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

Comparing libev/ev.pod (file contents):
Revision 1.350 by sf-exg, Mon Jan 10 08:36:41 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
…		…
582	=item C<EVBACKEND_PORT> (value 32, Solaris 10)	584	=item C<EVBACKEND_PORT> (value 32, Solaris 10)
583		585
584	This uses the Solaris 10 event port mechanism. As with everything on Solaris,	586	This uses the Solaris 10 event port mechanism. As with everything on Solaris,
585	it's really slow, but it still scales very well (O(active_fds)).	587	it's really slow, but it still scales very well (O(active_fds)).
586		588
587	Please note that Solaris event ports can deliver a lot of spurious
588	notifications, so you need to use non-blocking I/O or other means to avoid
589	blocking when no data (or space) is available.
590
591	While this backend scales well, it requires one system call per active	589	While this backend scales well, it requires one system call per active
592	file descriptor per loop iteration. For small and medium numbers of file	590	file descriptor per loop iteration. For small and medium numbers of file
593	descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend	591	descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
594	might perform better.	592	might perform better.
595		593
596	On the positive side, with the exception of the spurious readiness	594	On the positive side, this backend actually performed fully to
597	notifications, this backend actually performed fully to specification
598	in all tests and is fully embeddable, which is a rare feat among the	595	specification in all tests and is fully embeddable, which is a rare feat
599	OS-specific backends (I vastly prefer correctness over speed hacks).	596	among the OS-specific backends (I vastly prefer correctness over speed
		597	hacks).
		598
		599	On the negative side, the interface is I<bizarre> - so bizarre that
		600	even sun itself gets it wrong in their code examples: The event polling
		601	function sometimes returning events to the caller even though an error
		602	occurred, but with no indication whether it has done so or not (yes, it's
		603	even documented that way) - deadly for edge-triggered interfaces where
		604	you absolutely have to know whether an event occurred or not because you
		605	have to re-arm the watcher.
		606
		607	Fortunately libev seems to be able to work around these idiocies.
600		608
601	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
602	C<EVBACKEND_POLL>.	610	C<EVBACKEND_POLL>.
603		611
604	=item C<EVBACKEND_ALL>	612	=item C<EVBACKEND_ALL>
…		…
1349	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
1350	functions that do not need a watcher.	1358	functions that do not need a watcher.
1351		1359
1352	=back	1360	=back
1353		1361
1354	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER	1362	See also the L<ASSOCIATING CUSTOM DATA WITH A WATCHER> and L<BUILDING YOUR
1355		1363	OWN COMPOSITE WATCHERS> idioms.
1356	Each watcher has, by default, a member C<void *data> that you can change
1357	and read at any time: libev will completely ignore it. This can be used
1358	to associate arbitrary data with your watcher. If you need more data and
1359	don't want to allocate memory and store a pointer to it in that data
1360	member, you can also "subclass" the watcher type and provide your own
1361	data:
1362
1363	struct my_io
1364	{
1365	ev_io io;
1366	int otherfd;
1367	void *somedata;
1368	struct whatever *mostinteresting;
1369	};
1370
1371	...
1372	struct my_io w;
1373	ev_io_init (&w.io, my_cb, fd, EV_READ);
1374
1375	And since your callback will be called with a pointer to the watcher, you
1376	can cast it back to your own type:
1377
1378	static void my_cb (struct ev_loop loop, ev_io w_, int revents)
1379	{
1380	struct my_io w = (struct my_io )w_;
1381	...
1382	}
1383
1384	More interesting and less C-conformant ways of casting your callback type
1385	instead have been omitted.
1386
1387	Another common scenario is to use some data structure with multiple
1388	embedded watchers:
1389
1390	struct my_biggy
1391	{
1392	int some_data;
1393	ev_timer t1;
1394	ev_timer t2;
1395	}
1396
1397	In this case getting the pointer to C<my_biggy> is a bit more
1398	complicated: Either you store the address of your C<my_biggy> struct
1399	in the C<data> member of the watcher (for woozies), or you need to use
1400	some pointer arithmetic using C<offsetof> inside your watchers (for real
1401	programmers):
1402
1403	#include <stddef.h>
1404
1405	static void
1406	t1_cb (EV_P_ ev_timer *w, int revents)
1407	{
1408	struct my_biggy big = (struct my_biggy *)
1409	(((char *)w) - offsetof (struct my_biggy, t1));
1410	}
1411
1412	static void
1413	t2_cb (EV_P_ ev_timer *w, int revents)
1414	{
1415	struct my_biggy big = (struct my_biggy *)
1416	(((char *)w) - offsetof (struct my_biggy, t2));
1417	}
1418		1364
1419	=head2 WATCHER STATES	1365	=head2 WATCHER STATES
1420		1366
1421	There are various watcher states mentioned throughout this manual -	1367	There are various watcher states mentioned throughout this manual -
1422	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
…		…
1608	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
1609	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
1610	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
1611	required if you know what you are doing).	1557	required if you know what you are doing).
