Provided by: libpoe-perl_1.3670-1_all bug

NAME

       POE::Kernel - an event-based application kernel in Perl

SYNOPSIS

         use POE; # auto-includes POE::Kernel and POE::Session

         POE::Session->create(
           inline_states => {
             _start => sub { $_[KERNEL]->yield("next") },
             next   => sub {
               print "tick...\n";
               $_[KERNEL]->delay(next => 1);
             },
           },
         );

         POE::Kernel->run();
         exit;

       In the spirit of Perl, there are a lot of other ways to use POE.

DESCRIPTION

       POE::Kernel is the heart of POE.  It provides the lowest-level features: non-blocking
       multiplexed I/O, timers, and signal watchers are the most significant.  Everything else is
       built upon this foundation.

       POE::Kernel is not an event loop in itself.  For that it uses one of several available
       POE::Loop interface modules.  See CPAN for modules in the POE::Loop namespace.

       POE's documentation assumes the reader understands the @_ offset constants (KERNEL, HEAP,
       ARG0, etc.).  The curious or confused reader will find more detailed explanation in
       POE::Session.

USING POE

   Literally Using POE
       POE.pm is little more than a class loader.  It implements some magic to cut down on the
       setup work.

       Parameters to "use POE" are not treated as normal imports.  Rather, they're abbreviated
       modules to be included along with POE.

         use POE qw(Component::Client::TCP).

       As you can see, the leading "POE::" can be omitted this way.

       POE.pm also includes POE::Kernel and POE::Session by default.  These two modules are used
       by nearly all POE-based programs.  So the above example is actually the equivalent of:

         use POE;
         use POE::Kernel;
         use POE::Session;
         use POE::Component::Client::TCP;

   Using POE::Kernel
       POE::Kernel needs to know which event loop you want to use.  This is supported in three
       different ways:

       The first way is to use an event loop module before using POE::Kernel (or POE, which loads
       POE::Kernel for you):

         use Tk; # or one of several others
         use POE::Kernel.

       POE::Kernel scans the list of modules already loaded, and it loads an appropriate
       POE::Loop adapter if it finds a known event loop.

       The next way is to explicitly load the POE::Loop class you want:

         use POE qw(Loop::Gtk);

       Finally POE::Kernel's "import()" supports more programmer-friendly configuration:

         use POE::Kernel { loop => "Gtk" };
         use POE::Session;

   Anatomy of a POE-Based Application
       Programs using POE work like any other.  They load required modules, perform some setup,
       run some code, and eventually exit.  Halting Problem notwithstanding.

       A POE-based application loads some modules, sets up one or more sessions, runs the code in
       those sessions, and eventually exits.

         use POE;
         POE::Session->create( ... map events to code here ... );
         POE::Kernel->run();
         exit;

   POE::Kernel singleton
       The POE::Kernel is a singleton object; there can be only one POE::Kernel instance within a
       process.  This allows many object methods to also be package methods.

   Sessions
       POE implements isolated compartments called sessions.  Sessions play the role of tasks or
       threads within POE.  POE::Kernel acts as POE's task scheduler, doling out timeslices to
       each session by invoking callbacks within them.

       Callbacks are not preemptive.  As long as one is running, no others will be dispatched.
       This is known as cooperative multitasking.  Each session must cooperate by returning to
       the central dispatching kernel.

       Cooperative multitasking vastly simplifies data sharing, since no two pieces of code may
       alter data at once.

       A session may also take exclusive control of a program's time, if necessary, by simply not
       returning in a timely fashion.  It's even possible to write completely blocking programs
       that use POE as a state machine rather than a cooperative dispatcher.

       Every POE-based application needs at least one session.  Code cannot run within POE
       without being a part of some session.  Likewise, a threaded program always has a "thread
       zero".

       Sessions in POE::Kernel should not be confused with POE::Session even though the two are
       inextricably associated.  POE::Session adapts POE::Kernel's dispatcher to a particular
       calling convention.  Other POE::Session classes exist on the CPAN.  Some radically alter
       the way event handlers are called.  <http://search.cpan.org/search?query=poe+session>.

   Resources
       Resources are events and things which may create new events, such as timers, I/O watchers,
       and even other sessions.

       POE::Kernel tracks resources on behalf of its active sessions.  It generates events
       corresponding to these resources' activity, notifying sessions when it's time to do
       things.

       The conversation goes something like this:

         Session: Be a dear, Kernel, and let me know when someone clicks on
                  this widget.  Thanks so much!

         [TIME PASSES]  [SFX: MOUSE CLICK]

         Kernel: Right, then.  Someone's clicked on your widget.
                 Here you go.

       Furthermore, since the Kernel keeps track of everything sessions do, it knows when a
       session has run out of tasks to perform.  When this happens, the Kernel emits a "_stop"
       event at the dead session so it can clean up and shutdown.

         Kernel: Please switch off the lights and lock up; it's time to go.

       Likewise, if a session stops on its own and there still are opened resource watchers, the
       Kernel knows about them and cleans them up on the session's behalf.  POE excels at long-
       running services because it so meticulously tracks and cleans up resources.

       POE::Resources and the POE::Resource classes implement each kind of resource, which are
       summarized here and covered in greater detail later.

       Events.
         An event is a message to a sessions.  Posting an event keeps both the sender and the
         receiver alive until after the event has been dispatched.  This is only guaranteed if
         both the sender and receiver are in the same process.  Inter-Kernel message passing add-
         ons may have other guarantees.  Please see their documentation for details.

         The rationale is that the event is in play, so the receiver must remain active for it to
         be dispatched.  The sender remains alive in case the receiver would like to send back a
         response.

         Posted events cannot be preemptively canceled.  They tend to be short-lived in practice,
         so this generally isn't an issue.

       Timers.
         Timers allow an application to send a message to the future. Once set, a timer will keep
         the destination session active until it goes off and the resulting event is dispatched.

       Aliases.
         Session aliases are an application-controlled way of addressing a session.  Aliases act
         as passive event watchers.  As long as a session has an alias, some other session may
         send events to that session by that name.  Aliases keep sessions alive as long as a
         process has active sessions.

         If the only sessions remaining are being kept alive solely by their aliases, POE::Kernel
         will send them a terminal "IDLE" signal.  In most cases this will terminate the
         remaining sessions and allow the program to exit.  If the sessions remain in memory
         without waking up on the "IDLE" signal, POE::Kernel sends them a non-maskable "ZOMBIE"
         signal.  They are then forcibly removed, and the program will finally exit.

       I/O watchers.
         A session will remain active as long as a session is paying attention to some external
         data source or sink. See select_read and select_write.

       Child sessions.
         A session acting as a parent of one or more other sessions will remain active until all
         the child sessions stop.  This may be bypassed by detaching the children from the
         parent.

       Child processes.
         Child process are watched by sig_child().  The sig_child() watcher will keep the
         watching session active until the child process has been reaped by POE::Kernel and the
         resulting event has been dispatched.

         All other signal watchers, including using "sig" to watch for "CHLD", do not keep their
         sessions active.  If you need a session to remain active when it's only watching for
         signals, have it set an alias or one of its own public reference counters.

       Public reference counters.
         A session will remain active as long as it has one or more nonzero public (or external)
         reference counter.

   Session Lifespans
       "Session" as a term is somewhat overloaded.  There are two related concepts that share the
       name.  First there is the class POE::Session, and objects created with it or related
       classes.  Second there is a data structure within POE::Kernel that tracks the POE::Session
       objects in play and the various resources owned by each.

       The way POE's garbage collector works is that a session object gives itself to POE::Kernel
       at creation time.  The Kernel then holds onto that object as long as resources exist that
       require the session to remain alive.  When all of these resources are destroyed or
       released, the session object has nothing left to trigger activity.  POE::Kernel notifies
       the object it's through, and cleans up its internal session context.  The session object
       is released, and self-destructs in the normal Perlish fashion.

       Sessions may be stopped even if they have active resources.  For example, a session may
       fail to handle a terminal signal.  In this case, POE::Kernel forces the session to stop,
       and all resources associated with the session are preemptively released.

   Events
       An event is a message that is sent from one part of the POE application to another.  An
       event consists of the event's name, optional event-specific parameters and OOB
       information.  An event may be sent from the kernel, from a wheel or from a session.

       An application creates an event with "post", "yield", "call" or even "signal".
       POE::Kernel creates events in response external stimulus (signals, select, etc).

       Event Handlers

       An event is handled by a function called an event handler, which is some code that is
       designated to be called when a particular event is dispatched.  See "Event Handler
       Management" and POE::Session.

       The term state is often used in place of event handler, especially when treating sessions
       as event driven state machines.

       Handlers are always called in scalar context for asynchronous events (i.e. via post()).
       Synchronous events, invoked with call(), are handled in the same context that call() was
       called.

       Event handlers may not directly return references to objects in the "POE" namespace.
       POE::Kernel will stringify these references to prevent timing issues with certain objects'
       destruction.  For example, this error handler would cause errors because a deleted wheel
       would not be destructed when one might think:

         sub handle_error {
           warn "Got an error";
           delete $_[HEAP]{wheel};
         }

       The delete() call returns the deleted wheel member, which is then returned implicitly by
       handle_error().

   Using POE with Other Event Loops
       POE::Kernel supports any number of event loops.  Two are included in the base
       distribution.  Historically, POE included other loops but they were moved into a separate
       distribution.  You can find them and other loops on the CPAN.

       POE's public interfaces remain the same regardless of the event loop being used.  Since
       most graphical toolkits include some form of event loop, back-end code should be portable
       to all of them.

       POE's cooperation with other event loops lets POE be embedded into other software.  The
       common underlying event loop drives both the application and POE.  For example, by using
       POE::Loop::Glib, one can embed POE into Vim, irssi, and so on.  Application scripts can
       then take advantage of POE::Component::Client::HTTP (and everything else) to do large-
       scale work without blocking the rest of the program.

       Because this is Perl, there are multiple ways to load an alternate event loop.  The
       simplest way is to load the event loop before loading POE::Kernel.

         use Gtk;
         use POE;

       Remember that POE loads POE::Kernel internally.

       POE::Kernel examines the modules loaded before it and detects that Gtk has been loaded.
       If POE::Loop::Gtk is available, POE loads and hooks it into POE::Kernel automatically.

