANGUP)
{
// Send the shutdown to all of our pool peers
this->putq (message);
break;
}
// Cast the pointer to our specialized Message_Block. We could
// have done this at the getq() call but I wanted to be explicit
// about what we're doing here
message_block = (Message_Block*)message;
/*
Since we left alone the ACE_Data_Block used by the
Message_Block we have chosen to use it to send arbitrary data
as well.
*/
const char *cp = message_block->rd_ptr ();
// Don't forget to skip the NULL we inserted
message_block->rd_ptr (strlen (cp) + 1);
/*
Get the Unit_Of_Work pointer out of our specialized
Message_Block. Since the methods of interest are virtual, we
don't have to know what kind of work we're to do.
*/
unit_of_work = message_block->data();
/*
Invoke a couple of method calls on the object we constructed.
*/
unit_of_work->who_am_i ();
unit_of_work->what_am_i ();
ACE_DEBUG ((LM_DEBUG, "(%P|%t) Block 0x%x contains (%s)\n", (void *) message, cp));
/*
Pretend that the work takes a little time to process. This
prevents one thread from getting all of the action. In a real
system you wouldn't need to do this since the work really
would take time to complete.
*/
ACE_OS::sleep (ACE_Time_Value (0, 5000));
/*
Release the message block and allow the unit of work to also go
away.
*/
message->release ();
}
return (0);
}
</PRE>
<HR WIDTH="100%">
<P>
Like main() this is actually simpler than the previous tutorial. It's
much cleaner to carry around a pointer to the object we're working
with than to try copying data.
<P>
The only complication is the new ACE_Barrier. It's a pretty simple
object that makes it easy for you to synch threads in this way. You
could do some fancy tricks with mutexes, counters & semaphores but why
bother when the Barrier already exists.
<P>
<HR WIDTH="100%">
<P>
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