POE::Loop(3) User Contributed Perl Documentation POE::Loop(3)

POE::Loop - documentation for POE's event loop bridge interface

$kernel->loop_watch_filehandle($handle, $mode);
$kernel->loop_ignore_filehandle($handle, $mode);
$kernel->loop_pause_filehandle($handle, $mode);
$kernel->loop_resume_filehandle($handle, $mode);

POE::Loop is a virtual base class that defines a standard event loop interface. POE::Loop subclasses mix into POE::Kernel and implement the features needed to manage underlying event loops in a consistent fashion. This documentation covers the interface, which is shared by all subclasses.

As POE::Kernel loads, it searches through %INC for event loop modules. POE::Kernel loads the most appropriate POE::Loop subclass for the event loop it finds. The subclass slots its methods into POE::Kernel, completing the class at load time. POE and POE::Kernel provide ways to state the desired event loop in case the auto-detection makes a mistake or the developer prefers to be explicit. See "Using POE with Other Event Loops" in POE::Kernel for instructions on how to actually use POE with other event loops, event loop naming conventions, and other details.

POE::Loop subclasses exist for many of the event loops Perl supports: select(), IO::Poll, WxWindows, EV, Glib, Event, and so on. See CPAN for a full list.

As previously noted, POE::Loop subclasses provide additional methods to POE::Kernel and are not proper objects in themselves.

Each POE::Loop subclass first defines its own namespace and version within it. This way CPAN and other things can track its version. They then switch to the POE::Kernel package to define their additional methods.

POE::Loop is designed as a mix-in class because Perl imposed a performance penalty for method inheritance at the time the class was designed. This could be changed in the future, but it will require cascaded changes in several other classes.

Here is a skeleton of a POE::Loop subclass:

use strict;
# YourToolkit bridge for POE::Kernel;
package POE::Loop::YourToolkit;
use vars qw($VERSION);
$VERSION = '1.000'; # NOTE - Should be #.### (three decimal places)
package POE::Kernel;
# Define private lexical data here.
# Implement the POE::Loop interface here.
=head1 NAME
... documentation goes here ...

POE::Loop's public interface is divided into four parts: administrative methods, signal handler methods, time management methods, and filehandle watcher methods. Each group and its members will be described in detail shortly.

POE::Loop subclasses use lexical variables to keep track of things. Exact implementation is left up to the subclass' author. POE::Loop::Select keeps its bit vectors for select() calls in class-scoped (static) lexical variables. POE::Loop::Gtk tracks a single time watcher and multiple file watchers there.

Bridges often employ private methods as callbacks from their event loops. The Event, Gtk, and Tk bridges do this. Private callback names should begin with "_loop_" to avoid colliding with other methods.

Developers should look at existing bridges to get a feel for things. The "-m" flag for perldoc will show a module in its entirety.

perldoc -m POE::Loop::Select
perldoc -m POE::Loop::Gtk

These methods initialize and finalize an event loop, run the loop to process events, and halt it.


Initialize the event loop. Graphical toolkits especially need some sort of init() call or sequence to set up. For example, Tk requires a widget to be created before any events will be processed, and the program's user interface will be considered destroyed if that widget is closed.

sub loop_initialize {
  my $self = shift;
  $poe_main_window = Tk::MainWindow->new();
  die "could not create a main Tk window" unless defined $poe_main_window;

POE::Loop::Select initializes its select() bit vectors.

sub loop_initialize {
  @loop_vectors = ( '', '', '' );
  vec($loop_vectors[MODE_RD], 0, 1) = 0;
  vec($loop_vectors[MODE_WR], 0, 1) = 0;
  vec($loop_vectors[MODE_EX], 0, 1) = 0;


Finalize the event loop. Most event loops do not require anything here since they have already stopped by the time loop_finalize() is called. However, this is a good place to check that a bridge has not leaked memory or data. This example comes from POE::Loop::Event.

sub loop_finalize {
  my $self = shift;
  foreach my $fd (0..$#fileno_watcher) {
    next unless defined $fileno_watcher[$fd];
    foreach my $mode (MODE_RD, MODE_WR, MODE_EX) {
        "Mode $mode watcher for fileno $fd is defined during loop finalize"
      ) if defined $fileno_watcher[$fd]->[$mode];


Wait for time to pass or new events to occur, and dispatch any events that become due. If the underlying event loop does this through callbacks, then loop_do_timeslice() will either provide minimal glue or do nothing.