       It's less mysterious to load the appropriate POE::Loop class directly. Their names follow
       the format "POE::Loop::$loop_module_name", where $loop_module_name is the name of the
       event loop module after each "::" has been substituted with an underscore. It can be
       abbreviated using POE's loader magic.

         use POE qw( Loop::Event_Lib );

       POE also recognizes XS loops, they reside in the "POE::XS::Loop::$loop_module_name"
       namespace.  Using them may give you a performance improvement on your platform, as the
       eventloop are some of the hottest code in the system.  As always, benchmark your
       application against various loops to see which one is best for your workload and platform.

         use POE qw( XS::Loop::EPoll );

       Please don't load the loop modules directly, because POE will not have a chance to
       initialize it's internal structures yet. Code written like this will throw errors on
       startup. It might look like a bug in POE, but it's just the way POE is designed.

         use POE::Loop::IO_Poll;
         use POE;

       POE::Kernel also supports configuration directives on its own "use" line.  A loop
       explicitly specified this way will override the search logic.

         use POE::Kernel { loop => "Glib" };

       Finally, one may specify the loop class by setting the POE::Loop or POE::XS:Loop class
       name in the POE_EVENT_LOOP environment variable.  This mechanism was added for tests that
       need to specify the loop from a distance.

         BEGIN { $ENV{POE_EVENT_LOOP} = "POE::XS::Loop::Poll" }
         use POE;

       Of course this may also be set from your shell:

         % export POE_EVENT_LOOP='POE::XS::Loop::Poll'
         % make test

       Many external event loops support their own callback mechanisms.  POE::Session's
       "postback()" and "callback()" methods return plain Perl code references that will generate
       POE events when called.  Applications can pass these code references to event loops for
       use as callbacks.

       POE's distribution includes two event loop interfaces.  CPAN holds several more:

       POE::Loop::Select (bundled)

       By default POE uses its select() based loop to drive its event system.  This is perhaps
       the least efficient loop, but it is also the most portable.  POE optimizes for correctness
       above all.

       POE::Loop::IO_Poll (bundled)

       The IO::Poll event loop provides an alternative that theoretically scales better than
       select().

       POE::Loop::Event (separate distribution)

       This event loop provides interoperability with other modules that use Event.  It may also
       provide a performance boost because Event is written in a compiled language.
       Unfortunately, this makes Event less portable than Perl's built-in select().

       POE::Loop::Gtk (separate distribution)

       This event loop allows programs to work under the Gtk graphical toolkit.

       POE::Loop::Tk (separate distribution)

       This event loop allows programs to work under the Tk graphical toolkit.  Tk has some
       restrictions that require POE to behave oddly.

       Tk's event loop will not run unless one or more widgets are created.  POE must therefore
       create such a widget before it can run. POE::Kernel exports $poe_main_window so that the
       application developer may use the widget (which is a MainWindow), since POE doesn't need
       it other than for dispatching events.

       Creating and using a different MainWindow often has an undesired outcome.

       POE::Loop::EV (separate distribution)

       POE::Loop::EV allows POE-based programs to use the EV event library with little or no
       change.

       POE::Loop::Glib (separate distribution)

       POE::Loop::Glib allows POE-based programs to use Glib with little or no change.  It also
       supports embedding POE-based programs into applications that already use Glib.  For
       example, we have heard that POE has successfully embedded into vim, irssi and xchat via
       this loop.

       POE::Loop::Kqueue (separate distribution)

       POE::Loop::Kqueue allows POE-based programs to transparently use the BSD kqueue event
       library on operating systems that support it.

       POE::Loop::Prima (separate distribution)

       POE::Loop::Prima allows POE-based programs to use Prima's event loop with little or no
       change.  It allows POE libraries to be used within Prima applications.

       POE::Loop::Wx (separate distribution)

       POE::Loop::Wx allows POE-based programs to use Wx's event loop with little or no change.
       It allows POE libraries to be used within Wx applications, such as Padre.

       POE::XS::Loop::EPoll (separate distribution)

       POE::XS::Loop::EPoll allows POE components to transparently use the EPoll event library on
       operating systems that support it.

       POE::XS::Loop::Poll (separate distribution)

       POE::XS::Loop::Poll is a higher-performance C-based libpoll event loop.  It replaces some
       of POE's hot Perl code with C for better performance.

       Other Event Loops (separate distributions)

       POE may be extended to handle other event loops.  Developers are invited to work with us
       to support their favorite loops.

PUBLIC METHODS

       POE::Kernel encapsulates a lot of features.  The documentation for each set of features is
       grouped by purpose.

   Kernel Management and Accessors
       ID

       ID() currently returns POE::Kernel's unique identifier.  Every Kernel instance is assigned
       a globally unique ID at birth.  has_forked() alters the ID so that each forked process has
       a unique one, too.

         % perl -wl -MPOE -e 'print $poe_kernel->ID'
         macbookpoe.local-4d5305de-0000e6b8-00000001

       The content of these IDs may change from time to time.  Your code should not depend upon
       the current format.

       Deprecation Warning 2011-02-09

       Your code should not depend upon ID() remaining unique.  The uniqueness will be removed in
       a future release of POE.  If you require unique IDs, please see one of the fine GUID
       and/or UUID modules on the CPAN:

         http://search.cpan.org/search?query=GUID&mode=dist
         http://search.cpan.org/search?query=UUID&mode=dist

       POE doesn't require globally or universally unique kernel IDs.  The creation and
       maintenance of these IDs adds overhead to POE::Kernel's has_forked() method.  Other
       modules do it better, upon demand, without incurring overhead for those who don't need
       them.

       run

       run() runs POE::Kernel's event dispatcher.  It will not return until all sessions have
       ended.  run() is a class method so a POE::Kernel reference is not needed to start a
       program's execution.

         use POE;
         POE::Session->create( ... ); # one or more
         POE::Kernel->run();          # set them all running
         exit;

       POE implements the Reactor pattern at its core.  Events are dispatched to functions and
       methods through callbacks.  The code behind run() waits for and dispatches events.

       run() will not return until every session has ended.  This includes sessions that were
       created while run() was running.

       POE::Kernel will print a strong message if a program creates sessions but fails to call
       run().  Prior to this warning, we received tons of bug reports along the lines of "my POE
       program isn't doing anything".  It turned out that people forgot to start an event
       dispatcher, so events were never dispatched.

       If the lack of a run() call is deliberate, perhaps because some other event loop already
       has control, you can avoid the message by calling it before creating a session.  run() at
       that point will initialize POE and return immediately.  POE::Kernel will be satisfied that
       run() was called, although POE will not have actually taken control of the event loop.

         use POE;
         POE::Kernel->run(); # silence the warning
         POE::Session->create( ... );
         exit;

       Note, however, that this varies from one event loop to another.  If a particular POE::Loop
       implementation doesn't support it, that's probably a bug.  Please file a bug report with
       the owner of the relevant POE::Loop module.

       run_one_timeslice

       run_one_timeslice() dispatches any events that are due to be delivered.  These events
       include timers that are due, asynchronous messages that need to be delivered, signals that
       require handling, and notifications for files with pending I/O.  Do not rely too much on
       event ordering.  run_one_timeslice() is defined by the underlying event loop, and its
       timing may vary.

       run() is implemented similar to

         run_one_timeslice() while $session_count > 0;

       run_one_timeslice() can be used to keep running POE::Kernel's dispatcher while emulating
       blocking behavior.  The pattern is implemented with a flag that is set when some
       asynchronous event occurs.  A loop calls run_one_timeslice() until that flag is set.  For
       example:

         my $done = 0;

         sub handle_some_event {
           $done = 1;
         }

         $kernel->run_one_timeslice() while not $done;

       Do be careful.  The above example will spin if POE::Kernel is done but $done is never set.
       The loop will never be done, even though there's nothing left that will set $done.

       run_while SCALAR_REF

       run_while() is an experimental version of run_one_timeslice() that will only return when
       there are no more active sessions, or the value of the referenced scalar becomes false.

       Here's a version of the run_one_timeslice() example using run_while() instead:

         my $job_count = 3;

         sub handle_some_event {
           $job_count--;
         }

         $kernel->run_while(\$job_count);

       has_forked

           my $pid = fork();
           die "Unable to fork" unless defined $pid;
           unless( $pid ) {
               $poe_kernel->has_forked;
           }

       Inform the kernel that it is now running in a new process.  This allows the kernel to
       reset some internal data to adjust to the new situation.

       has_forked() must be called in the child process if you wish to run the same kernel.
       However, if you want the child process to have new kernel, you must call "stop" instead.

       Note: POE's internals will detect if a fork occurred before "run()" and will call
       "has_forked()" automatically. If you are unsure whether you need to call it or not, please
       enable "ASSERT_USAGE" and POE will emit a warning if it's necessary.

       stop

       stop() causes POE::Kernel->run() to return early.  It does this by emptying the event
       queue, freeing all used resources, and stopping every active session.  stop() is not meant
       to be used lightly.  Proceed with caution.

       Caveats:

       The session that calls stop() will not be fully DESTROYed until it returns.  Invoking an
       event handler in the session requires a reference to that session, and weak references are
       prohibited in POE for backward compatibility reasons, so it makes sense that the last
       session won't be garbage collected right away.

       Sessions are not notified about their destruction.  If anything relies on _stop being
       delivered, it will break and/or leak memory.

       stop() is still considered experimental.  It was added to improve fork() support for
       POE::Wheel::Run.  If it proves unfixably problematic, it will be removed without much
       notice.

       stop() is advanced magic.  Programmers who think they need it are invited to become
       familiar with its source.

       See "Running POE::Kernel in the Child" in POE::Wheel::Run for an example of how to use
       this facility.

   Asynchronous Messages (FIFO Events)
       Asynchronous messages are events that are dispatched in the order in which they were
       enqueued (the first one in is the first one out, otherwise known as first-in/first-out, or
       FIFO order).  These methods enqueue new messages for delivery.  The act of enqueuing a
       message keeps the sender alive at least until the message is delivered.

       post DESTINATION, EVENT_NAME [, PARAMETER_LIST]

       post() enqueues a message to be dispatched to a particular DESTINATION session.  The
       message will be handled by the code associated with EVENT_NAME.  If a PARAMETER_LIST is
       included, its values will also be passed along.

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->post( $_[SESSION], "event_name", 0 );
             },
             event_name => sub {
               print "$_[ARG0]\n";
               $_[KERNEL]->post( $_[SESSION], "event_name", $_[ARG0] + 1 );
             },
           }
         );

       post() returns a Boolean value indicating whether the message was successfully enqueued.
       If post() returns false, $! is set to explain the failure:

       ESRCH ("No such process") - The DESTINATION session did not exist at the time post() was
       called.

       yield EVENT_NAME [, PARAMETER_LIST]

       yield() is a shortcut for post() where the destination session is the same as the sender.
       This example is equivalent to the one for post():

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->yield( "event_name", 0 );
             },
             event_name => sub {
               print "$_[ARG0]\n";
               $_[KERNEL]->yield( "event_name", $_[ARG0] + 1 );
             },
           }
         );

       As with post(), yield() returns right away, and the enqueued EVENT_NAME is dispatched
       later.  This may be confusing if you're already familiar with threading.

       yield() should always succeed, so it does not return a meaningful value.