1612		1558
1613	If you cannot use non-blocking mode, then force the use of a
1614	known-to-be-good backend (at the time of this writing, this includes only
1615	C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). The same applies to file
1616	descriptors for which non-blocking operation makes no sense (such as
1617	files) - libev doesn't guarantee any specific behaviour in that case.
1618
1619	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
1620	receive "spurious" readiness notifications, that is your callback might	1560	receive "spurious" readiness notifications, that is, your callback might
1621	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
1622	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
1623	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
1624	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
1625	it is best to always use non-blocking I/O: An extra C<read>(2) returning
1626	C<EAGAIN> is far preferable to a program hanging until some data arrives.	1565	preferable to a program hanging until some data arrives.
1627		1566
1628	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
1629	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
1630	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
1631	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
1632	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
1633	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
1634	indefinitely.	1573	indefinitely.
1635		1574
1636	But really, best use non-blocking mode.	1575	But really, best use non-blocking mode.
1637		1576
…		…
1665		1604
1666	There is no workaround possible except not registering events	1605	There is no workaround possible except not registering events
1667	for potentially C<dup ()>'ed file descriptors, or to resort to	1606	for potentially C<dup ()>'ed file descriptors, or to resort to
1668	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.	1607	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
1669		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
1670	=head3 The special problem of fork	1642	=head3 The special problem of fork
1671		1643
1672	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
1673	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
1674	it in the child.	1646	it in the child if you want to continue to use it in the child.
1675		1647
1676	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
1677	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
1678	enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or	1650	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
1679	C<EVBACKEND_POLL>.
1680		1651
1681	=head3 The special problem of SIGPIPE	1652	=head3 The special problem of SIGPIPE
1682		1653
1683	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>:
1684	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
…		…
3423		3394
3424	This section explains some common idioms that are not immediately	3395	This section explains some common idioms that are not immediately
3425	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
3426	section only contains stuff that wouldn't fit anywhere else.	3397	section only contains stuff that wouldn't fit anywhere else.
3427		3398
3428	=over 4	3399	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
3429		3400
3430	=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
3431		3468
3432	Often (especially in GUI toolkits) there are places where you have	3469	Often (especially in GUI toolkits) there are places where you have
3433	I<modal> interaction, which is most easily implemented by recursively	3470	I<modal> interaction, which is most easily implemented by recursively
3434	invoking C<ev_run>.	3471	invoking C<ev_run>.
3435		3472
…		…
3464	exit_main_loop = 1;	3501	exit_main_loop = 1;
3465		3502
3466	// exit both	3503	// exit both
3467	exit_main_loop = exit_nested_loop = 1;	3504	exit_main_loop = exit_nested_loop = 1;
3468		3505
3469	=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.
3470		3701
3471		3702
3472	=head1 LIBEVENT EMULATION	3703	=head1 LIBEVENT EMULATION
3473		3704
3474	Libev offers a compatibility emulation layer for libevent. It cannot	3705	Libev offers a compatibility emulation layer for libevent. It cannot
…		…
4404	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:
4405		4636
4406	#include "ev_cpp.h"	4637	#include "ev_cpp.h"
4407	#include "ev.c"	4638	#include "ev.c"
4408		4639
4409	=head1 INTERACTION WITH OTHER PROGRAMS OR LIBRARIES	4640	=head1 INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT
4410		4641
4411	=head2 THREADS AND COROUTINES	4642	=head2 THREADS AND COROUTINES
4412		4643
4413	=head3 THREADS	4644	=head3 THREADS
4414		4645
…		…
4465	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
4466	watcher callback into the event loop interested in the signal.	4697	watcher callback into the event loop interested in the signal.
4467		4698
4468	=back	4699	=back
4469		4700
4470	=head4 THREAD LOCKING EXAMPLE	4701	See also L<THREAD LOCKING EXAMPLE>.
4471
4472	Here is a fictitious example of how to run an event loop in a different
4473	thread than where callbacks are being invoked and watchers are
4474	created/added/removed.
4475
4476	For a real-world example, see the C<EV::Loop::Async> perl module,
4477	which uses exactly this technique (which is suited for many high-level
4478	languages).
4479
4480	The example uses a pthread mutex to protect the loop data, a condition
4481	variable to wait for callback invocations, an async watcher to notify the
4482	event loop thread and an unspecified mechanism to wake up the main thread.
