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62 62
63 // unloop was called, so exit 63 // unloop was called, so exit
64 return 0; 64 return 0;
65 } 65 }
66 66
67=head1 DESCRIPTION 67=head1 ABOUT THIS DOCUMENT
68
69This document documents the libev software package.
68 70
69The newest version of this document is also available as an html-formatted 71The newest version of this document is also available as an html-formatted
70web page you might find easier to navigate when reading it for the first 72web page you might find easier to navigate when reading it for the first
71time: L<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>. 73time: L<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
74
75While this document tries to be as complete as possible in documenting
76libev, its usage and the rationale behind its design, it is not a tutorial
77on event-based programming, nor will it introduce event-based programming
78with libev.
79
80Familarity with event based programming techniques in general is assumed
81throughout this document.
82
83=head1 ABOUT LIBEV
72 84
73Libev is an event loop: you register interest in certain events (such as a 85Libev is an event loop: you register interest in certain events (such as a
74file descriptor being readable or a timeout occurring), and it will manage 86file descriptor being readable or a timeout occurring), and it will manage
75these event sources and provide your program with events. 87these event sources and provide your program with events.
76 88
110name C<loop> (which is always of type C<ev_loop *>) will not have 122name C<loop> (which is always of type C<ev_loop *>) will not have
111this argument. 123this argument.
112 124
113=head2 TIME REPRESENTATION 125=head2 TIME REPRESENTATION
114 126
115Libev represents time as a single floating point number, representing the 127Libev represents time as a single floating point number, representing
116(fractional) number of seconds since the (POSIX) epoch (somewhere near 128the (fractional) number of seconds since the (POSIX) epoch (somewhere
117the beginning of 1970, details are complicated, don't ask). This type is 129near the beginning of 1970, details are complicated, don't ask). This
118called C<ev_tstamp>, which is what you should use too. It usually aliases 130type is called C<ev_tstamp>, which is what you should use too. It usually
119to the C<double> type in C, and when you need to do any calculations on 131aliases to the C<double> type in C. When you need to do any calculations
120it, you should treat it as some floating point value. Unlike the name 132on it, you should treat it as some floating point value. Unlike the name
121component C<stamp> might indicate, it is also used for time differences 133component C<stamp> might indicate, it is also used for time differences
122throughout libev. 134throughout libev.
123 135
124=head1 ERROR HANDLING 136=head1 ERROR HANDLING
125 137
632 644
633This function is rarely useful, but when some event callback runs for a 645This function is rarely useful, but when some event callback runs for a
634very long time without entering the event loop, updating libev's idea of 646very long time without entering the event loop, updating libev's idea of
635the current time is a good idea. 647the current time is a good idea.
636 648
637See also "The special problem of time updates" in the C<ev_timer> section. 649See also L<The special problem of time updates> in the C<ev_timer> section.
638 650
639=item ev_suspend (loop) 651=item ev_suspend (loop)
640 652
641=item ev_resume (loop) 653=item ev_resume (loop)
642 654
1083integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI> 1095integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI>
1084(default: C<-2>). Pending watchers with higher priority will be invoked 1096(default: C<-2>). Pending watchers with higher priority will be invoked
1085before watchers with lower priority, but priority will not keep watchers 1097before watchers with lower priority, but priority will not keep watchers
1086from being executed (except for C<ev_idle> watchers). 1098from being executed (except for C<ev_idle> watchers).
1087 1099
1088See L<
1089
1090This means that priorities are I<only> used for ordering callback
1091invocation after new events have been received. This is useful, for
1092example, to reduce latency after idling, or more often, to bind two
1093watchers on the same event and make sure one is called first.
1094
1095If you need to suppress invocation when higher priority events are pending 1100If you need to suppress invocation when higher priority events are pending
1096you need to look at C<ev_idle> watchers, which provide this functionality. 1101you need to look at C<ev_idle> watchers, which provide this functionality.
1097 1102
1098You I<must not> change the priority of a watcher as long as it is active or 1103You I<must not> change the priority of a watcher as long as it is active or
1099pending. 1104pending.
1100
1101The default priority used by watchers when no priority has been set is
1102always C<0>, which is supposed to not be too high and not be too low :).
1103 1105
1104Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is 1106Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is
1105fine, as long as you do not mind that the priority value you query might 1107fine, as long as you do not mind that the priority value you query might
1106or might not have been clamped to the valid range. 1108or might not have been clamped to the valid range.
1109
1110The default priority used by watchers when no priority has been set is
1111always C<0>, which is supposed to not be too high and not be too low :).
1112
1113See L<WATCHER PRIORITY MODELS>, below, for a more thorough treatment of
1114priorities.
