[ViewVC] Diff of: cvs/libev/ev.pod

Comparing libev/ev.pod (file contents):
Revision 1.352 by root, Mon Jan 10 14:30:15 2011 UTC vs.
Revision 1.358 by sf-exg, Tue Jan 11 08:43:48 2011 UTC

…		…
506	employing an additional generation counter and comparing that against the	506	employing an additional generation counter and comparing that against the
507	events to filter out spurious ones, recreating the set when required. Last	507	events to filter out spurious ones, recreating the set when required. Last
508	not least, it also refuses to work with some file descriptors which work	508	not least, it also refuses to work with some file descriptors which work
509	perfectly fine with C<select> (files, many character devices...).	509	perfectly fine with C<select> (files, many character devices...).
510		510
511	Epoll is truly the train wreck analog among event poll mechanisms.	511	Epoll is truly the train wreck analog among event poll mechanisms,
		512	a frankenpoll, cobbled together in a hurry, no thought to design or
		513	interaction with others.
512		514
513	While stopping, setting and starting an I/O watcher in the same iteration	515	While stopping, setting and starting an I/O watcher in the same iteration
514	will result in some caching, there is still a system call per such	516	will result in some caching, there is still a system call per such
515	incident (because the same I<file descriptor> could point to a different	517	incident (because the same I<file descriptor> could point to a different
516	I<file description> now), so its best to avoid that. Also, C<dup ()>'ed	518	I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
…		…
595	hacks).	597	hacks).
596		598
597	On the negative side, the interface is I<bizarre> - so bizarre that	599	On the negative side, the interface is I<bizarre> - so bizarre that
598	even sun itself gets it wrong in their code examples: The event polling	600	even sun itself gets it wrong in their code examples: The event polling
599	function sometimes returning events to the caller even though an error	601	function sometimes returning events to the caller even though an error
600	occured, but with no indication whether it has done so or not (yes, it's	602	occurred, but with no indication whether it has done so or not (yes, it's
601	even documented that way) - deadly for edge-triggered interfaces where	603	even documented that way) - deadly for edge-triggered interfaces where
602	you absolutely have to know whether an event occured or not because you	604	you absolutely have to know whether an event occurred or not because you
603	have to re-arm the watcher.	605	have to re-arm the watcher.
604		606
605	Fortunately libev seems to be able to work around these idiocies.	607	Fortunately libev seems to be able to work around these idiocies.
606		608
607	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as	609	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
…		…
1355	See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related	1357	See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related
1356	functions that do not need a watcher.	1358	functions that do not need a watcher.
1357		1359
1358	=back	1360	=back
1359		1361
1360	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER	1362	See also the L<ASSOCIATING CUSTOM DATA WITH A WATCHER> and L<BUILDING YOUR
1361		1363	OWN COMPOSITE WATCHERS> idioms.
1362	Each watcher has, by default, a member C<void *data> that you can change
1363	and read at any time: libev will completely ignore it. This can be used
1364	to associate arbitrary data with your watcher. If you need more data and
1365	don't want to allocate memory and store a pointer to it in that data
1366	member, you can also "subclass" the watcher type and provide your own
1367	data:
1368
1369	struct my_io
1370	{
1371	ev_io io;
1372	int otherfd;
1373	void *somedata;
1374	struct whatever *mostinteresting;
1375	};
1376
1377	...
1378	struct my_io w;
1379	ev_io_init (&w.io, my_cb, fd, EV_READ);
1380
1381	And since your callback will be called with a pointer to the watcher, you
1382	can cast it back to your own type:
1383
1384	static void my_cb (struct ev_loop loop, ev_io w_, int revents)
1385	{
1386	struct my_io w = (struct my_io )w_;
1387	...
1388	}
1389
1390	More interesting and less C-conformant ways of casting your callback type
1391	instead have been omitted.
