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

Comparing libev/ev.pod (file contents):
Revision 1.427 by root, Sun Apr 28 14:57:12 2013 UTC vs.
Revision 1.469 by root, Sat Jun 3 08:53:03 2023 UTC

+=encoding utf-8
 =head1 NAME
 libev - a high performance full-featured event loop written in C
 =head1 SYNOPSIS
 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
 When libev detects a usage error such as a negative timer interval, then
 it will print a diagnostic message and abort (via the C<assert> mechanism,
 so C<NDEBUG> will disable this checking): these are programming errors in
 the libev caller and need to be fixed there.
+Via the C<EV_FREQUENT> macro you can compile in and/or enable extensive
+consistency checking code inside libev that can be used to check for
+internal inconsistencies, suually caused by application bugs.
-Libev also has a few internal error-checking C<assert>ions, and also has
+Libev also has a few internal error-checking C<assert>ions. These do not
-extensive consistency checking code. These do not trigger under normal
-circumstances, as they indicate either a bug in libev or worse.
+trigger under normal circumstances, as they indicate either a bug in libev
+or worse.
 =head1 GLOBAL FUNCTIONS
 These functions can be called anytime, even before initialising the
 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)
 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>
 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<EVFLAG_NOTIMERFD>
+When this flag is specified, the libev will avoid using a C<timerfd> to
+detect time jumps. It will still be able to detect time jumps, but takes
+longer and has a lower accuracy in doing so, but saves a file descriptor
+per loop.
+The current implementation only tries to use a C<timerfd> when the first
+C<ev_periodic> watcher is started and falls back on other methods if it
+cannot be created, but this behaviour might change in the future.
 =item C<EVBACKEND_SELECT>  (value 1, portable select backend)
 This is your standard select(2) backend. Not I<completely> standard, as
 libev tries to roll its own fd_set with no limits on the number of fds,
 This backend maps C<EV_READ> to C<POLLIN | POLLERR | POLLHUP>, and
 C<EV_WRITE> to C<POLLOUT | POLLERR | POLLHUP>.
 =item C<EVBACKEND_EPOLL>   (value 4, Linux)
-Use the linux-specific epoll(7) interface (for both pre- and post-2.6.9
+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, but
 it scales phenomenally better. While poll and select usually scale like
 O(total_fds) where total_fds is the total number of fds (or the highest
 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 (but libev
+only tries to use it in 4.19+).
+This is another Linux train wreck of an event interface.
+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, forcing you to fall back to epoll, inheriting all its design
+issues.
+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>. If no AIO
+requests are left, this backend will be skipped during initialisation, and
+will switch to epoll when the loop is active.
+Most problematic in practice, 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 many 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 all these problem, the current version of libev uses its
+epoll backend as a fallback for file descriptor types that do not work. Or
+falls back completely to epoll if the kernel acts up.
+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)
-Kqueue deserves special mention, as at the time of this writing, it
+Kqueue deserves special mention, as at the time this backend was
-was broken on all BSDs except NetBSD (usually it doesn't work reliably
+implemented, it was broken on all BSDs except NetBSD (usually it doesn't
-with anything but sockets and pipes, except on Darwin, where of course
+work reliably with anything but sockets and pipes, except on Darwin,
-it's completely useless). Unlike epoll, however, whose brokenness
+where of course it's completely useless). Unlike epoll, however, whose
-is by design, these kqueue bugs can (and eventually will) be fixed
+brokenness is by design, these kqueue bugs can be (and mostly have been)
-without API changes to existing programs. For this reason it's not being
+fixed without API changes to existing programs. For this reason it's not
-"auto-detected" unless you explicitly specify it in the flags (i.e. using
+being "auto-detected" on all platforms unless you explicitly specify it
-C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
+in the flags (i.e. using C<EVBACKEND_KQUEUE>) or libev was compiled on a
-system like NetBSD.
+known-to-be-good (-enough) system like NetBSD.
 You still can embed kqueue into a normal poll or select backend and use it
 only for sockets (after having made sure that sockets work with kqueue on
 the target platform). See C<ev_embed> watchers for more info.
 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 (you
-might have to leak fd's on fork, but it's more sane than epoll) and it
+might have to leak fds on fork, but it's more sane than epoll) and it
 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
 and is not embeddable, which would limit the usefulness of this backend
 immensely.
 =item C<EVBACKEND_PORT>    (value 32, Solaris 10)
-This uses the Solaris 10 event port mechanism. As with everything on Solaris,
+This uses the Solaris 10 event port mechanism. As with everything on
-it's really slow, but it still scales very well (O(active_fds)).
+Solaris, it's really slow, but it still scales very well (O(active_fds)).
