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Coro(3)		      User Contributed Perl Documentation	       Coro(3)

NAME
       Coro - the only real threads in perl

SYNOPSIS
	 use Coro;

	 async {
	    # some asynchronous	thread of execution
	    print "2\n";
	    cede; # yield back to main
	    print "4\n";
	 };
	 print "1\n";
	 cede; # yield to coro
	 print "3\n";
	 cede; # and again

	 # use locking
	 my $lock = new	Coro::Semaphore;
	 my $locked;

	 $lock->down;
	 $locked = 1;
	 $lock->up;

DESCRIPTION
       For a tutorial-style introduction, please read the Coro::Intro manpage.
       This manpage mainly contains reference information.

       This module collection manages continuations in general,	most often in
       the form	of cooperative threads (also called coros, or simply "coro" in
       the documentation). They	are similar to kernel threads but don't	(in
       general)	run in parallel	at the same time even on SMP machines. The
       specific	flavor of thread offered by this module	also guarantees	you
       that it will not	switch between threads unless necessary, at easily-
       identified points in your program, so locking and parallel access are
       rarely an issue,	making thread programming much safer and easier	than
       using other thread models.

       Unlike the so-called "Perl threads" (which are not actually real
       threads but only	the windows process emulation (see section of same
       name for	more details) ported to	UNIX, and as such act as processes),
       Coro provides a full shared address space, which	makes communication
       between threads very easy. And coro threads are fast, too: disabling
       the Windows process emulation code in your perl and using Coro can
       easily result in	a two to four times speed increase for your programs.
       A parallel matrix multiplication	benchmark (very	communication-
       intensive) runs over 300	times faster on	a single core than perls
       pseudo-threads on a quad	core using all four cores.

       Coro achieves that by supporting	multiple running interpreters that
       share data, which is especially useful to code pseudo-parallel
       processes and for event-based programming, such as multiple HTTP-GET
       requests	running	concurrently. See Coro::AnyEvent to learn more on how
       to integrate Coro into an event-based environment.

       In this module, a thread	is defined as "callchain + lexical variables +
       some package variables +	C stack), that is, a thread has	its own
       callchain, its own set of lexicals and its own set of perls most
       important global	variables (see Coro::State for more configuration and
       background info).

       See also	the "SEE ALSO" section at the end of this document - the Coro
       module family is	quite large.

CORO THREAD LIFE CYCLE
       During the long and exciting (or	not) life of a coro thread, it goes
       through a number	of states:

       1. Creation
	   The first thing in the life of a coro thread	is its creation	-
	   obviously. The typical way to create	a thread is to call the	"async
	   BLOCK" function:

	      async {
		 # thread code goes here
	      };

	   You can also	pass arguments,	which are put in @_:

	      async {
		 print $_[1]; #	prints 2
	      }	1, 2, 3;

	   This	creates	a new coro thread and puts it into the ready queue,
	   meaning it will run as soon as the CPU is free for it.

	   "async" will	return a Coro object - you can store this for future
	   reference or	ignore it - a thread that is running, ready to run or
	   waiting for some event is alive on its own.

	   Another way to create a thread is to	call the "new" constructor
	   with	a code-reference:

	      new Coro sub {
		 # thread code goes here
	      }, @optional_arguments;

	   This	is quite similar to calling "async", but the important
	   difference is that the new thread is	not put	into the ready queue,
	   so the thread will not run until somebody puts it there. "async"
	   is, therefore, identical to this sequence:

	      my $coro = new Coro sub {
		 # thread code goes here
	      };
	      $coro->ready;
	      return $coro;

       2. Startup
	   When	a new coro thread is created, only a copy of the code
	   reference and the arguments are stored, no extra memory for stacks
	   and so on is	allocated, keeping the coro thread in a	low-memory
	   state.

	   Only	when it	actually starts	executing will all the resources be
	   finally allocated.

	   The optional	arguments specified at coro creation are available in
	   @_, similar to function calls.

       3. Running / Blocking
	   A lot can happen after the coro thread has started running. Quite
	   usually, it will not	run to the end in one go (because you could
	   use a function instead), but	it will	give up	the CPU	regularly
	   because it waits for	external events.

	   As long as a	coro thread runs, its Coro object is available in the
	   global variable $Coro::current.

	   The low-level way to	give up	the CPU	is to call the scheduler,
	   which selects a new coro thread to run:

	      Coro::schedule;

	   Since running threads are not in the	ready queue, calling the
	   scheduler without doing anything else will block the	coro thread
	   forever - you need to arrange either	for the	coro to	put woken up
	   (readied) by	some other event or some other thread, or you can put
	   it into the ready queue before scheduling:

	      #	this is	exactly	what Coro::cede	does
	      $Coro::current->ready;
	      Coro::schedule;

	   All the higher-level	synchronisation	methods	(Coro::Semaphore,
	   Coro::rouse_*...) are actually implemented via "->ready" and
	   "Coro::schedule".

	   While the coro thread is running it also might get assigned a
	   C-level thread, or the C-level thread might be unassigned from it,
	   as the Coro runtime wishes. A C-level thread	needs to be assigned
	   when	your perl thread calls into some C-level function and that
	   function in turn calls perl and perl	then wants to switch
	   coroutines. This happens most often when you	run an event loop and
	   block in the	callback, or when perl itself calls some function such
	   as "AUTOLOAD" or methods via	the "tie" mechanism.