1392
1393	Another common scenario is to use some data structure with multiple
1394	embedded watchers:
1395
1396	struct my_biggy
1397	{
1398	int some_data;
1399	ev_timer t1;
1400	ev_timer t2;
1401	}
1402
1403	In this case getting the pointer to C<my_biggy> is a bit more
1404	complicated: Either you store the address of your C<my_biggy> struct
1405	in the C<data> member of the watcher (for woozies), or you need to use
1406	some pointer arithmetic using C<offsetof> inside your watchers (for real
1407	programmers):
1408
1409	#include <stddef.h>
1410
1411	static void
1412	t1_cb (EV_P_ ev_timer *w, int revents)
1413	{
1414	struct my_biggy big = (struct my_biggy *)
1415	(((char *)w) - offsetof (struct my_biggy, t1));
1416	}
1417
1418	static void
1419	t2_cb (EV_P_ ev_timer *w, int revents)
1420	{
1421	struct my_biggy big = (struct my_biggy *)
1422	(((char *)w) - offsetof (struct my_biggy, t2));
1423	}
1424		1364
1425	=head2 WATCHER STATES	1365	=head2 WATCHER STATES
1426		1366
1427	There are various watcher states mentioned throughout this manual -	1367	There are various watcher states mentioned throughout this manual -
1428	active, pending and so on. In this section these states and the rules to	1368	active, pending and so on. In this section these states and the rules to
…		…
1614	In general you can register as many read and/or write event watchers per	1554	In general you can register as many read and/or write event watchers per
1615	fd as you want (as long as you don't confuse yourself). Setting all file	1555	fd as you want (as long as you don't confuse yourself). Setting all file
1616	descriptors to non-blocking mode is also usually a good idea (but not	1556	descriptors to non-blocking mode is also usually a good idea (but not
1617	required if you know what you are doing).	1557	required if you know what you are doing).
1618		1558
1619	If you cannot use non-blocking mode, then force the use of a
1620	known-to-be-good backend (at the time of this writing, this includes only
1621	C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). The same applies to file
1622	descriptors for which non-blocking operation makes no sense (such as
1623	files) - libev doesn't guarantee any specific behaviour in that case.
1624
1625	Another thing you have to watch out for is that it is quite easy to	1559	Another thing you have to watch out for is that it is quite easy to
1626	receive "spurious" readiness notifications, that is your callback might	1560	receive "spurious" readiness notifications, that is, your callback might
1627	be called with C<EV_READ> but a subsequent C<read>(2) will actually block	1561	be called with C<EV_READ> but a subsequent C<read>(2) will actually block
1628	because there is no data. Not only are some backends known to create a	1562	because there is no data. It is very easy to get into this situation even
1629	lot of those (for example Solaris ports), it is very easy to get into	1563	with a relatively standard program structure. Thus it is best to always
1630	this situation even with a relatively standard program structure. Thus	1564	use non-blocking I/O: An extra C<read>(2) returning C<EAGAIN> is far
1631	it is best to always use non-blocking I/O: An extra C<read>(2) returning
1632	C<EAGAIN> is far preferable to a program hanging until some data arrives.	1565	preferable to a program hanging until some data arrives.
1633		1566
1634	If you cannot run the fd in non-blocking mode (for example you should	1567	If you cannot run the fd in non-blocking mode (for example you should
1635	not play around with an Xlib connection), then you have to separately	1568	not play around with an Xlib connection), then you have to separately
1636	re-test whether a file descriptor is really ready with a known-to-be good	1569	re-test whether a file descriptor is really ready with a known-to-be good
1637	interface such as poll (fortunately in our Xlib example, Xlib already	1570	interface such as poll (fortunately in the case of Xlib, it already does
1638	does this on its own, so its quite safe to use). Some people additionally	1571	this on its own, so its quite safe to use). Some people additionally
1639	use C<SIGALRM> and an interval timer, just to be sure you won't block	1572	use C<SIGALRM> and an interval timer, just to be sure you won't block
1640	indefinitely.	1573	indefinitely.
1641		1574
1642	But really, best use non-blocking mode.	1575	But really, best use non-blocking mode.
1643		1576
…		…
1671		1604
1672	There is no workaround possible except not registering events	1605	There is no workaround possible except not registering events
1673	for potentially C<dup ()>'ed file descriptors, or to resort to	1606	for potentially C<dup ()>'ed file descriptors, or to resort to
1674	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.	1607	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
1675		1608
		1609	=head3 The special problem of files
		1610
		1611	Many people try to use C<select> (or libev) on file descriptors
		1612	representing files, and expect it to become ready when their program
		1613	doesn't block on disk accesses (which can take a long time on their own).
