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

Comparing libev/ev.pod (file contents):
Revision 1.369 by root, Mon May 30 18:34:28 2011 UTC vs.
Revision 1.451 by root, Mon Jun 24 00:19:26 2019 UTC

+=encoding utf-8
 =head1 NAME
 libev - a high performance full-featured event loop written in C
 =head1 SYNOPSIS
 =head1 WHAT TO READ WHEN IN A HURRY
 This manual tries to be very detailed, but unfortunately, this also makes
 it very long. If you just want to know the basics of libev, I suggest
-reading L<ANATOMY OF A WATCHER>, then the L<EXAMPLE PROGRAM> above and
+reading L</ANATOMY OF A WATCHER>, then the L</EXAMPLE PROGRAM> above and
-look up the missing functions in L<GLOBAL FUNCTIONS> and the C<ev_io> and
+look up the missing functions in L</GLOBAL FUNCTIONS> and the C<ev_io> and
-C<ev_timer> sections in L<WATCHER TYPES>.
+C<ev_timer> sections in L</WATCHER TYPES>.
 =head1 ABOUT LIBEV
 Libev is an event loop: you register interest in certain events (such as a
 file descriptor being readable or a timeout occurring), and it will manage
 details of the event, and then hand it over to libev by I<starting> the
 watcher.
 =head2 FEATURES
-Libev supports C<select>, C<poll>, the Linux-specific C<epoll>, the
+Libev supports C<select>, C<poll>, the Linux-specific aio and C<epoll>
-BSD-specific C<kqueue> and the Solaris-specific event port mechanisms
+interfaces, the BSD-specific C<kqueue> and the Solaris-specific event port
-for file descriptor events (C<ev_io>), the Linux C<inotify> interface
+mechanisms for file descriptor events (C<ev_io>), the Linux C<inotify>
-(for C<ev_stat>), Linux eventfd/signalfd (for faster and cleaner
+interface (for C<ev_stat>), Linux eventfd/signalfd (for faster and cleaner
 inter-thread wakeup (C<ev_async>)/signal handling (C<ev_signal>)) relative
 timers (C<ev_timer>), absolute timers with customised rescheduling
 (C<ev_periodic>), synchronous signals (C<ev_signal>), process status
 change events (C<ev_child>), and event watchers dealing with the event
 loop mechanism itself (C<ev_idle>, C<ev_embed>, C<ev_prepare> and
 =item ev_tstamp ev_time ()
 Returns the current time as libev would use it. Please note that the
 C<ev_now> function is usually faster and also often returns the timestamp
 you actually want to know. Also interesting is the combination of
-C<ev_update_now> and C<ev_now>.
+C<ev_now_update> 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 ()
 the current system, you would need to look at C<ev_embeddable_backends ()
 & ev_supported_backends ()>, likewise for recommended ones.
 See the description of C<ev_embed> watchers for more info.
-=item ev_set_allocator (void *(*cb)(void *ptr, long size))
+=item ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())
 Sets the allocation function to use (the prototype is similar - the
 semantics are identical to the C<realloc> C89/SuS/POSIX function). It is
 used to allocate and free memory (no surprises here). If it returns zero
 when memory needs to be allocated (C<size != 0>), the library might abort
 You could override this function in high-availability programs to, say,
 free some memory if it cannot allocate memory, to use a special allocator,
 or even to sleep a while and retry until some memory is available.
+Example: The following is the C<realloc> function that libev itself uses
+which should work with C<realloc> and C<free> functions of all kinds and
+is probably a good basis for your own implementation.
+   static void *
+   ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
+   {
+     if (size)
+       return realloc (ptr, size);
+     free (ptr);
+     return 0;
+   }
 Example: Replace the libev allocator with one that waits a bit and then
-retries (example requires a standards-compliant C<realloc>).
+retries.
    static void *
    persistent_realloc (void *ptr, size_t size)
    {
+     if (!size)
+       {
+         free (ptr);
+         return 0;
+       }
      for (;;)
        {
          void *newptr = realloc (ptr, size);
          if (newptr)
    }
    ...
    ev_set_allocator (persistent_realloc);
-=item ev_set_syserr_cb (void (*cb)(const char *msg))
+=item ev_set_syserr_cb (void (*cb)(const char *msg) throw ())
 Set the callback function to call on a retryable system call error (such
 as failed select, poll, epoll_wait). The message is a printable string
 indicating the system call or subsystem causing the problem. If this
 callback is set, then libev will expect it to remedy the situation, no
 If this flag bit is or'ed into the flag value (or the program runs setuid
 or setgid) then libev will I<not> look at the environment variable
 C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will
 override the flags completely if it is found in the environment. This is
-useful to try out specific backends to test their performance, or to work
+useful to try out specific backends to test their performance, to work
-around bugs.
+around bugs, or to make libev threadsafe (accessing environment variables
+cannot be done in a threadsafe way, but usually it works if no other
+thread modifies them).
 =item C<EVFLAG_FORKCHECK>
 Instead of calling C<ev_loop_fork> manually after a fork, you can also
 make libev check for a fork in each iteration by enabling this flag.
 This works by calling C<getpid ()> on every iteration of the loop,
 and thus this might slow down your event loop if you do a lot of loop
 iterations and little real work, but is usually not noticeable (on my
-GNU/Linux system for example, C<getpid> is actually a simple 5-insn sequence
+GNU/Linux system for example, C<getpid> is actually a simple 5-insn
-without a system call and thus I<very> fast, but my GNU/Linux system also has
+sequence without a system call and thus I<very> fast, but my GNU/Linux
-C<pthread_atfork> which is even faster).
+system also has C<pthread_atfork> which is even faster). (Update: glibc
+versions 2.25 apparently removed the C<getpid> optimisation again).
 The big advantage of this flag is that you can forget about fork (and
-forget about forgetting to tell libev about forking) when you use this
+forget about forgetting to tell libev about forking, although you still
-flag.
+have to ignore C<SIGPIPE>) when you use this flag.
 This flag setting cannot be overridden or specified in the C<LIBEV_FLAGS>
 environment variable.
 =item C<EVFLAG_NOINOTIFY>
 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.
 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
 All this means that, in practice, C<EVBACKEND_SELECT> can be as fast or
 faster than epoll for maybe up to a hundred file descriptors, depending on
 the usage. So sad.
 While nominally embeddable in other event loops, this feature is broken in
-all kernel versions tested so far.
+a lot of kernel revisions, but probably(!) works in current versions.
+This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
+C<EVBACKEND_POLL>.
+=item C<EVBACKEND_LINUXAIO>   (value 64, Linux)
+Use the linux-specific linux aio (I<not> C<< aio(7) >> but C<<
+io_submit(2) >>) event interface available in post-4.18 kernels.
+If this backend works for you (as of this writing, it was very
+experimental), it is the best event interface available on linux and might
+be well worth enabling it - if it isn't available in your kernel this will
+be detected and this backend will be skipped.
+This backend can batch oneshot requests and supports a user-space ring
+buffer to receive events. It also doesn't suffer from most of the design
+problems of epoll (such as not being able to remove event sources from the
+epoll set), and generally sounds too good to be true. Because, this being
+the linux kernel, of course it suffers from a whole new set of limitations.