       4. Termination
	   Many	threads	actually terminate after some time. There are a	number
	   of ways to terminate	a coro thread, the simplest is returning from
	   the top-level code reference:

	      async {
		 # after returning from	here, the coro thread is terminated
	      };

	      async {
		 return	if 0.5 <  rand;	# terminate a little earlier, maybe
		 print "got a chance to	print this\n";
		 # or here
	      };

	   Any values returned from the	coroutine can be recovered using
	   "->join":

	      my $coro = async {
		 "hello, world\n" # return a string
	      };

	      my $hello_world =	$coro->join;

	      print $hello_world;

	   Another way to terminate is to call "Coro::terminate", which	at any
	   subroutine call nesting level:

	      async {
		 Coro::terminate "return value 1", "return value 2";
	      };

	   Yet another way is to "->cancel" (or	"->safe_cancel") the coro
	   thread from another thread:

	      my $coro = async {
		 exit 1;
	      };

	      $coro->cancel; # also accepts values for ->join to retrieve

	   Cancellation	can be dangerous - it's	a bit like calling "exit"
	   without actually exiting, and might leave C libraries and XS
	   modules in a	weird state. Unlike other thread implementations,
	   however, Coro is exceptionally safe with regards to cancellation,
	   as perl will	always be in a consistent state, and for those cases
	   where you want to do	truly marvellous things	with your coro while
	   it is being cancelled - that	is, make sure all cleanup code is
	   executed from the thread being cancelled - there is even a
	   "->safe_cancel" method.

	   So, cancelling a thread that	runs in	an XS event loop might not be
	   the best idea, but any other	combination that deals with perl only
	   (cancelling when a thread is	in a "tie" method or an	"AUTOLOAD" for
	   example) is safe.

	   Last	not least, a coro thread object	that isn't referenced is
	   "->cancel"'ed automatically - just like other objects in Perl. This
	   is not such a common	case, however -	a running thread is
	   referencedy by $Coro::current, a thread ready to run	is referenced
	   by the ready	queue, a thread	waiting	on a lock or semaphore is
	   referenced by being in some wait list and so	on. But	a thread that
	   isn't in any	of those queues	gets cancelled:

	      async {
		 schedule; # cede to other coros, don't	go into	the ready queue
	      };

	      cede;
	      #	now the	async above is destroyed, as it	is not referenced by anything.

	   A slightly embellished example might	make it	clearer:

	      async {
		 my $guard = Guard::guard { print "destroyed\n"	};
		 schedule while	1;
	      };

	      cede;

	   Superficially one might not expect any output - since the "async"
	   implements an endless loop, the $guard will not be cleaned up.
	   However, since the thread object returned by	"async"	is not stored
	   anywhere, the thread	is initially referenced	because	it is in the
	   ready queue,	when it	runs it	is referenced by $Coro::current, but
	   when	it calls "schedule", it	gets "cancel"ed	causing	the guard
	   object to be	destroyed (see the next	section), and printing its
	   message.

	   If this seems a bit drastic,	remember that this only	happens	when
	   nothing references the thread anymore, which	means there is no way
	   to further execute it, ever.	The only options at this point are
	   leaking the thread, or cleaning it up, which	brings us to...

       5. Cleanup
	   Threads will	allocate various resources. Most but not all will be
	   returned when a thread terminates, during clean-up.

	   Cleanup is quite similar to throwing	an uncaught exception: perl
	   will	work its way up	through	all subroutine calls and blocks. On
	   its way, it will release all	"my" variables,	undo all "local"'s and
	   free	any other resources truly local	to the thread.

	   So, a common	way to free resources is to keep them referenced only
	   by my variables:

	      async {
		 my $big_cache = new Cache ...;
	      };

	   If there are	no other references, then the $big_cache object	will
	   be freed when the thread terminates,	regardless of how it does so.

	   What	it does	"NOT" do is unlock any Coro::Semaphores	or similar
	   resources, but that's where the "guard" methods come	in handy:

	      my $sem =	new Coro::Semaphore;

	      async {
		 my $lock_guard	= $sem->guard;
		 # if we return, or die	or get cancelled, here,
		 # then	the semaphore will be "up"ed.
	      };

	   The "Guard::guard" function comes in	handy for any custom cleanup
	   you might want to do	(but you cannot	switch to other	coroutines
	   from	those code blocks):

	      async {
		 my $window = new Gtk2::Window "toplevel";
		 # The window will not be cleaned up automatically, even when $window
		 # gets	freed, so use a	guard to ensure	its destruction
		 # in case of an error:
		 my $window_guard = Guard::guard { $window->destroy };

		 # we are safe here
	      };

	   Last	not least, "local" can often be	handy, too, e.g. when
	   temporarily replacing the coro thread description:

	      sub myfunction {
		 local $Coro::current->{desc} =	"inside	myfunction(@_)";

		 # if we return	or die here, the description will be restored
	      }

       6. Viva La Zombie Muerte
	   Even	after a	thread has terminated and cleaned up its resources,
	   the Coro object still is there and stores the return	values of the
	   thread.

