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

Comparing libev/ev.pod (file contents):
Revision 1.357 by root, Tue Jan 11 02:15:58 2011 UTC vs.
Revision 1.379 by root, Tue Jul 12 23:32:10 2011 UTC

      ev_timer_start (loop, &timeout_watcher);
      // now wait for events to arrive
      ev_run (loop, 0);
-     // unloop was called, so exit
+     // break was called, so exit
      return 0;
    }
 =head1 ABOUT THIS DOCUMENT
 you actually want to know. Also interesting is the combination of
 C<ev_update_now> and C<ev_now>.
 =item ev_sleep (ev_tstamp interval)
-Sleep for the given interval: The current thread will be blocked until
+Sleep for the given interval: The current thread will be blocked
-either it is interrupted or the given time interval has passed. Basically
+until either it is interrupted or the given time interval has
+passed (approximately - it might return a bit earlier even if not
+interrupted). Returns immediately if C<< interval <= 0 >>.
-this is a sub-second-resolution C<sleep ()>.
+Basically this is a sub-second-resolution C<sleep ()>.
+The range of the C<interval> is limited - libev only guarantees to work
+with sleep times of up to one day (C<< interval <= 86400 >>).
 =item int ev_version_major ()
 =item int ev_version_minor ()
 example) that can't properly initialise their signal masks.
 =item C<EVFLAG_NOSIGMASK>
 When this flag is specified, then libev will avoid to modify the signal
-mask. Specifically, this means you ahve to make sure signals are unblocked
+mask. Specifically, this means you have to make sure signals are unblocked
 when you want to receive them.
 This behaviour is useful when you want to do your own signal handling, or
 want to handle signals only in specific threads and want to avoid libev
 unblocking the signals.
+It's also required by POSIX in a threaded program, as libev calls
+C<sigprocmask>, whose behaviour is officially unspecified.
 This flag's behaviour will become the default in future versions of libev.
 =item C<EVBACKEND_SELECT>  (value 1, portable select backend)
 =item C<EVBACKEND_EPOLL>   (value 4, Linux)
 Use the linux-specific epoll(7) interface (for both pre- and post-2.6.9
 kernels).
-For few fds, this backend is a bit little slower than poll and select,
+For few fds, this backend is a bit little slower than poll and select, but
-but it scales phenomenally better. While poll and select usually scale
+it scales phenomenally better. While poll and select usually scale like
-like O(total_fds) where n is the total number of fds (or the highest fd),
+O(total_fds) where total_fds is the total number of fds (or the highest
-epoll scales either O(1) or O(active_fds).
+fd), epoll scales either O(1) or O(active_fds).
 The epoll mechanism deserves honorable mention as the most misdesigned
 of the more advanced event mechanisms: mere annoyances include silently
 dropping file descriptors, requiring a system call per change per file
 descriptor (and unnecessary guessing of parameters), problems with dup,
 0.1ms) and so on. The biggest issue is fork races, however - if a program
 forks then I<both> parent and child process have to recreate the epoll
 set, which can take considerable time (one syscall per file descriptor)
 and is of course hard to detect.
-Epoll is also notoriously buggy - embedding epoll fds I<should> work, but
+Epoll is also notoriously buggy - embedding epoll fds I<should> work,
-of course I<doesn't>, and epoll just loves to report events for totally
+but of course I<doesn't>, and epoll just loves to report events for
-I<different> file descriptors (even already closed ones, so one cannot
+totally I<different> file descriptors (even already closed ones, so
-even remove them from the set) than registered in the set (especially
+one cannot even remove them from the set) than registered in the set
-on SMP systems). Libev tries to counter these spurious notifications by
+(especially on SMP systems). Libev tries to counter these spurious
-employing an additional generation counter and comparing that against the
+notifications by employing an additional generation counter and comparing
-events to filter out spurious ones, recreating the set when required. Last
+that against the events to filter out spurious ones, recreating the set
+when required. Epoll also erroneously rounds down timeouts, but gives you
+no way to know when and by how much, so sometimes you have to busy-wait
+because epoll returns immediately despite a nonzero timeout. And last
 not least, it also refuses to work with some file descriptors which work
 perfectly fine with C<select> (files, many character devices...).
