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

Comparing libev/ev.pod (file contents):
Revision 1.344 by root, Wed Nov 10 13:41:48 2010 UTC vs.
Revision 1.357 by root, Tue Jan 11 02:15:58 2011 UTC

…		…
299	}	299	}
300		300
301	...	301	...
302	ev_set_syserr_cb (fatal_error);	302	ev_set_syserr_cb (fatal_error);
303		303
		304	=item ev_feed_signal (int signum)
		305
		306	This function can be used to "simulate" a signal receive. It is completely
		307	safe to call this function at any time, from any context, including signal
		308	handlers or random threads.
		309
		310	Its main use is to customise signal handling in your process, especially
		311	in the presence of threads. For example, you could block signals
		312	by default in all threads (and specifying C<EVFLAG_NOSIGMASK> when
		313	creating any loops), and in one thread, use C<sigwait> or any other
		314	mechanism to wait for signals, then "deliver" them to libev by calling
		315	C<ev_feed_signal>.
		316
304	=back	317	=back
305		318
306	=head1 FUNCTIONS CONTROLLING EVENT LOOPS	319	=head1 FUNCTIONS CONTROLLING EVENT LOOPS
307		320
308	An event loop is described by a C<struct ev_loop *> (the C<struct> is	321	An event loop is described by a C<struct ev_loop *> (the C<struct> is
…		…
418	threads that are not interested in handling them.	431	threads that are not interested in handling them.
419		432
420	Signalfd will not be used by default as this changes your signal mask, and	433	Signalfd will not be used by default as this changes your signal mask, and
421	there are a lot of shoddy libraries and programs (glib's threadpool for	434	there are a lot of shoddy libraries and programs (glib's threadpool for
422	example) that can't properly initialise their signal masks.	435	example) that can't properly initialise their signal masks.
		436
		437	=item C<EVFLAG_NOSIGMASK>
		438
		439	When this flag is specified, then libev will avoid to modify the signal
		440	mask. Specifically, this means you ahve to make sure signals are unblocked
		441	when you want to receive them.
		442
		443	This behaviour is useful when you want to do your own signal handling, or
		444	want to handle signals only in specific threads and want to avoid libev
		445	unblocking the signals.
		446
		447	This flag's behaviour will become the default in future versions of libev.
423		448
424	=item C<EVBACKEND_SELECT> (value 1, portable select backend)	449	=item C<EVBACKEND_SELECT> (value 1, portable select backend)
425		450
426	This is your standard select(2) backend. Not I<completely> standard, as	451	This is your standard select(2) backend. Not I<completely> standard, as
427	libev tries to roll its own fd_set with no limits on the number of fds,	452	libev tries to roll its own fd_set with no limits on the number of fds,
…		…
481	employing an additional generation counter and comparing that against the	506	employing an additional generation counter and comparing that against the
482	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
483	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
484	perfectly fine with C<select> (files, many character devices...).	509	perfectly fine with C<select> (files, many character devices...).
485		510
486	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.
487		514
488	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
489	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
490	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
491	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
…		…
557	=item C<EVBACKEND_PORT> (value 32, Solaris 10)	584	=item C<EVBACKEND_PORT> (value 32, Solaris 10)
558		585
559	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,
560	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)).
561		588
562	Please note that Solaris event ports can deliver a lot of spurious
563	notifications, so you need to use non-blocking I/O or other means to avoid
564	blocking when no data (or space) is available.
565
566	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
567	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
568	descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend	591	descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
569	might perform better.	592	might perform better.
570		593
571	On the positive side, with the exception of the spurious readiness	594	On the positive side, this backend actually performed fully to
572	notifications, this backend actually performed fully to specification
573	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
574	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.