For example, loop_do_timeslice() for POE::Loop::Select sets up and calls select(). If any files or other resources become active, it enqueues events for them. Finally, it triggers dispatch for any events are due.

On the other hand, the Gtk event loop handles all this, so loop_do_timeslice() is empty for the Gtk bridge.

A sample loop_do_timeslice() implementation is not presented here because it would either be quite large or empty. See each POE::Loop::IO_Poll or Select for large ones. Event and Gtk are empty.

The bridges for Poll and Select for large ones. The ones for Event and Gtk are empty, and Tk's (in POE::Loop::TkCommon) is rather small.


Run an event loop until POE has no more sessions to handle events. This method tends to be quite small, and it is often implemented in terms of loop_do_timeslice(). For example, POE::Loop::IO_Poll implements it:

sub loop_run {
  my $self = shift;
  while ($self->_data_ses_count()) {

This method is even more trivial when an event loop handles it. This is from the Gtk bridge:

sub loop_run {
  unless (defined $_watcher_timer) {
    $_watcher_timer = Gtk->idle_add(\&_loop_resume_timer);


loop_halt() does what it says: It halts POE's underlying event loop. It tends to be either trivial for external event loops or empty for ones that are implemented in the bridge itself (IO_Poll, Select).

For example, the loop_run() method in the Poll bridge exits when sessions have run out, so its loop_halt() method is empty:

sub loop_halt {
  # does nothing

Gtk, however, needs to be stopped because it does not know when POE is done.

sub loop_halt {

These methods enable and disable signal watchers. They are used by POE::Resource::Signals to manage an event loop's signal watchers.

Most event loops use Perl's %SIG to watch for signals. This is so common that POE::Loop::PerlSignals implements the interface on behalf of other subclasses.

loop_watch_signal SIGNAL_NAME

Watch for a given SIGNAL_NAME. SIGNAL_NAME is the version found in %SIG, which tends to be the operating signal's name with the leading "SIG" removed.

POE::Loop::PerlSignals' implementation adds callbacks to %SIG except for CHLD/CLD, which begins a waitpid() polling loop instead.

As of this writing, all of the POE::Loop subclasses register their signal handlers through POE::Loop::PerlSignals.

There are three types of signal handlers:

CHLD/CLD handlers, when managed by the bridges themselves, poll for exited children. POE::Kernel does most of this, but loop_watch_signal() still needs to start the process.

PIPE handlers. The PIPE signal event must be sent to the session that is active when the signal occurred.

Everything else. Signal events for everything else are sent to POE::Kernel, where they are distributed to every session.

The loop_watch_signal() methods tends to be very long, so an example is not presented here. The Event and Select bridges have good examples, though.

loop_ignore_signal SIGNAL_NAME

Stop watching SIGNAL_NAME. POE::Loop::PerlSignals does this by resetting the %SIG for the SIGNAL_NAME to a sane value.

$SIG{CHLD} is left alone so as to avoid interfering with system() and other things.

SIGPIPE is generally harmless since POE generates events for this condition. Therefore $SIG{PIPE} is set to "IGNORE" when it's not being handled.

All other signal handlers default to "DEFAULT" when not in use.

loop_attach_uidestroy WIDGET

POE, when used with a graphical toolkit, should shut down when the user interface is closed. loop_attach_uidestroy() is used to shut down POE when a particular WIDGET is destroyed.