		1614
		1615	However, this cannot ever work in the "expected" way - you get a readiness
		1616	notification as soon as the kernel knows whether and how much data is
		1617	there, and in the case of open files, that's always the case, so you
		1618	always get a readiness notification instantly, and your read (or possibly
		1619	write) will still block on the disk I/O.
		1620
		1621	Another way to view it is that in the case of sockets, pipes, character
		1622	devices and so on, there is another party (the sender) that delivers data
		1623	on its own, but in the case of files, there is no such thing: the disk
		1624	will not send data on its own, simply because it doesn't know what you
		1625	wish to read - you would first have to request some data.
		1626
		1627	Since files are typically not-so-well supported by advanced notification
		1628	mechanism, libev tries hard to emulate POSIX behaviour with respect
		1629	to files, even though you should not use it. The reason for this is
		1630	convenience: sometimes you want to watch STDIN or STDOUT, which is
		1631	usually a tty, often a pipe, but also sometimes files or special devices
		1632	(for example, C<epoll> on Linux works with F</dev/random> but not with
		1633	F</dev/urandom>), and even though the file might better be served with
		1634	asynchronous I/O instead of with non-blocking I/O, it is still useful when
		1635	it "just works" instead of freezing.
		1636
		1637	So avoid file descriptors pointing to files when you know it (e.g. use
		1638	libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or
		1639	when you rarely read from a file instead of from a socket, and want to
		1640	reuse the same code path.
		1641
1676	=head3 The special problem of fork	1642	=head3 The special problem of fork
1677		1643
1678	Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit	1644	Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
1679	useless behaviour. Libev fully supports fork, but needs to be told about	1645	useless behaviour. Libev fully supports fork, but needs to be told about
1680	it in the child.	1646	it in the child if you want to continue to use it in the child.
1681		1647
1682	To support fork in your programs, you either have to call	1648	To support fork in your child processes, you have to call C<ev_loop_fork
1683	C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,	1649	()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to
1684	enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or	1650	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
1685	C<EVBACKEND_POLL>.
1686		1651
1687	=head3 The special problem of SIGPIPE	1652	=head3 The special problem of SIGPIPE
1688		1653
1689	While not really specific to libev, it is easy to forget about C<SIGPIPE>:	1654	While not really specific to libev, it is easy to forget about C<SIGPIPE>:
1690	when writing to a pipe whose other end has been closed, your program gets	1655	when writing to a pipe whose other end has been closed, your program gets
…		…
3429		3394
3430	This section explains some common idioms that are not immediately	3395	This section explains some common idioms that are not immediately
3431	obvious. Note that examples are sprinkled over the whole manual, and this	3396	obvious. Note that examples are sprinkled over the whole manual, and this
3432	section only contains stuff that wouldn't fit anywhere else.	3397	section only contains stuff that wouldn't fit anywhere else.
3433		3398
3434	=over 4	3399	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
3435		3400
3436	=item Model/nested event loop invocations and exit conditions.	3401	Each watcher has, by default, a C<void *data> member that you can read
		3402	or modify at any time: libev will completely ignore it. This can be used
		3403	to associate arbitrary data with your watcher. If you need more data and
		3404	don't want to allocate memory separately and store a pointer to it in that
		3405	data member, you can also "subclass" the watcher type and provide your own
		3406	data:
		3407
		3408	struct my_io
		3409	{
		3410	ev_io io;
		3411	int otherfd;
		3412	void *somedata;
		3413	struct whatever *mostinteresting;
		3414	};
		3415
		3416	...
		3417	struct my_io w;
		3418	ev_io_init (&w.io, my_cb, fd, EV_READ);
		3419
		3420	And since your callback will be called with a pointer to the watcher, you
		3421	can cast it back to your own type:
		3422
		3423	static void my_cb (struct ev_loop loop, ev_io w_, int revents)
		3424	{
		3425	struct my_io w = (struct my_io )w_;
		3426	...
		3427	}
		3428
		3429	More interesting and less C-conformant ways of casting your callback
		3430	function type instead have been omitted.