 While this backend scales well, it requires one system call per active
 file descriptor per loop iteration. For small and medium numbers of file
 descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
 might perform better.
 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
 sense, so e.g. C<ev_is_active> might still return true. It is your
 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
    - Queue all expired timers.
    - Queue all expired periodics.
    - Queue all idle watchers with priority higher than that of pending events.
    - Queue all check watchers.
    - Call all queued watchers in reverse order (i.e. check watchers first).
-     Signals and child watchers are implemented as I/O watchers, and will
+     Signals, async and child watchers are implemented as I/O watchers, and
-     be handled here by queueing them when their watcher gets executed.
+     will be handled here by queueing them when their watcher gets executed.
    - If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
      were used, or there are no active watchers, goto FINISH, otherwise
      continue with step LOOP.
    FINISH:
    - Reset the ev_break status iff it was EVBREAK_ONE.
 with a watcher-specific start function (C<< ev_TYPE_start (loop, watcher
 *) >>), and you can stop watching for events at any time by calling the
 corresponding stop function (C<< ev_TYPE_stop (loop, watcher *) >>.
 As long as your watcher is active (has been started but not stopped) you
-must not touch the values stored in it. Most specifically you must never
+must not touch the values stored in it except when explicitly documented
-reinitialise it or call its C<ev_TYPE_set> macro.
+otherwise. Most specifically you must never reinitialise it or call its
+C<ev_TYPE_set> macro.
 Each and every callback receives the event loop pointer as first, the
 registered watcher structure as second, and a bitset of received events as
 third argument.
 =item bool ev_is_active (ev_TYPE *watcher)
 Returns a true value iff the watcher is active (i.e. it has been started
 and not yet been stopped). As long as a watcher is active you must not modify
-it.
+it unless documented otherwise.
+Obviously, it is safe to call this on an active watcher, or actually any
+watcher that is initialised.
 =item bool ev_is_pending (ev_TYPE *watcher)
 Returns a true value iff the watcher is pending, (i.e. it has outstanding
 events but its callback has not yet been invoked). As long as a watcher
 is pending (but not active) you must not call an init function on it (but
 C<ev_TYPE_set> is safe), you must not change its priority, and you must
 make sure the watcher is available to libev (e.g. you cannot C<free ()>
 it).
+It is safe to call this on any watcher in any state as long as it is
+initialised.
 =item callback ev_cb (ev_TYPE *watcher)
 Returns the callback currently set on the watcher.
 =item ev_set_cb (ev_TYPE *watcher, callback)
 from being executed (except for C<ev_idle> watchers).
 If you need to suppress invocation when higher priority events are pending
 you need to look at C<ev_idle> watchers, which provide this functionality.
-You I<must not> change the priority of a watcher as long as it is active or
+You I<must not> change the priority of a watcher as long as it is active
-pending.
+or pending. Reading the priority with C<ev_priority> is fine in any state.
 Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is
 fine, as long as you do not mind that the priority value you query might
 or might not have been clamped to the valid range.
 =item ev_feed_event (loop, ev_TYPE *watcher, int revents)
 Feeds the given event set into the event loop, as if the specified event
 had happened for the specified watcher (which must be a pointer to an
-initialised but not necessarily started event watcher). Obviously you must
+initialised but not necessarily started event watcher, though it can be
-not free the watcher as long as it has pending events.
+active). Obviously you must not free the watcher as long as it has pending
+events.
 Stopping the watcher, letting libev invoke it, or calling
 C<ev_clear_pending> will clear the pending event, even if the watcher was
 not started in the first place.
 =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
-this state it cannot be accessed (except in a few documented ways), moved,
+this state it cannot be accessed (except in a few documented ways, such as
-freed or anything else - the only legal thing is to keep a pointer to it,
+stoping it), moved, freed or anything else - the only legal thing is to
-and call libev functions on it that are documented to work on active watchers.
+keep a pointer to it, and call libev functions on it that are documented
+to work on active watchers.
+As a rule of thumb, before accessing a member or calling any function on
+a watcher, it should be stopped (or freshly initialised). If that is not
+convenient, you can check the documentation for that function or member to
+see if it is safe to use on an active watcher.
 =item pending
 If a watcher is active and libev determines that an event it is interested
-in has occurred (such as a timer expiring), it will become pending. It will
+in has occurred (such as a timer expiring), it will become pending. It
-stay in this pending state until either it is stopped or its callback is
+will stay in this pending state until either it is explicitly stopped or
-about to be invoked, so it is not normally pending inside the watcher
+its callback is about to be invoked, so it is not normally pending inside
-callback.
+the watcher callback.