The shutdown is done by firing a UIDESTROY signal when the WIDGET's closure or destruction callback is invoked. UIDESTROY guarantees the program will shut down by virtue of being terminal and non-maskable.

loop_attach_uidestroy() is only meaningful in POE::Loop subclasses that tie into user interfaces. All other subclasses leave the method empty.

Here's Gtk's:

sub loop_attach_uidestroy {
  my ($self, $window) = @_;
    delete_event => sub {
      if ($self->_data_ses_count()) {
          $self, $self,
          __FILE__, __LINE__, undef, monotime(), -__LINE__
      return 0;

These methods enable and disable a time watcher or alarm in the underlying event loop. POE only requires one, which is reused or re-created as necessary.

Most event loops trigger callbacks when time has passed. It is the bridge's responsibility to register and unregister a callback as needed. When invoked, the callback should dispatch events that have become due and possibly set up a new callback for the next event to be dispatched.

The time management methods may accept NEXT_EVENT_TIME. This is the time the next event will become due, in UNIX epoch time. NEXT_EVENT_TIME is a real number and may have sub-second accuracy. It is the bridge's responsibility to convert this value into something the underlying event loop requires.

loop_resume_time_watcher NEXT_EVENT_TIME

Resume an already active time watcher. It is used with loop_pause_time_watcher() to provide less expensive timer toggling for frequent use cases. As mentioned above, NEXT_EVENT_TIME is in UNIX epoch time and may have sub-second accuracy.

loop_resume_time_watcher() is used by bridges that set them watchers in the underlying event loop. For example, POE::Loop::Gtk implements it this way:

sub loop_resume_time_watcher {
  my ($self, $next_time) = @_;
  $next_time -= time();
  $next_time *= 1000;
  $next_time = 0 if $next_time < 0;
  $_watcher_timer = Gtk->timeout_add(
    $next_time, \&_loop_event_callback

This method is usually empty in bridges that implement their own event loops.

loop_reset_time_watcher NEXT_EVENT_TIME

Reset a time watcher, often by stopping or destroying an existing one and creating a new one in its place. It is often a wrapper for loop_resume_time_watcher() that first destroys an existing watcher. For example, POE::Loop::Gkt's implementation:

sub loop_reset_time_watcher {
  my ($self, $next_time) = @_;
  undef $_watcher_timer;


Pause a time watcher without destroying it, if the underlying event loop supports such a thing. POE::Loop::Event does support it:

sub loop_pause_time_watcher {
  $_watcher_timer or return;

These methods enable and disable file activity watchers. There are four methods: loop_watch_filehandle(), loop_ignore_filehandle(), loop_pause_filehandle(), and loop_resume_filehandle(). The "pause" and "resume" methods are lightweight versions of "ignore" and "watch", respectively.

All the methods take the same two parameters: a file HANDLE and a file access MODE. Modes may be MODE_RD, MODE_WR, or MODE_EX. These constants are defined by POE::Kernel and correspond to the semantics of POE::Kernel's select_read(), select_write(), and select_expedite() methods.

POE calls MODE_EX "expedited" because it often signals that a file is ready for out-of-band information. Not all event loops handle MODE_EX. For example, Tk:

sub loop_watch_filehandle {
  my ($self, $handle, $mode) = @_;
  my $fileno = fileno($handle);
  my $tk_mode;
  if ($mode == MODE_RD) {
    $tk_mode = 'readable';
  elsif ($mode == MODE_WR) {
    $tk_mode = 'writable';
  else {
    # The Tk documentation implies by omission that expedited
    # filehandles aren't, uh, handled.  This is part 1 of 2.
    confess "Tk does not support expedited filehandles";
  # ... rest omitted ....

loop_watch_filehandle FILE_HANDLE, IO_MODE

Watch a FILE_HANDLE for activity in a given IO_MODE. Depending on the underlying event loop, a watcher or callback will be registered for the FILE_HANDLE. Activity in the specified IO_MODE (read, write, or out of band) will trigger emission of the proper event in application space.