		3431
		3432	=head2 BUILDING YOUR OWN COMPOSITE WATCHERS
		3433
		3434	Another common scenario is to use some data structure with multiple
		3435	embedded watchers, in effect creating your own watcher that combines
		3436	multiple libev event sources into one "super-watcher":
		3437
		3438	struct my_biggy
		3439	{
		3440	int some_data;
		3441	ev_timer t1;
		3442	ev_timer t2;
		3443	}
		3444
		3445	In this case getting the pointer to C<my_biggy> is a bit more
		3446	complicated: Either you store the address of your C<my_biggy> struct in
		3447	the C<data> member of the watcher (for woozies or C++ coders), or you need
		3448	to use some pointer arithmetic using C<offsetof> inside your watchers (for
		3449	real programmers):
		3450
		3451	#include <stddef.h>
		3452
		3453	static void
		3454	t1_cb (EV_P_ ev_timer *w, int revents)
		3455	{
		3456	struct my_biggy big = (struct my_biggy *)
		3457	(((char *)w) - offsetof (struct my_biggy, t1));
		3458	}
		3459
		3460	static void
		3461	t2_cb (EV_P_ ev_timer *w, int revents)
		3462	{
		3463	struct my_biggy big = (struct my_biggy *)
		3464	(((char *)w) - offsetof (struct my_biggy, t2));
		3465	}
		3466
		3467	=head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS
3437		3468
3438	Often (especially in GUI toolkits) there are places where you have	3469	Often (especially in GUI toolkits) there are places where you have
3439	I<modal> interaction, which is most easily implemented by recursively	3470	I<modal> interaction, which is most easily implemented by recursively
3440	invoking C<ev_run>.	3471	invoking C<ev_run>.
3441		3472
…		…
3470	exit_main_loop = 1;	3501	exit_main_loop = 1;
3471		3502
3472	// exit both	3503	// exit both
3473	exit_main_loop = exit_nested_loop = 1;	3504	exit_main_loop = exit_nested_loop = 1;
3474		3505
3475	=back	3506	=head2 THREAD LOCKING EXAMPLE
		3507
		3508	Here is a fictitious example of how to run an event loop in a different
		3509	thread than where callbacks are being invoked and watchers are
		3510	created/added/removed.
		3511
		3512	For a real-world example, see the C<EV::Loop::Async> perl module,
		3513	which uses exactly this technique (which is suited for many high-level
		3514	languages).
		3515
		3516	The example uses a pthread mutex to protect the loop data, a condition
		3517	variable to wait for callback invocations, an async watcher to notify the
		3518	event loop thread and an unspecified mechanism to wake up the main thread.
		3519
		3520	First, you need to associate some data with the event loop:
		3521
		3522	typedef struct {
		3523	mutex_t lock; /* global loop lock */
		3524	ev_async async_w;
		3525	thread_t tid;
		3526	cond_t invoke_cv;
		3527	} userdata;
		3528
		3529	void prepare_loop (EV_P)
		3530	{
		3531	// for simplicity, we use a static userdata struct.
		3532	static userdata u;
		3533
		3534	ev_async_init (&u->async_w, async_cb);
		3535	ev_async_start (EV_A_ &u->async_w);
		3536
		3537	pthread_mutex_init (&u->lock, 0);
		3538	pthread_cond_init (&u->invoke_cv, 0);
		3539
		3540	// now associate this with the loop
		3541	ev_set_userdata (EV_A_ u);
		3542	ev_set_invoke_pending_cb (EV_A_ l_invoke);
		3543	ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
		3544
		3545	// then create the thread running ev_loop
		3546	pthread_create (&u->tid, 0, l_run, EV_A);
		3547	}
		3548
		3549	The callback for the C<ev_async> watcher does nothing: the watcher is used
		3550	solely to wake up the event loop so it takes notice of any new watchers
		3551	that might have been added:
		3552
		3553	static void
		3554	async_cb (EV_P_ ev_async *w, int revents)
		3555	{
		3556	// just used for the side effects
		3557	}
		3558
		3559	The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
		3560	protecting the loop data, respectively.