-The watcher might or might not be active while it is pending (for example,
+Generally, the watcher might or might not be active while it is pending
-an expired non-repeating timer can be pending but no longer active). If it
+(for example, an expired non-repeating timer can be pending but no longer
-is stopped, it can be freely accessed (e.g. by calling C<ev_TYPE_set>),
+active). If it is pending but not active, it can be freely accessed (e.g.
-but it is still property of the event loop at this time, so cannot be
+by calling C<ev_TYPE_set>), but it is still property of the event loop at
-moved, freed or reused. And if it is active the rules described in the
+this time, so cannot be moved, freed or reused. And if it is active the
-previous item still apply.
+rules described in the previous item still apply.
+Explicitly stopping a watcher will also clear the pending state
+unconditionally, so it is safe to stop a watcher and then free it.
 It is also possible to feed an event on a watcher that is not active (e.g.
 via C<ev_feed_event>), in which case it becomes pending without being
 active.
 Many event loops support I<watcher priorities>, which are usually small
 integers that influence the ordering of event callback invocation
 between watchers in some way, all else being equal.
-In libev, Watcher priorities can be set using C<ev_set_priority>. See its
+In libev, watcher priorities can be set using C<ev_set_priority>. See its
 description for the more technical details such as the actual priority
 range.
 There are two common ways how these these priorities are being interpreted
 by event loops:
 This section describes each watcher in detail, but will not repeat
 information given in the last section. Any initialisation/set macros,
 functions and members specific to the watcher type are explained.
-Members are additionally marked with either I<[read-only]>, meaning that,
+Most members are additionally marked with either I<[read-only]>, meaning
-while the watcher is active, you can look at the member and expect some
+that, while the watcher is active, you can look at the member and expect
-sensible content, but you must not modify it (you can modify it while the
+some sensible content, but you must not modify it (you can modify it while
-watcher is stopped to your hearts content), or I<[read-write]>, which
+the watcher is stopped to your hearts content), or I<[read-write]>, which
-means you can expect it to have some sensible content while the watcher
+means you can expect it to have some sensible content while the watcher is
-is active, but you can also modify it. Modifying it may not do something
+active, but you can also modify it (within the same thread as the event
+loop, i.e. without creating data races). Modifying it may not do something
 sensible or take immediate effect (or do anything at all), but libev will
 not crash or malfunction in any way.
+In any case, the documentation for each member will explain what the
+effects are, and if there are any additional access restrictions.
 =head2 C<ev_io> - is this file descriptor readable or writable?
 I/O watchers check whether a file descriptor is readable or writable
 in each iteration of the event loop, or, more precisely, when reading
 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, linuxaio, iouring) 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>.
 =item ev_io_init (ev_io *, callback, int fd, int events)
 =item ev_io_set (ev_io *, int fd, int events)
 Configures an C<ev_io> watcher. The C<fd> is the file descriptor to
-receive events for and C<events> is either C<EV_READ>, C<EV_WRITE> or
+receive events for and C<events> is either C<EV_READ>, C<EV_WRITE>, both
-C<EV_READ | EV_WRITE>, to express the desire to receive the given events.
+C<EV_READ | EV_WRITE> or C<0>, to express the desire to receive the given
+events.
-=item int fd [read-only]
+Note that setting the C<events> to C<0> and starting the watcher is
+supported, but not specially optimized - if your program sometimes happens
+to generate this combination this is fine, but if it is easy to avoid
+starting an io watcher watching for no events you should do so.
-The file descriptor being watched.
+=item ev_io_modify (ev_io *, int events)
+Similar to C<ev_io_set>, but only changes the requested events. Using this
+might be faster with some backends, as libev can assume that the C<fd>
+still refers to the same underlying file description, something it cannot
+do when using C<ev_io_set>.
+=item int fd [no-modify]
+The file descriptor being watched. While it can be read at any time, you
+must not modify this member even when the watcher is stopped - always use
+C<ev_io_set> for that.
-=item int events [read-only]
+=item int events [no-modify]
-The events being watched.
+The set of events the fd is being watched for, among other flags. Remember
+that this is a bit set - to test for C<EV_READ>, use C<< w->events &
+EV_READ >>, and similarly for C<EV_WRITE>.
+As with C<fd>, you must not modify this member even when the watcher is
+stopped, always use C<ev_io_set> or C<ev_io_modify> for that.
 =back
 =head3 Examples
 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
 =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
 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
 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
 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);
 and calls it in the wrong process, the fork handlers will be invoked, too,
 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
 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
 event loop thread and an unspecified mechanism to wake up the main thread.