   Synchronous Messages
       It is sometimes necessary for code to be invoked right away.  For example, some resources
       must be serviced right away, or they'll faithfully continue reporting their readiness.
       These reports would appear as a stream of duplicate events.  Synchronous events can also
       prevent data from going stale between the time an event is enqueued and the time it's
       delivered.

       Synchronous event handlers preempt POE's event queue, so they should perform simple tasks
       of limited duration.  Synchronous events that need to do more than just service a resource
       should pass the resource's information to an asynchronous handler.  Otherwise synchronous
       operations will occur out of order in relation to asynchronous events.  It's very easy to
       have race conditions or break causality this way, so try to avoid it unless you're okay
       with the consequences.

       POE provides these ways to call message handlers right away.

       call DESTINATION, EVENT_NAME [, PARAMETER_LIST]

       call()'s semantics are nearly identical to post()'s.  call() invokes a DESTINATION's
       handler associated with an EVENT_NAME.  An optional PARAMETER_LIST will be passed along to
       the message's handler.  The difference, however, is that the handler will be invoked
       immediately, even before call() returns.

       call() returns the value returned by the EVENT_NAME handler.  It can do this because the
       handler is invoked before call() returns.  call() can therefore be used as an accessor,
       although there are better ways to accomplish simple accessor behavior.

         POE::Session->create(
           inline_states => {
             _start => sub {
               print "Got: ", $_[KERNEL]->call($_[SESSION], "do_now"), "\n";
             },
             do_now => sub {
               return "some value";
             }
           }
         );

       The POE::Wheel classes uses call() to synchronously deliver I/O notifications.  This
       avoids a host of race conditions.

       call() may fail in the same way and for the same reasons as post().  On failure, $! is set
       to some nonzero value indicating why.  Since call() may return undef as a matter of
       course, it's recommended that $! be checked for the error condition as well as the
       explanation.

       ESRCH ("No such process") - The DESTINATION session did not exist at the time post() was
       called.

   Timer Events (Delayed Messages)
       It's often useful to wait for a certain time or until a certain amount of time has passed.
       POE supports this with events that are deferred until either an absolute time ("alarms")
       or until a certain duration of time has elapsed ("delays").

       Timer interfaces are further divided into two groups.  One group identifies timers by the
       names of their associated events.  Another group identifies timers by a unique identifier
       returned by the timer constructors.  Technically, the two are both name-based, but the
       "identifier-based" timers provide a second, more specific handle to identify individual
       timers.

       Timers may only be set up for the current session.  This design was modeled after alarm()
       and SIGALRM, which only affect the current UNIX process.  Each session has a separate
       namespace for timer names.  Timer methods called in one session cannot affect the timers
       in another.  As you may have noticed, quite a lot of POE's API is designed to prevent
       sessions from interfering with each other.

       The best way to simulate deferred inter-session messages is to send an immediate message
       that causes the destination to set a timer.  The destination's timer then defers the
       action requested of it.  This way is preferred because the time spent communicating the
       request between sessions may not be trivial, especially if the sessions are separated by a
       network.  The destination can determine how much time remains on the requested timer and
       adjust its wait time accordingly.

       Name-Based Timers

       Name-based timers are identified by the event names used to set them.  It is possible for
       different sessions to use the same timer event names, since each session is a separate
       compartment with its own timer namespace.  It is possible for a session to have multiple
       timers for a given event, but results may be surprising.  Be careful to use the right
       timer methods.

       The name-based timer methods are alarm(), alarm_add(), delay(), and delay_add().

       alarm EVENT_NAME [, EPOCH_TIME [, PARAMETER_LIST] ]

       alarm() clears all existing timers in the current session with the same EVENT_NAME.  It
       then sets a new timer, named EVENT_NAME, that will fire EVENT_NAME at the current session
       when EPOCH_TIME has been reached.  An optional PARAMETER_LIST may be passed along to the
       timer's handler.

       Omitting the EPOCH_TIME and subsequent parameters causes alarm() to clear the EVENT_NAME
       timers in the current session without setting a new one.

       EPOCH_TIME is the UNIX epoch time.  You know, seconds since midnight, 1970-01-01.  POE
       uses Time::HiRes::time(), which allows EPOCH_TIME to be (or include) fractional seconds.

       POE supports fractional seconds, but accuracy falls off steeply after 1/100 second.
       Mileage will vary depending on your CPU speed and your OS time resolution.

       Be sure to use Time::HiRes::time() rather than Perl's built-in time() if sub-second
       accuracy matters at all.  The built-in time() returns floor(Time::HiRes::time()), which is
       nearly always some fraction of a second in the past.  For example the high-resolution time
       might be 1200941422.89996.  At that same instant, time() would be 1200941422.  An alarm
       for time() + 0.5 would be 0.39996 seconds in the past, so it would be dispatched
       immediately (if not sooner).

       POE's event queue is time-ordered, so a timer due before time() will be delivered ahead of
       other events but not before timers with even earlier due times.  Therefore an alarm() with
       an EPOCH_TIME before time() jumps ahead of the queue.

       All timers are implemented identically internally, regardless of how they are set.
       alarm() will therefore blithely clear timers set by other means.

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->alarm( tick => time() + 1, 0 );
             },
             tick => sub {
               print "tick $_[ARG0]\n";
               $_[KERNEL]->alarm( tock => time() + 1, $_[ARG0] + 1 );
             },
             tock => sub {
               print "tock $_[ARG0]\n";
               $_[KERNEL]->alarm( tick => time() + 1, $_[ARG0] + 1 );
             },
           }
         );

       alarm() returns 0 on success or a true value on failure.  Usually EINVAL to signal an
       invalid parameter, such as an undefined EVENT_NAME.

       alarm_add EVENT_NAME, EPOCH_TIME [, PARAMETER_LIST]

       alarm_add() is used to add a new alarm timer named EVENT_NAME without clearing existing
       timers.  EPOCH_TIME is a required parameter.  Otherwise the semantics are identical to
       alarm().

       A program may use alarm_add() without first using alarm().

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->alarm_add( tick => time() + 1.0, 1_000_000 );
               $_[KERNEL]->alarm_add( tick => time() + 1.5, 2_000_000 );
             },
             tick => sub {
               print "tick $_[ARG0]\n";
               $_[KERNEL]->alarm_add( tock => time() + 1, $_[ARG0] + 1 );
             },
             tock => sub {
               print "tock $_[ARG0]\n";
               $_[KERNEL]->alarm_add( tick => time() + 1, $_[ARG0] + 1 );
             },
           }
         );

       alarm_add() returns 0 on success or EINVAL if EVENT_NAME or EPOCH_TIME is undefined.

       delay EVENT_NAME [, DURATION_SECONDS [, PARAMETER_LIST] ]

       delay() clears all existing timers in the current session with the same EVENT_NAME.  It
       then sets a new timer, named EVENT_NAME, that will fire EVENT_NAME at the current session
       when DURATION_SECONDS have elapsed from "now".  An optional PARAMETER_LIST may be passed
       along to the timer's handler.

       Omitting the DURATION_SECONDS and subsequent parameters causes delay() to clear the
       EVENT_NAME timers in the current session without setting a new one.

       DURATION_SECONDS may be or include fractional seconds.  As with all of POE's timers,
       accuracy falls off steeply after 1/100 second.  Mileage will vary depending on your CPU
       speed and your OS time resolution.

       POE's event queue is time-ordered, so a timer due before time() will be delivered ahead of
       other events but not before timers with even earlier due times.  Therefore a delay () with
       a zero or negative DURATION_SECONDS jumps ahead of the queue.

       delay() may be considered a shorthand form of alarm(), but there are subtle differences in
       timing issues.  This code is roughly equivalent to the alarm() example.

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->delay( tick => 1, 0 );
             },
             tick => sub {
               print "tick $_[ARG0]\n";
               $_[KERNEL]->delay( tock => 1, $_[ARG0] + 1 );
             },
             tock => sub {
               print "tock $_[ARG0]\n";
               $_[KERNEL]->delay( tick => 1, $_[ARG0] + 1 );
             },
           }
         );

       delay() returns 0 on success or a reason for failure: EINVAL if EVENT_NAME is undefined.

       delay_add EVENT_NAME, DURATION_SECONDS [, PARAMETER_LIST]

       delay_add() is used to add a new delay timer named EVENT_NAME without clearing existing
       timers.  DURATION_SECONDS is a required parameter.  Otherwise the semantics are identical
       to delay().

       A program may use delay_add() without first using delay().

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->delay_add( tick => 1.0, 1_000_000 );
               $_[KERNEL]->delay_add( tick => 1.5, 2_000_000 );
             },
             tick => sub {
               print "tick $_[ARG0]\n";
               $_[KERNEL]->delay_add( tock => 1, $_[ARG0] + 1 );
             },
             tock => sub {
               print "tock $_[ARG0]\n";
               $_[KERNEL]->delay_add( tick => 1, $_[ARG0] + 1 );
             },
           }
         );

       delay_add() returns 0 on success or EINVAL if EVENT_NAME or EPOCH_TIME is undefined.

       Identifier-Based Timers

       A second way to manage timers is through identifiers.  Setting an alarm or delay with the
       "identifier" methods allows a program to manipulate several timers with the same name in
       the same session.  As covered in alarm() and delay() however, it's possible to mix named
       and identified timer calls, but the consequences may not always be expected.

       alarm_set EVENT_NAME, EPOCH_TIME [, PARAMETER_LIST]

       alarm_set() sets an alarm, returning a unique identifier that can be used to adjust or
       remove the alarm later.  Unlike alarm(), it does not first clear existing timers with the
       same EVENT_NAME.  Otherwise the semantics are identical to alarm().

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[HEAP]{alarm_id} = $_[KERNEL]->alarm_set(
                 party => time() + 1999
               );
               $_[KERNEL]->delay(raid => 1);
             },
             raid => sub {
               $_[KERNEL]->alarm_remove( delete $_[HEAP]{alarm_id} );
             },
           }
         );

       alarm_set() returns false if it fails and sets $! with the explanation.  $! will be EINVAL
       if EVENT_NAME or TIME is undefined.

       alarm_adjust ALARM_ID, DELTA_SECONDS

       alarm_adjust() adjusts an existing timer's due time by DELTA_SECONDS, which may be
       positive or negative.  It may even be zero, but that's not as useful.  On success, it
       returns the timer's new due time since the start of the UNIX epoch.

       It's possible to alarm_adjust() timers created by delay_set() as well as alarm_set().

       This example moves an alarm's due time ten seconds earlier.

         use POSIX qw(strftime);

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[HEAP]{alarm_id} = $_[KERNEL]->alarm_set(
                 party => time() + 1999
               );
               $_[KERNEL]->delay(postpone => 1);
             },
             postpone => sub {
               my $new_time = $_[KERNEL]->alarm_adjust(
                 $_[HEAP]{alarm_id}, -10
               );
               print(
                 "Now we're gonna party like it's ",
                 strftime("%F %T", gmtime($new_time)), "\n"
               );
             },
           }
         );

       alarm_adjust() returns Boolean false if it fails, setting $! to the reason why.  $! may be
       EINVAL if ALARM_ID or DELTA_SECONDS are undefined.  It may be ESRCH if ALARM_ID no longer
       refers to a pending timer.  $! may also contain EPERM if ALARM_ID is valid but belongs to
       a different session.

       alarm_remove ALARM_ID

       alarm_remove() removes the alarm identified by ALARM_ID.  ALARM_ID comes from a previous
       alarm_set() or delay_set() call.

