<!-- page08.html,v 1.12 2002/02/25 18:13:55 jwillemsen Exp --> <HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <META NAME="GENERATOR" CONTENT="Mozilla/4.04 [en] (X11; I; Linux 2.0.32 i486) [Netscape]"> <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> <HR> <P>Finally, <A HREF="thread_pool.cpp">thread_pool.cpp</A> where we have the Thread_Pool object implementation. <P> Remember back in <A HREF="../006/page01.html">Tutorial 6</A> when I was talking about <i>THR_NEW_LWP</i>? Look closely and you'll see it here. It's bitwise OR'd with <i>THR_DETACHED</i> just to keep things interesting. <P> <HR WIDTH="100%"> <PRE> <font color=red>// page08.html,v 1.12 2002/02/25 18:13:55 jwillemsen Exp</font> <font color=blue>#include</font> "<font color=green>thread_pool.h</font>" <font color=red>/* We need this header so that we can invoke handle_input() on the objects we dequeue. */</font> <font color=blue>#include</font> "<A HREF="../../../ace/Event_Handler.h">ace/Event_Handler.h</A>" <font color=red>/* All we do here is initialize our active thread counter. */</font> <font color=#008888>Thread_Pool::Thread_Pool</font> (void) : active_threads_ (0) { } <font color=red>/* Our start() method is a thin disguise around the ACE_Task<> activate() method. By hiding activate() in this way, the users of Thread_Pool don't have to worry about the thread configuration flags. */</font> int <font color=#008888>Thread_Pool::start</font> (int pool_size) { return this->activate (THR_NEW_LWP|THR_DETACHED, pool_size); } <font color=red>/* Closing the thread pool can be a tricky exercise. I've decided to take an easy approach and simply enqueue a secret message for each thread we have active. */</font> int <font color=#008888>Thread_Pool::stop</font> (void) { <font color=red>/* Find out how many threads are currently active */</font> int counter = active_threads_.value (); <font color=red>/* For each one of the active threads, enqueue a "<font color=green>null</font>" event handler. Below, we'll teach our svc() method that "<font color=green>null</font>" means "<font color=green>shutdown</font>". */</font> while (counter--) this->enqueue (0); <font color=red>/* As each svc() method exits, it will decrement the active thread counter. We just wait here for it to reach zero. Since we don't know how long it will take, we sleep for a quarter of a second between tries. */</font> while (active_threads_.value ()) <font color=#008888>ACE_OS::sleep</font> (ACE_Time_Value (0, 250000)); return(0); } <font color=red>/* When an object wants to do work in the pool, it should call the enqueue() method. We introduce the ACE_Message_Block here but, unfortunately, we seriously misuse it. */</font> int <font color=#008888>Thread_Pool::enqueue</font> (ACE_Event_Handler *handler) { <font color=red>/* An ACE_Message_Block is a chunk of data. You put them into an ACE_Message_Queue. ACE_Task<> has an ACE_Message_Queue built in. In fact, the parameter to ACE_Task<> is passed directly to ACE_Message_Queue. If you look back at our header file you'll see that we used ACE_MT_SYNCH as the parameter indicating that we want MultiThread Synch safety. This allows us to safely put ACE_Message_Block objects into the message queue in one thread and take them out in another. */</font> <font color=red>/* An ACE_Message_Block wants to have char* data. We don't have that. We could cast our ACE_Event_Handler* directly to a char* but I wanted to be more explicit. Since casting pointers around is a dangerous thing, I've gone out of my way here to be very clear about what we're doing. First: Cast the handler pointer to a void pointer. You can't do any useful work on a void pointer, so this is a clear message that we're making the pointer unusable. Next: Cast the void pointer to a char pointer that the ACE_Message_Block will accept. */</font> void *v_data = (void *) handler; char *c_data = (char *) v_data; ACE_Message_Block *mb; <font color=red>/* Construct a new ACE_Message_Block. For efficiency, you might want to preallocate a stack of these and reuse them. For simplicity, I'll just create what I need as I need it. */</font> ACE_NEW_RETURN (mb, ACE_Message_Block (c_data), -1); <font color=red>/* Our putq() method is a wrapper around one of the enqueue methods of the ACE_Message_Queue that we own. Like all good methods, it returns -1 if it fails for some reason. */</font> if (this->putq (mb) == -1) { <font color=red>/* Another trait of the ACE_Message_Block objects is that they are reference counted. Since they're designed to be passed around between various objects in several threads we can't just delete them whenever we feel like it. The release() method is similar to the destroy() method we've used elsewhere. It watches the reference count and will delete the object when possible. */</font> mb->release (); return -1; } return 0; } <font color=red>/* The "<font color=green>guard</font>" concept is very powerful and used throughout multi-threaded applications. A guard normally does some operation on an object at construction and the "<font color=green>opposite</font>" operation at destruction. For instance, when you guard a mutex (lock) object, the guard will acquire the lock on construction and release