575		608
576	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
577	C<EVBACKEND_POLL>.	610	C<EVBACKEND_POLL>.
578		611
579	=item C<EVBACKEND_ALL>	612	=item C<EVBACKEND_ALL>
580		613
581	Try all backends (even potentially broken ones that wouldn't be tried	614	Try all backends (even potentially broken ones that wouldn't be tried
582	with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as	615	with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
583	C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.	616	C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.
584		617
585	It is definitely not recommended to use this flag.	618	It is definitely not recommended to use this flag, use whatever
		619	C<ev_recommended_backends ()> returns, or simply do not specify a backend
		620	at all.
		621
		622	=item C<EVBACKEND_MASK>
		623
		624	Not a backend at all, but a mask to select all backend bits from a
		625	C<flags> value, in case you want to mask out any backends from a flags
		626	value (e.g. when modifying the C<LIBEV_FLAGS> environment variable).
586		627
587	=back	628	=back
588		629
589	If one or more of the backend flags are or'ed into the flags value,	630	If one or more of the backend flags are or'ed into the flags value,
590	then only these backends will be tried (in the reverse order as listed	631	then only these backends will be tried (in the reverse order as listed
…		…
867	running when nothing else is active.	908	running when nothing else is active.
868		909
869	ev_signal exitsig;	910	ev_signal exitsig;
870	ev_signal_init (&exitsig, sig_cb, SIGINT);	911	ev_signal_init (&exitsig, sig_cb, SIGINT);
871	ev_signal_start (loop, &exitsig);	912	ev_signal_start (loop, &exitsig);
872	evf_unref (loop);	913	ev_unref (loop);
873		914
874	Example: For some weird reason, unregister the above signal handler again.	915	Example: For some weird reason, unregister the above signal handler again.
875		916
876	ev_ref (loop);	917	ev_ref (loop);
877	ev_signal_stop (loop, &exitsig);	918	ev_signal_stop (loop, &exitsig);
…		…
1316	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
1317	functions that do not need a watcher.	1358	functions that do not need a watcher.
1318		1359
1319	=back	1360	=back
1320		1361
1321	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER	1362	See also the L<ASSOCIATING CUSTOM DATA WITH A WATCHER> and L<BUILDING YOUR
1322		1363	OWN COMPOSITE WATCHERS> idioms.
1323	Each watcher has, by default, a member C<void *data> that you can change
1324	and read at any time: libev will completely ignore it. This can be used
1325	to associate arbitrary data with your watcher. If you need more data and
1326	don't want to allocate memory and store a pointer to it in that data
1327	member, you can also "subclass" the watcher type and provide your own
1328	data:
1329
1330	struct my_io
1331	{
1332	ev_io io;
1333	int otherfd;
1334	void *somedata;
1335	struct whatever *mostinteresting;
1336	};
1337
1338	...
1339	struct my_io w;
1340	ev_io_init (&w.io, my_cb, fd, EV_READ);
1341
1342	And since your callback will be called with a pointer to the watcher, you
1343	can cast it back to your own type:
1344
1345	static void my_cb (struct ev_loop loop, ev_io w_, int revents)
1346	{
1347	struct my_io w = (struct my_io )w_;
1348	...
1349	}
1350
1351	More interesting and less C-conformant ways of casting your callback type
1352	instead have been omitted.
1353
1354	Another common scenario is to use some data structure with multiple
1355	embedded watchers:
1356
1357	struct my_biggy
1358	{
1359	int some_data;
1360	ev_timer t1;
1361	ev_timer t2;
1362	}
1363
1364	In this case getting the pointer to C<my_biggy> is a bit more
1365	complicated: Either you store the address of your C<my_biggy> struct
1366	in the C<data> member of the watcher (for woozies), or you need to use
1367	some pointer arithmetic using C<offsetof> inside your watchers (for real
1368	programmers):
1369
1370	#include <stddef.h>
1371
1372	static void
1373	t1_cb (EV_P_ ev_timer *w, int revents)
1374	{
1375	struct my_biggy big = (struct my_biggy *)
1376	(((char *)w) - offsetof (struct my_biggy, t1));
1377	}
1378
1379	static void
1380	t2_cb (EV_P_ ev_timer *w, int revents)
1381	{
1382	struct my_biggy big = (struct my_biggy *)
1383	(((char *)w) - offsetof (struct my_biggy, t2));
1384	}
1385		1364
1386	=head2 WATCHER STATES	1365	=head2 WATCHER STATES
1387		1366
1388	There are various watcher states mentioned throughout this manual -	1367	There are various watcher states mentioned throughout this manual -
1389	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
…		…
1575	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
1576	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
1577	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
1578	required if you know what you are doing).	1557	required if you know what you are doing).