       Upon success, alarm_remove() returns something true based on its context.  In a list
       context, it returns three things: The removed alarm's event name, the UNIX time it was due
       to go off, and a reference to the PARAMETER_LIST (if any) assigned to the timer when it
       was created.  If necessary, the timer can be re-set with this information.

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[HEAP]{alarm_id} = $_[KERNEL]->alarm_set(
                 party => time() + 1999
               );
               $_[KERNEL]->delay(raid => 1);
             },
             raid => sub {
               my ($name, $time, $param) = $_[KERNEL]->alarm_remove(
                 $_[HEAP]{alarm_id}
               );
               print(
                 "Removed alarm for event $name due at $time with @$param\n"
               );

               # Or reset it, if you'd like.  Possibly after modification.
               $_[KERNEL]->alarm_set($name, $time, @$param);
             },
           }
         );

       In a scalar context, it returns a reference to a list of the three things above.

         # Remove and reset an alarm.
         my $alarm_info = $_[KERNEL]->alarm_remove( $alarm_id );
         my $new_id = $_[KERNEL]->alarm_set(
           $alarm_info[0], $alarm_info[1], @{$alarm_info[2]}
         );

       Upon failure, however, alarm_remove() returns a Boolean false value and sets $! with the
       reason why the call failed:

       EINVAL ("Invalid argument") indicates a problem with one or more parameters, usually an
       undefined ALARM_ID.

       ESRCH ("No such process") indicates that ALARM_ID did not refer to a pending alarm.

       EPERM ("Operation not permitted").  A session cannot remove an alarm it does not own.

       alarm_remove_all

       alarm_remove_all() removes all the pending timers for the current session, regardless of
       creation method or type.  This method takes no arguments.  It returns information about
       the alarms that were removed, either as a list of alarms or a list reference depending
       whether alarm_remove_all() is called in scalar or list context.

       Each removed alarm's information is identical to the format explained in alarm_remove().

         sub some_event_handler {
           my @removed_alarms = $_[KERNEL]->alarm_remove_all();
           foreach my $alarm (@removed_alarms) {
             my ($name, $time, $param) = @$alarm;
             ...;
           }
         }

       delay_set EVENT_NAME, DURATION_SECONDS [, PARAMETER_LIST]

       delay_set() sets a timer for DURATION_SECONDS in the future.  The timer will be dispatched
       to the code associated with EVENT_NAME in the current session.  An optional PARAMETER_LIST
       will be passed through to the handler.  It returns the same sort of things that
       alarm_set() does.

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->delay_set("later", 5, "hello", "world");
             },
             later => sub {
               print "@_[ARG0..#$_]\n";
             }
           }
         );

       delay_adjust ALARM_ID, SECONDS_FROM_NOW

       delay_adjust() changes a timer's due time to be SECONDS_FROM_NOW.  It's useful for
       refreshing watchdog- or timeout-style timers.  On success it returns the new absolute UNIX
       time the timer will be due.

       It's possible for delay_adjust() to adjust timers created by alarm_set() as well as
       delay_set().

         use POSIX qw(strftime);

         POE::Session->create(
           inline_states => {
             # Setup.
             # ... omitted.

             got_input => sub {
               my $new_time = $_[KERNEL]->delay_adjust(
                 $_[HEAP]{input_timeout}, 60
               );
               print(
                 "Refreshed the input timeout.  Next may occur at ",
                 strftime("%F %T", gmtime($new_time)), "\n"
               );
             },
           }
         );

       On failure it returns Boolean false and sets $! to a reason for the failure.  See the
       explanation of $! for alarm_adjust().

       delay_remove is not needed

       There is no delay_remove().  Timers are all identical internally, so alarm_remove() will
       work with timer IDs returned by delay_set().

       delay_remove_all is not needed

       There is no delay_remove_all().  Timers are all identical internally, so
       alarm_remove_all() clears them all regardless how they were created.

       Comparison

       Below is a table to help compare the various delayed message-sending methods

         +-----------+------------------+---------------------+------------+
         |           | time argument    | clears other events | returns on |
         | method    | passed to method | of the same name    | success    |
         +-----------+------------------+---------------------+------------+
         | delay_set | seconds from now | N                   | alarm_id   |
         | delay     | seconds from now | Y                   | 0 (false)  |
         | alarm_set | unix epoch time  | N                   | alarm_id   |
         | alarm     | unix epoch time  | Y                   | 0 (false)  |
         +-----------+------------------+---------------------+------------+

   Session Identifiers (IDs and Aliases)
       A session may be referred to by its object references (either blessed or stringified), a
       session ID, or one or more symbolic names we call aliases.

       Every session is represented by an object, so session references are fairly
       straightforward.  POE::Kernel may reference these objects.  For instance, post() may use
       $_[SENDER] as a destination:

         POE::Session->create(
           inline_states => {
             _start => sub { $_[KERNEL]->alias_set("echoer") },
             ping => sub {
               $_[KERNEL]->post( $_[SENDER], "pong", @_[ARG0..$#_] );
             }
           }
         );

       POE also recognized stringified Session objects for convenience and as a form of weak
       reference.  Here $_[SENDER] is wrapped in quotes to stringify it:

         POE::Session->create(
           inline_states => {
             _start => sub { $_[KERNEL]->alias_set("echoer") },
             ping => sub {
               $_[KERNEL]->post( "$_[SENDER]", "pong", @_[ARG0..$#_] );
             }
           }
         );

       Every session is assigned a unique ID at creation time.  No two active sessions will have
       the same ID, but IDs may be reused over time.  The combination of a kernel ID and a
       session ID should be sufficient as a global unique identifier.

         POE::Session->create(
           inline_states => {
             _start => sub { $_[KERNEL]->alias_set("echoer") },
             ping => sub {
               $_[KERNEL]->delay(
                 pong_later => rand(5), $_[SENDER]->ID, @_[ARG0..$#_]
               );
             },
             pong_later => sub {
               $_[KERNEL]->post( $_[ARG0], "pong", @_[ARG1..$#_] );
             }
           }
         );

       Kernels also maintain a global session namespace or dictionary from which may be used to
       map a symbolic aliases to a session. Once an alias is mapping has been created, that alias
       may be used to refer to the session wherever a session may be specified.

       In the previous examples, each echoer service has set an "echoer" alias.  Another session
       can post a ping request to the echoer session by using that alias rather than a session
       object or ID.  For example:

         POE::Session->create(
           inline_states => {
             _start => sub { $_[KERNEL]->post(echoer => ping => "whee!" ) },
             pong => sub { print "@_[ARG0..$#_]\n" }
           }
         );

       A session with an alias will not stop until all other activity has stopped.  Aliases are
       treated as a kind of event watcher.  Events come from active sessions.  Aliases therefore
       become useless when there are no active sessions left.  Rather than leaving the program
       running in a "zombie" state, POE detects this deadlock condition and triggers a cleanup.
       See "Signal Classes" for more information.

       alias_set ALIAS

       alias_set() maps an ALIAS in POE::Kernel's dictionary to the current session. The ALIAS
       may then be used nearly everywhere a session reference, stringified reference, or ID is
       expected.

       Sessions may have more than one alias.  Each alias must be defined in a separate
       alias_set() call.  A single alias may not refer to more than one session.

       Multiple alias examples are above.

       alias_set() returns 0 on success, or a nonzero failure indicator: EEXIST ("File exists")
       indicates that the alias is already assigned to to a different session.

       alias_remove ALIAS

       alias_remove() removes an ALIAS for the current session from POE::Kernel's dictionary.
       The ALIAS will no longer refer to the current session.  This does not negatively affect
       events already posted to POE's queue.  Alias resolution occurs at post() time, not at
       delivery time.

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[KERNEL]->alias_set("short_window");
               $_[KERNEL]->delay(close_window => 1);
             },
             close_window => {
               $_[KERNEL]->alias_remove("short_window");
             }
           }
         );

       alias_remove() returns 0 on success or a nonzero failure code:  ESRCH ("No such process")
       indicates that the ALIAS is not currently in POE::Kernel's dictionary.  EPERM ("Operation
       not permitted") means that the current session may not remove the ALIAS because it is in
       use by some other session.

       alias_resolve ALIAS

       alias_resolve() returns a session reference corresponding to a given ALIAS.  Actually, the
       ALIAS may be a stringified session reference, a session ID, or an alias previously
       registered by alias_set().

       One use for alias_resolve() is to detect whether another session has gone away:

         unless (defined $_[KERNEL]->alias_resolve("Elvis")) {
           print "Elvis has left the building.\n";
         }

       As previously mentioned, alias_resolve() returns a session reference or undef on failure.
       Failure also sets $! to ESRCH ("No such process") when the ALIAS is not currently in
       POE::Kernel's.

       alias_list [SESSION_REFERENCE]

       alias_list() returns a list of aliases associated with a specific SESSION, or with the
       current session if SESSION is omitted.  alias_list() returns an empty list if the
       requested SESSION has no aliases.

       SESSION may be a session reference (blessed or stringified), a session ID, or a session
       alias.

         POE::Session->create(
           inline_states => {
             $_[KERNEL]->alias_set("mi");
             print(
               "The names I call myself: ",
               join(", ", $_[KERNEL]->alias_list()),
               "\n"
             );
           }
         );

       ID_id_to_session SESSION_ID

       ID_id_to_session() translates a session ID into a session reference.  It's a special-
       purpose subset of alias_resolve(), so it's a little faster and somewhat less flexible.

         unless (defined $_[KERNEL]->ID_id_to_session($session_id)) {
           print "Session $session_id doesn't exist.\n";
         }

       ID_id_to_session() returns undef if a lookup failed.  $! will be set to ESRCH ("No such
       process").

       ID_session_to_id SESSION_REFERENCE

       ID_session_to_id() converts a blessed or stringified SESSION_REFERENCE into a session ID.
       It's more practical for stringified references, as programs can call the POE::Session ID()
       method on the blessed ones.  These statements are equivalent:

         $id = $_[SENDER]->ID();
         $id = $_[KERNEL]->ID_session_to_id($_[SENDER]);
         $id = $_[KERNEL]->ID_session_to_id("$_[SENDER]");

       As with other POE::Kernel lookup methods, ID_session_to_id() returns undef on failure,
       setting $! to ESRCH ("No such process").