		3561
		3562	static void
		3563	l_release (EV_P)
		3564	{
		3565	userdata *u = ev_userdata (EV_A);
		3566	pthread_mutex_unlock (&u->lock);
		3567	}
		3568
		3569	static void
		3570	l_acquire (EV_P)
		3571	{
		3572	userdata *u = ev_userdata (EV_A);
		3573	pthread_mutex_lock (&u->lock);
		3574	}
		3575
		3576	The event loop thread first acquires the mutex, and then jumps straight
		3577	into C<ev_run>:
		3578
		3579	void *
		3580	l_run (void *thr_arg)
		3581	{
		3582	struct ev_loop loop = (struct ev_loop )thr_arg;
		3583
		3584	l_acquire (EV_A);
		3585	pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
		3586	ev_run (EV_A_ 0);
		3587	l_release (EV_A);
		3588
		3589	return 0;
		3590	}
		3591
		3592	Instead of invoking all pending watchers, the C<l_invoke> callback will
		3593	signal the main thread via some unspecified mechanism (signals? pipe
		3594	writes? C<Async::Interrupt>?) and then waits until all pending watchers
		3595	have been called (in a while loop because a) spurious wakeups are possible
		3596	and b) skipping inter-thread-communication when there are no pending
		3597	watchers is very beneficial):
		3598
		3599	static void
		3600	l_invoke (EV_P)
		3601	{
		3602	userdata *u = ev_userdata (EV_A);
		3603
		3604	while (ev_pending_count (EV_A))
		3605	{
		3606	wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
		3607	pthread_cond_wait (&u->invoke_cv, &u->lock);
		3608	}
		3609	}
		3610
		3611	Now, whenever the main thread gets told to invoke pending watchers, it
		3612	will grab the lock, call C<ev_invoke_pending> and then signal the loop
		3613	thread to continue:
		3614
		3615	static void
		3616	real_invoke_pending (EV_P)
		3617	{
		3618	userdata *u = ev_userdata (EV_A);
		3619
		3620	pthread_mutex_lock (&u->lock);
		3621	ev_invoke_pending (EV_A);
		3622	pthread_cond_signal (&u->invoke_cv);
		3623	pthread_mutex_unlock (&u->lock);
		3624	}
		3625
		3626	Whenever you want to start/stop a watcher or do other modifications to an
		3627	event loop, you will now have to lock:
		3628
		3629	ev_timer timeout_watcher;
		3630	userdata *u = ev_userdata (EV_A);
		3631
		3632	ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
		3633
		3634	pthread_mutex_lock (&u->lock);
		3635	ev_timer_start (EV_A_ &timeout_watcher);
		3636	ev_async_send (EV_A_ &u->async_w);
		3637	pthread_mutex_unlock (&u->lock);
		3638
		3639	Note that sending the C<ev_async> watcher is required because otherwise
		3640	an event loop currently blocking in the kernel will have no knowledge
		3641	about the newly added timer. By waking up the loop it will pick up any new
		3642	watchers in the next event loop iteration.
		3643
		3644	=head2 THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS
		3645
		3646	While the overhead of a callback that e.g. schedules a thread is small, it
		3647	is still an overhead. If you embed libev, and your main usage is with some
		3648	kind of threads or coroutines, you might want to customise libev so that
		3649	doesn't need callbacks anymore.
		3650
		3651	Imagine you have coroutines that you can switch to using a function
		3652	C<switch_to (coro)>, that libev runs in a coroutine called C<libev_coro>
		3653	and that due to some magic, the currently active coroutine is stored in a
		3654	global called C<current_coro>. Then you can build your own "wait for libev
		3655	event" primitive by changing C<EV_CB_DECLARE> and C<EV_CB_INVOKE> (note
		3656	the differing C<;> conventions):
		3657
		3658	#define EV_CB_DECLARE(type) struct my_coro *cb;
		3659	#define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
		3660
		3661	That means instead of having a C callback function, you store the
		3662	coroutine to switch to in each watcher, and instead of having libev call
		3663	your callback, you instead have it switch to that coroutine.