it on destruction. In this way, your method can simply let the guard go out of scope and know that the lock is released. Guards aren't only useful for locks however. In this application I've created two guard objects for quite a different purpose. */</font> <font color=red>/* The Counter_Guard is constructed with a reference to the thread pool's active thread counter. The guard increments the counter when it is created and decrements it at destruction. By creating one of these in svc(), I know that the counter will be decremented no matter how or where svc() returns. */</font> class Counter_Guard { public: Counter_Guard (<font color=#008888>Thread_Pool::counter_t</font> &counter) : counter_ (counter) { ++counter_; } ~Counter_Guard (void) { --counter_; } protected: <font color=#008888>Thread_Pool::counter_t</font> &counter_; }; <font color=red>/* My Message_Block_Guard is also a little non-traditional. It doesn't do anything in the constructor but it's destructor ensures that the message block's release() method is called. This is a cheap way to prevent a memory leak if I need an additional exit point in svc(). */</font> class Message_Block_Guard { public: Message_Block_Guard (ACE_Message_Block *&mb) : mb_ (mb) { } ~Message_Block_Guard (void) { mb_->release (); } protected: ACE_Message_Block *&mb_; }; <font color=red>/* Now we come to the svc() method. As I said, this is being executed in each thread of the Thread_Pool. Here, we pull messages off of our built-in ACE_Message_Queue and cause them to do work. */</font> int <font color=#008888>Thread_Pool::svc</font> (void) { <font color=red>/* The getq() method takes a reference to a pointer. So... we need a pointer to give it a reference to. */</font> ACE_Message_Block *mb; <font color=red>/* Create the guard for our active thread counter object. No matter where we choose to return() from svc(), we now know that the counter will be decremented. */</font> Counter_Guard counter_guard (active_threads_); <font color=red>/* Get messages from the queue until we have a failure. There's no real good reason for failure so if it happens, we leave immediately. */</font> while (this->getq (mb) != -1) { <font color=red>/* A successful getq() will cause "<font color=green>mb</font>" to point to a valid refernce-counted ACE_Message_Block. We use our guard object here so that we're sure to call the release() method of that message block and reduce it's reference count. Once the count reaches zero, it will be deleted. */</font> Message_Block_Guard message_block_guard (mb); <font color=red>/* As noted before, the ACE_Message_Block stores it's data as a char*. We pull that out here and later turn it into an ACE_Event_Handler* */</font> char *c_data = mb->base (); <font color=red>/* We've chosen to use a "<font color=green>null</font>" value as an indication to leave. If the data we got from the queue is not null then we have some work to do. */</font> if (c_data) { <font color=red>/* Once again, we go to great lengths to emphasize the fact that we're casting pointers around in rather impolite ways. We could have cast the char* directly to an ACE_Event_Handler* but then folks might think that's an OK thing to do. (Note: The correct way to use an ACE_Message_Block is to write data into it. What I should have done was create a message block big enough to hold an event handler pointer and then written the pointer value into the block. When we got here, I would have to read that data back into a pointer. While politically correct, it is also a lot of work. If you're careful you can get away with casting pointers around.) */</font> void *v_data = (void *) c_data; ACE_Event_Handler *handler = (ACE_Event_Handler *) v_data; <font color=red>/* Now that we finally have an event handler pointer, invoke it's handle_input() method. Since we don't know it's handle, we just give it a default. That's OK because we know that we're not using the handle in the method anyway. */</font> if (handler->handle_input (ACE_INVALID_HANDLE) == -1) { <font color=red>/* Tell the handler that it's time to go home. The "<font color=green>normal</font>" method for shutting down a handler whose handler failed is to invoke handle_close(). This will take care of cleaning it up for us. Notice how we use the handler's get_handle() method to populate it's "<font color=green>handle</font>" parameter. Convenient isn't it? */</font> handler->handle_close (handler->get_handle (), 0); <font color=red>/* Also notice that we don't exit the svc() method here! The first time I did this, I was exiting. After a few clients disconnect you have an empty thread pool. Hard to do any more work after that... */</font> } } else <font color=red>/* If we get here, we were given a message block with "<font color=green>null</font>" data. That is our signal to leave, so we return(0) to leave gracefully. */</font> return 0; <font color=red>// Ok, shutdown request</font> <font color=red>// message_block_guard goes out of scope here and releases the</font> <font color=red>// message_block instance.</font> } return 0; } </PRE> <P><HR WIDTH="100%"> <CENTER>[<A HREF="../online-tutorials.html">Tutorial Index</A>] [<A HREF="page09.html">Continue This Tutorial</A>]</CENTER>