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

Comparing libev/ev.pod (file contents):
Revision 1.203 by root, Sun Oct 26 00:52:51 2008 UTC vs.
Revision 1.217 by root, Mon Nov 17 03:37:08 2008 UTC

…		…
8		8
9	=head2 EXAMPLE PROGRAM	9	=head2 EXAMPLE PROGRAM
10		10
11	// a single header file is required	11	// a single header file is required
12	#include <ev.h>	12	#include <ev.h>
		13
		14	#include <stdio.h> // for puts
13		15
14	// every watcher type has its own typedef'd struct	16	// every watcher type has its own typedef'd struct
15	// with the name ev_TYPE	17	// with the name ev_TYPE
16	ev_io stdin_watcher;	18	ev_io stdin_watcher;
17	ev_timer timeout_watcher;	19	ev_timer timeout_watcher;
…		…
298	If you don't know what event loop to use, use the one returned from this	300	If you don't know what event loop to use, use the one returned from this
299	function.	301	function.
300		302
301	Note that this function is I<not> thread-safe, so if you want to use it	303	Note that this function is I<not> thread-safe, so if you want to use it
302	from multiple threads, you have to lock (note also that this is unlikely,	304	from multiple threads, you have to lock (note also that this is unlikely,
303	as loops cannot bes hared easily between threads anyway).	305	as loops cannot be shared easily between threads anyway).
304		306
305	The default loop is the only loop that can handle C<ev_signal> and	307	The default loop is the only loop that can handle C<ev_signal> and
306	C<ev_child> watchers, and to do this, it always registers a handler	308	C<ev_child> watchers, and to do this, it always registers a handler
307	for C<SIGCHLD>. If this is a problem for your application you can either	309	for C<SIGCHLD>. If this is a problem for your application you can either
308	create a dynamic loop with C<ev_loop_new> that doesn't do that, or you	310	create a dynamic loop with C<ev_loop_new> that doesn't do that, or you
…		…
384	=item C<EVBACKEND_EPOLL> (value 4, Linux)	386	=item C<EVBACKEND_EPOLL> (value 4, Linux)
385		387
386	For few fds, this backend is a bit little slower than poll and select,	388	For few fds, this backend is a bit little slower than poll and select,
387	but it scales phenomenally better. While poll and select usually scale	389	but it scales phenomenally better. While poll and select usually scale
388	like O(total_fds) where n is the total number of fds (or the highest fd),	390	like O(total_fds) where n is the total number of fds (or the highest fd),
389	epoll scales either O(1) or O(active_fds). The epoll design has a number	391	epoll scales either O(1) or O(active_fds).
390	of shortcomings, such as silently dropping events in some hard-to-detect	392
391	cases and requiring a system call per fd change, no fork support and bad	393	The epoll mechanism deserves honorable mention as the most misdesigned
392	support for dup.	394	of the more advanced event mechanisms: mere annoyances include silently
		395	dropping file descriptors, requiring a system call per change per file
		396	descriptor (and unnecessary guessing of parameters), problems with dup and
		397	so on. The biggest issue is fork races, however - if a program forks then
		398	I<both> parent and child process have to recreate the epoll set, which can
		399	take considerable time (one syscall per file descriptor) and is of course
		400	hard to detect.
		401
		402	Epoll is also notoriously buggy - embedding epoll fds I<should> work, but
		403	of course I<doesn't>, and epoll just loves to report events for totally
		404	I<different> file descriptors (even already closed ones, so one cannot
		405	even remove them from the set) than registered in the set (especially
		406	on SMP systems). Libev tries to counter these spurious notifications by
		407	employing an additional generation counter and comparing that against the
		408	events to filter out spurious ones, recreating the set when required.