+For one, it is not easily embeddable (but probably could be done using
+an event fd at some extra overhead). It also is subject to a system wide
+limit that can be configured in F</proc/sys/fs/aio-max-nr> - each loop
+currently requires C<61> of this number. If no aio requests are left, this
+backend will be skipped during initialisation.
+Most problematic in practise, however, is that not all file descriptors
+work with it. For example, in linux 5.1, tcp sockets, pipes, event fds,
+files, F</dev/null> and a few others are supported, but ttys do not work
+properly (a known bug that the kernel developers don't care about, see
+L<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
+(yet?) a generic event polling interface.
+Overall, it seems the linux developers just don't want it to have a
+generic event handling mechanism other than C<select> or C<poll>.
+To work around the fd type problem, the current version of libev uses
+epoll as a fallback for file deescriptor types that do not work. Epoll
+is used in, kind of, slow mode that hopefully avoids most of its design
+problems and requires 1-3 extra syscalls per active fd every iteration.
 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
 C<EVBACKEND_POLL>.
 =item C<EVBACKEND_KQUEUE>  (value 8, most BSD clones)
 It scales in the same way as the epoll backend, but the interface to the
 kernel is more efficient (which says nothing about its actual speed, of
 course). While stopping, setting and starting an I/O watcher does never
 cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to
-two event changes per incident. Support for C<fork ()> is very bad (but
+two event changes per incident. Support for C<fork ()> is very bad (you
-sane, unlike epoll) and it drops fds silently in similarly hard-to-detect
+might have to leak fd's on fork, but it's more sane than epoll) and it
-cases
+drops fds silently in similarly hard-to-detect cases.
 This backend usually performs well under most conditions.
 While nominally embeddable in other event loops, this doesn't work
 everywhere, so you might need to test for this. And since it is broken
 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>.
 Example: Use whatever libev has to offer, but make sure that kqueue is
 used if available.
    struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
+Example: Similarly, on linux, you mgiht want to take advantage of the
+linux aio backend if possible, but fall back to something else if that
+isn't available.
+   struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
 =item ev_loop_destroy (loop)
 Destroys an event loop object (frees all memory and kernel state
 etc.). None of the active event watchers will be stopped in the normal
 If you need dynamically allocated loops it is better to use C<ev_loop_new>
 and C<ev_loop_destroy>.
 =item ev_loop_fork (loop)
-This function sets a flag that causes subsequent C<ev_run> iterations to
+This function sets a flag that causes subsequent C<ev_run> iterations
-reinitialise the kernel state for backends that have one. Despite the
+to reinitialise the kernel state for backends that have one. Despite
-name, you can call it anytime, but it makes most sense after forking, in
+the name, you can call it anytime you are allowed to start or stop
-the child process. You I<must> call it (or use C<EVFLAG_FORKCHECK>) in the
+watchers (except inside an C<ev_prepare> callback), but it makes most
+sense after forking, in the child process. You I<must> call it (or use
-child before resuming or calling C<ev_run>.
+C<EVFLAG_FORKCHECK>) in the child before resuming or calling C<ev_run>.
+In addition, if you want to reuse a loop (via this function or
+C<EVFLAG_FORKCHECK>), you I<also> have to ignore C<SIGPIPE>.
 Again, you I<have> to call it on I<any> loop that you want to re-use after
 a fork, I<even if you do not plan to use the loop in the parent>. This is
 because some kernel interfaces *cough* I<kqueue> *cough* do funny things
 during fork.
 On the other hand, you only need to call this function in the child
 This function is rarely useful, but when some event callback runs for a
 very long time without entering the event loop, updating libev's idea of
 the current time is a good idea.
-See also L<The special problem of time updates> in the C<ev_timer> section.
+See also L</The special problem of time updates> in the C<ev_timer> section.
 =item ev_suspend (loop)
 =item ev_resume (loop)
 without a previous call to C<ev_suspend>.
 Calling C<ev_suspend>/C<ev_resume> has the side effect of updating the
 event loop time (see C<ev_now_update>).
-=item ev_run (loop, int flags)
+=item bool ev_run (loop, int flags)
 Finally, this is it, the event handler. This function usually is called
 after you have initialised all your watchers and you want to start
 handling events. It will ask the operating system for any new events, call
-the watcher callbacks, an then repeat the whole process indefinitely: This
+the watcher callbacks, and then repeat the whole process indefinitely: This
 is why event loops are called I<loops>.
 If the flags argument is specified as C<0>, it will keep handling events
 until either no event watchers are active anymore or C<ev_break> was
 called.
+The return value is false if there are no more active watchers (which
+usually means "all jobs done" or "deadlock"), and true in all other cases
+(which usually means " you should call C<ev_run> again").
 Please note that an explicit C<ev_break> is usually better than
 relying on all watchers to be stopped when deciding when a program has
 finished (especially in interactive programs), but having a program
 that automatically loops as long as it has to and no longer by virtue
 of relying on its watchers stopping correctly, that is truly a thing of
 beauty.
-This function is also I<mostly> exception-safe - you can break out of
+This function is I<mostly> exception-safe - you can break out of a
-a C<ev_run> call by calling C<longjmp> in a callback, throwing a C++
+C<ev_run> call by calling C<longjmp> in a callback, throwing a C++
 exception and so on. This does not decrement the C<ev_depth> value, nor
 will it clear any outstanding C<EVBREAK_ONE> breaks.
 A flags value of C<EVRUN_NOWAIT> will look for new events, will handle
 those events and any already outstanding ones, but will not wait and
 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
 invoke the actual watchers inside another context (another thread etc.).
 If you want to reset the callback, use C<ev_invoke_pending> as new
 callback.
-=item ev_set_loop_release_cb (loop, void (*release)(EV_P), void (*acquire)(EV_P))
+=item ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())
 Sometimes you want to share the same loop between multiple threads. This
 can be done relatively simply by putting mutex_lock/unlock calls around
 each call to a libev function.
 However, C<ev_run> can run an indefinite time, so it is not feasible
 to wait for it to return. One way around this is to wake up the event
-loop via C<ev_break> and C<av_async_send>, another way is to set these
+loop via C<ev_break> and C<ev_async_send>, another way is to set these
 I<release> and I<acquire> callbacks on the loop.
 When set, then C<release> will be called just before the thread is
 suspended waiting for new events, and C<acquire> is called just
 afterwards.
 =item C<EV_PREPARE>
 =item C<EV_CHECK>
-All C<ev_prepare> watchers are invoked just I<before> C<ev_run> starts
+All C<ev_prepare> watchers are invoked just I<before> C<ev_run> starts to
-to gather new events, and all C<ev_check> watchers are invoked just after
+gather new events, and all C<ev_check> watchers are queued (not invoked)
-C<ev_run> has gathered them, but before it invokes any callbacks for any
+just after C<ev_run> has gathered them, but before it queues any callbacks
+for any received events. That means C<ev_prepare> watchers are the last
+watchers invoked before the event loop sleeps or polls for new events, and
+C<ev_check> watchers will be invoked before any other watchers of the same
+or lower priority within an event loop iteration.
-received events. Callbacks of both watcher types can start and stop as
+Callbacks of both watcher types can start and stop as many watchers as
-many watchers as they want, and all of them will be taken into account
+they want, and all of them will be taken into account (for example, a
-(for example, a C<ev_prepare> watcher might start an idle watcher to keep
+C<ev_prepare> watcher might start an idle watcher to keep C<ev_run> from
-C<ev_run> from blocking).