POE::Loop::Select sets the fileno()'s bit in the proper select() bit vector. It also keeps track of which file descriptors are active.

sub loop_watch_filehandle {
  my ($self, $handle, $mode) = @_;
  my $fileno = fileno($handle);
  vec($loop_vectors[$mode], $fileno, 1) = 1;
  $loop_filenos{$fileno} |= (1<<$mode);

loop_ignore_filehandle FILE_HANDLE, IO_MODE

Stop watching the FILE_HANDLE in a given IO_MODE. Stops (and possibly destroys) an event watcher corresponding to the FILE_HANDLE and IO_MODE.

POE::Loop::IO_Poll's loop_ignore_filehandle() manages descriptor/mode bits for its _poll() method here. It also performs some cleanup if a descriptor is no longer being watched after this ignore call.

sub loop_ignore_filehandle {
  my ($self, $handle, $mode) = @_;
  my $fileno = fileno($handle);
  my $type = mode_to_poll($mode);
  my $current = $poll_fd_masks{$fileno} || 0;
  my $new = $current & ~$type;
  if (TRACE_FILES) {
        "<fh> Ignore $fileno: " .
        ": Current mask: 0x%02X - removing 0x%02X = 0x%02X\n",
        $current, $type, $new
  if ($new) {
    $poll_fd_masks{$fileno} = $new;
  else {
    delete $poll_fd_masks{$fileno};

loop_pause_filehandle FILE_HANDLE, IO_MODE

This is a lightweight form of loop_ignore_filehandle(). It is used along with loop_resume_filehandle() to temporarily toggle a watcher's state for a FILE_HANDLE in a particular IO_MODE.

Some event loops, such as Event.pm, support their file watchers being disabled and re-enabled without the need to destroy and re-create the watcher objects.

sub loop_pause_filehandle {
  my ($self, $handle, $mode) = @_;
  my $fileno = fileno($handle);

By comparison, Event's loop_ignore_filehandle() method cancels and destroys the watcher object.

sub loop_ignore_filehandle {
  my ($self, $handle, $mode) = @_;
  my $fileno = fileno($handle);
  if (defined $fileno_watcher[$fileno]->[$mode]) {
    undef $fileno_watcher[$fileno]->[$mode];

Ignoring and re-creating watchers is relatively expensive, so POE::Kernel's select_pause_read() and select_resume_read() methods (and the corresponding ones for write and expedite) use the faster versions.

loop_resume_filehandle FILE_HANDLE, IO_MODE

This is a lightweight form of loop_watch_filehandle(). It is used along with loop_pause_filehandle() to temporarily toggle a watcher's state for a FILE_HANDLE in a particular IO_MODE.

This is a rehash of "Using POE with Other Event Loops" in POE::Kernel.

Firstly, if a POE::Loop subclass is manually loaded before POE::Kernel, then that will be used. End of story.

If one isn't, POE::Kernel searches for an external event loop module in %INC. For each module in %INC, corresponding POE::XS::Loop and POE::Loop subclasses are tried.

For example, if IO::Poll is loaded, POE::Kernel tries

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

This is relatively expensive, but it ensures that POE::Kernel can find new POE::Loop subclasses without defining them in a central registry.

POE::Loop::Select is the fallback event loop. It's loaded if no other event loop can be found in %INC.

It can't be repeated often enough that event loops must be loaded before POE::Kernel. Otherwise they will not be present in %INC, and POE::Kernel will not detect them.

POE, POE::Loop::Event, POE::Loop::Gtk, POE::Loop::IO_Poll, POE::Loop::Select, POE::Loop::Tk.

POE::Test::Loops is POE's event loop tests released as a separate, reusable distribution. POE::Loop authors are encouraged to use the tests for their own distributions.

None known.

Please see POE for more information about authors, contributors, and POE's licensing.

2024-07-13 perl v5.38.2