393		409
394	While stopping, setting and starting an I/O watcher in the same iteration	410	While stopping, setting and starting an I/O watcher in the same iteration
395	will result in some caching, there is still a system call per such incident	411	will result in some caching, there is still a system call per such
396	(because the fd could point to a different file description now), so its	412	incident (because the same I<file descriptor> could point to a different
397	best to avoid that. Also, C<dup ()>'ed file descriptors might not work	413	I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
398	very well if you register events for both fds.	414	file descriptors might not work very well if you register events for both
399		415	file descriptors.
400	Please note that epoll sometimes generates spurious notifications, so you
401	need to use non-blocking I/O or other means to avoid blocking when no data
402	(or space) is available.
403		416
404	Best performance from this backend is achieved by not unregistering all	417	Best performance from this backend is achieved by not unregistering all
405	watchers for a file descriptor until it has been closed, if possible,	418	watchers for a file descriptor until it has been closed, if possible,
406	i.e. keep at least one watcher active per fd at all times. Stopping and	419	i.e. keep at least one watcher active per fd at all times. Stopping and
407	starting a watcher (without re-setting it) also usually doesn't cause	420	starting a watcher (without re-setting it) also usually doesn't cause
408	extra overhead.	421	extra overhead. A fork can both result in spurious notifications as well
		422	as in libev having to destroy and recreate the epoll object, which can
		423	take considerable time and thus should be avoided.
		424
		425	All this means that, in practice, C<EVBACKEND_SELECT> can be as fast or
		426	faster than epoll for maybe up to a hundred file descriptors, depending on
		427	the usage. So sad.
409		428
410	While nominally embeddable in other event loops, this feature is broken in	429	While nominally embeddable in other event loops, this feature is broken in
411	all kernel versions tested so far.	430	all kernel versions tested so far.
412		431
413	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as	432	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
414	C<EVBACKEND_POLL>.	433	C<EVBACKEND_POLL>.
415		434
416	=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)	435	=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)
417		436
418	Kqueue deserves special mention, as at the time of this writing, it was	437	Kqueue deserves special mention, as at the time of this writing, it
419	broken on all BSDs except NetBSD (usually it doesn't work reliably with	438	was broken on all BSDs except NetBSD (usually it doesn't work reliably
420	anything but sockets and pipes, except on Darwin, where of course it's	439	with anything but sockets and pipes, except on Darwin, where of course
421	completely useless). For this reason it's not being "auto-detected" unless	440	it's completely useless). Unlike epoll, however, whose brokenness
422	you explicitly specify it in the flags (i.e. using C<EVBACKEND_KQUEUE>) or	441	is by design, these kqueue bugs can (and eventually will) be fixed
423	libev was compiled on a known-to-be-good (-enough) system like NetBSD.	442	without API changes to existing programs. For this reason it's not being
		443	"auto-detected" unless you explicitly specify it in the flags (i.e. using
		444	C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
		445	system like NetBSD.
424		446
425	You still can embed kqueue into a normal poll or select backend and use it	447	You still can embed kqueue into a normal poll or select backend and use it
426	only for sockets (after having made sure that sockets work with kqueue on	448	only for sockets (after having made sure that sockets work with kqueue on
427	the target platform). See C<ev_embed> watchers for more info.	449	the target platform). See C<ev_embed> watchers for more info.
428		450
429	It scales in the same way as the epoll backend, but the interface to the	451	It scales in the same way as the epoll backend, but the interface to the
430	kernel is more efficient (which says nothing about its actual speed, of	452	kernel is more efficient (which says nothing about its actual speed, of
431	course). While stopping, setting and starting an I/O watcher does never	453	course). While stopping, setting and starting an I/O watcher does never
432	cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to	454	cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to
433	two event changes per incident. Support for C<fork ()> is very bad and it	455	two event changes per incident. Support for C<fork ()> is very bad (but
434	drops fds silently in similarly hard-to-detect cases.	456	sane, unlike epoll) and it drops fds silently in similarly hard-to-detect
		457	cases
435		458
436	This backend usually performs well under most conditions.	459	This backend usually performs well under most conditions.