	   When	there are no other references, it will simply be cleaned up
	   and freed.

	   If there areany references, the Coro	object will stay around, and
	   you can call	"->join" as many times as you wish to retrieve the
	   result values:

	      async {
		 print "hi\n";
		 1
	      };

	      #	run the	async above, and free everything before	returning
	      #	from Coro::cede:
	      Coro::cede;

	      {
		 my $coro = async {
		    print "hi\n";
		    1
		 };

		 # run the async above,	and clean up, but do not free the coro
		 # object:
		 Coro::cede;

		 # optionally retrieve the result values
		 my @results = $coro->join;

		 # now $coro goes out of scope,	and presumably gets freed
	      };

GLOBAL VARIABLES
       $Coro::main
	   This	variable stores	the Coro object	that represents	the main
	   program. While you can "ready" it and do most other things you can
	   do to coro, it is mainly useful to compare again $Coro::current, to
	   see whether you are running in the main program or not.

       $Coro::current
	   The Coro object representing	the current coro (the last coro	that
	   the Coro scheduler switched to). The	initial	value is $Coro::main
	   (of course).

	   This	variable is strictly read-only.	You can	take copies of the
	   value stored	in it and use it as any	other Coro object, but you
	   must	not otherwise modify the variable itself.

       $Coro::idle
	   This	variable is mainly useful to integrate Coro into event loops.
	   It is usually better	to rely	on Coro::AnyEvent or Coro::EV, as this
	   is pretty low-level functionality.

	   This	variable stores	a Coro object that is put into the ready queue
	   when	there are no other ready threads (without invoking any ready
	   hooks).

	   The default implementation dies with	"FATAL:	deadlock detected.",
	   followed by a thread	listing, because the program has no other way
	   to continue.

	   This	hook is	overwritten by modules such as "Coro::EV" and
	   "Coro::AnyEvent" to wait on an external event that hopefully	wakes
	   up a	coro so	the scheduler can run it.

	   See Coro::EV	or Coro::AnyEvent for examples of using	this
	   technique.

SIMPLE CORO CREATION
       async { ... } [@args...]
	   Create a new	coro and return	its Coro object	(usually unused). The
	   coro	will be	put into the ready queue, so it	will start running
	   automatically on the	next scheduler run.

	   The first argument is a codeblock/closure that should be executed
	   in the coro.	When it	returns	argument returns the coro is
	   automatically terminated.

	   The remaining arguments are passed as arguments to the closure.

	   See the "Coro::State::new" constructor for info about the coro
	   environment in which	coro are executed.

	   Calling "exit" in a coro will do the	same as	calling	exit outside
	   the coro. Likewise, when the	coro dies, the program will exit, just
	   as it would in the main program.

	   If you do not want that, you	can provide a default "die" handler,
	   or simply avoid dieing (by use of "eval").

	   Example: Create a new coro that just	prints its arguments.

	      async {
		 print "@_\n";
	      }	1,2,3,4;

       async_pool { ...	} [@args...]
	   Similar to "async", but uses	a coro pool, so	you should not call
	   terminate or	join on	it (although you are allowed to), and you get
	   a coro that might have executed other code already (which can be
	   good	or bad :).

	   On the plus side, this function is about twice as fast as creating
	   (and	destroying) a completely new coro, so if you need a lot	of
	   generic coros in quick successsion, use "async_pool", not "async".

	   The code block is executed in an "eval" context and a warning will
	   be issued in	case of	an exception instead of	terminating the
	   program, as "async" does. As	the coro is being reused, stuff	like
	   "on_destroy"	will not work in the expected way, unless you call
	   terminate or	cancel,	which somehow defeats the purpose of pooling
	   (but	is fine	in the exceptional case).

	   The priority	will be	reset to 0 after each run, all "swap_sv" calls
	   will	be undone, tracing will	be disabled, the description will be
	   reset and the default output	filehandle gets	restored, so you can
	   change all these. Otherwise the coro	will be	re-used	"as-is": most
	   notably if you change other per-coro	global stuff such as $/	you
	   must	needs revert that change, which	is most	simply done by using
	   local as in:	"local $/".

	   The idle pool size is limited to 8 idle coros (this can be adjusted
	   by changing $Coro::POOL_SIZE), but there can	be as many non-idle
	   coros as required.

	   If you are concerned	about pooled coros growing a lot because a
	   single "async_pool" used a lot of stackspace	you can	e.g.
	   "async_pool { terminate }" once per second or so to slowly
	   replenish the pool. In addition to that, when the stacks used by a
	   handler grows larger	than 32kb (adjustable via $Coro::POOL_RSS) it
	   will	also be	destroyed.

STATIC METHODS
       Static methods are actually functions that implicitly operate on	the
       current coro.

       schedule
	   Calls the scheduler.	The scheduler will find	the next coro that is
	   to be run from the ready queue and switches to it. The next coro to
	   be run is simply the	one with the highest priority that is longest
	   in its ready	queue. If there	is no coro ready, it will call the
	   $Coro::idle hook.

	   Please note that the	current	coro will not be put into the ready
	   queue, so calling this function usually means you will never	be
	   called again	unless something else (e.g. an event handler) calls
	   "->ready", thus waking you up.