1579		1558
1580	If you cannot use non-blocking mode, then force the use of a
1581	known-to-be-good backend (at the time of this writing, this includes only
1582	C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). The same applies to file
1583	descriptors for which non-blocking operation makes no sense (such as
1584	files) - libev doesn't guarantee any specific behaviour in that case.
1585
1586	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
1587	receive "spurious" readiness notifications, that is your callback might	1560	receive "spurious" readiness notifications, that is, your callback might
1588	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
1589	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
1590	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
1591	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
1592	it is best to always use non-blocking I/O: An extra C<read>(2) returning
1593	C<EAGAIN> is far preferable to a program hanging until some data arrives.	1565	preferable to a program hanging until some data arrives.
1594		1566
1595	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
1596	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
1597	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
1598	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
1599	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
1600	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
1601	indefinitely.	1573	indefinitely.
1602		1574
1603	But really, best use non-blocking mode.	1575	But really, best use non-blocking mode.
1604		1576
…		…
1632		1604
1633	There is no workaround possible except not registering events	1605	There is no workaround possible except not registering events
1634	for potentially C<dup ()>'ed file descriptors, or to resort to	1606	for potentially C<dup ()>'ed file descriptors, or to resort to
1635	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.	1607	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
1636		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 it's own, but in the case of files, there is no such thing: the disk
		1624	will not send data on it's 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
1637	=head3 The special problem of fork	1642	=head3 The special problem of fork
1638		1643
1639	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
1640	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
1641	it in the child.	1646	it in the child if you want to continue to use it in the child.
1642		1647
1643	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
1644	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
1645	enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or	1650	C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
1646	C<EVBACKEND_POLL>.
1647		1651
1648	=head3 The special problem of SIGPIPE	1652	=head3 The special problem of SIGPIPE
1649		1653
1650	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>:
1651	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
…		…
2319	I<has> to modify the signal mask, at least temporarily.	2323	I<has> to modify the signal mask, at least temporarily.
2320		2324
2321	So I can't stress this enough: I<If you do not reset your signal mask when	2325	So I can't stress this enough: I<If you do not reset your signal mask when
2322	you expect it to be empty, you have a race condition in your code>. This	2326	you expect it to be empty, you have a race condition in your code>. This
2323	is not a libev-specific thing, this is true for most event libraries.	2327	is not a libev-specific thing, this is true for most event libraries.
		2328
		2329	=head3 The special problem of threads signal handling
		2330
		2331	POSIX threads has problematic signal handling semantics, specifically,
		2332	a lot of functionality (sigfd, sigwait etc.) only really works if all
		2333	threads in a process block signals, which is hard to achieve.
		2334
		2335	When you want to use sigwait (or mix libev signal handling with your own
		2336	for the same signals), you can tackle this problem by globally blocking
		2337	all signals before creating any threads (or creating them with a fully set
		2338	sigprocmask) and also specifying the C<EVFLAG_NOSIGMASK> when creating
		2339	loops. Then designate one thread as "signal receiver thread" which handles
		2340	these signals. You can pass on any signals that libev might be interested
		2341	in by calling C<ev_feed_signal>.
2324		2342
2325	=head3 Watcher-Specific Functions and Data Members	2343	=head3 Watcher-Specific Functions and Data Members
2326		2344
2327	=over 4	2345	=over 4
2328		2346
…		…
3175	it by calling C<ev_async_send>, which is thread- and signal safe.	3193	it by calling C<ev_async_send>, which is thread- and signal safe.