1107 1115
1108=item ev_invoke (loop, ev_TYPE *watcher, int revents) 1116=item ev_invoke (loop, ev_TYPE *watcher, int revents)
1109 1117
1110Invoke the C<watcher> with the given C<loop> and C<revents>. Neither 1118Invoke the C<watcher> with the given C<loop> and C<revents>. Neither
1111C<loop> nor C<revents> need to be valid as long as the watcher callback 1119C<loop> nor C<revents> need to be valid as long as the watcher callback
1176 #include <stddef.h> 1184 #include <stddef.h>
1177 1185
1178 static void 1186 static void
1179 t1_cb (EV_P_ ev_timer *w, int revents) 1187 t1_cb (EV_P_ ev_timer *w, int revents)
1180 { 1188 {
1181 struct my_biggy big = (struct my_biggy * 1189 struct my_biggy big = (struct my_biggy *)
1182 (((char *)w) - offsetof (struct my_biggy, t1)); 1190 (((char *)w) - offsetof (struct my_biggy, t1));
1183 } 1191 }
1184 1192
1185 static void 1193 static void
1186 t2_cb (EV_P_ ev_timer *w, int revents) 1194 t2_cb (EV_P_ ev_timer *w, int revents)
1187 { 1195 {
1188 struct my_biggy big = (struct my_biggy * 1196 struct my_biggy big = (struct my_biggy *)
1189 (((char *)w) - offsetof (struct my_biggy, t2)); 1197 (((char *)w) - offsetof (struct my_biggy, t2));
1190 } 1198 }
1199
1200=head2 WATCHER PRIORITY MODELS
1201
1202Many event loops support I<watcher priorities>, which are usually small
1203integers that influence the ordering of event callback invocation
1204between watchers in some way, all else being equal.
1205
1206In libev, Watcher priorities can be set using C<ev_set_priority>. See its
1207description for the more technical details such as the actual priority
1208range.
1209
1210There are two common ways how these these priorities are being interpreted
1211by event loops:
1212
1213In the more common lock-out model, higher priorities "lock out" invocation
1214of lower priority watchers, which means as long as higher priority
1215watchers receive events, lower priority watchers are not being invoked.
1216
1217The less common only-for-ordering model uses priorities solely to order
1218callback invocation within a single event loop iteration: Higher priority
1219watchers are invoked before lower priority ones, but they all get invoked
1220before polling for new events.
1221
1222Libev uses the second (only-for-ordering) model for all its watchers
1223except for idle watchers (which use the lock-out model).
1224
1225The rationale behind this is that implementing the lock-out model for
1226watchers is not well supported by most kernel interfaces, and most event
1227libraries will just poll for the same events again and again as long as
1228their callbacks have not been executed, which is very inefficient in the
1229common case of one high-priority watcher locking out a mass of lower
1230priority ones.
1231
1232Static (ordering) priorities are most useful when you have two or more
1233watchers handling the same resource: a typical usage example is having an
1234C<ev_io> watcher to receive data, and an associated C<ev_timer> to handle
1235timeouts. Under load, data might be received while the program handles
1236other jobs, but since timers normally get invoked first, the timeout
1237handler will be executed before checking for data. In that case, giving
1238the timer a lower priority than the I/O watcher ensures that I/O will be
1239handled first even under adverse conditions (which is usually, but not
1240always, what you want).
1241
1242Since idle watchers use the "lock-out" model, meaning that idle watchers
1243will only be executed when no same or higher priority watchers have
1244received events, they can be used to implement the "lock-out" model when
1245required.
1246
1247For example, to emulate how many other event libraries handle priorities,
1248you can associate an C<ev_idle> watcher to each such watcher, and in
1249the normal watcher callback, you just start the idle watcher. The real
1250processing is done in the idle watcher callback. This causes libev to
1251continously poll and process kernel event data for the watcher, but when
1252the lock-out case is known to be rare (which in turn is rare :), this is
1253workable.
1254
1255Usually, however, the lock-out model implemented that way will perform
1256miserably under the type of load it was designed to handle. In that case,
1257it might be preferable to stop the real watcher before starting the
1258idle watcher, so the kernel will not have to process the event in case
1259the actual processing will be delayed for considerable time.
1260
1261Here is an example of an I/O watcher that should run at a strictly lower
1262priority than the default, and which should only process data when no
1263other events are pending:
1264
1265 ev_idle idle; // actual processing watcher
1266 ev_io io; // actual event watcher
1267
1268 static void
1269 io_cb (EV_P_ ev_io *w, int revents)
1270 {
1271 // stop the I/O watcher, we received the event, but
1272 // are not yet ready to handle it.