	   This	makes "schedule" the generic method to use to block the
	   current coro	and wait for events: first you remember	the current
	   coro	in a variable, then arrange for	some callback of yours to call
	   "->ready" on	that once some event happens, and last you call
	   "schedule" to put yourself to sleep.	Note that a lot	of things can
	   wake	your coro up, so you need to check whether the event indeed
	   happened, e.g. by storing the status	in a variable.

	   See HOW TO WAIT FOR A CALLBACK, below, for some ways	to wait	for
	   callbacks.

       cede
	   "Cede" to other coros. This function	puts the current coro into the
	   ready queue and calls "schedule", which has the effect of giving up
	   the current "timeslice" to other coros of the same or higher
	   priority. Once your coro gets its turn again	it will	automatically
	   be resumed.

	   This	function is often called "yield" in other languages.

       Coro::cede_notself
	   Works like cede, but	is not exported	by default and will cede to
	   any coro, regardless	of priority. This is useful sometimes to
	   ensure progress is made.

       terminate [arg...]
	   Terminates the current coro with the	given status values (see
	   cancel). The	values will not	be copied, but referenced directly.

       Coro::on_enter BLOCK, Coro::on_leave BLOCK
	   These function install enter	and leave winders in the current
	   scope. The enter block will be executed when	on_enter is called and
	   whenever the	current	coro is	re-entered by the scheduler, while the
	   leave block is executed whenever the	current	coro is	blocked	by the
	   scheduler, and also when the	containing scope is exited (by
	   whatever means, be it exit, die, last etc.).

	   Neither invoking the	scheduler, nor exceptions, are allowed within
	   those BLOCKs. That means: do	not even think about calling "die"
	   without an eval, and	do not even think of entering the scheduler in
	   any way.

	   Since both BLOCKs are tied to the current scope, they will
	   automatically be removed when the current scope exits.

	   These functions implement the same concept as "dynamic-wind"	in
	   scheme does,	and are	useful when you	want to	localise some resource
	   to a	specific coro.

	   They	slow down thread switching considerably	for coros that use
	   them	(about 40% for a BLOCK with a single assignment, so thread
	   switching is	still reasonably fast if the handlers are fast).

	   These functions are best understood by an example: The following
	   function will change	the current timezone to
	   "Antarctica/South_Pole", which requires a call to "tzset", but by
	   using "on_enter" and	"on_leave", which remember/change the current
	   timezone and	restore	the previous value, respectively, the timezone
	   is only changed for the coro	that installed those handlers.

	      use POSIX	qw(tzset);

	      async {
		 my $old_tz; # store outside TZ	value here

		 Coro::on_enter	{
		    $old_tz = $ENV{TZ};	# remember the old value

		    $ENV{TZ} = "Antarctica/South_Pole";
		    tzset; # enable new	value
		 };

		 Coro::on_leave	{
		    $ENV{TZ} = $old_tz;
		    tzset; # restore old value
		 };

		 # at this place, the timezone is Antarctica/South_Pole,
		 # without disturbing the TZ of	any other coro.
	      };

	   This	can be used to localise	about any resource (locale, uid,
	   current working directory etc.) to a	block, despite the existence
	   of other coros.

	   Another interesting example implements time-sliced multitasking
	   using interval timers (this could obviously be optimised, but does
	   the job):

	      #	"timeslice" the	given block
	      sub timeslice(&) {
		 use Time::HiRes ();

		 Coro::on_enter	{
		    # on entering the thread, we set an	VTALRM handler to cede
		    $SIG{VTALRM} = sub { cede };
		    # and then start the interval timer
		    Time::HiRes::setitimer &Time::HiRes::ITIMER_VIRTUAL, 0.01, 0.01;
		 };
		 Coro::on_leave	{
		    # on leaving the thread, we	stop the interval timer	again
		    Time::HiRes::setitimer &Time::HiRes::ITIMER_VIRTUAL, 0, 0;
		 };

		 &{+shift};
	      }

	      #	use like this:
	      timeslice	{
		 # The following is an endless loop that would normally
		 # monopolise the process. Since it runs in a timesliced
		 # environment,	it will	regularly cede to other	threads.
		 while () { }
	      };

       killall
	   Kills/terminates/cancels all	coros except the currently running
	   one.

	   Note	that while this	will try to free some of the main interpreter
	   resources if	the calling coro isn't the main	coro, but one cannot
	   free	all of them, so	if a coro that is not the main coro calls this
	   function, there will	be some	one-time resource leak.

CORO OBJECT METHODS
       These are the methods you can call on coro objects (or to create	them).

       new Coro	\&sub [, @args...]
	   Create a new	coro and return	it. When the sub returns, the coro
	   automatically terminates as if "terminate" with the returned	values
	   were	called.	To make	the coro run you must first put	it into	the
	   ready queue by calling the ready method.

	   See "async" and "Coro::State::new" for additional info about	the
	   coro	environment.

       $success	= $coro->ready
	   Put the given coro into the end of its ready	queue (there is	one
	   queue for each priority) and	return true. If	the coro is already in
	   the ready queue, do nothing and return false.

	   This	ensures	that the scheduler will	resume this coro automatically
	   once	all the	coro of	higher priority	and all	coro of	the same
	   priority that were put into the ready queue earlier have been
	   resumed.