+blocking).
 =item C<EV_EMBED>
 The embedded event loop specified in the C<ev_embed> watcher needs attention.
 =item callback ev_cb (ev_TYPE *watcher)
 Returns the callback currently set on the watcher.
-=item ev_cb_set (ev_TYPE *watcher, callback)
+=item ev_set_cb (ev_TYPE *watcher, callback)
 Change the callback. You can change the callback at virtually any time
 (modulo threads).
 =item ev_set_priority (ev_TYPE *watcher, int priority)
 or might not have been clamped to the valid range.
 The default priority used by watchers when no priority has been set is
 always C<0>, which is supposed to not be too high and not be too low :).
-See L<WATCHER PRIORITY MODELS>, below, for a more thorough treatment of
+See L</WATCHER PRIORITY MODELS>, below, for a more thorough treatment of
 priorities.
 =item ev_invoke (loop, ev_TYPE *watcher, int revents)
 Invoke the C<watcher> with the given C<loop> and C<revents>. Neither
 See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related
 functions that do not need a watcher.
 =back
-See also the L<ASSOCIATING CUSTOM DATA WITH A WATCHER> and L<BUILDING YOUR
+See also the L</ASSOCIATING CUSTOM DATA WITH A WATCHER> and L</BUILDING YOUR
 OWN COMPOSITE WATCHERS> idioms.
 =head2 WATCHER STATES
 There are various watcher states mentioned throughout this manual -
 transition between them will be described in more detail - and while these
 rules might look complicated, they usually do "the right thing".
 =over 4
-=item initialiased
+=item initialised
-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 an event loop. It can be moved around, freed, reused etc. at
 But really, best use non-blocking mode.
 =head3 The special problem of disappearing file descriptors
-Some backends (e.g. kqueue, epoll) need to be told about closing a file
+Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
-descriptor (either due to calling C<close> explicitly or any other means,
+a file descriptor (either due to calling C<close> explicitly or any other
-such as C<dup2>). The reason is that you register interest in some file
+means, such as C<dup2>). The reason is that you register interest in some
-descriptor, but when it goes away, the operating system will silently drop
+file descriptor, but when it goes away, the operating system will silently
-this interest. If another file descriptor with the same number then is
+drop this interest. If another file descriptor with the same number then
-registered with libev, there is no efficient way to see that this is, in
+is registered with libev, there is no efficient way to see that this is,
-fact, a different file descriptor.
+in fact, a different file descriptor.
 To avoid having to explicitly tell libev about such cases, libev follows
 the following policy:  Each time C<ev_io_set> is being called, libev
 will assume that this is potentially a new file descriptor, otherwise
 it is assumed that the file descriptor stays the same. That means that
 when you rarely read from a file instead of from a socket, and want to
 reuse the same code path.
 =head3 The special problem of fork
-Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
+Some backends (epoll, kqueue, probably linuxaio) do not support C<fork ()>
-useless behaviour. Libev fully supports fork, but needs to be told about
+at all or exhibit useless behaviour. Libev fully supports fork, but needs
-it in the child if you want to continue to use it in the child.
+to be told about it in the child if you want to continue to use it in the
+child.
 To support fork in your child processes, you have to call C<ev_loop_fork
 ()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to
 C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
 detecting time jumps is hard, and some inaccuracies are unavoidable (the
 monotonic clock option helps a lot here).
 The callback is guaranteed to be invoked only I<after> its timeout has
 passed (not I<at>, so on systems with very low-resolution clocks this
-might introduce a small delay). If multiple timers become ready during the
+might introduce a small delay, see "the special problem of being too
+early", below). If multiple timers become ready during the same loop
-same loop iteration then the ones with earlier time-out values are invoked
+iteration then the ones with earlier time-out values are invoked before
-before ones of the same priority with later time-out values (but this is
+ones of the same priority with later time-out values (but this is no
-no longer true when a callback calls C<ev_run> recursively).
+longer true when a callback calls C<ev_run> recursively).
 =head3 Be smart about timeouts
 Many real-world problems involve some kind of timeout, usually for error
 recovery. A typical example is an HTTP request - if the other side hangs,
 In this case, it would be more efficient to leave the C<ev_timer> alone,
 but remember the time of last activity, and check for a real timeout only
 within the callback:
+   ev_tstamp timeout = 60.;
    ev_tstamp last_activity; // time of last activity
+   ev_timer timer;
    static void
    callback (EV_P_ ev_timer *w, int revents)
    {
-     ev_tstamp now     = ev_now (EV_A);
+     // calculate when the timeout would happen
-     ev_tstamp timeout = last_activity + 60.;
+     ev_tstamp after = last_activity - ev_now (EV_A) + timeout;
-     // if last_activity + 60. is older than now, we did time out
+     // if negative, it means we the timeout already occurred
-     if (timeout < now)
+     if (after < 0.)
        {
          // timeout occurred, take action
        }
      else
        {
-         // callback was invoked, but there was some activity, re-arm
+         // callback was invoked, but there was some recent
-         // the watcher to fire in last_activity + 60, which is
+         // activity. simply restart the timer to time out
-         // guaranteed to be in the future, so "again" is positive:
+         // after "after" seconds, which is the earliest time
-         w->repeat = timeout - now;
+         // the timeout can occur.
+         ev_timer_set (w, after, 0.);
-         ev_timer_again (EV_A_ w);
+         ev_timer_start (EV_A_ w);
        }
    }
-To summarise the callback: first calculate the real timeout (defined
+To summarise the callback: first calculate in how many seconds the
-as "60 seconds after the last activity"), then check if that time has
+timeout will occur (by calculating the absolute time when it would occur,
-been reached, which means something I<did>, in fact, time out. Otherwise
+C<last_activity + timeout>, and subtracting the current time, C<ev_now
-the callback was invoked too early (C<timeout> is in the future), so
+(EV_A)> from that).
-re-schedule the timer to fire at that future time, to see if maybe we have
-a timeout then.
-Note how C<ev_timer_again> is used, taking advantage of the
+If this value is negative, then we are already past the timeout, i.e. we
-C<ev_timer_again> optimisation when the timer is already running.
+timed out, and need to do whatever is needed in this case.
+Otherwise, we now the earliest time at which the timeout would trigger,
+and simply start the timer with this timeout value.
+In other words, each time the callback is invoked it will check whether
+the timeout occurred. If not, it will simply reschedule itself to check
+again at the earliest time it could time out. Rinse. Repeat.
 This scheme causes more callback invocations (about one every 60 seconds
 minus half the average time between activity), but virtually no calls to
 libev to change the timeout.
-To start the timer, simply initialise the watcher and set C<last_activity>
+To start the machinery, simply initialise the watcher and set
-to the current time (meaning we just have some activity :), then call the
+C<last_activity> to the current time (meaning there was some activity just
-callback, which will "do the right thing" and start the timer:
+now), then call the callback, which will "do the right thing" and start
+the timer:
+   last_activity = ev_now (EV_A);
-   ev_init (timer, callback);
+   ev_init (&timer, callback);
-   last_activity = ev_now (loop);
+   callback (EV_A_ &timer, 0);
-   callback (loop, timer, EV_TIMER);
-And when there is some activity, simply store the current time in
+When there is some activity, simply store the current time in
 C<last_activity>, no libev calls at all:
+   if (activity detected)
-   last_activity = ev_now (loop);
+     last_activity = ev_now (EV_A);
+When your timeout value changes, then the timeout can be changed by simply
+providing a new value, stopping the timer and calling the callback, which
+will again do the right thing (for example, time out immediately :).