   I/O Watchers (Selects)
       No event system would be complete without the ability to asynchronously watch for I/O
       events.  POE::Kernel implements the lowest level watchers, which are called "selects"
       because they were historically implemented using Perl's built-in select(2) function.

       Applications handle I/O readiness events by performing some activity on the underlying
       filehandle.  Read-readiness might be handled by reading from the handle.  Write-readiness
       by writing to it.

       All I/O watcher events include two parameters.  "ARG0" contains the handle that is ready
       for work.  "ARG1" contains an integer describing what's ready.

         sub handle_io {
           my ($handle, $mode) = @_[ARG0, ARG1];
           print "File $handle is ready for ";
           if ($mode == 0) {
             print "reading";
           }
           elsif ($mode == 1) {
             print "writing";
           }
           elsif ($mode == 2) {
             print "out-of-band reading";
           }
           else {
             die "unknown mode $mode";
           }
           print "\n";
           # ... do something here
         }

       The remaining parameters, @_[ARG2..$%_], contain additional parameters that were passed to
       the POE::Kernel method that created the watcher.

       POE::Kernel conditions filehandles to be 8-bit clean and non-blocking.  Programs that need
       them conditioned differently should set them up after starting POE I/O watchers. If you
       are running a Perl older than 5.8.1 and is using tied filehandles, you need to set non-
       blocking mode yourself as IO::Handle does not work well.  See
       <https://rt.cpan.org/Ticket/Display.html?id=67545> for more info.

       I/O watchers will prevent sessions from stopping.

       select_read FILE_HANDLE [, EVENT_NAME [, ADDITIONAL_PARAMETERS] ]

       select_read() starts or stops the current session from watching for incoming data on a
       given FILE_HANDLE.  The watcher is started if EVENT_NAME is specified, or stopped if it's
       not.  ADDITIONAL_PARAMETERS, if specified, will be passed to the EVENT_NAME handler as
       @_[ARG2..$#_].

         POE::Session->create(
           inline_states => {
             _start => sub {
               $_[HEAP]{socket} = IO::Socket::INET->new(
                 PeerAddr => "localhost",
                 PeerPort => 25,
               );
               $_[KERNEL]->select_read( $_[HEAP]{socket}, "got_input" );
               $_[KERNEL]->delay(timed_out => 1);
             },
             got_input => sub {
               my $socket = $_[ARG0];
               while (sysread($socket, my $buf = "", 8192)) {
                 print $buf;
               }
             },
             timed_out => sub {
               $_[KERNEL]->select_read( delete $_[HEAP]{socket} );
             },
           }
         );

       select_read() does not return anything significant.

       select_write FILE_HANDLE [, EVENT_NAME [, ADDITIONAL_PARAMETERS] ]

       select_write() follows the same semantics as select_read(), but it starts or stops a
       watcher that looks for write-readiness.  That is, when EVENT_NAME is delivered, it means
       that FILE_HANDLE is ready to be written to.

       select_write() does not return anything significant.

       select_expedite FILE_HANDLE [, EVENT_NAME [, ADDITIONAL_PARAMETERS] ]

       select_expedite() does the same sort of thing as select_read() and select_write(), but it
       watches a FILE_HANDLE for out-of-band data ready to be input from a FILE_HANDLE.  Hardly
       anybody uses this, but it exists for completeness' sake.

       An EVENT_NAME event will be delivered whenever the FILE_HANDLE can be read from out-of-
       band.  Out-of-band data is considered "expedited" because it is often ahead of a socket's
       normal data.

       select_expedite() does not return anything significant.

       select_pause_read FILE_HANDLE

       select_pause_read() is a lightweight way to pause a FILE_HANDLE input watcher without
       performing all the bookkeeping of a select_read().  It's used with select_resume_read() to
       implement input flow control.

       Input that occurs on FILE_HANDLE will backlog in the operating system buffers until
       select_resume_read() is called.

       A side effect of bypassing the select_read() bookkeeping is that a paused FILE_HANDLE will
       not prematurely stop the current session.

       select_pause_read() does not return anything significant.

       select_resume_read FILE_HANDLE

       select_resume_read() resumes a FILE_HANDLE input watcher that was previously paused by
       select_pause_read().  See select_pause_read() for more discussion on lightweight input
       flow control.

       Data backlogged in the operating system due to a select_pause_read() call will become
       available after select_resume_read() is called.

       select_resume_read() does not return anything significant.

       select_pause_write FILE_HANDLE

       select_pause_write() pauses a FILE_HANDLE output watcher the same way select_pause_read()
       does for input.  Please see select_pause_read() for further discussion.

       select_resume_write FILE_HANDLE

       select_resume_write() resumes a FILE_HANDLE output watcher the same way that
       select_resume_read() does for input.  See select_resume_read() for further discussion.

       select FILE_HANDLE [, EV_READ [, EV_WRITE [, EV_EXPEDITE [, ARGS] ] ] ]

       POE::Kernel's select() method sets or clears a FILE_HANDLE's read, write and expedite
       watchers at once.  It's a little more expensive than calling select_read(), select_write()
       and select_expedite() manually, but it's significantly more convenient.

       Defined event names enable their corresponding watchers, and undefined event names disable
       them.  This turns off all the watchers for a FILE_HANDLE:

         sub stop_io {
           $_[KERNEL]->select( $_[HEAP]{file_handle} );
         }

       This statement:

         $_[KERNEL]->select( $file_handle, undef, "write_event", undef, @stuff );

       is equivalent to:

         $_[KERNEL]->select_read( $file_handle );
         $_[KERNEL]->select_write( $file_handle, "write_event", @stuff );
         $_[KERNEL]->select_expedite( $file_handle );

       POE::Kernel's select() should not be confused with Perl's built-in select() function.

       As with the other I/O watcher methods, select() does not return a meaningful value.

   Session Management
       Sessions are dynamic.  They may be created and destroyed during a program's lifespan.
       When a session is created, it becomes the "child" of the current session.  The creator --
       the current session -- becomes its "parent" session.  This is loosely modeled after UNIX
       processes.

       The most common session management is done by creating new sessions and allowing them to
       eventually stop.

       Every session has a parent, even the very first session created.  Sessions without obvious
       parents are children of the program's POE::Kernel instance.

       Child sessions will keep their parents active.  See "Session Lifespans" for more about why
       sessions stay alive.

       The parent/child relationship tree also governs the way many signals are dispatched.  See
       "Common Signal Dispatching" for more information on that.

       Session Management Events (_start, _stop, _parent, _child)

       POE::Kernel provides four session management events: _start, _stop, _parent and _child.
       They are invoked synchronously whenever a session is newly created or just about to be
       destroyed.

       _start
         _start should be familiar by now.  POE dispatches the _start event to initialize a
         session after it has been registered under POE::Kernel.  What is not readily apparent,
         however, is that it is invoked before the POE::Session constructor returns.

         Within the _start handler, the event's sender is the session that created the new
         session.  Otherwise known as the new session's parent.  Sessions created before
         POE::Kernel->run() is called will be descendents of the program's POE::Kernel singleton.

         The _start handler's return value is passed to the parent session in a _child event,
         along with the notification that the parent's new child was created successfully.  See
         the discussion of _child for more details.

           POE::Session->create(
             inline_states => { _start=> \&_start },
             args => [ $some, $args ]
           );

           sub _start {
             my ( $some, $args ) = @_[ ARG0, ARG1 ];
             # ....
           }

       _stop
         _stop is a little more mysterious.  POE calls a _stop handler when a session is
         irrevocably about to be destroyed.  Part of session destruction is the forcible
         reclamation of its resources (events, timers, message events, etc.) so it's not possible
         to post() a message from _stop's handler.  A program is free to try, but the event will
         be destroyed before it has a chance to be dispatched.

         the _stop handler's return value is passed to the parent's _child event.  See _child for
         more details.

         _stop is usually invoked when a session has no further reason to live, although signals
         may cause them to stop sooner.

         The corresponding _child handler is invoked synchronously just after _stop returns.

       _parent
         _parent is used to notify a child session when its parent has changed.  This usually
         happens when a session is first created.  It can also happen when a child session is
         detached from its parent. See detach_child and "detach_myself".

         _parent's ARG0 contains the session's previous parent, and ARG1 contains its new parent.

           sub _parent {
             my ( $old_parent, $new_parent ) = @_[ ARG0, ARG1 ];
             print(
               "Session ", $_[SESSION]->ID,
               " parent changed from session ", $old_parent->ID,
               " to session ", $new_parent->ID,
               "\n"
             );
           }

       _child
         _child notifies one session when a child session has been created, destroyed, or
         reassigned to or from another parent.  It's usually dispatched when sessions are created
         or destroyed.  It can also happen when a session is detached from its parent.

         _child includes some information in the "arguments" portion of @_.  Typically ARG0, ARG1
         and ARG2, but these may be overridden by a different POE::Session class:

         ARG0 contains a string describing what has happened to the child.  The string may be
         'create' (the child session has been created), 'gain' (the child has been given by
         another session), or 'lose' (the child session has stopped or been given away).

         In all cases, ARG1 contains a reference to the child session.

         In the 'create' case, ARG2 holds the value returned by the child session's _start
         handler.  Likewise, ARG2 holds the _stop handler's return value for the 'lose' case.

           sub _child {
             my( $reason, $child ) = @_[ ARG0, ARG1 ];
             if( $reason eq 'create' ) {
               my $retval = $_[ ARG2 ];
             }
             # ...
           }

       The events are delivered in specific orders.

       When a new session is created:

       1.  The session's constructor is called.

       2.  The session is put into play.  That is, POE::Kernel enters the session into its
           bookkeeping.

       3.  The new session receives _start.

       4.  The parent session receives _child ('create'), the new session reference, and the new
           session's _start's return value.

       5.  The session's constructor returns.

       When an old session stops:

       1.  If the session has children of its own, they are given to the session's parent.  This
           triggers one or more _child ('gain') events in the parent, and a _parent in each
           child.

       2.  Once divested of its children, the stopping session receives a _stop event.

       3.  The stopped session's parent receives a _child ('lose') event with the departing
           child's reference and _stop handler's return value.

       4.  The stopped session is removed from play, as are all its remaining resources.

       5.  The parent session is checked for idleness.  If so, garbage collection will commence
           on it, and it too will be stopped

       When a session is detached from its parent:

       1.  The parent session of the session being detached is notified with a _child ('lose')
           event.  The _stop handler's return value is undef since the child is not actually
           stopping.

       2.  The detached session is notified with a _parent event that its new parent is
           POE::Kernel itself.

       3.  POE::Kernel's bookkeeping data is adjusted to reflect the change of parentage.

       4.  The old parent session is checked for idleness.  If so, garbage collection will
           commence on it, and it too will be stopped

       Session Management Methods

       These methods allow sessions to be detached from their parents in the rare cases where the
       parent/child relationship gets in the way.

       detach_child CHILD_SESSION

       detach_child() detaches a particular CHILD_SESSION from the current session.  On success,
       the CHILD_SESSION will become a child of the POE::Kernel instance, and detach_child() will
       return true.  On failure however, detach_child() returns false and sets $! to explain the
       nature of the failure:

       ESRCH ("No such process").
           The CHILD_SESSION is not a valid session.

