libev/ev.pod

=head1 NAME

libev - a high performance full-featured event loop written in C

=head1 SYNOPSIS

   #include <ev.h>

=head2 EXAMPLE PROGRAM

   // a single header file is required
   #include <ev.h>

   #include <stdio.h> // for puts

   // every watcher type has its own typedef'd struct
   // with the name ev_TYPE
   ev_io stdin_watcher;
   ev_timer timeout_watcher;

   // all watcher callbacks have a similar signature
   // this callback is called when data is readable on stdin
   static void
   stdin_cb (EV_P_ ev_io *w, int revents)
   {
     puts ("stdin ready");
     // for one-shot events, one must manually stop the watcher
     // with its corresponding stop function.
     ev_io_stop (EV_A_ w);

     // this causes all nested ev_loop's to stop iterating
     ev_unloop (EV_A_ EVUNLOOP_ALL);
   }

   // another callback, this time for a time-out
   static void
   timeout_cb (EV_P_ ev_timer *w, int revents)
   {
     puts ("timeout");
     // this causes the innermost ev_loop to stop iterating
     ev_unloop (EV_A_ EVUNLOOP_ONE);
   }

   int
   main (void)
   {
     // use the default event loop unless you have special needs
     struct ev_loop *loop = ev_default_loop (0);

     // initialise an io watcher, then start it
     // this one will watch for stdin to become readable
     ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
     ev_io_start (loop, &stdin_watcher);

     // initialise a timer watcher, then start it
     // simple non-repeating 5.5 second timeout
     ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
     ev_timer_start (loop, &timeout_watcher);

     // now wait for events to arrive
     ev_loop (loop, 0);

     // unloop was called, so exit
     return 0;
   }

=head1 ABOUT THIS DOCUMENT

This document documents the libev software package.

The newest version of this document is also available as an html-formatted
web page you might find easier to navigate when reading it for the first
time: L<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.

While this document tries to be as complete as possible in documenting
libev, its usage and the rationale behind its design, it is not a tutorial
on event-based programming, nor will it introduce event-based programming
with libev.

Familiarity with event based programming techniques in general is assumed
throughout this document.

=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
these event sources and provide your program with events.

To do this, it must take more or less complete control over your process
(or thread) by executing the I<event loop> handler, and will then
communicate events via a callback mechanism.

You register interest in certain events by registering so-called I<event
watchers>, which are relatively small C structures you initialise with the
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
BSD-specific C<kqueue> and the Solaris-specific event port mechanisms
for file descriptor events (C<ev_io>), the Linux C<inotify> 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
C<ev_check> watchers) as well as file watchers (C<ev_stat>) and even
limited support for fork events (C<ev_fork>).

It also is quite fast (see this
L<benchmark|http://libev.schmorp.de/bench.html> comparing it to libevent
for example).

=head2 CONVENTIONS

Libev is very configurable. In this manual the default (and most common)
configuration will be described, which supports multiple event loops. For
more info about various configuration options please have a look at
B<EMBED> section in this manual. If libev was configured without support
for multiple event loops, then all functions taking an initial argument of
name C<loop> (which is always of type C<struct ev_loop *>) will not have
this argument.

=head2 TIME REPRESENTATION

Libev represents time as a single floating point number, representing
the (fractional) number of seconds since the (POSIX) epoch (in practise
somewhere near the beginning of 1970, details are complicated, don't
ask). This type is called C<ev_tstamp>, which is what you should use
too. It usually aliases to the C<double> type in C. When you need to do
any calculations on it, you should treat it as some floating point value.

Unlike the name component C<stamp> might indicate, it is also used for
time differences (e.g. delays) throughout libev.

=head1 ERROR HANDLING

Libev knows three classes of errors: operating system errors, usage errors
and internal errors (bugs).

When libev catches an operating system error it cannot handle (for example
a system call indicating a condition libev cannot fix), it calls the callback
set via C<ev_set_syserr_cb>, which is supposed to fix the problem or
abort. The default is to print a diagnostic message and to call C<abort
()>.

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.

Libev also has a few internal error-checking C<assert>ions, and also has
extensive consistency checking code. These do not 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
library in any way.

=over 4

=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.

=item ev_sleep (ev_tstamp interval)

Sleep for the given interval: The current thread will be blocked until
either it is interrupted or the given time interval has passed. Basically
this is a sub-second-resolution C<sleep ()>.

=item int ev_version_major ()

=item int ev_version_minor ()

You can find out the major and minor ABI version numbers of the library
you linked against by calling the functions C<ev_version_major> and
C<ev_version_minor>. If you want, you can compare against the global
symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
version of the library your program was compiled against.

These version numbers refer to the ABI version of the library, not the
release version.

Usually, it's a good idea to terminate if the major versions mismatch,
as this indicates an incompatible change. Minor versions are usually
compatible to older versions, so a larger minor version alone is usually
not a problem.

Example: Make sure we haven't accidentally been linked against the wrong
version (note, however, that this will not detect ABI mismatches :).

   assert (("libev version mismatch",
            ev_version_major () == EV_VERSION_MAJOR
            && ev_version_minor () >= EV_VERSION_MINOR));

=item unsigned int ev_supported_backends ()

Return the set of all backends (i.e. their corresponding C<EV_BACKEND_*>
value) compiled into this binary of libev (independent of their
availability on the system you are running on). See C<ev_default_loop> for
a description of the set values.