-Epoll is truly the train wreck analog among event poll mechanisms,
+Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
-a frankenpoll, cobbled together in a hurry, no thought to design or
+cobbled together in a hurry, no thought to design or interaction with
-interaction with others.
+others. Oh, the pain, will it ever stop...
 While stopping, setting and starting an I/O watcher in the same iteration
 will result in some caching, there is still a system call per such
 incident (because the same I<file descriptor> could point to a different
 I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
 among the OS-specific backends (I vastly prefer correctness over speed
 hacks).
 On the negative side, the interface is I<bizarre> - so bizarre that
 even sun itself gets it wrong in their code examples: The event polling
-function sometimes returning events to the caller even though an error
+function sometimes returns events to the caller even though an error
 occurred, but with no indication whether it has done so or not (yes, it's
-even documented that way) - deadly for edge-triggered interfaces where
+even documented that way) - deadly for edge-triggered interfaces where you
-you absolutely have to know whether an event occurred or not because you
+absolutely have to know whether an event occurred or not because you have
-have to re-arm the watcher.
+to re-arm the watcher.
 Fortunately libev seems to be able to work around these idiocies.
 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
 C<EVBACKEND_POLL>.
 This is useful if you are waiting for some external event in conjunction
 with something not expressible using other libev watchers (i.e. "roll your
 own C<ev_run>"). However, a pair of C<ev_prepare>/C<ev_check> watchers is
 usually a better approach for this kind of thing.
-Here are the gory details of what C<ev_run> does:
+Here are the gory details of what C<ev_run> does (this is for your
+understanding, not a guarantee that things will work exactly like this in
+future versions):
    - Increment loop depth.
    - Reset the ev_break status.
    - Before the first iteration, call any pending watchers.
    LOOP:
 anymore.
    ... queue jobs here, make sure they register event watchers as long
    ... as they still have work to do (even an idle watcher will do..)
    ev_run (my_loop, 0);
-   ... jobs done or somebody called unloop. yeah!
+   ... jobs done or somebody called break. yeah!
 =item ev_break (loop, how)
 Can be used to make a call to C<ev_run> return early (but only after it
 has processed all outstanding events). The C<how> argument must be either
 overhead for the actual polling but can deliver many events at once.
 By setting a higher I<io collect interval> you allow libev to spend more
 time collecting I/O events, so you can handle more events per iteration,
 at the cost of increasing latency. Timeouts (both C<ev_periodic> and
-C<ev_timer>) will be not affected. Setting this to a non-null value will
+C<ev_timer>) will not be affected. Setting this to a non-null value will
 introduce an additional C<ev_sleep ()> call into most loop iterations. The
 sleep time ensures that libev will not poll for I/O events more often then
-once per this interval, on average.
+once per this interval, on average (as long as the host time resolution is
+good enough).
 Likewise, by setting a higher I<timeout collect interval> you allow libev
 to spend more time collecting timeouts, at the expense of increased
 latency/jitter/inexactness (the watcher callback will be called
 later). C<ev_io> watchers will not be affected. Setting this to a non-null
 =over 4
 =item initialiased
-Before a watcher can be registered with the event looop it has to be
+Before a watcher can be registered with the event loop it has to be
 initialised. This can be done with a call to C<ev_TYPE_init>, or calls to
 C<ev_init> followed by the watcher-specific C<ev_TYPE_set> function.
-In this state it is simply some block of memory that is suitable for use
+In this state it is simply some block of memory that is suitable for
-in an event loop. It can be moved around, freed, reused etc. at will.
+use in an event loop. It can be moved around, freed, reused etc. at
+will - as long as you either keep the memory contents intact, or call
+C<ev_TYPE_init> again.