437		460
438	While nominally embeddable in other event loops, this doesn't work	461	While nominally embeddable in other event loops, this doesn't work
439	everywhere, so you might need to test for this. And since it is broken	462	everywhere, so you might need to test for this. And since it is broken
…		…
468	might perform better.	491	might perform better.
469		492
470	On the positive side, with the exception of the spurious readiness	493	On the positive side, with the exception of the spurious readiness
471	notifications, this backend actually performed fully to specification	494	notifications, this backend actually performed fully to specification
472	in all tests and is fully embeddable, which is a rare feat among the	495	in all tests and is fully embeddable, which is a rare feat among the
473	OS-specific backends.	496	OS-specific backends (I vastly prefer correctness over speed hacks).
474		497
475	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as	498	This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
476	C<EVBACKEND_POLL>.	499	C<EVBACKEND_POLL>.
477		500
478	=item C<EVBACKEND_ALL>	501	=item C<EVBACKEND_ALL>
…		…
635	the loop.	658	the loop.
636		659
637	A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if	660	A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
638	necessary) and will handle those and any already outstanding ones. It	661	necessary) and will handle those and any already outstanding ones. It
639	will block your process until at least one new event arrives (which could	662	will block your process until at least one new event arrives (which could
640	be an event internal to libev itself, so there is no guarentee that a	663	be an event internal to libev itself, so there is no guarantee that a
641	user-registered callback will be called), and will return after one	664	user-registered callback will be called), and will return after one
642	iteration of the loop.	665	iteration of the loop.
643		666
644	This is useful if you are waiting for some external event in conjunction	667	This is useful if you are waiting for some external event in conjunction
645	with something not expressible using other libev watchers (i.e. "roll your	668	with something not expressible using other libev watchers (i.e. "roll your
…		…
1398	else	1421	else
1399	{	1422	{
1400	// callback was invoked, but there was some activity, re-arm	1423	// callback was invoked, but there was some activity, re-arm
1401	// the watcher to fire in last_activity + 60, which is	1424	// the watcher to fire in last_activity + 60, which is
1402	// guaranteed to be in the future, so "again" is positive:	1425	// guaranteed to be in the future, so "again" is positive:
1403	w->again = timeout - now;	1426	w->repeat = timeout - now;
1404	ev_timer_again (EV_A_ w);	1427	ev_timer_again (EV_A_ w);
1405	}	1428	}
1406	}	1429	}
1407		1430
1408	To summarise the callback: first calculate the real timeout (defined	1431	To summarise the callback: first calculate the real timeout (defined
…		…
1910		1933
1911		1934
1912	=head2 C<ev_stat> - did the file attributes just change?	1935	=head2 C<ev_stat> - did the file attributes just change?
1913		1936
1914	This watches a file system path for attribute changes. That is, it calls	1937	This watches a file system path for attribute changes. That is, it calls
1915	C<stat> regularly (or when the OS says it changed) and sees if it changed	1938	C<stat> on that path in regular intervals (or when the OS says it changed)
1916	compared to the last time, invoking the callback if it did.	1939	and sees if it changed compared to the last time, invoking the callback if
		1940	it did.
1917		1941
1918	The path does not need to exist: changing from "path exists" to "path does	1942	The path does not need to exist: changing from "path exists" to "path does
1919	not exist" is a status change like any other. The condition "path does	1943	not exist" is a status change like any other. The condition "path does not
1920	not exist" is signified by the C<st_nlink> field being zero (which is	1944	exist" (or more correctly "path cannot be stat'ed") is signified by the
1921	otherwise always forced to be at least one) and all the other fields of	1945	C<st_nlink> field being zero (which is otherwise always forced to be at
1922	the stat buffer having unspecified contents.	1946	least one) and all the other fields of the stat buffer having unspecified
		1947	contents.
1923		1948
1924	The path I<should> be absolute and I<must not> end in a slash. If it is	1949	The path I<must not> end in a slash or contain special components such as
		1950	C<.> or C<..>. The path I<should> be absolute: If it is relative and
1925	relative and your working directory changes, the behaviour is undefined.	1951	your working directory changes, then the behaviour is undefined.