+   timeout = new_value;
+   ev_timer_stop (EV_A_ &timer);
+   callback (EV_A_ &timer, 0);
 This technique is slightly more complex, but in most cases where the
 time-out is unlikely to be triggered, much more efficient.
-Changing the timeout is trivial as well (if it isn't hard-coded in the
-callback :) - just change the timeout and invoke the callback, which will
-fix things for you.
 =item 4. Wee, just use a double-linked list for your timeouts.
 If there is not one request, but many thousands (millions...), all
 employing some kind of timeout with the same timeout value, then one can
 Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
 rather complicated, but extremely efficient, something that really pays
 off after the first million or so of active timers, i.e. it's usually
 overkill :)
+=head3 The special problem of being too early
+If you ask a timer to call your callback after three seconds, then
+you expect it to be invoked after three seconds - but of course, this
+cannot be guaranteed to infinite precision. Less obviously, it cannot be
+guaranteed to any precision by libev - imagine somebody suspending the
+process with a STOP signal for a few hours for example.
+So, libev tries to invoke your callback as soon as possible I<after> the
+delay has occurred, but cannot guarantee this.
+A less obvious failure mode is calling your callback too early: many event
+loops compare timestamps with a "elapsed delay >= requested delay", but
+this can cause your callback to be invoked much earlier than you would
+expect.
+To see why, imagine a system with a clock that only offers full second
+resolution (think windows if you can't come up with a broken enough OS
+yourself). If you schedule a one-second timer at the time 500.9, then the
+event loop will schedule your timeout to elapse at a system time of 500
+(500.9 truncated to the resolution) + 1, or 501.
+If an event library looks at the timeout 0.1s later, it will see "501 >=
+501" and invoke the callback 0.1s after it was started, even though a
+one-second delay was requested - this is being "too early", despite best
+intentions.
+This is the reason why libev will never invoke the callback if the elapsed
+delay equals the requested delay, but only when the elapsed delay is
+larger than the requested delay. In the example above, libev would only invoke
+the callback at system time 502, or 1.1s after the timer was started.
+So, while libev cannot guarantee that your callback will be invoked
+exactly when requested, it I<can> and I<does> guarantee that the requested
+delay has actually elapsed, or in other words, it always errs on the "too
+late" side of things.
 =head3 The special problem of time updates
-Establishing the current time is a costly operation (it usually takes at
+Establishing the current time is a costly operation (it usually takes
-least two system calls): EV therefore updates its idea of the current
+at least one system call): EV therefore updates its idea of the current
 time only before and after C<ev_run> collects new events, which causes a
 growing difference between C<ev_now ()> and C<ev_time ()> when handling
 lots of events in one iteration.
 The relative timeouts are calculated relative to the C<ev_now ()>
 time. This is usually the right thing as this timestamp refers to the time
 of the event triggering whatever timeout you are modifying/starting. If
 you suspect event processing to be delayed and you I<need> to base the
-timeout on the current time, use something like this to adjust for this:
+timeout on the current time, use something like the following to adjust
+for it:
-   ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
+   ev_timer_set (&timer, after + (ev_time () - ev_now ()), 0.);
 If the event loop is suspended for a long time, you can also force an
 update of the time returned by C<ev_now ()> by calling C<ev_now_update
-()>.
+()>, although that will push the event time of all outstanding events
+further into the future.
+=head3 The special problem of unsynchronised clocks
+Modern systems have a variety of clocks - libev itself uses the normal
+"wall clock" clock and, if available, the monotonic clock (to avoid time
+jumps).
+Neither of these clocks is synchronised with each other or any other clock
+on the system, so C<ev_time ()> might return a considerably different time
+than C<gettimeofday ()> or C<time ()>. On a GNU/Linux system, for example,
+a call to C<gettimeofday> might return a second count that is one higher
+than a directly following call to C<time>.
+The moral of this is to only compare libev-related timestamps with
+C<ev_time ()> and C<ev_now ()>, at least if you want better precision than
+a second or so.
+One more problem arises due to this lack of synchronisation: if libev uses
+the system monotonic clock and you compare timestamps from C<ev_time>
+or C<ev_now> from when you started your timer and when your callback is
+invoked, you will find that sometimes the callback is a bit "early".
+This is because C<ev_timer>s work in real time, not wall clock time, so
+libev makes sure your callback is not invoked before the delay happened,
+I<measured according to the real time>, not the system clock.
+If your timeouts are based on a physical timescale (e.g. "time out this
+connection after 100 seconds") then this shouldn't bother you as it is
+exactly the right behaviour.
+If you want to compare wall clock/system timestamps to your timers, then
+you need to use C<ev_periodic>s, as these are based on the wall clock
+time, where your comparisons will always generate correct results.
 =head3 The special problems of suspended animation
 When you leave the server world it is quite customary to hit machines that
 can suspend/hibernate - what happens to the clocks during such a suspend?
 =item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
 =item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
-Configure the timer to trigger after C<after> seconds. If C<repeat>
+Configure the timer to trigger after C<after> seconds (fractional and
-is C<0.>, then it will automatically be stopped once the timeout is
+negative values are supported). If C<repeat> is C<0.>, then it will
-reached. If it is positive, then the timer will automatically be
+automatically be stopped once the timeout is reached. If it is positive,
-configured to trigger again C<repeat> seconds later, again, and again,
+then the timer will automatically be configured to trigger again C<repeat>
-until stopped manually.
+seconds later, again, and again, until stopped manually.
 The timer itself will do a best-effort at avoiding drift, that is, if
 you configure a timer to trigger every 10 seconds, then it will normally
 trigger at exactly 10 second intervals. If, however, your program cannot
 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:
+repeating. It basically works like calling C<ev_timer_stop>, updating the
+timeout to the C<repeat> value and calling C<ev_timer_start>.
+The exact semantics are as in the following rules, all of which will be
+applied to the watcher:
+=over 4
-If the timer is pending, its pending status is cleared.
+=item If the timer is pending, the pending status is always cleared.
-If the timer is started but non-repeating, stop it (as if it timed out).
+=item If the timer is started but non-repeating, stop it (as if it timed
+out, without invoking it).
-If the timer is repeating, either start it if necessary (with the
+=item If the timer is repeating, make the C<repeat> value the new timeout
-C<repeat> value), or reset the running timer to the C<repeat> value.
+and start the timer, if necessary.
+=back
-This sounds a bit complicated, see L<Be smart about timeouts>, above, for a
+This sounds a bit complicated, see L</Be smart about timeouts>, above, for a
 usage example.
 =item ev_tstamp ev_timer_remaining (loop, ev_timer *)
 Returns the remaining time until a timer fires. If the timer is active,
 Periodic watchers are also timers of a kind, but they are very versatile
 (and unfortunately a bit complex).