 =item started/running/active
 Once a watcher has been started with a call to C<ev_TYPE_start> it becomes
 property of the event loop, and is actively waiting for events. While in
 latter will clear any pending state the watcher might be in, regardless
 of whether it was active or not, so stopping a watcher explicitly before
 freeing it is often a good idea.
 While stopped (and not pending) the watcher is essentially in the
-initialised state, that is it can be reused, moved, modified in any way
+initialised state, that is, it can be reused, moved, modified in any way
-you wish.
+you wish (but when you trash the memory block, you need to C<ev_TYPE_init>
+it again).
 =back
 =head2 WATCHER PRIORITY MODELS
 always get a readiness notification instantly, and your read (or possibly
 write) will still block on the disk I/O.
 Another way to view it is that in the case of sockets, pipes, character
 devices and so on, there is another party (the sender) that delivers data
-on it's own, but in the case of files, there is no such thing: the disk
+on its own, but in the case of files, there is no such thing: the disk
-will not send data on it's own, simply because it doesn't know what you
+will not send data on its own, simply because it doesn't know what you
 wish to read - you would first have to request some data.
 Since files are typically not-so-well supported by advanced notification
 mechanism, libev tries hard to emulate POSIX behaviour with respect
 to files, even though you should not use it. The reason for this is
 keep up with the timer (because it takes longer than those 10 seconds to
 do stuff) the timer will not fire more than once per event loop iteration.
 =item ev_timer_again (loop, ev_timer *)
-This will act as if the timer timed out and restart it again if it is
+This will act as if the timer timed out and restarts it again if it is
 repeating. The exact semantics are:
 If the timer is pending, its pending status is cleared.
 If the timer is started but non-repeating, stop it (as if it timed out).
 Another way to think about it (for the mathematically inclined) is that
 C<ev_periodic> will try to run the callback in this mode at the next possible
 time where C<time = offset (mod interval)>, regardless of any time jumps.
-For numerical stability it is preferable that the C<offset> value is near
+The C<interval> I<MUST> be positive, and for numerical stability, the
-C<ev_now ()> (the current time), but there is no range requirement for
+interval value should be higher than C<1/8192> (which is around 100
-this value, and in fact is often specified as zero.
+microseconds) and C<offset> should be higher than C<0> and should have
+at most a similar magnitude as the current time (say, within a factor of
+ten). Typical values for offset are, in fact, C<0> or something between
+C<0> and C<interval>, which is also the recommended range.
 Note also that there is an upper limit to how often a timer can fire (CPU
 speed for example), so if C<interval> is very small then timing stability
 will of course deteriorate. Libev itself tries to be exact to be about one
 millisecond (if the OS supports it and the machine is fast enough).
 =head3 The special problem of inheritance over fork/execve/pthread_create
 Both the signal mask (C<sigprocmask>) and the signal disposition
 (C<sigaction>) are unspecified after starting a signal watcher (and after
 stopping it again), that is, libev might or might not block the signal,
-and might or might not set or restore the installed signal handler.
+and might or might not set or restore the installed signal handler (but
+see C<EVFLAG_NOSIGMASK>).
 While this does not matter for the signal disposition (libev never
 sets signals to C<SIG_IGN>, so handlers will be reset to C<SIG_DFL> on
 C<execve>), this matters for the signal mask: many programs do not expect
 certain signals to be blocked.
    atexit (program_exits);
 =head2 C<ev_async> - how to wake up an event loop
-In general, you cannot use an C<ev_run> from multiple threads or other
+In general, you cannot use an C<ev_loop> from multiple threads or other
 asynchronous sources such as signal handlers (as opposed to multiple event
 loops - those are of course safe to use in different threads).
 Sometimes, however, you need to wake up an event loop you do not control,
 for example because it belongs to another thread. This is what C<ev_async>
 C<ev_async_sent> calls). In fact, you could use signal watchers as a kind
 of "global async watchers" by using a watcher on an otherwise unused
 signal, and C<ev_feed_signal> to signal this watcher from another thread,
 even without knowing which loop owns the signal.
-Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not
-just the default loop.