1926		1952
1927	Since there is no standard kernel interface to do this, the portable	1953	Since there is no portable change notification interface available, the
1928	implementation simply calls C<stat (2)> regularly on the path to see if	1954	portable implementation simply calls C<stat(2)> regularly on the path
1929	it changed somehow. You can specify a recommended polling interval for	1955	to see if it changed somehow. You can specify a recommended polling
1930	this case. If you specify a polling interval of C<0> (highly recommended!)	1956	interval for this case. If you specify a polling interval of C<0> (highly
1931	then a I<suitable, unspecified default> value will be used (which	1957	recommended!) then a I<suitable, unspecified default> value will be used
1932	you can expect to be around five seconds, although this might change	1958	(which you can expect to be around five seconds, although this might
1933	dynamically). Libev will also impose a minimum interval which is currently	1959	change dynamically). Libev will also impose a minimum interval which is
1934	around C<0.1>, but thats usually overkill.	1960	currently around C<0.1>, but that's usually overkill.
1935		1961
1936	This watcher type is not meant for massive numbers of stat watchers,	1962	This watcher type is not meant for massive numbers of stat watchers,
1937	as even with OS-supported change notifications, this can be	1963	as even with OS-supported change notifications, this can be
1938	resource-intensive.	1964	resource-intensive.
1939		1965
1940	At the time of this writing, the only OS-specific interface implemented	1966	At the time of this writing, the only OS-specific interface implemented
1941	is the Linux inotify interface (implementing kqueue support is left as	1967	is the Linux inotify interface (implementing kqueue support is left as an
1942	an exercise for the reader. Note, however, that the author sees no way	1968	exercise for the reader. Note, however, that the author sees no way of
1943	of implementing C<ev_stat> semantics with kqueue).	1969	implementing C<ev_stat> semantics with kqueue, except as a hint).
1944		1970
1945	=head3 ABI Issues (Largefile Support)	1971	=head3 ABI Issues (Largefile Support)
1946		1972
1947	Libev by default (unless the user overrides this) uses the default	1973	Libev by default (unless the user overrides this) uses the default
1948	compilation environment, which means that on systems with large file	1974	compilation environment, which means that on systems with large file
1949	support disabled by default, you get the 32 bit version of the stat	1975	support disabled by default, you get the 32 bit version of the stat
1950	structure. When using the library from programs that change the ABI to	1976	structure. When using the library from programs that change the ABI to
1951	use 64 bit file offsets the programs will fail. In that case you have to	1977	use 64 bit file offsets the programs will fail. In that case you have to
1952	compile libev with the same flags to get binary compatibility. This is	1978	compile libev with the same flags to get binary compatibility. This is
1953	obviously the case with any flags that change the ABI, but the problem is	1979	obviously the case with any flags that change the ABI, but the problem is
1954	most noticeably disabled with ev_stat and large file support.	1980	most noticeably displayed with ev_stat and large file support.
1955		1981
1956	The solution for this is to lobby your distribution maker to make large	1982	The solution for this is to lobby your distribution maker to make large
1957	file interfaces available by default (as e.g. FreeBSD does) and not	1983	file interfaces available by default (as e.g. FreeBSD does) and not
1958	optional. Libev cannot simply switch on large file support because it has	1984	optional. Libev cannot simply switch on large file support because it has
1959	to exchange stat structures with application programs compiled using the	1985	to exchange stat structures with application programs compiled using the
1960	default compilation environment.	1986	default compilation environment.
1961		1987
1962	=head3 Inotify and Kqueue	1988	=head3 Inotify and Kqueue
1963		1989
1964	When C<inotify (7)> support has been compiled into libev (generally	1990	When C<inotify (7)> support has been compiled into libev and present at
1965	only available with Linux 2.6.25 or above due to bugs in earlier	1991	runtime, it will be used to speed up change detection where possible. The
1966	implementations) and present at runtime, it will be used to speed up	1992	inotify descriptor will be created lazily when the first C<ev_stat>
1967	change detection where possible. The inotify descriptor will be created	1993	watcher is being started.