       $coro->suspend
	   Suspends the	specified coro.	A suspended coro works just like any
	   other coro, except that the scheduler will not select a suspended
	   coro	for execution.

	   Suspending a	coro can be useful when	you want to keep the coro from
	   running, but	you don't want to destroy it, or when you want to
	   temporarily freeze a	coro (e.g. for debugging) to resume it later.

	   A scenario for the former would be to suspend all (other) coros
	   after a fork	and keep them alive, so	their destructors aren't
	   called, but new coros can be	created.

       $coro->resume
	   If the specified coro was suspended,	it will	be resumed. Note that
	   when	the coro was in	the ready queue	when it	was suspended, it
	   might have been unreadied by	the scheduler, so an activation	might
	   have	been lost.

	   To avoid this, it is	best to	put a suspended	coro into the ready
	   queue unconditionally, as every synchronisation mechanism must
	   protect itself against spurious wakeups, and	the one	in the Coro
	   family certainly do that.

       $state->is_new
	   Returns true	iff this Coro object is	"new", i.e. has	never been run
	   yet.	Those states basically consist of only the code	reference to
	   call	and the	arguments, but consumes	very little other resources.
	   New states will automatically get assigned a	perl interpreter when
	   they	are transferred	to.

       $state->is_zombie
	   Returns true	iff the	Coro object has	been cancelled,	i.e.  its
	   resources freed because they	were "cancel"'ed, "terminate"'d,
	   "safe_cancel"'ed or simply went out of scope.

	   The name "zombie" stems from	UNIX culture, where a process that has
	   exited and only stores and exit status and no other resources is
	   called a "zombie".

       $is_ready = $coro->is_ready
	   Returns true	iff the	Coro object is in the ready queue. Unless the
	   Coro	object gets destroyed, it will eventually be scheduled by the
	   scheduler.

       $is_running = $coro->is_running
	   Returns true	iff the	Coro object is currently running. Only one
	   Coro	object can ever	be in the running state	(but it	currently is
	   possible to have multiple running Coro::States).

       $is_suspended = $coro->is_suspended
	   Returns true	iff this Coro object has been suspended. Suspended
	   Coros will not ever be scheduled.

       $coro->cancel ($arg...)
	   Terminate the given Coro thread and make it return the given
	   arguments as	status (default: an empty list). Never returns if the
	   Coro	is the current Coro.

	   This	is a rather brutal way to free a coro, with some limitations -
	   if the thread is inside a C callback	that doesn't expect to be
	   canceled, bad things	can happen, or if the cancelled	thread insists
	   on running complicated cleanup handlers that	rely on	its thread
	   context, things will	not work.

	   Any cleanup code being run (e.g. from "guard" blocks, destructors
	   and so on) will be run without a thread context, and	is not allowed
	   to switch to	other threads. A common	mistake	is to call "->cancel"
	   from	a destructor called by die'ing inside the thread to be
	   cancelled for example.

	   On the plus side, "->cancel"	will always clean up the thread, no
	   matter what.	 If your cleanup code is complex or you	want to	avoid
	   cancelling a	C-thread that doesn't know how to clean	up itself, it
	   can be better to "->throw" an exception, or use "->safe_cancel".

	   The arguments to "->cancel" are not copied, but instead will	be
	   referenced directly (e.g. if	you pass $var and after	the call
	   change that variable, then you might	change the return values
	   passed to e.g. "join", so don't do that).

	   The resources of the	Coro are usually freed (or destructed) before
	   this	call returns, but this can be delayed for an indefinite	amount
	   of time, as in some cases the manager thread	has to run first to
	   actually destruct the Coro object.

       $coro->safe_cancel ($arg...)
	   Works mostly	like "->cancel", but is	inherently "safer", and
	   consequently, can fail with an exception in cases the thread	is not
	   in a	cancellable state. Essentially,	"->safe_cancel"	is a
	   "->cancel" with extra checks	before canceling.

	   It works a bit like throwing	an exception that cannot be caught -
	   specifically, it will clean up the thread from within itself, so
	   all cleanup handlers	(e.g. "guard" blocks) are run with full	thread
	   context and can block if they wish. The downside is that there is
	   no guarantee	that the thread	can be cancelled when you call this
	   method, and therefore, it might fail. It is also considerably
	   slower than "cancel"	or "terminate".

	   A thread is in a safe-cancellable state if it either	has never been
	   run yet, has	already	been canceled/terminated or otherwise
	   destroyed, or has no	C context attached and is inside an SLF
	   function.

	   The first two states	are trivial - a	thread that hasnot started or
	   has already finished	is safe	to cancel.

	   The last state basically means that the thread isn't	currently
	   inside a perl callback called from some C function (usually via
	   some	XS modules) and	isn't currently	executing inside some C
	   function itself (via	Coro's XS API).

	   This	call returns true when it could	cancel the thread, or croaks
	   with	an error otherwise (i.e. it either returns true	or doesn't
	   return at all).

	   Why the weird interface? Well, there	are two	common models on how
	   and when to cancel things. In the first, you	have the expectation
	   that	your coro thread can be	cancelled when you want	to cancel it -
	   if the thread isn't cancellable, this would be a bug	somewhere, so
	   "->safe_cancel" croaks to notify of the bug.