 First, you need to associate some data with the event loop:
    typedef struct {
-     mutex_t lock; /* global loop lock */
+     pthread_mutex_t lock; /* global loop lock */
+     pthread_t tid;
+     pthread_cond_t invoke_cv;
      ev_async async_w;
-     thread_t tid;
-     cond_t invoke_cv;
    } userdata;
    void prepare_loop (EV_P)
    {
       // for simplicity, we use a static userdata struct.
       static userdata u;
-      ev_async_init (&u->async_w, async_cb);
+      ev_async_init (&u.async_w, async_cb);
-      ev_async_start (EV_A_ &u->async_w);
+      ev_async_start (EV_A_ &u.async_w);
-      pthread_mutex_init (&u->lock, 0);
+      pthread_mutex_init (&u.lock, 0);
-      pthread_cond_init (&u->invoke_cv, 0);
+      pthread_cond_init (&u.invoke_cv, 0);
       // now associate this with the loop
-      ev_set_userdata (EV_A_ u);
+      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_run
-      pthread_create (&u->tid, 0, l_run, EV_A);
+      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
 that might have been added:
 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"
 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
+to libev: exceptions may be thrown only from watcher callbacks, all other
-other callbacks (allocator, syserr, loop acquire/release and periodic
+callbacks (allocator, syserr, loop acquire/release and periodic reschedule
-reschedule callbacks) must not throw exceptions, and might need a C<throw
+callbacks) must not throw exceptions, and might need a C<noexcept>
-()> specification. If you have code that needs to be compiled as both C
+specification. If you have code that needs to be compiled as both C and
-and C++ you can use the C<EV_THROW> macro for this:
+C++ you can use the C<EV_NOEXCEPT> macro for this:
    static void
-   fatal_error (const char *msg) EV_THROW
+   fatal_error (const char *msg) EV_NOEXCEPT
    {
      perror (msg);
      abort ();
    }
 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
      void operator() (ev::io &w, int revents)
      {
        ...
      }
    }
    myfunctor f;
    ev::io w;
    w.set (&f);
 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.
+For C<ev::io> watchers there is an additional C<set> method that acepts a
+new event mask only, and internally calls C<ev_io_modify>.
 =item w->start ()
 Starts the watcher. Note that there is no C<loop> argument, as the
 constructor already stores the event loop.
    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_iouring.c    only when the linux io_uring 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
 available and will probe for kernel support at runtime. This will improve
 C<ev_signal> and C<ev_async> performance and reduce resource consumption.
 If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
 2.7 or newer, otherwise disabled.
+=item EV_USE_SIGNALFD
+If defined to be C<1>, then libev will assume that C<signalfd ()> is
+available and will probe for kernel support at runtime. This enables
+the use of EVFLAG_SIGNALFD for faster and simpler signal handling. If
+undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
+2.7 or newer, otherwise disabled.
+=item EV_USE_TIMERFD
+If defined to be C<1>, then libev will assume that C<timerfd ()> is
+available and will probe for kernel support at runtime. This allows
+libev to detect time jumps accurately. If undefined, it will be enabled
+if the headers indicate GNU/Linux + Glibc 2.8 or newer and define
+C<TFD_TIMER_CANCEL_ON_SET>, otherwise disabled.
+=item EV_USE_EVENTFD
+If defined to be C<1>, then libev will assume that C<eventfd ()> is
+available and will probe for kernel support at runtime. This will improve
+C<ev_signal> and C<ev_async> performance and reduce resource consumption.
+If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
+2.7 or newer, otherwise disabled.
 =item EV_USE_SELECT
 If undefined or defined to be C<1>, libev will compile in support for the
 C<select>(2) backend. No attempt at auto-detection will be done: if no
 other method takes over, select will be it. Otherwise the select backend
 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 (C<EV_USE_EPOLL> must also be enabled). If undefined, it will be
+enabled on linux, otherwise disabled.
+=item EV_USE_IOURING
+If defined to be C<1>, libev will compile in support for the Linux
+io_uring backend (C<EV_USE_EPOLL> must also be enabled). Due to it's
+current 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,
 called. If set to C<2>, then the internal verification code will be
 called once per loop, which can slow down libev. If set to C<3>, then the
 verification code will be called very frequently, which will slow down
 libev considerably.
+Verification errors are reported via C's C<assert> mechanism, so if you
+disable that (e.g. by defining C<NDEBUG>) then no errors will be reported.
 The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
 will be C<0>.
 =item EV_COMMON
 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.
 =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:

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Comparing libev/ev.pod (file contents): Revision 1.427 by root, Sun Apr 28 14:57:12 2013 UTC vs. Revision 1.469 by root, Sat Jun 3 08:53:03 2023 UTC

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Comparing libev/ev.pod (file contents):
Revision 1.427 by root, Sun Apr 28 14:57:12 2013 UTC vs.
Revision 1.469 by root, Sat Jun 3 08:53:03 2023 UTC