1968	lazily when the first C<ev_stat> watcher is being started.
1969		1994
1970	Inotify presence does not change the semantics of C<ev_stat> watchers	1995	Inotify presence does not change the semantics of C<ev_stat> watchers
1971	except that changes might be detected earlier, and in some cases, to avoid	1996	except that changes might be detected earlier, and in some cases, to avoid
1972	making regular C<stat> calls. Even in the presence of inotify support	1997	making regular C<stat> calls. Even in the presence of inotify support
1973	there are many cases where libev has to resort to regular C<stat> polling,	1998	there are many cases where libev has to resort to regular C<stat> polling,
1974	but as long as the path exists, libev usually gets away without polling.	1999	but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
		2000	many bugs), the path exists (i.e. stat succeeds), and the path resides on
		2001	a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
		2002	xfs are fully working) libev usually gets away without polling.
1975		2003
1976	There is no support for kqueue, as apparently it cannot be used to	2004	There is no support for kqueue, as apparently it cannot be used to
1977	implement this functionality, due to the requirement of having a file	2005	implement this functionality, due to the requirement of having a file
1978	descriptor open on the object at all times, and detecting renames, unlinks	2006	descriptor open on the object at all times, and detecting renames, unlinks
1979	etc. is difficult.	2007	etc. is difficult.
1980		2008
		2009	=head3 C<stat ()> is a synchronous operation
		2010
		2011	Libev doesn't normally do any kind of I/O itself, and so is not blocking
		2012	the process. The exception are C<ev_stat> watchers - those call C<stat
		2013	()>, which is a synchronous operation.
		2014
		2015	For local paths, this usually doesn't matter: unless the system is very
		2016	busy or the intervals between stat's are large, a stat call will be fast,
		2017	as the path data is suually in memory already (except when starting the
		2018	watcher).
		2019
		2020	For networked file systems, calling C<stat ()> can block an indefinite
		2021	time due to network issues, and even under good conditions, a stat call
		2022	often takes multiple milliseconds.
		2023
		2024	Therefore, it is best to avoid using C<ev_stat> watchers on networked
		2025	paths, although this is fully supported by libev.
		2026
1981	=head3 The special problem of stat time resolution	2027	=head3 The special problem of stat time resolution
1982		2028
1983	The C<stat ()> system call only supports full-second resolution portably, and	2029	The C<stat ()> system call only supports full-second resolution portably,
1984	even on systems where the resolution is higher, most file systems still	2030	and even on systems where the resolution is higher, most file systems
1985	only support whole seconds.	2031	still only support whole seconds.
1986		2032
1987	That means that, if the time is the only thing that changes, you can	2033	That means that, if the time is the only thing that changes, you can
1988	easily miss updates: on the first update, C<ev_stat> detects a change and	2034	easily miss updates: on the first update, C<ev_stat> detects a change and
1989	calls your callback, which does something. When there is another update	2035	calls your callback, which does something. When there is another update
1990	within the same second, C<ev_stat> will be unable to detect unless the	2036	within the same second, C<ev_stat> will be unable to detect unless the
…		…
2629	=over 4	2675	=over 4
2630		2676
2631	=item ev_async_init (ev_async *, callback)	2677	=item ev_async_init (ev_async *, callback)
2632		2678
2633	Initialises and configures the async watcher - it has no parameters of any	2679	Initialises and configures the async watcher - it has no parameters of any
2634	kind. There is a C<ev_asynd_set> macro, but using it is utterly pointless,	2680	kind. There is a C<ev_async_set> macro, but using it is utterly pointless,
2635	trust me.	2681	trust me.