       EPERM ("Operation not permitted").
           The CHILD_SESSION exists, but it is not a child of the current session.

       detach_child() will generate "_parent" and/or "_child" events to the appropriate sessions.
       See Session Management Events for a detailed explanation of these events.  See above for
       the order the events are generated.

       detach_myself

       detach_myself() detaches the current session from its current parent.  The new parent will
       be the running POE::Kernel instance.  It returns true on success.  On failure it returns
       false and sets $! to explain the nature of the failure:

       EPERM ("Operation not permitted").
           The current session is already a child of POE::Kernel, so it may not be detached.

       detach_child() will generate "_parent" and/or "_child" events to the appropriate sessions.
       See Session Management Events for a detailed explanation of these events.  See above for
       the order the events are generated.

   Signals
       POE::Kernel provides methods through which a program can register interest in signals that
       come along, can deliver its own signals without resorting to system calls, and can
       indicate that signals have been handled so that default behaviors are not necessary.

       Signals are action at a distance by nature, and their implementation requires widespread
       synchronization between sessions (and reentrancy in the dispatcher, but that's an
       implementation detail).  Perfecting the semantics has proven difficult, but POE tries to
       do the Right Thing whenever possible.

       POE does not register %SIG handlers for signals until sig() is called to watch for them.
       Therefore a signal's default behavior occurs for unhandled signals.  That is, SIGINT will
       gracelessly stop a program, SIGWINCH will do nothing, SIGTSTP will pause a program, and so
       on.

       Signal Classes

       There are three signal classes.  Each class defines a default behavior for the signal and
       whether the default can be overridden.  They are:

       Benign, advisory, or informative signals

       These are three names for the same signal class.  Signals in this class notify a session
       of an event but do not terminate the session if they are not handled.

       It is possible for an application to create its own benign signals.  See "signal" below.

       Terminal signals

       Terminal signals will kill sessions if they are not handled by a "sig_handled"() call.
       The OS signals that usually kill or dump a process are considered terminal in POE, but
       they never trigger a coredump.  These are: HUP, INT, QUIT and TERM.

       There are two terminal signals created by and used within POE:

       DIE "DIE" notifies sessions that a Perl exception has occurred.  See "Exception Handling"
           for details.

       IDLE
           The "IDLE" signal is used to notify leftover sessions that a program has run out of
           things to do.

       Nonmaskable signals

       Nonmaskable signals are terminal regardless whether sig_handled() is called.  The term
       comes from "NMI", the non-maskable CPU interrupt usually generated by an unrecoverable
       hardware exception.

       Sessions that receive a non-maskable signal will unavoidably stop.  POE implements two
       non-maskable signals:

       ZOMBIE
           This non-maskable signal is fired if a program has received an "IDLE" signal but
           neither restarted nor exited.  The program has become a zombie (that is, it's neither
           dead nor alive, and only exists to consume braaaains ...er...  memory).  The "ZOMBIE"
           signal acts like a cricket bat to the head, bringing the zombie down, for good.

       UIDESTROY
           This non-maskable signal indicates that a program's user interface has been closed,
           and the program should take the user's hint and buzz off as well.  It's usually
           generated when a particular GUI widget is closed.

       Common Signal Dispatching

       Most signals are not dispatched to a single session.  POE's session lineage (parents and
       children) form a sort of family tree.  When a signal is sent to a session, it first passes
       through any children (and grandchildren, and so on) that are also interested in the
       signal.

       In the case of terminal signals, if any of the sessions a signal passes through calls
       "sig_handled"(), then the signal is considered taken care of.  However if none of them do,
       then the entire session tree rooted at the destination session is terminated.  For
       example, consider this tree of sessions:

         POE::Kernel
           Session 2
             Session 4
             Session 5
           Session 3
             Session 6
             Session 7

       POE::Kernel is the parent of sessions 2 and 3.  Session 2 is the parent of sessions 4 and
       5.  And session 3 is the parent of 6 and 7.

       A signal sent to Session 2 may also be dispatched to session 4 and 5 because they are 2's
       children.  Sessions 4 and 5 will only receive the signal if they have registered the
       appropriate watcher.  If the signal is terminal, and none of the signal watchers in
       sessions 2, 4 and 5 called "sig_handled()", all 3 sessions will be terminated.

       The program's POE::Kernel instance is considered to be a session for the purpose of signal
       dispatch.  So any signal sent to POE::Kernel will propagate through every interested
       session in the entire program.  This is in fact how OS signals are handled: A global
       signal handler is registered to forward the signal to POE::Kernel.

       Signal Semantics

       All signals come with the signal name in ARG0.  The signal name is as it appears in %SIG,
       with one exception: Child process signals are always "CHLD" even if the current operating
       system recognizes them as "CLD".

       Certain signals have special semantics:

       SIGCHLD

       SIGCLD

       Both "SIGCHLD" and "SIGCLD" indicate that a child process has exited or been terminated by
       some signal.  The actual signal name varies between operating systems, but POE uses "CHLD"
       regardless.

       Interest in "SIGCHLD" is registered using the "sig_child" method.  The "sig"() method also
       works, but it's not as nice.

       The "SIGCHLD" event includes three parameters:

       ARG0
           "ARG0" contains the string 'CHLD' (even if the OS calls it SIGCLD, SIGMONKEY, or
           something else).

       ARG1
           "ARG1" contains the process ID of the finished child process.

       ARG2
           And "ARG2" holds the value of $? for the finished process.

       Example:

         sub sig_CHLD {
           my( $name, $PID, $exit_val ) = @_[ ARG0, ARG1, ARG2 ];
           # ...
         }

       SIGPIPE

       SIGPIPE is rarely used since POE provides events that do the same thing.  Nevertheless
       SIGPIPE is supported if you need it.  Unlike most events, however, SIGPIPE is dispatched
       directly to the active session when it's caught.  Barring race conditions, the active
       session should be the one that caused the OS to send the signal in the first place.

       The SIGPIPE signal will still propagate to child sessions.

       ARG0 is "PIPE".  There is no other information associated with this signal.

       SIGWINCH

       Window resizes can generate a large number of signals very quickly.  This may not be a
       problem when using perl 5.8.0 or later, but earlier versions may not take kindly to such
       abuse.  You have been warned.

       ARG0 is "WINCH".  There is no other information associated with this signal.

       Exception Handling

       POE::Kernel provides only one form of exception handling: the "DIE" signal.

       When exception handling is enabled (the default), POE::Kernel wraps state invocation in
       "eval{}".  If the event handler raises an exception, generally with "die", POE::Kernel
       will dispatch a "DIE" signal to the event's destination session.

       "ARG0" is the signal name, "DIE".

       "ARG1" is a hashref describing the exception:

       error_str
           The text of the exception.  In other words, $@.

       dest_session
           Session object of the state that the raised the exception.  In other words,
           $_[SESSION] in the function that died.

       event
           Name of the event that died.

       source_session
           Session object that sent the original event.  That is, $_[SENDER] in the function that
           died.

       from_state
           State from which the original event was sent.  That is, $_[CALLER_STATE] in the
           function that died.

       file
           Name of the file the event was sent from.  That is, $_[CALLER_FILE] in the function
           that died.

       line
           Line number the event was sent from.  That is, $_[CALLER_LINE] in the function that
           died.

       Note that the preceding discussion assumes you are using POE::Session's call semantics.

       Note that the "DIE" signal is sent to the session that raised the exception, not the
       session that sent the event that caused the exception to be raised.

         sub _start {
           $poe_kernel->sig( DIE => 'sig_DIE' );
           $poe_kernel->yield( 'some_event' );
         }

         sub some_event {
           die "I didn't like that!";
         }

         sub sig_DIE {
           my( $sig, $ex ) = @_[ ARG0, ARG1 ];
           # $sig is 'DIE'
           # $ex is the exception hash
           warn "$$: error in $ex->{event}: $ex->{error_str}";
           $poe_kernel->sig_handled();

           # Send the signal to session that sent the original event.
           if( $ex->{source_session} ne $_[SESSION] ) {
             $poe_kernel->signal( $ex->{source_session}, 'DIE', $sig, $ex );
           }
         }

       POE::Kernel's built-in exception handling can be disabled by setting the
       "POE::Kernel::CATCH_EXCEPTIONS" constant to zero.  As with other compile-time
       configuration constants, it must be set before POE::Kernel is compiled:

         BEGIN {
           package POE::Kernel;
           use constant CATCH_EXCEPTIONS => 0;
         }
         use POE;

       or

         sub POE::Kernel::CATCH_EXCEPTIONS () { 0 }
         use POE;

   Signal Watcher Methods
       And finally the methods themselves.

       sig SIGNAL_NAME [, EVENT_NAME [, LIST] ]

       sig() registers or unregisters an EVENT_NAME event for a particular SIGNAL_NAME, with an
       optional LIST of parameters that will be passed to the signal's handler---after any data
       that comes wit the signal.

       If EVENT_NAME is defined, the signal handler is registered.  Otherwise it's unregistered.

       Each session can register only one handler per SIGNAL_NAME.  Subsequent registrations will
       replace previous ones.  Multiple sessions may however watch the same signal.

       SIGNAL_NAMEs are generally the same as members of %SIG, with two exceptions.  First, "CLD"
       is an alias for "CHLD" (although see "sig_child").  And second, it's possible to send and
       handle signals created by the application and have no basis in the operating system.

         sub handle_start {
           $_[KERNEL]->sig( INT => "event_ui_shutdown" );
           $_[KERNEL]->sig( bat => "holy_searchlight_batman" );
           $_[KERNEL]->sig( signal => "main_screen_turn_on" );
         }

       The operating system may never be able to generate the last two signals, but a POE session
       can by using POE::Kernel's "signal"() method.

       Later on the session may decide not to handle the signals:

         sub handle_ui_shutdown {
           $_[KERNEL]->sig( "INT" );
           $_[KERNEL]->sig( "bat" );
           $_[KERNEL]->sig( "signal" );
         }

       More than one session may register interest in the same signal, and a session may clear
       its own signal watchers without affecting those in other sessions.

       sig() does not return a meaningful value.

       sig_child PROCESS_ID [, EVENT_NAME [, LIST] ]

       sig_child() is a convenient way to deliver an EVENT_NAME event when a particular
       PROCESS_ID has exited.  An optional LIST of parameters will be passed to the signal
       handler after the waitpid() information.

       The watcher can be cleared at any time by calling sig_child() with just the PROCESS_ID.