3176		3194
3177	This functionality is very similar to C<ev_signal> watchers, as signals,	3195	This functionality is very similar to C<ev_signal> watchers, as signals,
3178	too, are asynchronous in nature, and signals, too, will be compressed	3196	too, are asynchronous in nature, and signals, too, will be compressed
3179	(i.e. the number of callback invocations may be less than the number of	3197	(i.e. the number of callback invocations may be less than the number of
3180	C<ev_async_sent> calls).	3198	C<ev_async_sent> calls). In fact, you could use signal watchers as a kind
		3199	of "global async watchers" by using a watcher on an otherwise unused
		3200	signal, and C<ev_feed_signal> to signal this watcher from another thread,
		3201	even without knowing which loop owns the signal.
3181		3202
3182	Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not	3203	Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not
3183	just the default loop.	3204	just the default loop.
3184		3205
3185	=head3 Queueing	3206	=head3 Queueing
…		…
3361	Feed an event on the given fd, as if a file descriptor backend detected	3382	Feed an event on the given fd, as if a file descriptor backend detected
3362	the given events it.	3383	the given events it.
3363		3384
3364	=item ev_feed_signal_event (loop, int signum)	3385	=item ev_feed_signal_event (loop, int signum)
3365		3386
3366	Feed an event as if the given signal occurred (C<loop> must be the default	3387	Feed an event as if the given signal occurred. See also C<ev_feed_signal>,
3367	loop!).	3388	which is async-safe.
3368		3389
3369	=back	3390	=back
		3391
		3392
		3393	=head1 COMMON OR USEFUL IDIOMS (OR BOTH)
		3394
		3395	This section explains some common idioms that are not immediately
		3396	obvious. Note that examples are sprinkled over the whole manual, and this
		3397	section only contains stuff that wouldn't fit anywhere else.
		3398
		3399	=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
		3400
		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
		3468
		3469	Often (especially in GUI toolkits) there are places where you have
		3470	I<modal> interaction, which is most easily implemented by recursively
		3471	invoking C<ev_run>.
		3472
		3473	This brings the problem of exiting - a callback might want to finish the
		3474	main C<ev_run> call, but not the nested one (e.g. user clicked "Quit", but
		3475	a modal "Are you sure?" dialog is still waiting), or just the nested one
		3476	and not the main one (e.g. user clocked "Ok" in a modal dialog), or some
		3477	other combination: In these cases, C<ev_break> will not work alone.
		3478
		3479	The solution is to maintain "break this loop" variable for each C<ev_run>
		3480	invocation, and use a loop around C<ev_run> until the condition is
		3481	triggered, using C<EVRUN_ONCE>:
		3482
		3483	// main loop
		3484	int exit_main_loop = 0;
		3485
		3486	while (!exit_main_loop)
		3487	ev_run (EV_DEFAULT_ EVRUN_ONCE);
		3488
		3489	// in a model watcher
		3490	int exit_nested_loop = 0;
		3491
		3492	while (!exit_nested_loop)
		3493	ev_run (EV_A_ EVRUN_ONCE);
		3494
		3495	To exit from any of these loops, just set the corresponding exit variable:
		3496
		3497	// exit modal loop
		3498	exit_nested_loop = 1;
		3499
		3500	// exit main program, after modal loop is finished
		3501	exit_main_loop = 1;
		3502
		3503	// exit both
		3504	exit_main_loop = exit_nested_loop = 1;
		3505
		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.
3370		3701
3371		3702
3372	=head1 LIBEVENT EMULATION	3703	=head1 LIBEVENT EMULATION
3373		3704
3374	Libev offers a compatibility emulation layer for libevent. It cannot	3705	Libev offers a compatibility emulation layer for libevent. It cannot
3375	emulate the internals of libevent, so here are some usage hints:	3706	emulate the internals of libevent, so here are some usage hints:
3376		3707
3377	=over 4	3708	=over 4
		3709
		3710	=item * Only the libevent-1.4.1-beta API is being emulated.