		3664
		3665	A coroutine might now wait for an event with a function called
		3666	C<wait_for_event>. (the watcher needs to be started, as always, but it doesn't
		3667	matter when, or whether the watcher is active or not when this function is
		3668	called):
		3669
		3670	void
		3671	wait_for_event (ev_watcher *w)
		3672	{
		3673	ev_cb_set (w) = current_coro;
		3674	switch_to (libev_coro);
		3675	}
		3676
		3677	That basically suspends the coroutine inside C<wait_for_event> and
		3678	continues the libev coroutine, which, when appropriate, switches back to
		3679	this or any other coroutine. I am sure if you sue this your own :)
		3680
		3681	You can do similar tricks if you have, say, threads with an event queue -
		3682	instead of storing a coroutine, you store the queue object and instead of
		3683	switching to a coroutine, you push the watcher onto the queue and notify
		3684	any waiters.
		3685
		3686	To embed libev, see L<EMBEDDING>, but in short, it's easiest to create two
		3687	files, F<my_ev.h> and F<my_ev.c> that include the respective libev files:
		3688
		3689	// my_ev.h
		3690	#define EV_CB_DECLARE(type) struct my_coro *cb;
		3691	#define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb);
		3692	#include "../libev/ev.h"
		3693
		3694	// my_ev.c
		3695	#define EV_H "my_ev.h"
		3696	#include "../libev/ev.c"
		3697
		3698	And then use F<my_ev.h> when you would normally use F<ev.h>, and compile
		3699	F<my_ev.c> into your project. When properly specifying include paths, you
		3700	can even use F<ev.h> as header file name directly.
3476		3701
3477		3702
3478	=head1 LIBEVENT EMULATION	3703	=head1 LIBEVENT EMULATION
3479		3704
3480	Libev offers a compatibility emulation layer for libevent. It cannot	3705	Libev offers a compatibility emulation layer for libevent. It cannot
…		…
4410	And a F<ev_cpp.C> implementation file that contains libev proper and is compiled:	4635	And a F<ev_cpp.C> implementation file that contains libev proper and is compiled:
4411		4636
4412	#include "ev_cpp.h"	4637	#include "ev_cpp.h"
4413	#include "ev.c"	4638	#include "ev.c"
4414		4639
4415	=head1 INTERACTION WITH OTHER PROGRAMS OR LIBRARIES	4640	=head1 INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT
4416		4641
4417	=head2 THREADS AND COROUTINES	4642	=head2 THREADS AND COROUTINES
4418		4643
4419	=head3 THREADS	4644	=head3 THREADS
4420		4645
…		…
4471	default loop and triggering an C<ev_async> watcher from the default loop	4696	default loop and triggering an C<ev_async> watcher from the default loop
4472	watcher callback into the event loop interested in the signal.	4697	watcher callback into the event loop interested in the signal.
4473		4698
4474	=back	4699	=back
4475		4700
4476	=head4 THREAD LOCKING EXAMPLE	4701	See also L<THREAD LOCKING EXAMPLE>.
4477
4478	Here is a fictitious example of how to run an event loop in a different
4479	thread than where callbacks are being invoked and watchers are
4480	created/added/removed.
4481
4482	For a real-world example, see the C<EV::Loop::Async> perl module,
4483	which uses exactly this technique (which is suited for many high-level
4484	languages).
4485
4486	The example uses a pthread mutex to protect the loop data, a condition
4487	variable to wait for callback invocations, an async watcher to notify the
4488	event loop thread and an unspecified mechanism to wake up the main thread.
4489
4490	First, you need to associate some data with the event loop:
4491
4492	typedef struct {
4493	mutex_t lock; /* global loop lock */
4494	ev_async async_w;
4495	thread_t tid;
4496	cond_t invoke_cv;
4497	} userdata;
4498
4499	void prepare_loop (EV_P)
4500	{
4501	// for simplicity, we use a static userdata struct.