       A session may register as many sig_child() handlers as necessary, but a session may only
       have one per PROCESS_ID.

       sig_child() watchers are one-shot.  They automatically unregister themselves once the
       EVENT_NAME has been delivered.  There's no point in continuing to watch for a signal that
       will never come again.  Other signal handlers persist until they are cleared.

       sig_child() watchers keep a session alive for as long as they are active.  This is unique
       among POE's signal watchers.

       Programs that wish to reliably reap child processes should be sure to call sig_child()
       before returning from the event handler that forked the process.  Otherwise POE::Kernel
       may have an opportunity to call waitpid() before an appropriate event watcher has been
       registered.

       Programs that reap processes with waitpid() must clear POE's watchers for the same process
       IDs, otherwise POE will wait indefinitely for processes that never send signals.

       sig_child() does not return a meaningful value.

         sub forked_parent {
           my( $heap, $pid, $details ) = @_[ HEAP, ARG0, ARG1 ];
           $poe_kernel->sig_child( $pid, 'sig_child', $details );
         }

         sub sig_child {
           my( $heap, $sig, $pid, $exit_val, $details ) = @_[ HEAP, ARG0..ARG3 ];
           my $details = delete $heap->{ $pid };
           warn "$$: Child $pid exited"
           # .... also, $details has been passed from forked_parent()
           # through sig_child()
         }

       sig_handled

       sig_handled() informs POE::Kernel that the currently dispatched signal has been handled by
       the currently active session. If the signal is terminal, the sig_handled() call prevents
       POE::Kernel from stopping the sessions that received the signal.

       A single signal may be dispatched to several sessions.  Only one needs to call
       sig_handled() to prevent the entire group from being stopped.  If none of them call it,
       however, then they are all stopped together.

       sig_handled() does not return a meaningful value.

         sub _start {
           $_[KERNEL]->sig( INT => 'sig_INT' );
         }

         sub sig_INT {
           warn "$$ SIGINT";
           $_[KERNEL]->sig_handled();
         }

       signal SESSION, SIGNAL_NAME [, ARGS_LIST]

       signal() posts a SIGNAL_NAME signal to a specific SESSION with an optional ARGS_LIST that
       will be passed to every interested handler.  As mentioned elsewhere, the signal may be
       delivered to SESSION's children, grandchildren, and so on.  And if SESSION is the
       POE::Kernel itself, then all interested sessions will receive the signal.

       It is possible to send a signal in POE that doesn't exist in the operating system.
       signal() places the signal directly into POE's event queue as if they came from the
       operating system, but they are not limited to signals recognized by kill().  POE uses a
       few of these fictitious signals for its own global notifications.

       For example:

         sub some_event_handler {
           # Turn on all main screens.
           $_[KERNEL]->signal( $_[KERNEL], "signal" );
         }

       signal() returns true on success.  On failure, it returns false after setting $! to
       explain the nature of the failure:

       ESRCH ("No such process")
           The SESSION does not exist.

       Because all sessions are a child of POE::Kernel, sending a signal to the kernel will
       propagate the signal to all sessions.  This is a cheap form of multicast.

         $_[KERNEL]->signal( $_[KERNEL], 'shutdown' );

       signal_ui_destroy WIDGET_OBJECT

       signal_ui_destroy() associates the destruction of a particular WIDGET_OBJECT with the
       complete destruction of the program's user interface.  When the WIDGET_OBJECT destructs,
       POE::Kernel issues the non-maskable UIDESTROY signal, which quickly triggers mass
       destruction of all active sessions.  POE::Kernel->run() returns shortly thereafter.

         sub setup_ui {
           $_[HEAP]{main_widget} = Gtk->new("toplevel");
           # ... populate the main widget here ...
           $_[KERNEL]->signal_ui_destroy( $_[HEAP]{main_widget} );
         }

       Detecting widget destruction is specific to each toolkit.

   Event Handler Management
       Event handler management methods let sessions hot swap their event handlers at run time.
       For example, the POE::Wheel objects use state() to dynamically mix their own event
       handlers into the sessions that create them.

       These methods only affect the current session; it would be rude to change another
       session's handlers.

       There is only one method in this group.  Since it may be called in several different ways,
       it may be easier to understand if each is documented separately.

       state EVENT_NAME [, CODE_REFERNCE]

       state() sets or removes a handler for EVENT_NAME in the current session.  The function
       referred to by CODE_REFERENCE will be called whenever EVENT_NAME events are dispatched to
       the current session.  If CODE_REFERENCE is omitted, the handler for EVENT_NAME will be
       removed.

       A session may only have one handler for a given EVENT_NAME.  Subsequent attempts to set an
       EVENT_NAME handler will replace earlier handlers with the same name.

         # Stop paying attention to input.  Say goodbye, and
         # trigger a socket close when the message is sent.
         sub send_final_response {
           $_[HEAP]{wheel}->put("KTHXBYE");
           $_[KERNEL]->state( 'on_client_input' );
           $_[KERNEL]->state( on_flush => \&close_connection );
         }

       state EVENT_NAME [, OBJECT_REFERENCE [, OBJECT_METHOD_NAME] ]

       Set or remove a handler for EVENT_NAME in the current session.  If an OBJECT_REFERENCE is
       given, that object will handle the event.  An optional OBJECT_METHOD_NAME may be provided.
       If the method name is not given, POE will look for a method matching the EVENT_NAME
       instead.  If the OBJECT_REFERENCE is omitted, the handler for EVENT_NAME will be removed.

       A session may only have one handler for a given EVENT_NAME.  Subsequent attempts to set an
       EVENT_NAME handler will replace earlier handlers with the same name.

         $_[KERNEL]->state( 'some_event', $self );
         $_[KERNEL]->state( 'other_event', $self, 'other_method' );

       state EVENT_NAME [, CLASS_NAME [, CLASS_METHOD_NAME] ]

       This form of state() call is virtually identical to that of the object form.

       Set or remove a handler for EVENT_NAME in the current session.  If an CLASS_NAME is given,
       that class will handle the event.  An optional CLASS_METHOD_NAME may be provided.  If the
       method name is not given, POE will look for a method matching the EVENT_NAME instead.  If
       the CLASS_NAME is omitted, the handler for EVENT_NAME will be removed.

       A session may only have one handler for a given EVENT_NAME.  Subsequent attempts to set an
       EVENT_NAME handler will replace earlier handlers with the same name.

         $_[KERNEL]->state( 'some_event', __PACKAGE__ );
         $_[KERNEL]->state( 'other_event', __PACKAGE__, 'other_method' );

   Public Reference Counters
       The methods in this section manipulate reference counters on the current session or
       another session.

       Each session has a namespace for user-manipulated reference counters.  These namespaces
       are associated with the target SESSION_ID for the reference counter methods, not the
       caller.  Nothing currently prevents one session from decrementing a reference counter that
       was incremented by another, but this behavior is not guaranteed to remain.  For now, it's
       up to the users of these methods to choose obscure counter names to avoid conflicts.

       Reference counting is a big part of POE's magic.  Various objects (mainly event watchers
       and components) hold references to the sessions that own them.  "Session Lifespans"
       explains the concept in more detail.

       The ability to keep a session alive is sometimes useful in an application or library.  For
       example, a component may hold a public reference to another session while it processes a
       request from that session.  In doing so, the component guarantees that the requester is
       still around when a response is eventually ready.  Keeping a reference to the session's
       object is not enough.  POE::Kernel has its own internal reference counting mechanism.

       refcount_increment SESSION_ID, COUNTER_NAME

       refcount_increment() increases the value of the COUNTER_NAME reference counter for the
       session identified by a SESSION_ID.  To discourage the use of session references, the
       refcount_increment() target session must be specified by its session ID.

       The target session will not stop until the value of any and all of its COUNTER_NAME
       reference counters are zero.  (Actually, it may stop in some cases, such as failing to
       handle a terminal signal.)

       Negative reference counters are legal.  They still must be incremented back to zero before
       a session is eligible for stopping.

         sub handle_request {
           # Among other things, hold a reference count on the sender.
           $_[KERNEL]->refcount_increment( $_[SENDER]->ID, "pending request");
           $_[HEAP]{requesters}{$request_id} = $_[SENDER]->ID;
         }

       For this to work, the session needs a way to remember the $_[SENDER]->ID for a given
       request.  Customarily the session generates a request ID and uses that to track the
       request until it is fulfilled.

       refcount_increment() returns the resulting reference count (which may be zero) on success.
       On failure, it returns undef and sets $! to be the reason for the error.

       ESRCH: The SESSION_ID does not refer to a currently active session.

       refcount_decrement SESSION_ID, COUNTER_NAME

       refcount_decrement() reduces the value of the COUNTER_NAME reference counter for the
       session identified by a SESSION_ID.  It is the counterpoint for refcount_increment().
       Please see refcount_increment() for more context.

         sub finally_send_response {
           # Among other things, release the reference count for the
           # requester.
           my $requester_id = delete $_[HEAP]{requesters}{$request_id};
           $_[KERNEL]->refcount_decrement( $requester_id, "pending request");
         }

       The requester's $_[SENDER]->ID is remembered and removed from the heap (lest there be
       memory leaks).  It's used to decrement the reference counter that was incremented at the
       start of the request.

       refcount_decrement() returns the resulting reference count (which may be zero) on success.
       On failure, it returns undef, and $! will be set to the reason for the failure:

       ESRCH: The SESSION_ID does not refer to a currently active session.

       It is not possible to discover currently active public references.  See POE::API::Peek.

   Kernel State Accessors
       POE::Kernel provides a few accessors into its massive brain so that library developers may
       have convenient access to necessary data without relying on their callers to provide it.

       These accessors expose ways to break session encapsulation.  Please use them sparingly and
       carefully.

       get_active_session

       get_active_session() returns a reference to the session that is currently running, or a
       reference to the program's POE::Kernel instance if no session is running at that moment.
       The value is equivalent to POE::Session's $_[SESSION].

       This method was added for libraries that need $_[SESSION] but don't want to include it as
       a parameter in their APIs.

         sub some_housekeeping {
           my( $self ) = @_;
           my $session = $poe_kernel->get_active_session;
           # do some housekeeping on $session
         }

       get_active_event

       get_active_event() returns the name of the event currently being dispatched.  It returns
       an empty string when called outside event dispatch.  The value is equivalent to
       POE::Session's $_[STATE].

         sub waypoint {
           my( $message ) = @_;
           my $event = $poe_kernel->get_active_event;
           print STDERR "$$:$event:$mesage\n";
         }

       get_event_count

       get_event_count() returns the number of events pending in POE's event queue.  It is
       exposed for POE::Loop class authors.  It may be deprecated in the future.

       get_next_event_time

       get_next_event_time() returns the time the next event is due, in a form compatible with
       the UNIX time() function.  It is exposed for POE::Loop class authors.  It may be
       deprecated in the future.

       poe_kernel_loop

       poe_kernel_loop() returns the name of the POE::Loop class that is used to detect and
       dispatch events.