	   In the second model you sometimes want to ask nicely	to cancel a
	   thread, but if it's not a good time,	well, then don't cancel. This
	   can be done relatively easy like this:

	      if (! eval { $coro->safe_cancel }) {
		 warn "unable to cancel	thread:	$@";
	      }

	   However, what you never should do is	first try to cancel "safely"
	   and if that fails, cancel the "hard"	way with "->cancel". That
	   makes no sense: either you rely on being able to execute cleanup
	   code	in your	thread context,	or you don't. If you do, then
	   "->safe_cancel" is the only way, and	if you don't, then "->cancel"
	   is always faster and	more direct.

       $coro->schedule_to
	   Puts	the current coro to sleep (like	"Coro::schedule"), but instead
	   of continuing with the next coro from the ready queue, always
	   switch to the given coro object (regardless of priority etc.). The
	   readyness state of that coro	isn't changed.

	   This	is an advanced method for special cases	- I'd love to hear
	   about any uses for this one.

       $coro->cede_to
	   Like	"schedule_to", but puts	the current coro into the ready	queue.
	   This	has the	effect of temporarily switching	to the given coro, and
	   continuing some time	later.

	   This	is an advanced method for special cases	- I'd love to hear
	   about any uses for this one.

       $coro->throw ([$scalar])
	   If $throw is	specified and defined, it will be thrown as an
	   exception inside the	coro at	the next convenient point in time.
	   Otherwise clears the	exception object.

	   Coro	will check for the exception each time a schedule-like-
	   function returns, i.e. after	each "schedule", "cede",
	   "Coro::Semaphore->down", "Coro::Handle->readable" and so on.	Most
	   of those functions (all that	are part of Coro itself) detect	this
	   case	and return early in case an exception is pending.

	   The exception object	will be	thrown "as is" with the	specified
	   scalar in $@, i.e. if it is a string, no line number	or newline
	   will	be appended (unlike with "die").

	   This	can be used as a softer	means than either "cancel" or
	   "safe_cancel	"to ask	a coro to end itself, although there is	no
	   guarantee that the exception	will lead to termination, and if the
	   exception isn't caught it might well	end the	whole program.

	   You might also think	of "throw" as being the	moral equivalent of
	   "kill"ing a coro with a signal (in this case, a scalar).

       $coro->join
	   Wait	until the coro terminates and return any values	given to the
	   "terminate" or "cancel" functions. "join" can be called
	   concurrently	from multiple threads, and all will be resumed and
	   given the status return once	the $coro terminates.

       $coro->on_destroy (\&cb)
	   Registers a callback	that is	called when this coro thread gets
	   destroyed, that is, after its resources have	been freed but before
	   it is joined. The callback gets passed the terminate/cancel
	   arguments, if any, and must not die,	under any circumstances.

	   There can be	any number of "on_destroy" callbacks per coro, and
	   there is currently no way to	remove a callback once added.

       $oldprio	= $coro->prio ($newprio)
	   Sets	(or gets, if the argument is missing) the priority of the coro
	   thread. Higher priority coro	get run	before lower priority coros.
	   Priorities are small	signed integers	(currently -4 .. +3), that you
	   can refer to	using PRIO_xxx constants (use the import tag :prio to
	   get then):

	      PRIO_MAX > PRIO_HIGH > PRIO_NORMAL > PRIO_LOW > PRIO_IDLE	> PRIO_MIN
		  3    >     1	   >	  0	 >    -1    >	 -3	>    -4

	      #	set priority to	HIGH
	      current->prio (PRIO_HIGH);

	   The idle coro thread	($Coro::idle) always has a lower priority than
	   any existing	coro.

	   Changing the	priority of the	current	coro will take effect
	   immediately,	but changing the priority of a coro in the ready queue
	   (but	not running) will only take effect after the next schedule (of
	   that	coro). This is a bug that will be fixed	in some	future
	   version.

       $newprio	= $coro->nice ($change)
	   Similar to "prio", but subtract the given value from	the priority
	   (i.e.  higher values	mean lower priority, just as in	UNIX's nice
	   command).

       $olddesc	= $coro->desc ($newdesc)
	   Sets	(or gets in case the argument is missing) the description for
	   this	coro thread. This is just a free-form string you can associate
	   with	a coro.

	   This	method simply sets the "$coro->{desc}" member to the given
	   string. You can modify this member directly if you wish, and	in
	   fact, this is often preferred to indicate major processing states
	   that	can then be seen for example in	a Coro::Debug session:

	      sub my_long_function {
		 local $Coro::current->{desc} =	"now in	my_long_function";
		 ...
		 $Coro::current->{desc}	= "my_long_function: phase 1";
		 ...
		 $Coro::current->{desc}	= "my_long_function: phase 2";
		 ...
	      }

GLOBAL FUNCTIONS
       Coro::nready
	   Returns the number of coro that are currently in the	ready state,
	   i.e.	that can be switched to	by calling "schedule" directory	or
	   indirectly. The value 0 means that the only runnable	coro is	the
	   currently running one, so "cede" would have no effect, and
	   "schedule" would cause a deadlock unless there is an	idle handler
	   that	wakes up some coro.

       my $guard = Coro::guard { ... }
	   This	function still exists, but is deprecated. Please use the
	   "Guard::guard" function instead.

       unblock_sub { ... }
	   This	utility	function takes a BLOCK or code reference and
	   "unblocks" it, returning a new coderef. Unblocking means that
	   calling the new coderef will	return immediately without blocking,
	   returning nothing, while the	original code ref will be called (with
	   parameters) from within another coro.