 Unlike C<ev_timer>, periodic watchers are not based on real time (or
 relative time, the physical time that passes) but on wall clock time
-(absolute time, the thing you can read on your calender or clock). The
+(absolute time, the thing you can read on your calendar or clock). The
 difference is that wall clock time can run faster or slower than real
 time, and time jumps are not uncommon (e.g. when you adjust your
 wrist-watch).
 You can tell a periodic watcher to trigger after some specific point
 C<ev_timer>, which would still trigger roughly 10 seconds after starting
 it, as it uses a relative timeout).
 C<ev_periodic> watchers can also be used to implement vastly more complex
 timers, such as triggering an event on each "midnight, local time", or
-other complicated rules. This cannot be done with C<ev_timer> watchers, as
+other complicated rules. This cannot easily be done with C<ev_timer>
-those cannot react to time jumps.
+watchers, as those cannot react to time jumps.
 As with timers, the callback is guaranteed to be invoked only when the
 point in time where it is supposed to trigger has passed. If multiple
 timers become ready during the same loop iteration then the ones with
 earlier time-out values are invoked before ones with later time-out values
 NOTE: I<< This callback must always return a time that is higher than or
 equal to the passed C<now> value >>.
 This can be used to create very complex timers, such as a timer that
-triggers on "next midnight, local time". To do this, you would calculate the
+triggers on "next midnight, local time". To do this, you would calculate
-next midnight after C<now> and return the timestamp value for this. How
+the next midnight after C<now> and return the timestamp value for
-you do this is, again, up to you (but it is not trivial, which is the main
+this. Here is a (completely untested, no error checking) example on how to
-reason I omitted it as an example).
+do this:
+   #include <time.h>
+   static ev_tstamp
+   my_rescheduler (ev_periodic *w, ev_tstamp now)
+   {
+     time_t tnow = (time_t)now;
+     struct tm tm;
+     localtime_r (&tnow, &tm);
+     tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
+     ++tm.tm_mday; // midnight next day
+     return mktime (&tm);
+   }
+Note: this code might run into trouble on days that have more then two
+midnights (beginning and end).
 =back
 =item ev_periodic_again (loop, ev_periodic *)
    ev_periodic hourly_tick;
    ev_periodic_init (&hourly_tick, clock_cb,
                      fmod (ev_now (loop), 3600.), 3600., 0);
    ev_periodic_start (loop, &hourly_tick);
 =head2 C<ev_signal> - signal me when a signal gets signalled!
 Signal watchers will trigger an event when the process receives a specific
 signal one or more times. Even though signals are very asynchronous, libev
 only within the same loop, i.e. you can watch for C<SIGINT> in your
 default loop and for C<SIGIO> in another loop, but you cannot watch for
 C<SIGINT> in both the default loop and another loop at the same time. At
 the moment, C<SIGCHLD> is permanently tied to the default loop.
-When the first watcher gets started will libev actually register something
+Only after the first watcher for a signal is started will libev actually
-with the kernel (thus it coexists with your own signal handlers as long as
+register something with the kernel. It thus coexists with your own signal
-you don't register any with libev for the same signal).
+handlers as long as you don't register any with libev for the same signal.
 If possible and supported, libev will install its handlers with
 C<SA_RESTART> (or equivalent) behaviour enabled, so system calls should
 not be unduly interrupted. If you have a problem with system calls getting
 interrupted by signals you can block all signals in an C<ev_check> watcher
 =head2 C<ev_stat> - did the file attributes just change?
 This watches a file system path for attribute changes. That is, it calls
 C<stat> on that path in regular intervals (or when the OS says it changed)
-and sees if it changed compared to the last time, invoking the callback if
+and sees if it changed compared to the last time, invoking the callback
-it did.
+if it did. Starting the watcher C<stat>'s the file, so only changes that
+happen after the watcher has been started will be reported.
 The path does not need to exist: changing from "path exists" to "path does
 not exist" is a status change like any other. The condition "path does not
 exist" (or more correctly "path cannot be stat'ed") is signified by the
 C<st_nlink> field being zero (which is otherwise always forced to be at
 Apart from keeping your process non-blocking (which is a useful
 effect on its own sometimes), idle watchers are a good place to do
 "pseudo-background processing", or delay processing stuff to after the
 event loop has handled all outstanding events.
+=head3 Abusing an C<ev_idle> watcher for its side-effect
+As long as there is at least one active idle watcher, libev will never
+sleep unnecessarily. Or in other words, it will loop as fast as possible.
+For this to work, the idle watcher doesn't need to be invoked at all - the
+lowest priority will do.
+This mode of operation can be useful together with an C<ev_check> watcher,
+to do something on each event loop iteration - for example to balance load
+between different connections.
+See L</Abusing an ev_check watcher for its side-effect> for a longer
+example.
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 =item ev_idle_init (ev_idle *, callback)
 callback, free it. Also, use no error checking, as usual.
    static void
    idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
    {
+     // stop the watcher
+     ev_idle_stop (loop, w);
+     // now we can free it
      free (w);
      // now do something you wanted to do when the program has
      // no longer anything immediate to do.
    }
    ev_idle *idle_watcher = malloc (sizeof (ev_idle));
    ev_idle_start (loop, idle_watcher);
 =head2 C<ev_prepare> and C<ev_check> - customise your event loop!
-Prepare and check watchers are usually (but not always) used in pairs:
+Prepare and check watchers are often (but not always) used in pairs:
 prepare watchers get invoked before the process blocks and check watchers
 afterwards.
-You I<must not> call C<ev_run> or similar functions that enter
+You I<must not> call C<ev_run> (or similar functions that enter the
-the current event loop from either C<ev_prepare> or C<ev_check>
+current event loop) or C<ev_loop_fork> from either C<ev_prepare> or
-watchers. Other loops than the current one are fine, however. The
+C<ev_check> watchers. Other loops than the current one are fine,
-rationale behind this is that you do not need to check for recursion in
+however. The rationale behind this is that you do not need to check
-those watchers, i.e. the sequence will always be C<ev_prepare>, blocking,
+for recursion in those watchers, i.e. the sequence will always be
-C<ev_check> so if you have one watcher of each kind they will always be
+C<ev_prepare>, blocking, C<ev_check> so if you have one watcher of each
-called in pairs bracketing the blocking call.
+kind they will always be called in pairs bracketing the blocking call.
 Their main purpose is to integrate other event mechanisms into libev and
 their use is somewhat advanced. They could be used, for example, to track
 variable changes, implement your own watchers, integrate net-snmp or a
 coroutine library and lots more. They are also occasionally useful if
 with priority higher than or equal to the event loop and one coroutine
 of lower priority, but only once, using idle watchers to keep the event
 loop from blocking if lower-priority coroutines are active, thus mapping
 low-priority coroutines to idle/background tasks).
-It is recommended to give C<ev_check> watchers highest (C<EV_MAXPRI>)
+When used for this purpose, it is recommended to give C<ev_check> watchers
-priority, to ensure that they are being run before any other watchers
+highest (C<EV_MAXPRI>) priority, to ensure that they are being run before
-after the poll (this doesn't matter for C<ev_prepare> watchers).
+any other watchers after the poll (this doesn't matter for C<ev_prepare>
+watchers).