1273 ev_io_stop (EV_A_ w);
1274
1275 // start the idle watcher to ahndle the actual event.
1276 // it will not be executed as long as other watchers
1277 // with the default priority are receiving events.
1278 ev_idle_start (EV_A_ &idle);
1279 }
1280
1281 static void
1282 idle_cb (EV_P_ ev_idle *w, int revents)
1283 {
1284 // actual processing
1285 read (STDIN_FILENO, ...);
1286
1287 // have to start the I/O watcher again, as
1288 // we have handled the event
1289 ev_io_start (EV_P_ &io);
1290 }
1291
1292 // initialisation
1293 ev_idle_init (&idle, idle_cb);
1294 ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
1295 ev_io_start (EV_DEFAULT_ &io);
1296
1297In the "real" world, it might also be beneficial to start a timer, so that
1298low-priority connections can not be locked out forever under load. This
1299enables your program to keep a lower latency for important connections
1300during short periods of high load, while not completely locking out less
1301important ones.
1191 1302
1192 1303
1193=head1 WATCHER TYPES 1304=head1 WATCHER TYPES
1194 1305
1195This section describes each watcher in detail, but will not repeat 1306This section describes each watcher in detail, but will not repeat
1221descriptors to non-blocking mode is also usually a good idea (but not 1332descriptors to non-blocking mode is also usually a good idea (but not
1222required if you know what you are doing). 1333required if you know what you are doing).
1223 1334
1224If you cannot use non-blocking mode, then force the use of a 1335If you cannot use non-blocking mode, then force the use of a
1225known-to-be-good backend (at the time of this writing, this includes only 1336known-to-be-good backend (at the time of this writing, this includes only
1226C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). 1337C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). The same applies to file
1338descriptors for which non-blocking operation makes no sense (such as
1339files) - libev doesn't guarentee any specific behaviour in that case.
1227 1340
1228Another thing you have to watch out for is that it is quite easy to 1341Another thing you have to watch out for is that it is quite easy to
1229receive "spurious" readiness notifications, that is your callback might 1342receive "spurious" readiness notifications, that is your callback might
1230be called with C<EV_READ> but a subsequent C<read>(2) will actually block 1343be called with C<EV_READ> but a subsequent C<read>(2) will actually block
1231because there is no data. Not only are some backends known to create a 1344because there is no data. Not only are some backends known to create a
1352year, it will still time out after (roughly) one hour. "Roughly" because 1465year, it will still time out after (roughly) one hour. "Roughly" because
1353detecting time jumps is hard, and some inaccuracies are unavoidable (the 1466detecting time jumps is hard, and some inaccuracies are unavoidable (the
1354monotonic clock option helps a lot here). 1467monotonic clock option helps a lot here).
1355 1468
1356The callback is guaranteed to be invoked only I<after> its timeout has 1469The callback is guaranteed to be invoked only I<after> its timeout has
1357passed. If multiple timers become ready during the same loop iteration 1470passed (not I<at>, so on systems with very low-resolution clocks this
1358then the ones with earlier time-out values are invoked before ones with 1471might introduce a small delay). If multiple timers become ready during the
1472same loop iteration then the ones with earlier time-out values are invoked
1359later time-out values (but this is no longer true when a callback calls 1473before ones with later time-out values (but this is no longer true when a
1360C<ev_loop> recursively). 1474callback calls C<ev_loop> recursively).
1361 1475
1362=head3 Be smart about timeouts 1476=head3 Be smart about timeouts
1363 1477
1364Many real-world problems involve some kind of timeout, usually for error 1478Many real-world problems involve some kind of timeout, usually for error
1365recovery. A typical example is an HTTP request - if the other side hangs, 1479recovery. A typical example is an HTTP request - if the other side hangs,
2254 // no longer anything immediate to do. 2368 // no longer anything immediate to do.
2255 } 2369 }
2256 2370
2257 ev_idle *idle_watcher = malloc (sizeof (ev_idle)); 2371 ev_idle *idle_watcher = malloc (sizeof (ev_idle));
2258 ev_idle_init (idle_watcher, idle_cb); 2372 ev_idle_init (idle_watcher, idle_cb);
2259 ev_idle_start (loop, idle_cb); 2373 ev_idle_start (loop, idle_watcher);
2260 2374
2261 2375
2262=head2 C<ev_prepare> and C<ev_check> - customise your event loop! 2376=head2 C<ev_prepare> and C<ev_check> - customise your event loop!