	   The reason this function exists is that many	event libraries	(such
	   as the venerable Event module) are not thread-safe (a weaker	form
	   of reentrancy). This	means you must not block within	event
	   callbacks, otherwise	you might suffer from crashes or worse.	The
	   only	event library currently	known that is safe to use without
	   "unblock_sub" is EV (but you	might still run	into deadlocks if all
	   event loops are blocked).

	   Coro	will try to catch you when you block in	the event loop
	   ("FATAL: $Coro::idle	blocked	itself"), but this is just best	effort
	   and only works when you do not run your own event loop.

	   This	function allows	your callbacks to block	by executing them in
	   another coro	where it is safe to block. One example where blocking
	   is handy is when you	use the	Coro::AIO functions to save results to
	   disk, for example.

	   In short: simply use	"unblock_sub { ... }" instead of "sub {	... }"
	   when	creating event callbacks that want to block.

	   If your handler does	not plan to block (e.g.	simply sends a message
	   to another coro, or puts some other coro into the ready queue),
	   there is no reason to use "unblock_sub".

	   Note	that you also need to use "unblock_sub"	for any	other
	   callbacks that are indirectly executed by any C-based event loop.
	   For example,	when you use a module that uses	AnyEvent (and you use
	   Coro::AnyEvent) and it provides callbacks that are the result of
	   some	event callback,	then you must not block	either,	or use
	   "unblock_sub".

       $cb = rouse_cb
	   Create and return a "rouse callback". That's	a code reference that,
	   when	called,	will remember a	copy of	its arguments and notify the
	   owner coro of the callback.

	   Only	the first invocation will store	agruments and signal any
	   waiter - further calls will effectively be ignored, but it is ok to
	   try.

	   Also	see the	next function.

       @args = rouse_wait [$cb]
	   Wait	for the	specified rouse	callback to be invoked (or if the
	   argument is missing,	use the	most recently created callback in the
	   current coro).

	   As soon as the callback is invoked (or when the callback was
	   invoked before "rouse_wait"), it will return	the arguments
	   originally passed to	the rouse callback. In scalar context, that
	   means you get the last argument, just as if "rouse_wait" had	a
	   "return ($a1, $a2, $a3...)"	statement at the end.

	   You are only	allowed	to wait	once for a given rouse callback.

	   See the section HOW TO WAIT FOR A CALLBACK for an actual usage
	   example.

	   As of Coro 6.57, you	can reliably wait for a	rouse callback in a
	   different thread than from where it was created.

HOW TO WAIT FOR	A CALLBACK
       It is very common for a coro to wait for	some callback to be called.
       This occurs naturally when you use coro in an otherwise event-based
       program,	or when	you use	event-based libraries.

       These typically register	a callback for some event, and call that
       callback	when the event occurred. In a coro, however, you typically
       want to just wait for the event,	simplyifying things.

       For example "AnyEvent->child" registers a callback to be	called when a
       specific	child has exited:

	  my $child_watcher = AnyEvent->child (pid => $pid, cb => sub {	... });

       But from	within a coro, you often just want to write this:

	  my $status = wait_for_child $pid;

       Coro offers two functions specifically designed to make this easy,
       "rouse_cb" and "rouse_wait".

       The first function, "rouse_cb", generates and returns a callback	that,
       when invoked, will save its arguments and notify	the coro that created
       the callback.

       The second function, "rouse_wait", waits	for the	callback to be called
       (by calling "schedule" to go to sleep) and returns the arguments
       originally passed to the	callback.

       Using these functions, it becomes easy to write the "wait_for_child"
       function	mentioned above:

	  sub wait_for_child($)	{
	     my	($pid) = @_;

	     my	$watcher = AnyEvent->child (pid	=> $pid, cb => rouse_cb);

	     my	($rpid,	$rstatus) = rouse_wait;
	     $rstatus
	  }

       In the case where "rouse_cb" and	"rouse_wait" are not flexible enough,
       you can roll your own, using "schedule" and "ready":

	  sub wait_for_child($)	{
	     my	($pid) = @_;

	     # store the current coro in $current,
	     # and provide result variables for	the closure passed to ->child
	     my	$current = $Coro::current;
	     my	($done,	$rstatus);

	     # pass a closure to ->child
	     my	$watcher = AnyEvent->child (pid	=> $pid, cb => sub {
		$rstatus = $_[1]; # remember rstatus
		$done =	1;	  # mark $rstatus as valid
		$current->ready;  # wake up the	waiting	thread
	     });

	     # wait until the closure has been called
	     schedule while !$done;

	     $rstatus
	  }

BUGS/LIMITATIONS
       fork with pthread backend
	   When	Coro is	compiled using the pthread backend (which isn't
	   recommended but required on many BSDs as their libcs	are completely
	   broken), then coro will not survive a fork. There is	no known
	   workaround except to	fix your libc and use a	saner backend.

       perl process emulation ("threads")
	   This	module is not perl-pseudo-thread-safe. You should only ever
	   use this module from	the first thread (this requirement might be
	   removed in the future to allow per-thread schedulers, but
	   Coro::State does not	yet allow this). I recommend disabling thread
	   support and using processes,	as having the windows process
	   emulation enabled under unix	roughly	halves perl performance, even
	   when	not used.