 Also, C<ev_check> watchers (and C<ev_prepare> watchers, too) should not
 activate ("feed") events into libev. While libev fully supports this, they
 might get executed before other C<ev_check> watchers did their job. As
 C<ev_check> watchers are often used to embed other (non-libev) event
 loops those other event loops might be in an unusable state until their
 C<ev_check> watcher ran (always remind yourself to coexist peacefully with
 others).
+=head3 Abusing an C<ev_check> watcher for its side-effect
+C<ev_check> (and less often also C<ev_prepare>) watchers can also be
+useful because they are called once per event loop iteration. For
+example, if you want to handle a large number of connections fairly, you
+normally only do a bit of work for each active connection, and if there
+is more work to do, you wait for the next event loop iteration, so other
+connections have a chance of making progress.
+Using an C<ev_check> watcher is almost enough: it will be called on the
+next event loop iteration. However, that isn't as soon as possible -
+without external events, your C<ev_check> watcher will not be invoked.
+This is where C<ev_idle> watchers come in handy - all you need is a
+single global idle watcher that is active as long as you have one active
+C<ev_check> watcher. The C<ev_idle> watcher makes sure the event loop
+will not sleep, and the C<ev_check> watcher makes sure a callback gets
+invoked. Neither watcher alone can do that.
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 =over 4
 =item ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
-=item ev_embed_set (ev_embed *, callback, struct ev_loop *embedded_loop)
+=item ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)
 Configures the watcher to embed the given loop, which must be
 embeddable. If the callback is C<0>, then C<ev_embed_sweep> will be
 invoked automatically, otherwise it is the responsibility of the callback
 to invoke it (it will continue to be called until the sweep has been done,
 used).
    struct ev_loop *loop_hi = ev_default_init (0);
    struct ev_loop *loop_lo = 0;
    ev_embed embed;
    // see if there is a chance of getting one that works
    // (remember that a flags value of 0 means autodetection)
    loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
      ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
      : 0;
 C<loop_socket>. (One might optionally use C<EVFLAG_NOENV>, too).
    struct ev_loop *loop = ev_default_init (0);
    struct ev_loop *loop_socket = 0;
    ev_embed embed;
    if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
      if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
        {
          ev_embed_init (&embed, 0, loop_socket);
          ev_embed_start (loop, &embed);
 =head2 C<ev_fork> - the audacity to resume the event loop after a fork
 Fork watchers are called when a C<fork ()> was detected (usually because
 whoever is a good citizen cared to tell libev about it by calling
-C<ev_default_fork> or C<ev_loop_fork>). The invocation is done before the
+C<ev_loop_fork>). The invocation is done before the event loop blocks next
-event loop blocks next and before C<ev_check> watchers are being called,
+and before C<ev_check> watchers are being called, and only in the child
-and only in the child after the fork. If whoever good citizen calling
+after the fork. If whoever good citizen calling C<ev_default_fork> cheats
-C<ev_default_fork> cheats and calls it in the wrong process, the fork
+and calls it in the wrong process, the fork handlers will be invoked, too,
-handlers will be invoked, too, of course.
+of course.
 =head3 The special problem of life after fork - how is it possible?
-Most uses of C<fork()> consist of forking, then some simple calls to set
+Most uses of C<fork ()> consist of forking, then some simple calls to set
 up/change the process environment, followed by a call to C<exec()>. This
 sequence should be handled by libev without any problems.
 This changes when the application actually wants to do event handling
 in the child, or both parent in child, in effect "continuing" after the
 it by calling C<ev_async_send>, which is thread- and signal safe.
 This functionality is very similar to C<ev_signal> watchers, as signals,
 too, are asynchronous in nature, and signals, too, will be compressed
 (i.e. the number of callback invocations may be less than the number of
-C<ev_async_sent> calls). In fact, you could use signal watchers as a kind
+C<ev_async_send> 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
 Unlike C<ev_feed_event>, this call is safe to do from other threads,
 signal or similar contexts (see the discussion of C<EV_ATOMIC_T> in the
 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
 There are some other functions of possible interest. Described. Here. Now.
 =over 4
-=item ev_once (loop, int fd, int events, ev_tstamp timeout, callback)
+=item ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)
 This function combines a simple timer and an I/O watcher, calls your
 callback on whichever event happens first and automatically stops both
 watchers. This is useful if you want to wait for a single event on an fd
 or timeout without having to allocate/configure/start/stop/free one or
    ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
 =item ev_feed_fd_event (loop, int fd, int revents)
 Feed an event on the given fd, as if a file descriptor backend detected
-the given events it.
+the given events.
 =item ev_feed_signal_event (loop, int signum)
 Feed an event as if the given signal occurred. See also C<ev_feed_signal>,
 which is async-safe.
    {
      struct my_biggy big = (struct my_biggy *)
        (((char *)w) - offsetof (struct my_biggy, t2));
    }
+=head2 AVOIDING FINISHING BEFORE RETURNING
+Often you have structures like this in event-based programs:
+  callback ()
+  {
+    free (request);
+  }
+  request = start_new_request (..., callback);
+The intent is to start some "lengthy" operation. The C<request> could be
+used to cancel the operation, or do other things with it.
+It's not uncommon to have code paths in C<start_new_request> that
+immediately invoke the callback, for example, to report errors. Or you add
+some caching layer that finds that it can skip the lengthy aspects of the
+operation and simply invoke the callback with the result.
+The problem here is that this will happen I<before> C<start_new_request>
+has returned, so C<request> is not set.
+Even if you pass the request by some safer means to the callback, you
+might want to do something to the request after starting it, such as
+canceling it, which probably isn't working so well when the callback has
+already been invoked.
+A common way around all these issues is to make sure that
+C<start_new_request> I<always> returns before the callback is invoked. If
+C<start_new_request> immediately knows the result, it can artificially
+delay invoking the callback by using a C<prepare> or C<idle> watcher for
+example, or more sneakily, by reusing an existing (stopped) watcher and
+pushing it into the pending queue:
+   ev_set_cb (watcher, callback);
+   ev_feed_event (EV_A_ watcher, 0);
+This way, C<start_new_request> can safely return before the callback is
+invoked, while not delaying callback invocation too much.
 =head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS
 Often (especially in GUI toolkits) there are places where you have
 I<modal> interaction, which is most easily implemented by recursively
 invoking C<ev_run>.
 This brings the problem of exiting - a callback might want to finish the
 main C<ev_run> call, but not the nested one (e.g. user clicked "Quit", but
 a modal "Are you sure?" dialog is still waiting), or just the nested one
 and not the main one (e.g. user clocked "Ok" in a modal dialog), or some
-other combination: In these cases, C<ev_break> will not work alone.
+other combination: In these cases, a simple C<ev_break> will not work.
 The solution is to maintain "break this loop" variable for each C<ev_run>
 invocation, and use a loop around C<ev_run> until the condition is
 triggered, using C<EVRUN_ONCE>:
    int exit_main_loop = 0;
    while (!exit_main_loop)
      ev_run (EV_DEFAULT_ EVRUN_ONCE);
-   // in a model watcher
+   // in a modal watcher
    int exit_nested_loop = 0;
    while (!exit_nested_loop)
      ev_run (EV_A_ EVRUN_ONCE);
 called):
    void
    wait_for_event (ev_watcher *w)
    {
-     ev_cb_set (w) = current_coro;
+     ev_set_cb (w, current_coro);
      switch_to (libev_coro);
    }
 That basically suspends the coroutine inside C<wait_for_event> and
 continues the libev coroutine, which, when appropriate, switches back to
-this or any other coroutine. I am sure if you sue this your own :)
+this or any other coroutine.