   Session Helper Methods
       The methods in this group expose features for POE::Session class authors.

       session_alloc SESSION_OBJECT [, START_ARGS]

       session_alloc() allocates a session context within POE::Kernel for a newly created
       SESSION_OBJECT.  A list of optional START_ARGS will be passed to the session as part of
       the "_start" event.

       The SESSION_OBJECT is expected to follow a subset of POE::Session's interface.

       There is no session_free().  POE::Kernel determines when the session should stop and
       performs the necessary cleanup after dispatching _stop to the session.

   Miscellaneous Methods
       We don't know where to classify the methods in this section.

       new

       It is not necessary to call POE::Kernel's new() method.  Doing so will return the
       program's singleton POE::Kernel object, however.

PUBLIC EXPORTED VARIABLES

       POE::Kernel exports two variables for your coding enjoyment: $poe_kernel and
       $poe_main_window.  POE::Kernel is implicitly used by POE itself, so using POE gets you
       POE::Kernel (and its exports) for free.

       In more detail:

   $poe_kernel
       $poe_kernel contains a reference to the process' POE::Kernel singleton instance. It's
       mainly used for accessing POE::Kernel methods from places where $_[KERNEL] is not
       available.  It's most commonly used in helper libraries.

   $poe_main_window
       $poe_main_window is used by graphical toolkits that require at least one widget to be
       created before their event loops are usable.  This is currently only Tk.

       POE::Loop::Tk creates a main window to satisfy Tk's event loop.  The window is given to
       the application since POE has no other use for it.

       $poe_main_window is undefined in toolkits that don't require a widget to dispatch events.

       On a related note, POE will shut down if the widget in $poe_main_window is destroyed.
       This can be changed with POE::Kernel's "signal_ui_destroy" method.

DEBUGGING POE AND PROGRAMS USING IT

       POE includes quite a lot of debugging code, in the form of both fatal assertions and run-
       time traces.  They may be enabled at compile time, but there is no way to toggle them at
       run-time.  This was done to avoid run-time penalties in programs where debugging is not
       necessary.  That is, in most production cases.

       Traces are verbose reminders of what's going on within POE.  Each is prefixed with a four-
       character field describing the POE subsystem that generated it.

       Assertions (asserts) are quiet but deadly, both in performance (they cause a significant
       run-time performance hit) and because they cause fatal errors when triggered.

       The assertions and traces are useful for developing programs with POE, but they were
       originally added to debug POE itself.

       Each assertion and tracing group is enabled by setting a constant in the POE::Kernel
       namespace to a true value.

         BEGIN {
           package POE::Kernel;
           use constant ASSERT_DEFAULT => 1;
         }
         use POE;

       Or the old-fashioned (and more concise) "constant subroutine" method.  This doesn't need
       the "BEGIN{}" block since subroutine definitions are done at compile time.

         sub POE::Kernel::ASSERT_DEFAULT () { 1 }
         use POE;

       The switches must be defined as constants before POE::Kernel is first loaded.  Otherwise
       Perl's compiler will not see the constants when first compiling POE::Kernel, and the
       features will not be properly enabled.

       Assertions and traces may also be enabled by setting shell environment variables.  The
       environment variables are named after the POE::Kernel constants with a "POE_" prefix.

         POE_ASSERT_DEFAULT=1 POE_TRACE_DEFAULT=1 ./my_poe_program

       In alphabetical order:

   ASSERT_DATA
       ASSERT_DATA enables run-time data integrity checks within POE::Kernel and the classes that
       mix into it.  POE::Kernel tracks a lot of cross-referenced data, and this group of
       assertions ensures that it's consistent.

       Prefix: <dt>

       Environment variable: POE_ASSERT_DATA

   ASSERT_DEFAULT
       ASSERT_DEFAULT specifies the default value for assertions that are not explicitly enabled
       or disabled.  This is a quick and reliable way to make sure all assertions are on.

       No assertion uses ASSERT_DEFAULT directly, and this assertion flag has no corresponding
       output prefix.

       Turn on all assertions except ASSERT_EVENTS:

         sub POE::Kernel::ASSERT_DEFAULT () { 1 }
         sub POE::Kernel::ASSERT_EVENTS  () { 0 }
         use POE::Kernel;

       Prefix: (none)

       Environment variable: POE_ASSERT_DEFAULT

   ASSERT_EVENTS
       ASSERT_EVENTS mainly checks for attempts to dispatch events to sessions that don't exist.
       This assertion can assist in the debugging of strange, silent cases where event handlers
       are not called.

       Prefix: <ev>

       Environment variable: POE_ASSERT_EVENTS

   ASSERT_FILES
       ASSERT_FILES enables some run-time checks in POE's filehandle watchers and the code that
       manages them.

       Prefix: <fh>

       Environment variable: POE_ASSERT_FILES

   ASSERT_RETVALS
       ASSERT_RETVALS upgrades failure codes from POE::Kernel's methods from advisory return
       values to fatal errors.  Most programmers don't check the values these methods return, so
       ASSERT_RETVALS is a quick way to validate one's assumption that all is correct.

       Prefix: <rv>

       Environment variable: POE_ASSERT_RETVALS

   ASSERT_USAGE
       ASSERT_USAGE is the counterpoint to ASSERT_RETVALS.  It enables run-time checks that the
       parameters to POE::Kernel's methods are correct.  It's a quick (but not foolproof) way to
       verify a program's use of POE.

       Prefix: <us>

       Environment variable: POE_ASSERT_USAGE

   TRACE_DEFAULT
       TRACE_DEFAULT specifies the default value for traces that are not explicitly enabled or
       disabled.  This is a quick and reliable way to ensure your program generates copious
       output on the file named in TRACE_FILENAME or STDERR by default.

       To enable all traces except a few noisier ones:

         sub POE::Kernel::TRACE_DEFAULT () { 1 }
         sub POE::Kernel::TRACE_EVENTS  () { 0 }
         use POE::Kernel;

       Prefix: (none)

       Environment variable: POE_TRACE_DEFAULT

   TRACE_DESTROY
       TRACE_DESTROY causes every POE::Session object to dump the contents of its $_[HEAP] when
       Perl destroys it.  This trace was added to help developers find memory leaks in their
       programs.

       Prefix: A line that reads "----- Session $self Leak Check -----".

       Environment variable: POE_TRACE_DESTROY

   TRACE_EVENTS
       TRACE_EVENTS enables messages pertaining to POE's event queue's activities: when events
       are enqueued, dispatched or discarded, and more.  It's great for determining where events
       go and when.  Understandably this is one of POE's more verbose traces.

       Prefix: <ev>

       Environment variable: POE_TRACE_EVENTS

   TRACE_FILENAME
       TRACE_FILENAME specifies the name of a file where POE's tracing and assertion messages
       should go.  It's useful if you want the messages but have other plans for STDERR, which is
       where the messages go by default.

       POE's tests use this so the trace and assertion code can be instrumented during testing
       without spewing all over the terminal.

       Prefix: (none)

       Environment variable: POE_TRACE_FILENAME

   TRACE_FILES
       TRACE_FILES enables or disables traces in POE's filehandle watchers and the POE::Loop
       class that implements the lowest-level filehandle multiplexing.  This may be useful when
       tracking down strange behavior related to filehandles.

       Prefix: <fh>

       Environment variable: POE_TRACE_FILES

   TRACE_REFCNT
       TRACE_REFCNT governs whether POE::Kernel will trace sessions' reference counts.  As
       discussed in "Session Lifespans", POE does a lot of reference counting, and the current
       state of a session's reference counts determines whether the session lives or dies.  It's
       common for developers to wonder why a session stops too early or remains active too long.
       TRACE_REFCNT can help explain why.

       Prefix: <rc>

       Environment variable: POE_TRACE_REFCNT

   TRACE_RETVALS
       TRACE_RETVALS can enable carping whenever a POE::Kernel method is about to fail.  It's a
       non-fatal but noisier form of ASSERT_RETVALS.

       Prefix: <rv>

       Environment variable: POE_TRACE_RETVALS

   TRACE_SESSIONS
       TRACE_SESSIONS enables trace messages that pertain to session management.  Notice will be
       given when sessions are created or destroyed, and when the parent or child status of a
       session changes.

       Prefix: <ss>

       Environment variable: POE_TRACE_SESSIONS

   TRACE_SIGNALS
       TRACE_SIGNALS turns on (or off) traces in POE's signal handling subsystem.  Signal
       dispatch is one of POE's more complex parts, and the trace messages may help application
       developers understand signal propagation and timing.

       Prefix: <sg>

       Environment variable: POE_TRACE_SIGNALS

   USE_SIGCHLD
       Whether to use $SIG{CHLD} or to poll at an interval.

       This flag is enabled by default on Perl >= 5.8.1 as it has support for "safe signals".
       Please see perlipc for the gory details.

       You might want to disable this if you are running a version of Perl that is known to have
       bad signal handling, or if anything hijacks $SIG{CHLD}.  One module that is known to do
       this is Apache.

       Enabling this flag will cause child reaping to happen almost immediately, as opposed to
       once per "CHILD_POLLING_INTERVAL".

   CHILD_POLLING_INTERVAL
       The interval at which "wait" is called to determine if child processes need to be reaped
       and the "CHLD" signal emulated.

       Defaults to 1 second.

   USE_SIGNAL_PIPE
       The only safe way to handle signals is to implement a shared-nothing model.  POE builds a
       signal pipe that communicates between the signal handlers and the POE kernel loop in a
       safe and atomic manner.  The signal pipe is implemented with POE::Pipe::OneWay, using a
       "pipe" conduit on Unix.  Unfortunately, the signal pipe is not compatible with Windows and
       is not used on that platform.

       If you wish to revert to the previous unsafe signal behaviour, you must set
       "USE_SIGNAL_PIPE" to 0, or the environment variable "POE_USE_SIGNAL_PIPE".

   CATCH_EXCEPTIONS
       Whether or not POE should run event handler code in an eval { } and deliver the "DIE"
       signal on errors.

       See "Exception Handling".

ENVIRONMENT VARIABLES FOR TESTING

       POE's tests are lovely, dark and deep.  These environment variables allow testers to take
       roads less traveled.

   POE_DANTIC
       Windows and Perls built for it tend to be poor at doing UNIXy things, although they do
       try.  POE being very UNIXy itself must skip a lot of Windows tests.  The POE_DANTIC
       environment variable will, when true, enable all these tests.  It's intended to be used
       from time to time to see whether Windows has improved in some area.

SEE ALSO

       The SEE ALSO section in POE contains a table of contents covering the entire POE
       distribution.

BUGS

       ·   There is no mechanism in place to prevent external reference count names from
           clashing.

       ·   There is no mechanism to catch exceptions generated in another session.

AUTHORS & COPYRIGHTS

       Please see POE for more information about authors and contributors.