	   Attempts to use threads created in another emulated process will
	   crash ("cleanly", with a null pointer exception).

       coro switching is not signal safe
	   You must not	switch to another coro from within a signal handler
	   (only relevant with %SIG - most event libraries provide safe
	   signals), unless you	are sure you are not interrupting a Coro
	   function.

	   That	means you MUST NOT call	any function that might	"block"	the
	   current coro	- "cede", "schedule" "Coro::Semaphore->down" or
	   anything that calls those. Everything else, including calling
	   "ready", works.

WINDOWS	PROCESS	EMULATION
       A great many people seem	to be confused about ithreads (for example,
       Chip Salzenberg called me unintelligent,	incapable, stupid and
       gullible, while in the same mail	making rather confused statements
       about perl ithreads (for	example, that memory or	files would be
       shared),	showing	his lack of understanding of this area - if it is hard
       to understand for Chip, it is probably not obvious to everybody).

       What follows is an ultra-condensed version of my	talk about threads in
       scripting languages given on the	perl workshop 2009:

       The so-called "ithreads"	were originally	implemented for	two reasons:
       first, to (badly) emulate unix processes	on native win32	perls, and
       secondly, to replace the	older, real thread model ("5.005-threads").

       It does that by using threads instead of	OS processes. The difference
       between processes and threads is	that threads share memory (and other
       state, such as files) between threads within a single process, while
       processes do not	share anything (at least not semantically). That means
       that modifications done by one thread are seen by others, while
       modifications by	one process are	not seen by other processes.

       The "ithreads" work exactly like	that: when creating a new ithreads
       process,	all state is copied (memory is copied physically, files	and
       code is copied logically). Afterwards, it isolates all modifications.
       On UNIX,	the same behaviour can be achieved by using operating system
       processes, except that UNIX typically uses hardware built into the
       system to do this efficiently, while the	windows	process	emulation
       emulates	this hardware in software (rather efficiently, but of course
       it is still much	slower than dedicated hardware).

       As mentioned before, loading code, modifying code, modifying data
       structures and so on is only visible in the ithreads process doing the
       modification, not in other ithread processes within the same OS
       process.

       This is why "ithreads" do not implement threads for perl	at all,	only
       processes. What makes it	so bad is that on non-windows platforms, you
       can actually take advantage of custom hardware for this purpose (as
       evidenced by the	forks module, which gives you the (i-) threads API,
       just much faster).

       Sharing data is in the i-threads	model is done by transferring data
       structures between threads using	copying	semantics, which is very slow
       - shared	data simply does not exist. Benchmarks using i-threads which
       are communication-intensive show	extremely bad behaviour	with i-threads
       (in fact, so bad	that Coro, which cannot	take direct advantage of
       multiple	CPUs, is often orders of magnitude faster because it shares
       data using real threads,	refer to my talk for details).

       As summary, i-threads *use* threads to implement	processes, while the
       compatible forks	module *uses* processes	to emulate, uhm, processes.
       I-threads slow down every perl program when enabled, and	outside	of
       windows,	serve no (or little) practical purpose,	but disadvantages
       every single-threaded Perl program.

       This is the reason that I try to	avoid the name "ithreads", as it is
       misleading as it	implies	that it	implements some	kind of	thread model
       for perl, and prefer the	name "windows process emulation", which
       describes the actual use	and behaviour of it much better.

SEE ALSO
       Event-Loop integration: Coro::AnyEvent, Coro::EV, Coro::Event.

       Debugging: Coro::Debug.

       Support/Utility:	Coro::Specific,	Coro::Util.

       Locking and IPC:	Coro::Signal, Coro::Channel, Coro::Semaphore,
       Coro::SemaphoreSet, Coro::RWLock.

       I/O and Timers: Coro::Timer, Coro::Handle, Coro::Socket,	Coro::AIO.

       Compatibility with other	modules: Coro::LWP (but	see also
       AnyEvent::HTTP for a better-working alternative), Coro::BDB,
       Coro::Storable, Coro::Select.

       XS API: Coro::MakeMaker.

       Low level Configuration,	Thread Environment, Continuations:
       Coro::State.

AUTHOR/SUPPORT/CONTACT
	  Marc A. Lehmann <schmorp@schmorp.de>
	  http://software.schmorp.de/pkg/Coro.html

perl v5.32.0			  2020-07-29			       Coro(3)

NAME | SYNOPSIS | DESCRIPTION | CORO THREAD LIFE CYCLE | GLOBAL VARIABLES | SIMPLE CORO CREATION | STATIC METHODS | CORO OBJECT METHODS | GLOBAL FUNCTIONS | HOW TO WAIT FOR A CALLBACK | BUGS/LIMITATIONS | WINDOWS PROCESS EMULATION | SEE ALSO | AUTHOR/SUPPORT/CONTACT

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