 You can do similar tricks if you have, say, threads with an event queue -
 instead of storing a coroutine, you store the queue object and instead of
 switching to a coroutine, you push the watcher onto the queue and notify
 any waiters.
-To embed libev, see L<EMBEDDING>, but in short, it's easiest to create two
+To embed libev, see L</EMBEDDING>, but in short, it's easiest to create two
 files, F<my_ev.h> and F<my_ev.c> that include the respective libev files:
    // my_ev.h
    #define EV_CB_DECLARE(type)   struct my_coro *cb;
-   #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb);
+   #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
    #include "../libev/ev.h"
    // my_ev.c
    #define EV_H "my_ev.h"
    #include "../libev/ev.c"
 =back
 =head1 C++ SUPPORT
+=head2 C API
+The normal C API should work fine when used from C++: both ev.h and the
+libev sources can be compiled as C++. Therefore, code that uses the C API
+will work fine.
+Proper exception specifications might have to be added to callbacks passed
+to libev: exceptions may be thrown only from watcher callbacks, all other
+callbacks (allocator, syserr, loop acquire/release and periodic reschedule
+callbacks) must not throw exceptions, and might need a C<noexcept>
+specification. If you have code that needs to be compiled as both C and
+C++ you can use the C<EV_NOEXCEPT> macro for this:
+   static void
+   fatal_error (const char *msg) EV_NOEXCEPT
+   {
+     perror (msg);
+     abort ();
+   }
+   ...
+   ev_set_syserr_cb (fatal_error);
+The only API functions that can currently throw exceptions are C<ev_run>,
+C<ev_invoke>, C<ev_invoke_pending> and C<ev_loop_destroy> (the latter
+because it runs cleanup watchers).
+Throwing exceptions in watcher callbacks is only supported if libev itself
+is compiled with a C++ compiler or your C and C++ environments allow
+throwing exceptions through C libraries (most do).
+=head2 C++ API
 Libev comes with some simplistic wrapper classes for C++ that mainly allow
 you to use some convenience methods to start/stop watchers and also change
 the callback model to a model using method callbacks on objects.
 To use it,
    #include <ev++.h>
 This automatically includes F<ev.h> and puts all of its definitions (many
 of them macros) into the global namespace. All C++ specific things are
 put into the C<ev> namespace. It should support all the same embedding
 with C<operator ()> can be used as callbacks. Other types should be easy
 to add as long as they only need one additional pointer for context. If
 you need support for other types of functors please contact the author
 (preferably after implementing it).
+For all this to work, your C++ compiler either has to use the same calling
+conventions as your C compiler (for static member functions), or you have
+to embed libev and compile libev itself as C++.
 Here is a list of things available in the C<ev> namespace:
 =over 4
 =item C<ev::READ>, C<ev::WRITE> etc.
 =item C<ev::io>, C<ev::timer>, C<ev::periodic>, C<ev::idle>, C<ev::sig> etc.
 For each C<ev_TYPE> watcher in F<ev.h> there is a corresponding class of
 the same name in the C<ev> namespace, with the exception of C<ev_signal>
 which is called C<ev::sig> to avoid clashes with the C<signal> macro
-defines by many implementations.
+defined by many implementations.
 All of those classes have these methods:
 =over 4
      void operator() (ev::io &w, int revents)
      {
        ...
      }
    }
    myfunctor f;
    ev::io w;
    w.set (&f);
 Associates a different C<struct ev_loop> with this watcher. You can only
 do this when the watcher is inactive (and not pending either).
 =item w->set ([arguments])
-Basically the same as C<ev_TYPE_set>, with the same arguments. Either this
+Basically the same as C<ev_TYPE_set> (except for C<ev::embed> watchers>),
-method or a suitable start method must be called at least once. Unlike the
+with the same arguments. Either this method or a suitable start method
-C counterpart, an active watcher gets automatically stopped and restarted
+must be called at least once. Unlike the C counterpart, an active watcher
-when reconfiguring it with this method.
+gets automatically stopped and restarted when reconfiguring it with this
+method.
+For C<ev::embed> watchers this method is called C<set_embed>, to avoid
+clashing with the C<set (loop)> method.
 =item w->start ()
 Starts the watcher. Note that there is no C<loop> argument, as the
 constructor already stores the event loop.
 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/>.
 Brian Maher has written a partial interface to libev for lua (at the
 time of this writing, only C<ev_io> and C<ev_timer>), to be found at
 L<http://github.com/brimworks/lua-ev>.
+=item Javascript
+Node.js (L<http://nodejs.org>) uses libev as the underlying event library.
+=item Others
+There are others, and I stopped counting.
 =back
 =head1 MACRO MAGIC
 suitable for use with C<EV_A>.
 =item C<EV_DEFAULT>, C<EV_DEFAULT_>
 Similar to the other two macros, this gives you the value of the default
-loop, if multiple loops are supported ("ev loop default").
+loop, if multiple loops are supported ("ev loop default"). The default loop
+will be initialised if it isn't already initialised.
+For non-multiplicity builds, these macros do nothing, so you always have
+to initialise the loop somewhere.
 =item C<EV_DEFAULT_UC>, C<EV_DEFAULT_UC_>
 Usage identical to C<EV_DEFAULT> and C<EV_DEFAULT_>, but requires that the
 default loop has been initialised (C<UC> == unchecked). Their behaviour
    ev_vars.h
    ev_wrap.h
    ev_win32.c      required on win32 platforms only
-   ev_select.c     only when select backend is enabled (which is enabled by default)
+   ev_select.c     only when select backend is enabled
-   ev_poll.c       only when poll backend is enabled (disabled by default)
+   ev_poll.c       only when poll backend is enabled
-   ev_epoll.c      only when the epoll backend is enabled (disabled by default)
+   ev_epoll.c      only when the epoll backend is enabled
+   ev_linuxaio.c   only when the linux aio backend is enabled
-   ev_kqueue.c     only when the kqueue backend is enabled (disabled by default)
+   ev_kqueue.c     only when the kqueue backend is enabled
-   ev_port.c       only when the solaris port backend is enabled (disabled by default)
+   ev_port.c       only when the solaris port backend is enabled
 F<ev.c> includes the backend files directly when enabled, so you only need
 to compile this single file.
 =head3 LIBEVENT COMPATIBILITY API
 If programs implement their own fd to handle mapping on win32, then this
 macro can be used to override the C<close> function, useful to unregister
 file descriptors again. Note that the replacement function has to close
 the underlying OS handle.
+=item EV_USE_WSASOCKET
+If defined to be C<1>, libev will use C<WSASocket> to create its internal
+communication socket, which works better in some environments. Otherwise,
+the normal C<socket> function will be used, which works better in other
+environments.
 =item EV_USE_POLL
 If defined to be C<1>, libev will compile in support for the C<poll>(2)
 backend. Otherwise it will be enabled on non-win32 platforms. It
 takes precedence over select.