 =head3 Queueing
 C<ev_async> does not support queueing of data in any way. The reason
 is that the author does not know of a simple (or any) algorithm for a
 multiple-writer-single-reader queue that works in all cases and doesn't
 trust me.
 =item ev_async_send (loop, ev_async *)
 Sends/signals/activates the given C<ev_async> watcher, that is, feeds
-an C<EV_ASYNC> event on the watcher into the event loop. Unlike
+an C<EV_ASYNC> event on the watcher into the event loop, and instantly
+returns.
-C<ev_feed_event>, this call is safe to do from other threads, signal or
+Unlike C<ev_feed_event>, this call is safe to do from other threads,
-similar contexts (see the discussion of C<EV_ATOMIC_T> in the embedding
+signal or similar contexts (see the discussion of C<EV_ATOMIC_T> in the
-section below on what exactly this means).
+embedding section below on what exactly this means).
 Note that, as with other watchers in libev, multiple events might get
-compressed into a single callback invocation (another way to look at this
+compressed into a single callback invocation (another way to look at
-is that C<ev_async> watchers are level-triggered, set on C<ev_async_send>,
+this is that C<ev_async> watchers are level-triggered: they are set on
-reset when the event loop detects that).
+C<ev_async_send>, reset when the event loop detects that).
-This call incurs the overhead of a system call only once per event loop
+This call incurs the overhead of at most one extra system call per event
-iteration, so while the overhead might be noticeable, it doesn't apply to
+loop iteration, if the event loop is blocked, and no syscall at all if
-repeated calls to C<ev_async_send> for the same event loop.
+the event loop (or your program) is processing events. That means that
+repeated calls are basically free (there is no need to avoid calls for
+performance reasons) and that the overhead becomes smaller (typically
+zero) under load.
 =item bool = ev_async_pending (ev_async *)
 Returns a non-zero value when C<ev_async_send> has been called on the
 watcher but the event has not yet been processed (or even noted) by the
    exit_main_loop = exit_nested_loop = 1;
 =head2 THREAD LOCKING EXAMPLE
 Here is a fictitious example of how to run an event loop in a different
-thread than where callbacks are being invoked and watchers are
+thread from where callbacks are being invoked and watchers are
 created/added/removed.
 For a real-world example, see the C<EV::Loop::Async> perl module,
 which uses exactly this technique (which is suited for many high-level
 languages).
       // now associate this with the loop
       ev_set_userdata (EV_A_ u);
       ev_set_invoke_pending_cb (EV_A_ l_invoke);
       ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
-      // then create the thread running ev_loop
+      // then create the thread running ev_run
       pthread_create (&u->tid, 0, l_run, EV_A);
    }
 The callback for the C<ev_async> watcher does nothing: the watcher is used
 solely to wake up the event loop so it takes notice of any new watchers
 watchers in the constructor.
    class myclass
    {
      ev::io   io  ; void io_cb   (ev::io   &w, int revents);
-     ev::io2  io2 ; void io2_cb  (ev::io   &w, int revents);
+     ev::io   io2 ; void io2_cb  (ev::io   &w, int revents);
      ev::idle idle; void idle_cb (ev::idle &w, int revents);
      myclass (int fd)
      {
        io  .set <myclass, &myclass::io_cb  > (this);
 L<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>.
 =item D
 Leandro Lucarella has written a D language binding (F<ev.d>) for libev, to
-be found at L<http://proj.llucax.com.ar/wiki/evd>.
+be found at L<http://www.llucax.com.ar/proj/ev.d/index.html>.
 =item Ocaml
 Erkki Seppala has written Ocaml bindings for libev, to be found at
 L<http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.
 supported). It will also not define any of the structs usually found in
 F<event.h> that are not directly supported by the libev core alone.
 In standalone mode, libev will still try to automatically deduce the
 configuration, but has to be more conservative.
+=item EV_USE_FLOOR
+If defined to be C<1>, libev will use the C<floor ()> function for its
+periodic reschedule calculations, otherwise libev will fall back on a
+portable (slower) implementation. If you enable this, you usually have to
+link against libm or something equivalent. Enabling this when the C<floor>
+function is not available will fail, so the safe default is to not enable
+this.