		3711
		3712	This was the newest libevent version available when libev was implemented,
		3713	and is still mostly unchanged in 2010.
3378		3714
3379	=item * Use it by including <event.h>, as usual.	3715	=item * Use it by including <event.h>, as usual.
3380		3716
3381	=item * The following members are fully supported: ev_base, ev_callback,	3717	=item * The following members are fully supported: ev_base, ev_callback,
3382	ev_arg, ev_fd, ev_res, ev_events.	3718	ev_arg, ev_fd, ev_res, ev_events.
…		…
3417	Care has been taken to keep the overhead low. The only data member the C++	3753	Care has been taken to keep the overhead low. The only data member the C++
3418	classes add (compared to plain C-style watchers) is the event loop pointer	3754	classes add (compared to plain C-style watchers) is the event loop pointer
3419	that the watcher is associated with (or no additional members at all if	3755	that the watcher is associated with (or no additional members at all if
3420	you disable C<EV_MULTIPLICITY> when embedding libev).	3756	you disable C<EV_MULTIPLICITY> when embedding libev).
3421		3757
3422	Currently, functions, and static and non-static member functions can be	3758	Currently, functions, static and non-static member functions and classes
3423	used as callbacks. Other types should be easy to add as long as they only	3759	with C<operator ()> can be used as callbacks. Other types should be easy
3424	need one additional pointer for context. If you need support for other	3760	to add as long as they only need one additional pointer for context. If
3425	types of functors please contact the author (preferably after implementing	3761	you need support for other types of functors please contact the author
3426	it).	3762	(preferably after implementing it).
3427		3763
3428	Here is a list of things available in the C<ev> namespace:	3764	Here is a list of things available in the C<ev> namespace:
3429		3765
3430	=over 4	3766	=over 4
3431		3767
…		…
4299	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:
4300		4636
4301	#include "ev_cpp.h"	4637	#include "ev_cpp.h"
4302	#include "ev.c"	4638	#include "ev.c"
4303		4639
4304	=head1 INTERACTION WITH OTHER PROGRAMS OR LIBRARIES	4640	=head1 INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT
4305		4641
4306	=head2 THREADS AND COROUTINES	4642	=head2 THREADS AND COROUTINES
4307		4643
4308	=head3 THREADS	4644	=head3 THREADS
4309		4645
…		…
4360	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
4361	watcher callback into the event loop interested in the signal.	4697	watcher callback into the event loop interested in the signal.
4362		4698
4363	=back	4699	=back
4364		4700
4365	=head4 THREAD LOCKING EXAMPLE	4701	See also L<THREAD LOCKING EXAMPLE>.
4366
4367	Here is a fictitious example of how to run an event loop in a different
4368	thread than where callbacks are being invoked and watchers are
4369	created/added/removed.
4370
4371	For a real-world example, see the C<EV::Loop::Async> perl module,
4372	which uses exactly this technique (which is suited for many high-level
4373	languages).
4374
4375	The example uses a pthread mutex to protect the loop data, a condition
4376	variable to wait for callback invocations, an async watcher to notify the
4377	event loop thread and an unspecified mechanism to wake up the main thread.
4378
4379	First, you need to associate some data with the event loop:
4380
4381	typedef struct {
4382	mutex_t lock; /* global loop lock */
4383	ev_async async_w;
4384	thread_t tid;
4385	cond_t invoke_cv;
4386	} userdata;
4387
4388	void prepare_loop (EV_P)
4389	{
4390	// for simplicity, we use a static userdata struct.