2263 2377
2264Prepare and check watchers are usually (but not always) used in pairs: 2378Prepare and check watchers are usually (but not always) used in pairs:
2597event loop blocks next and before C<ev_check> watchers are being called, 2711event loop blocks next and before C<ev_check> watchers are being called,
2598and only in the child after the fork. If whoever good citizen calling 2712and only in the child after the fork. If whoever good citizen calling
2599C<ev_default_fork> cheats and calls it in the wrong process, the fork 2713C<ev_default_fork> cheats and calls it in the wrong process, the fork
2600handlers will be invoked, too, of course. 2714handlers will be invoked, too, of course.
2601 2715
2716=head3 The special problem of life after fork - how is it possible?
2717
2718Most uses of C<fork()> consist of forking, then some simple calls to ste
2719up/change the process environment, followed by a call to C<exec()>. This
2720sequence should be handled by libev without any problems.
2721
2722This changes when the application actually wants to do event handling
2723in the child, or both parent in child, in effect "continuing" after the
2724fork.
2725
2726The default mode of operation (for libev, with application help to detect
2727forks) is to duplicate all the state in the child, as would be expected
2728when I<either> the parent I<or> the child process continues.
2729
2730When both processes want to continue using libev, then this is usually the
2731wrong result. In that case, usually one process (typically the parent) is
2732supposed to continue with all watchers in place as before, while the other
2733process typically wants to start fresh, i.e. without any active watchers.
2734
2735The cleanest and most efficient way to achieve that with libev is to
2736simply create a new event loop, which of course will be "empty", and
2737use that for new watchers. This has the advantage of not touching more
2738memory than necessary, and thus avoiding the copy-on-write, and the
2739disadvantage of having to use multiple event loops (which do not support
2740signal watchers).
2741
2742When this is not possible, or you want to use the default loop for
2743other reasons, then in the process that wants to start "fresh", call
2744C<ev_default_destroy ()> followed by C<ev_default_loop (...)>. Destroying
2745the default loop will "orphan" (not stop) all registered watchers, so you
2746have to be careful not to execute code that modifies those watchers. Note
2747also that in that case, you have to re-register any signal watchers.
2748
2602=head3 Watcher-Specific Functions and Data Members 2749=head3 Watcher-Specific Functions and Data Members
2603 2750
2604=over 4 2751=over 4
2605 2752
2606=item ev_fork_init (ev_signal *, callback) 2753=item ev_fork_init (ev_signal *, callback)
3788way (note also that glib is the slowest event library known to man). 3935way (note also that glib is the slowest event library known to man).
3789 3936
3790There is no supported compilation method available on windows except 3937There is no supported compilation method available on windows except
3791embedding it into other applications. 3938embedding it into other applications.
3792 3939
3940Sensible signal handling is officially unsupported by Microsoft - libev
3941tries its best, but under most conditions, signals will simply not work.
3942
3793Not a libev limitation but worth mentioning: windows apparently doesn't 3943Not a libev limitation but worth mentioning: windows apparently doesn't
3794accept large writes: instead of resulting in a partial write, windows will 3944accept large writes: instead of resulting in a partial write, windows will
3795either accept everything or return C<ENOBUFS> if the buffer is too large, 3945either accept everything or return C<ENOBUFS> if the buffer is too large,
3796so make sure you only write small amounts into your sockets (less than a 3946so make sure you only write small amounts into your sockets (less than a
3797megabyte seems safe, but this apparently depends on the amount of memory 3947megabyte seems safe, but this apparently depends on the amount of memory
3801the abysmal performance of winsockets, using a large number of sockets 3951the abysmal performance of winsockets, using a large number of sockets
3802is not recommended (and not reasonable). If your program needs to use 3952is not recommended (and not reasonable). If your program needs to use
3803more than a hundred or so sockets, then likely it needs to use a totally 3953more than a hundred or so sockets, then likely it needs to use a totally
3804different implementation for windows, as libev offers the POSIX readiness 3954different implementation for windows, as libev offers the POSIX readiness
3805notification model, which cannot be implemented efficiently on windows 3955notification model, which cannot be implemented efficiently on windows
3806(Microsoft monopoly games). 3956(due to Microsoft monopoly games).
3807 3957
3808A typical way to use libev under windows is to embed it (see the embedding 3958A typical way to use libev under windows is to embed it (see the embedding
3809section for details) and use the following F<evwrap.h> header file instead 3959section for details) and use the following F<evwrap.h> header file instead
3810of F<ev.h>: 3960of F<ev.h>:
3811 3961
3847 3997
3848Early versions of winsocket's select only supported waiting for a maximum 3998Early versions of winsocket's select only supported waiting for a maximum
3849of C<64> handles (probably owning to the fact that all windows kernels 3999of C<64> handles (probably owning to the fact that all windows kernels
3850can only wait for C<64> things at the same time internally; Microsoft 4000can only wait for C<64> things at the same time internally; Microsoft
3851recommends spawning a chain of threads and wait for 63 handles and the 4001recommends spawning a chain of threads and wait for 63 handles and the
3852previous thread in each. Great). 4002previous thread in each. Sounds great!).