2636		2682
2637	=item ev_async_send (loop, ev_async *)	2683	=item ev_async_send (loop, ev_async *)
2638		2684
2639	Sends/signals/activates the given C<ev_async> watcher, that is, feeds	2685	Sends/signals/activates the given C<ev_async> watcher, that is, feeds
…		…
3072		3118
3073	#define EV_STANDALONE 1	3119	#define EV_STANDALONE 1
3074	#include "ev.h"	3120	#include "ev.h"
3075		3121
3076	Both header files and implementation files can be compiled with a C++	3122	Both header files and implementation files can be compiled with a C++
3077	compiler (at least, thats a stated goal, and breakage will be treated	3123	compiler (at least, that's a stated goal, and breakage will be treated
3078	as a bug).	3124	as a bug).
3079		3125
3080	You need the following files in your source tree, or in a directory	3126	You need the following files in your source tree, or in a directory
3081	in your include path (e.g. in libev/ when using -Ilibev):	3127	in your include path (e.g. in libev/ when using -Ilibev):
3082		3128
…		…
3554	loop, as long as you don't confuse yourself). The only exception is that	3600	loop, as long as you don't confuse yourself). The only exception is that
3555	you must not do this from C<ev_periodic> reschedule callbacks.	3601	you must not do this from C<ev_periodic> reschedule callbacks.
3556		3602
3557	Care has been taken to ensure that libev does not keep local state inside	3603	Care has been taken to ensure that libev does not keep local state inside
3558	C<ev_loop>, and other calls do not usually allow for coroutine switches as	3604	C<ev_loop>, and other calls do not usually allow for coroutine switches as
3559	they do not clal any callbacks.	3605	they do not call any callbacks.
3560		3606
3561	=head2 COMPILER WARNINGS	3607	=head2 COMPILER WARNINGS
3562		3608
3563	Depending on your compiler and compiler settings, you might get no or a	3609	Depending on your compiler and compiler settings, you might get no or a
3564	lot of warnings when compiling libev code. Some people are apparently	3610	lot of warnings when compiling libev code. Some people are apparently
…		…
3598	==2274== definitely lost: 0 bytes in 0 blocks.	3644	==2274== definitely lost: 0 bytes in 0 blocks.
3599	==2274== possibly lost: 0 bytes in 0 blocks.	3645	==2274== possibly lost: 0 bytes in 0 blocks.
3600	==2274== still reachable: 256 bytes in 1 blocks.	3646	==2274== still reachable: 256 bytes in 1 blocks.
3601		3647
3602	Then there is no memory leak, just as memory accounted to global variables	3648	Then there is no memory leak, just as memory accounted to global variables
3603	is not a memleak - the memory is still being refernced, and didn't leak.	3649	is not a memleak - the memory is still being referenced, and didn't leak.
3604		3650
3605	Similarly, under some circumstances, valgrind might report kernel bugs	3651	Similarly, under some circumstances, valgrind might report kernel bugs
3606	as if it were a bug in libev (e.g. in realloc or in the poll backend,	3652	as if it were a bug in libev (e.g. in realloc or in the poll backend,
3607	although an acceptable workaround has been found here), or it might be	3653	although an acceptable workaround has been found here), or it might be
3608	confused.	3654	confused.
…		…
3846	=back	3892	=back
3847		3893
3848		3894
3849	=head1 AUTHOR	3895	=head1 AUTHOR
3850		3896
3851	Marc Lehmann <libev@schmorp.de>.	3897	Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson.
3852		3898

Diff Legend

-–
+Removed lines
-+
+Added lines
-<
+Changed lines
->
+Changed lines

Comparing libev/ev.pod (file contents): Revision 1.203 by root, Sun Oct 26 00:52:51 2008 UTC vs. Revision 1.217 by root, Mon Nov 17 03:37:08 2008 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.203 by root, Sun Oct 26 00:52:51 2008 UTC vs.
Revision 1.217 by root, Mon Nov 17 03:37:08 2008 UTC