4502	static userdata u;
4503
4504	ev_async_init (&u->async_w, async_cb);
4505	ev_async_start (EV_A_ &u->async_w);
4506
4507	pthread_mutex_init (&u->lock, 0);
4508	pthread_cond_init (&u->invoke_cv, 0);
4509
4510	// now associate this with the loop
4511	ev_set_userdata (EV_A_ u);
4512	ev_set_invoke_pending_cb (EV_A_ l_invoke);
4513	ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
4514
4515	// then create the thread running ev_loop
4516	pthread_create (&u->tid, 0, l_run, EV_A);
4517	}
4518
4519	The callback for the C<ev_async> watcher does nothing: the watcher is used
4520	solely to wake up the event loop so it takes notice of any new watchers
4521	that might have been added:
4522
4523	static void
4524	async_cb (EV_P_ ev_async *w, int revents)
4525	{
4526	// just used for the side effects
4527	}
4528
4529	The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
4530	protecting the loop data, respectively.
4531
4532	static void
4533	l_release (EV_P)
4534	{
4535	userdata *u = ev_userdata (EV_A);
4536	pthread_mutex_unlock (&u->lock);
4537	}
4538
4539	static void
4540	l_acquire (EV_P)
4541	{
4542	userdata *u = ev_userdata (EV_A);
4543	pthread_mutex_lock (&u->lock);
4544	}
4545
4546	The event loop thread first acquires the mutex, and then jumps straight
4547	into C<ev_run>:
4548
4549	void *
4550	l_run (void *thr_arg)
4551	{
4552	struct ev_loop loop = (struct ev_loop )thr_arg;
4553
4554	l_acquire (EV_A);
4555	pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4556	ev_run (EV_A_ 0);
4557	l_release (EV_A);
4558
4559	return 0;
4560	}
4561
4562	Instead of invoking all pending watchers, the C<l_invoke> callback will
4563	signal the main thread via some unspecified mechanism (signals? pipe
4564	writes? C<Async::Interrupt>?) and then waits until all pending watchers
4565	have been called (in a while loop because a) spurious wakeups are possible
4566	and b) skipping inter-thread-communication when there are no pending
4567	watchers is very beneficial):
4568
4569	static void
4570	l_invoke (EV_P)
4571	{
4572	userdata *u = ev_userdata (EV_A);
4573
4574	while (ev_pending_count (EV_A))
4575	{
4576	wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4577	pthread_cond_wait (&u->invoke_cv, &u->lock);
4578	}
4579	}
4580
4581	Now, whenever the main thread gets told to invoke pending watchers, it
4582	will grab the lock, call C<ev_invoke_pending> and then signal the loop
4583	thread to continue:
4584
4585	static void
4586	real_invoke_pending (EV_P)
4587	{
4588	userdata *u = ev_userdata (EV_A);
4589
4590	pthread_mutex_lock (&u->lock);
4591	ev_invoke_pending (EV_A);
4592	pthread_cond_signal (&u->invoke_cv);
4593	pthread_mutex_unlock (&u->lock);
4594	}
4595
4596	Whenever you want to start/stop a watcher or do other modifications to an
4597	event loop, you will now have to lock:
4598
4599	ev_timer timeout_watcher;
4600	userdata *u = ev_userdata (EV_A);
4601
4602	ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4603
4604	pthread_mutex_lock (&u->lock);
4605	ev_timer_start (EV_A_ &timeout_watcher);
4606	ev_async_send (EV_A_ &u->async_w);
4607	pthread_mutex_unlock (&u->lock);
4608
4609	Note that sending the C<ev_async> watcher is required because otherwise
4610	an event loop currently blocking in the kernel will have no knowledge
4611	about the newly added timer. By waking up the loop it will pick up any new
4612	watchers in the next event loop iteration.
4613		4702
4614	=head3 COROUTINES	4703	=head3 COROUTINES
4615		4704
4616	Libev is very accommodating to coroutines ("cooperative threads"):	4705	Libev is very accommodating to coroutines ("cooperative threads"):
4617	libev fully supports nesting calls to its functions from different	4706	libev fully supports nesting calls to its functions from different

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Comparing libev/ev.pod (file contents): Revision 1.352 by root, Mon Jan 10 14:30:15 2011 UTC vs. Revision 1.358 by sf-exg, Tue Jan 11 08:43:48 2011 UTC

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Comparing libev/ev.pod (file contents):
Revision 1.352 by root, Mon Jan 10 14:30:15 2011 UTC vs.
Revision 1.358 by sf-exg, Tue Jan 11 08:43:48 2011 UTC