 If defined to be C<1>, libev will compile in support for the Linux
 C<epoll>(7) backend. Its availability will be detected at runtime,
 otherwise another method will be used as fallback. This is the preferred
 backend for GNU/Linux systems. If undefined, it will be enabled if the
 headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
+=item EV_USE_LINUXAIO
+If defined to be C<1>, libev will compile in support for the Linux
+aio backend. Due to it's currenbt limitations it has to be requested
+explicitly.  If undefined, it will be enabled on linux, otherwise
+disabled.
 =item EV_USE_KQUEUE
 If defined to be C<1>, libev will compile in support for the BSD style
 C<kqueue>(2) backend. Its actual availability will be detected at runtime,
 If defined to be C<1>, libev will compile in support for the Linux inotify
 interface to speed up C<ev_stat> watchers. Its actual availability will
 be detected at runtime. If undefined, it will be enabled if the headers
 indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
+=item EV_NO_SMP
+If defined to be C<1>, libev will assume that memory is always coherent
+between threads, that is, threads can be used, but threads never run on
+different cpus (or different cpu cores). This reduces dependencies
+and makes libev faster.
+=item EV_NO_THREADS
+If defined to be C<1>, libev will assume that it will never be called from
+different threads (that includes signal handlers), which is a stronger
+assumption than C<EV_NO_SMP>, above. This reduces dependencies and makes
+libev faster.
 =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 with respect to other threads or signal contexts. No
-type is easily found in the C language, so you can provide your own type
+such type is easily found in the C language, so you can provide your own
-that you know is safe for your purposes. It is used both for signal handler "locking"
+type that you know is safe for your purposes. It is used both for signal
-as well as for signal and thread safety in C<ev_async> watchers.
+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.
 =item EV_H (h)
 will have the C<struct ev_loop *> as first argument, and you can create
 additional independent event loops. Otherwise there will be no support
 for multiple event loops and there is no first event loop pointer
 argument. Instead, all functions act on the single default loop.
+Note that C<EV_DEFAULT> and C<EV_DEFAULT_> will no longer provide a
+default loop when multiplicity is switched off - you always have to
+initialise the loop manually in this case.
 =item EV_MINPRI
 =item EV_MAXPRI
 The range of allowed priorities. C<EV_MINPRI> must be smaller or equal to
    #define EV_USE_POLL 1
    #define EV_CHILD_ENABLE 1
    #define EV_ASYNC_ENABLE 1
 The actual value is a bitset, it can be a combination of the following
-values:
+values (by default, all of these are enabled):
 =over 4
 =item C<1> - faster/larger code
 code size by roughly 30% on amd64).
 When optimising for size, use of compiler flags such as C<-Os> with
 gcc is recommended, as well as C<-DNDEBUG>, as libev contains a number of
 assertions.
+The default is off when C<__OPTIMIZE_SIZE__> is defined by your compiler
+(e.g. gcc with C<-Os>).
 =item C<2> - faster/larger data structures
 Replaces the small 2-heap for timer management by a faster 4-heap, larger
 hash table sizes and so on. This will usually further increase code size
 and can additionally have an effect on the size of data structures at
 runtime.
+The default is off when C<__OPTIMIZE_SIZE__> is defined by your compiler
+(e.g. gcc with C<-Os>).
 =item C<4> - full API configuration
 This enables priorities (sets C<EV_MAXPRI>=2 and C<EV_MINPRI>=-2), and
 enables multiplicity (C<EV_MULTIPLICITY>=1).
 With an intelligent-enough linker (gcc+binutils are intelligent enough
 when you use C<-Wl,--gc-sections -ffunction-sections>) functions unused by
 your program might be left out as well - a binary starting a timer and an
 I/O watcher then might come out at only 5Kb.
+=item EV_API_STATIC
+If this symbol is defined (by default it is not), then all identifiers
+will have static linkage. This means that libev will not export any
+identifiers, and you cannot link against libev anymore. This can be useful
+when you embed libev, only want to use libev functions in a single file,
+and do not want its identifiers to be visible.
+To use this, define C<EV_API_STATIC> and include F<ev.c> in the file that
+wants to use libev.
+This option only works when libev is compiled with a C compiler, as C++
+doesn't support the required declaration syntax.
 =item EV_AVOID_STDIO
 If this is set to C<1> at compiletime, then libev will avoid using stdio
 functions (printf, scanf, perror etc.). This will increase the code size
 default loop and triggering an C<ev_async> watcher from the default loop
 watcher callback into the event loop interested in the signal.
 =back
-See also L<THREAD LOCKING EXAMPLE>.
+See also L</THREAD LOCKING EXAMPLE>.
 =head3 COROUTINES
 Libev is very accommodating to coroutines ("cooperative threads"):
 libev fully supports nesting calls to its functions from different
 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
 structure (guaranteed by POSIX but not by ISO C for example), but it also
 assumes that the same (machine) code can be used to call any watcher
 callback: The watcher callbacks have different type signatures, but libev
 calls them using an C<ev_watcher *> internally.
+=item null pointers and integer zero are represented by 0 bytes
+Libev uses C<memset> to initialise structs and arrays to C<0> bytes, and
+relies on this setting pointers and integers to null.
 =item pointer accesses must be thread-atomic
 Accessing a pointer value must be atomic, it must both be readable and
 writable in one piece - this is the case on all current architectures.
 thread" or will block signals process-wide, both behaviours would
 be compatible with libev. Interaction between C<sigprocmask> and
 C<pthread_sigmask> could complicate things, however.
 The most portable way to handle signals is to block signals in all threads
-except the initial one, and run the default loop in the initial thread as
+except the initial one, and run the signal handling loop in the initial
-well.
+thread as well.
 =item C<long> must be large enough for common memory allocation sizes
 To improve portability and simplify its API, libev uses C<long> internally
 instead of C<size_t> when allocating its data structures. On non-POSIX
 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 accuracy 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
 =over 4
 =item C<EV_COMPAT3> backwards compatibility mechanism
 The backward compatibility mechanism can be controlled by
-C<EV_COMPAT3>. See L<PREPROCESSOR SYMBOLS/MACROS> in the L<EMBEDDING>
+C<EV_COMPAT3>. See L</"PREPROCESSOR SYMBOLS/MACROS"> in the L</EMBEDDING>
 section.
 =item C<ev_default_destroy> and C<ev_default_fork> have been removed
 These calls can be replaced easily by their C<ev_loop_xxx> counterparts:
 =over 4
 =item active
 A watcher is active as long as it has been started and not yet stopped.
-See L<WATCHER STATES> for details.
+See L</WATCHER STATES> for details.
 =item application
 In this document, an application is whatever is using libev.
 watchers and events.
 =item pending
 A watcher is pending as soon as the corresponding event has been
-detected. See L<WATCHER STATES> for details.
+detected. See L</WATCHER STATES> for details.
 =item real time
 The physical time that is observed. It is apparently strictly monotonic :)

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Comparing libev/ev.pod (file contents): Revision 1.369 by root, Mon May 30 18:34:28 2011 UTC vs. Revision 1.451 by root, Mon Jun 24 00:19:26 2019 UTC

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Revision 1.369 by root, Mon May 30 18:34:28 2011 UTC vs.
Revision 1.451 by root, Mon Jun 24 00:19:26 2019 UTC