3853 4003
3854Newer versions support more handles, but you need to define C<FD_SETSIZE> 4004Newer versions support more handles, but you need to define C<FD_SETSIZE>
3855to some high number (e.g. C<2048>) before compiling the winsocket select 4005to some high number (e.g. C<2048>) before compiling the winsocket select
3856call (which might be in libev or elsewhere, for example, perl does its own 4006call (which might be in libev or elsewhere, for example, perl and many
3857select emulation on windows). 4007other interpreters do their own select emulation on windows).
3858 4008
3859Another limit is the number of file descriptors in the Microsoft runtime 4009Another limit is the number of file descriptors in the Microsoft runtime
3860libraries, which by default is C<64> (there must be a hidden I<64> fetish 4010libraries, which by default is C<64> (there must be a hidden I<64>
3861or something like this inside Microsoft). You can increase this by calling 4011fetish or something like this inside Microsoft). You can increase this
3862C<_setmaxstdio>, which can increase this limit to C<2048> (another 4012by calling C<_setmaxstdio>, which can increase this limit to C<2048>
3863arbitrary limit), but is broken in many versions of the Microsoft runtime 4013(another arbitrary limit), but is broken in many versions of the Microsoft
3864libraries.
3865
3866This might get you to about C<512> or C<2048> sockets (depending on 4014runtime libraries. This might get you to about C<512> or C<2048> sockets
3867windows version and/or the phase of the moon). To get more, you need to 4015(depending on windows version and/or the phase of the moon). To get more,
3868wrap all I/O functions and provide your own fd management, but the cost of 4016you need to wrap all I/O functions and provide your own fd management, but
3869calling select (O(n²)) will likely make this unworkable. 4017the cost of calling select (O(n²)) will likely make this unworkable.
3870 4018
3871=back 4019=back
3872 4020
3873=head2 PORTABILITY REQUIREMENTS 4021=head2 PORTABILITY REQUIREMENTS
3874 4022
3995involves iterating over all running async watchers or all signal numbers. 4143involves iterating over all running async watchers or all signal numbers.
3996 4144
3997=back 4145=back
3998 4146
3999 4147
4148=head1 GLOSSARY
4149
4150=over 4
4151
4152=item active
4153
4154A watcher is active as long as it has been started (has been attached to
4155an event loop) but not yet stopped (disassociated from the event loop).
4156
4157=item application
4158
4159In this document, an application is whatever is using libev.
4160
4161=item callback
4162
4163The address of a function that is called when some event has been
4164detected. Callbacks are being passed the event loop, the watcher that
4165received the event, and the actual event bitset.
4166
4167=item callback invocation
4168
4169The act of calling the callback associated with a watcher.
4170
4171=item event
4172
4173A change of state of some external event, such as data now being available
4174for reading on a file descriptor, time having passed or simply not having
4175any other events happening anymore.
4176
4177In libev, events are represented as single bits (such as C<EV_READ> or
4178C<EV_TIMEOUT>).
4179
4180=item event library
4181
4182A software package implementing an event model and loop.
4183
4184=item event loop
4185
4186An entity that handles and processes external events and converts them
4187into callback invocations.
4188
4189=item event model
4190
4191The model used to describe how an event loop handles and processes
4192watchers and events.
4193
4194=item pending
4195
4196A watcher is pending as soon as the corresponding event has been detected,
4197and stops being pending as soon as the watcher will be invoked or its
4198pending status is explicitly cleared by the application.
4199
4200A watcher can be pending, but not active. Stopping a watcher also clears
4201its pending status.
4202
4203=item real time
4204
4205The physical time that is observed. It is apparently strictly monotonic :)
4206
4207=item wall-clock time
4208
4209The time and date as shown on clocks. Unlike real time, it can actually
4210be wrong and jump forwards and backwards, e.g. when the you adjust your
4211clock.
4212
4213=item watcher
4214
4215A data structure that describes interest in certain events. Watchers need
4216to be started (attached to an event loop) before they can receive events.
4217
4218=item watcher invocation
4219
4220The act of calling the callback associated with a watcher.
4221
4222=back
4223
4000=head1 AUTHOR 4224=head1 AUTHOR
4001 4225
4002Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson. 4226Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson.
4003 4227

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