4391	static userdata u;
4392
4393	ev_async_init (&u->async_w, async_cb);
4394	ev_async_start (EV_A_ &u->async_w);
4395
4396	pthread_mutex_init (&u->lock, 0);
4397	pthread_cond_init (&u->invoke_cv, 0);
4398
4399	// now associate this with the loop
4400	ev_set_userdata (EV_A_ u);
4401	ev_set_invoke_pending_cb (EV_A_ l_invoke);
4402	ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
4403
4404	// then create the thread running ev_loop
4405	pthread_create (&u->tid, 0, l_run, EV_A);
4406	}
4407
4408	The callback for the C<ev_async> watcher does nothing: the watcher is used
4409	solely to wake up the event loop so it takes notice of any new watchers
4410	that might have been added:
4411
4412	static void
4413	async_cb (EV_P_ ev_async *w, int revents)
4414	{
4415	// just used for the side effects
4416	}
4417
4418	The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
4419	protecting the loop data, respectively.
4420
4421	static void
4422	l_release (EV_P)
4423	{
4424	userdata *u = ev_userdata (EV_A);
4425	pthread_mutex_unlock (&u->lock);
4426	}
4427
4428	static void
4429	l_acquire (EV_P)
4430	{
4431	userdata *u = ev_userdata (EV_A);
4432	pthread_mutex_lock (&u->lock);
4433	}
4434
4435	The event loop thread first acquires the mutex, and then jumps straight
4436	into C<ev_run>:
4437
4438	void *
4439	l_run (void *thr_arg)
4440	{
4441	struct ev_loop loop = (struct ev_loop )thr_arg;
4442
4443	l_acquire (EV_A);
4444	pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
4445	ev_run (EV_A_ 0);
4446	l_release (EV_A);
4447
4448	return 0;
4449	}
4450
4451	Instead of invoking all pending watchers, the C<l_invoke> callback will
4452	signal the main thread via some unspecified mechanism (signals? pipe
4453	writes? C<Async::Interrupt>?) and then waits until all pending watchers
4454	have been called (in a while loop because a) spurious wakeups are possible
4455	and b) skipping inter-thread-communication when there are no pending
4456	watchers is very beneficial):
4457
4458	static void
4459	l_invoke (EV_P)
4460	{
4461	userdata *u = ev_userdata (EV_A);
4462
4463	while (ev_pending_count (EV_A))
4464	{
4465	wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
4466	pthread_cond_wait (&u->invoke_cv, &u->lock);
4467	}
4468	}
4469
4470	Now, whenever the main thread gets told to invoke pending watchers, it
4471	will grab the lock, call C<ev_invoke_pending> and then signal the loop
4472	thread to continue:
4473
4474	static void
4475	real_invoke_pending (EV_P)
4476	{
4477	userdata *u = ev_userdata (EV_A);
4478
4479	pthread_mutex_lock (&u->lock);
4480	ev_invoke_pending (EV_A);
4481	pthread_cond_signal (&u->invoke_cv);
4482	pthread_mutex_unlock (&u->lock);
4483	}
4484
4485	Whenever you want to start/stop a watcher or do other modifications to an
4486	event loop, you will now have to lock:
4487
4488	ev_timer timeout_watcher;
4489	userdata *u = ev_userdata (EV_A);
4490
4491	ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
4492
4493	pthread_mutex_lock (&u->lock);
4494	ev_timer_start (EV_A_ &timeout_watcher);
4495	ev_async_send (EV_A_ &u->async_w);
4496	pthread_mutex_unlock (&u->lock);
4497
4498	Note that sending the C<ev_async> watcher is required because otherwise
4499	an event loop currently blocking in the kernel will have no knowledge
4500	about the newly added timer. By waking up the loop it will pick up any new
4501	watchers in the next event loop iteration.
4502		4702
4503	=head3 COROUTINES	4703	=head3 COROUTINES
4504		4704
4505	Libev is very accommodating to coroutines ("cooperative threads"):	4705	Libev is very accommodating to coroutines ("cooperative threads"):
4506	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 1.344 by root, Wed Nov 10 13:41:48 2010 UTC vs. Revision 1.357 by root, Tue Jan 11 02:15:58 2011 UTC

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Comparing libev/ev.pod (file contents):
Revision 1.344 by root, Wed Nov 10 13:41:48 2010 UTC vs.
Revision 1.357 by root, Tue Jan 11 02:15:58 2011 UTC