 =item EV_USE_MONOTONIC
 If defined to be C<1>, libev will try to detect the availability of the
 monotonic clock option at both compile time and runtime. Otherwise no
 indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
 =item EV_ATOMIC_T
 Libev requires an integer type (suitable for storing C<0> or C<1>) whose
-access is atomic with respect to other threads or signal contexts. No such
+access is atomic and serialised with respect to other threads or signal
-type is easily found in the C language, so you can provide your own type
+contexts. No such type is easily found in the C language, so you can
-that you know is safe for your purposes. It is used both for signal handler "locking"
+provide your own type that you know is safe for your purposes. It is used
-as well as for signal and thread safety in C<ev_async> watchers.
+both for signal handler "locking" as well as for signal and thread safety
+in C<ev_async> watchers.
 In the absence of this define, libev will use C<sig_atomic_t volatile>
-(from F<signal.h>), which is usually good enough on most platforms.
+(from F<signal.h>), which is usually good enough on most platforms,
+although strictly speaking using a type that also implies a memory fence
+is required.
 =item EV_H (h)
 The name of the F<ev.h> header file used to include it. The default if
 undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be
 requires, and its I/O model is fundamentally incompatible with the POSIX
 model. Libev still offers limited functionality on this platform in
 the form of the C<EVBACKEND_SELECT> backend, and only supports socket
 descriptors. This only applies when using Win32 natively, not when using
 e.g. cygwin. Actually, it only applies to the microsofts own compilers,
-as every compielr comes with a slightly differently broken/incompatible
+as every compiler comes with a slightly differently broken/incompatible
 environment.
 Lifting these limitations would basically require the full
 re-implementation of the I/O system. If you are into this kind of thing,
 then note that glib does exactly that for you in a very portable way (note
 The type C<double> is used to represent timestamps. It is required to
 have at least 51 bits of mantissa (and 9 bits of exponent), which is
 good enough for at least into the year 4000 with millisecond accuracy
 (the design goal for libev). This requirement is overfulfilled by
-implementations using IEEE 754, which is basically all existing ones. With
+implementations using IEEE 754, which is basically all existing ones.
-IEEE 754 doubles, you get microsecond accuracy until at least 2200.
+With IEEE 754 doubles, you get microsecond accuracy until at least the
+year 2255 (and millisecond accuray till the year 287396 - by then, libev
+is either obsolete or somebody patched it to use C<long double> or
+something like that, just kidding).
 =back
 If you know of other additional requirements drop me a note.
 =item Processing ev_async_send: O(number_of_async_watchers)
 =item Processing signals: O(max_signal_number)
 Sending involves a system call I<iff> there were no other C<ev_async_send>
-calls in the current loop iteration. Checking for async and signal events
+calls in the current loop iteration and the loop is currently
+blocked. Checking for async and signal events involves iterating over all
-involves iterating over all running async watchers or all signal numbers.
+running async watchers or all signal numbers.
 =back
 =head1 PORTING FROM LIBEV 3.X TO 4.X
 The physical time that is observed. It is apparently strictly monotonic :)
 =item wall-clock time
 The time and date as shown on clocks. Unlike real time, it can actually
-be wrong and jump forwards and backwards, e.g. when the you adjust your
+be wrong and jump forwards and backwards, e.g. when you adjust your
 clock.
 =item watcher
 A data structure that describes interest in certain events. Watchers need
 =back
 =head1 AUTHOR
 Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael
-Magnusson and Emanuele Giaquinta.
+Magnusson and Emanuele Giaquinta, and minor corrections by many others.

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Comparing libev/ev.pod (file contents): Revision 1.357 by root, Tue Jan 11 02:15:58 2011 UTC vs. Revision 1.379 by root, Tue Jul 12 23:32:10 2011 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.357 by root, Tue Jan 11 02:15:58 2011 UTC vs.
Revision 1.379 by root, Tue Jul 12 23:32:10 2011 UTC