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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 010</TITLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 010</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Passing chunks of data through an ACE_Message_Queue</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
Our <A HREF="task.cpp">Task</A> object definition:
<P>
<HR WIDTH="100%">
<PRE>
<font color=red>// $Id$</font>
<font color=blue>#include</font> "<font color=green>task.h</font>"
<font color=blue>#include</font> "<font color=green>block.h</font>"
<font color=red>/*
Set our housekeeping pointer to NULL and tell the user we exist.
*/</font>
<font color=#008888>Task::Task</font> (void)
: barrier_ (0)
{
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Task ctor 0x%x\n</font>", (void *) this));
}
<font color=red>/*
Take care of cleanup & tell the user we're going away.
*/</font>
<font color=#008888>Task::~Task</font> (void)
{
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Task dtor 0x%x\n</font>", (void *) this));
<font color=red>/*
Get our shutdown notification out of the queue and release it.
*/</font>
ACE_Message_Block * message;
this->getq(message);
message->release();
delete barrier_;
}
<font color=red>/*
Open the object to do work. We create the Barrier object and tell
it how many threads we'll be using. Next, we activate the Task
into the number of requested threads.
*/</font>
int <font color=#008888>Task::open</font> (int threads)
{
barrier_ = new ACE_Barrier (threads);
return this->activate (THR_NEW_LWP, threads);
}
<font color=red>/*
Tell the user we're closing and invoke the baseclass' close() to
take care of things.
*/</font>
int <font color=#008888>Task::close</font> (u_long flags)
{
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Task close 0x%x\n</font>", (void *) this));
return <font color=#008888>inherited::close</font> (flags);
}
<font color=red>/*
Our svc() method waits for work on the queue and then processes that work.
*/</font>
int <font color=#008888>Task::svc</font> (void)
{
<font color=red>/*
This will cause all of the threads to wait on this line until all
have invoked this method. The net result is that no thread in the
Task will get a shot at the queue until all of the threads are active.
There's no real need to do this but it's an easy intro into the use
of ACE_Barrier.
*/</font>
this->barrier_->wait ();
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Task 0x%x starts in thread %d\n</font>", (void *) this, <font color=#008888>ACE_Thread::self</font> ()));
<font color=red>/*
Remember that get() needs a reference to a pointer. To save stack
thrashing we'll go ahead and create a pointer outside of the almost-
infinite loop.
*/</font>
ACE_Message_Block *message;
while (1)
{
<font color=red>/*
Get a message from the queue.
*/</font>
if (this->getq (message) == -1)
{
ACE_ERROR_RETURN ((LM_ERROR, "<font color=green>%p\n</font>", "<font color=green>getq</font>"), -1);
}
<font color=red>/*
If we got the shutdown request, we need to go away.
*/</font>
if (message->msg_type () == <font color=#008888>ACE_Message_Block::MB_HANGUP</font>)
{
<font color=red>/*
Forward the request to any peer threads.
*/</font>
this->putq (message);
<font color=red>/*
Leave the infinite loop so that the thread exits.
*/</font>
break;
}
<font color=red>/*
The message queue stores char* data. We use rd_ptr() to get to
the beginning of the data.
*/</font>
const char *cp = message->rd_ptr ();
<font color=red>/*
Move the rd_ptr() past the data we read. This isn't real useful
here since we won't be reading any more from the block but it's
a good habit to get into.
*/</font>
message->rd_ptr( strlen(cp) );
<font color=red>/*
Display the block's address and data to the user.
*/</font>
ACE_DEBUG ((LM_DEBUG, "<font color=green>(%P|%t) Block 0x%x contains (%s)\n</font>", (void *) message, cp));
<font color=red>/*
Pretend that it takes a while to process the data.
*/</font>
<font color=#008888>ACE_OS::sleep</font> (ACE_Time_Value (0, 5000));
<font color=red>/*
Release the message block. Notice that we never delete a message block.
Blocks are reference counted & the release() method will take care of
the delete when there are no more references to the data.
*/</font>
message->release ();
}
return (0);
}
</PRE>
<HR WIDTH="100%">
<P>
This is all pretty straight-forward too. One gottcha we avoided was a memory leak
due to our shutdown message. Notice that svc() enqueues that block without bothering
to see if there are any more threads to dequeue it. Thats why our dtor can call getq()
without worrying about blocking infinitely: it knows the message block will be there.
<P><HR WIDTH="100%">
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