4483
4484	First, you need to associate some data with the event loop:
4485
4486	typedef struct {
4487	mutex_t lock; /* global loop lock */
4488	ev_async async_w;
4489	thread_t tid;
4490	cond_t invoke_cv;
4491	} userdata;
4492
4493	void prepare_loop (EV_P)
4494	{
4495	// for simplicity, we use a static userdata struct.
4496	static userdata u;
4497
4498	ev_async_init (&u->async_w, async_cb);
4499	ev_async_start (EV_A_ &u->async_w);
4500
4501	pthread_mutex_init (&u->lock, 0);
4502	pthread_cond_init (&u->invoke_cv, 0);
4503
4504	// now associate this with the loop
4505	ev_set_userdata (EV_A_ u);
4506	ev_set_invoke_pending_cb (EV_A_ l_invoke);
4507	ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
4508
4509	// then create the thread running ev_loop
4510	pthread_create (&u->tid, 0, l_run, EV_A);
4511	}
4512
4513	The callback for the C<ev_async> watcher does nothing: the watcher is used
4514	solely to wake up the event loop so it takes notice of any new watchers
4515	that might have been added:
4516
4517	static void
4518	async_cb (EV_P_ ev_async *w, int revents)
4519	{
4520	// just used for the side effects
4521	}
4522
4523	The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
4524	protecting the loop data, respectively.
4525
4526	static void
4527	l_release (EV_P)
4528	{
4529	userdata *u = ev_userdata (EV_A);
4530	pthread_mutex_unlock (&u->lock);
4531	}
4532
4533	static void
4534	l_acquire (EV_P)
4535	{
4536	userdata *u = ev_userdata (EV_A);
4537	pthread_mutex_lock (&u->lock);
4538	}
4539
4540	The event loop thread first acquires the mutex, and then jumps straight
4541	into C<ev_run>:
4542
4543	void *
4544	l_run (void *thr_arg)
4545	{
4546	struct ev_loop loop = (struct ev_loop )thr_arg;
4547
4548	l_acquire (EV_A);
4549	pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4550	ev_run (EV_A_ 0);
4551	l_release (EV_A);
4552
4553	return 0;
4554	}
4555
4556	Instead of invoking all pending watchers, the C<l_invoke> callback will
4557	signal the main thread via some unspecified mechanism (signals? pipe
4558	writes? C<Async::Interrupt>?) and then waits until all pending watchers
4559	have been called (in a while loop because a) spurious wakeups are possible
4560	and b) skipping inter-thread-communication when there are no pending
4561	watchers is very beneficial):
4562
4563	static void
4564	l_invoke (EV_P)
4565	{
4566	userdata *u = ev_userdata (EV_A);
4567
4568	while (ev_pending_count (EV_A))
4569	{
4570	wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4571	pthread_cond_wait (&u->invoke_cv, &u->lock);
4572	}
4573	}
4574
4575	Now, whenever the main thread gets told to invoke pending watchers, it
4576	will grab the lock, call C<ev_invoke_pending> and then signal the loop
4577	thread to continue:
4578
4579	static void
4580	real_invoke_pending (EV_P)
4581	{
4582	userdata *u = ev_userdata (EV_A);
4583
4584	pthread_mutex_lock (&u->lock);
4585	ev_invoke_pending (EV_A);
4586	pthread_cond_signal (&u->invoke_cv);
4587	pthread_mutex_unlock (&u->lock);
4588	}
4589
4590	Whenever you want to start/stop a watcher or do other modifications to an
4591	event loop, you will now have to lock:
4592
4593	ev_timer timeout_watcher;
4594	userdata *u = ev_userdata (EV_A);
4595
4596	ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4597
4598	pthread_mutex_lock (&u->lock);
4599	ev_timer_start (EV_A_ &timeout_watcher);
4600	ev_async_send (EV_A_ &u->async_w);
4601	pthread_mutex_unlock (&u->lock);
4602
4603	Note that sending the C<ev_async> watcher is required because otherwise
4604	an event loop currently blocking in the kernel will have no knowledge
4605	about the newly added timer. By waking up the loop it will pick up any new
4606	watchers in the next event loop iteration.
4607		4702
4608	=head3 COROUTINES	4703	=head3 COROUTINES
4609		4704
4610	Libev is very accommodating to coroutines ("cooperative threads"):	4705	Libev is very accommodating to coroutines ("cooperative threads"):
4611	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 1.350 by sf-exg, Mon Jan 10 08:36:41 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 1.350 by sf-exg, Mon Jan 10 08:36:41 2011 UTC vs.
Revision 1.358 by sf-exg, Tue Jan 11 08:43:48 2011 UTC