Example: make sure we have the epoll method, because yeah this is cool and
a must have and can we have a torrent of it please!!!11

   assert (("sorry, no epoll, no sex",
            ev_supported_backends () & EVBACKEND_EPOLL));

=item unsigned int ev_recommended_backends ()

Return the set of all backends compiled into this binary of libev and also
recommended for this platform. This set is often smaller than the one
returned by C<ev_supported_backends>, as for example kqueue is broken on
most BSDs and will not be auto-detected unless you explicitly request it
(assuming you know what you are doing). This is the set of backends that
libev will probe for if you specify no backends explicitly.

=item unsigned int ev_embeddable_backends ()

Returns the set of backends that are embeddable in other event loops. This
is the theoretical, all-platform, value. To find which backends
might be supported on 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)) [NOT REENTRANT]

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
or take some potentially destructive action.

Since some systems (at least OpenBSD and Darwin) fail to implement
correct C<realloc> semantics, libev will use a wrapper around the system
C<realloc> and C<free> functions by default.

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: Replace the libev allocator with one that waits a bit and then
retries (example requires a standards-compliant C<realloc>).

   static void *
   persistent_realloc (void *ptr, size_t size)
   {
     for (;;)
       {
         void *newptr = realloc (ptr, size);

         if (newptr)
           return newptr;

         sleep (60);
       }
   }

   ...
   ev_set_allocator (persistent_realloc);

=item ev_set_syserr_cb (void (*cb)(const char *msg)); [NOT REENTRANT]

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
matter what, when it returns. That is, libev will generally retry the
requested operation, or, if the condition doesn't go away, do bad stuff
(such as abort).

Example: This is basically the same thing that libev does internally, too.

   static void
   fatal_error (const char *msg)
   {
     perror (msg);
     abort ();
   }

   ...
   ev_set_syserr_cb (fatal_error);

=back

=head1 FUNCTIONS CONTROLLING THE EVENT LOOP

An event loop is described by a C<struct ev_loop *> (the C<struct>
is I<not> optional in this case, as there is also an C<ev_loop>
I<function>).

The library knows two types of such loops, the I<default> loop, which
supports signals and child events, and dynamically created loops which do
not.

=over 4

=item struct ev_loop *ev_default_loop (unsigned int flags)

This will initialise the default event loop if it hasn't been initialised
yet and return it. If the default loop could not be initialised, returns
false. If it already was initialised it simply returns it (and ignores the
flags. If that is troubling you, check C<ev_backend ()> afterwards).

If you don't know what event loop to use, use the one returned from this
function.

Note that this function is I<not> thread-safe, so if you want to use it
from multiple threads, you have to lock (note also that this is unlikely,
as loops cannot be shared easily between threads anyway).

The default loop is the only loop that can handle C<ev_signal> and
C<ev_child> watchers, and to do this, it always registers a handler
for C<SIGCHLD>. If this is a problem for your application you can either
create a dynamic loop with C<ev_loop_new> that doesn't do that, or you
can simply overwrite the C<SIGCHLD> signal handler I<after> calling
C<ev_default_init>.

The flags argument can be used to specify special behaviour or specific
backends to use, and is usually specified as C<0> (or C<EVFLAG_AUTO>).

The following flags are supported:

=over 4

=item C<EVFLAG_AUTO>

The default flags value. Use this if you have no clue (it's the right
thing, believe me).

=item C<EVFLAG_NOENV>

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
around bugs.

=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
without a system call and thus I<very> fast, but my GNU/Linux system also has
C<pthread_atfork> which is even faster).

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
flag.

This flag setting cannot be overridden or specified in the C<LIBEV_FLAGS>
environment variable.

=item C<EVFLAG_NOINOTIFY>

When this flag is specified, then libev will not attempt to use the
I<inotify> API for it's C<ev_stat> watchers. Apart from debugging and
testing, this flag can be useful to conserve inotify file descriptors, as
otherwise each loop using C<ev_stat> watchers consumes one inotify handle.

=item C<EVFLAG_SIGNALFD>

When this flag is specified, then libev will attempt to use the
I<signalfd> API for it's C<ev_signal> (and C<ev_child>) watchers. This API
delivers signals synchronously, which makes it both faster and might make
it possible to get the queued signal data. It can also simplify signal
handling with threads, as long as you properly block signals in your
threads that are not interested in handling them.

Signalfd will not be used by default as this changes your signal mask, and
there are a lot of shoddy libraries and programs (glib's threadpool for
example) that can't properly initialise their signal masks.

=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,
but if that fails, expect a fairly low limit on the number of fds when
using this backend. It doesn't scale too well (O(highest_fd)), but its
usually the fastest backend for a low number of (low-numbered :) fds.

To get good performance out of this backend you need a high amount of
parallelism (most of the file descriptors should be busy). If you are
writing a server, you should C<accept ()> in a loop to accept as many
connections as possible during one iteration. You might also want to have
a look at C<ev_set_io_collect_interval ()> to increase the amount of
readiness notifications you get per iteration.

This backend maps C<EV_READ> to the C<readfds> set and C<EV_WRITE> to the
C<writefds> set (and to work around Microsoft Windows bugs, also onto the
C<exceptfds> set on that platform).

=item C<EVBACKEND_POLL>    (value 2, poll backend, available everywhere except on windows)

And this is your standard poll(2) backend. It's more complicated
than select, but handles sparse fds better and has no artificial
limit on the number of fds you can use (except it will slow down
considerably with a lot of inactive fds). It scales similarly to select,
i.e. O(total_fds). See the entry for C<EVBACKEND_SELECT>, above, for
performance tips.

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
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 n is the total number of fds (or the highest fd),
epoll scales either O(1) or O(active_fds).

The epoll mechanism deserves honorable mention as the most misdesigned
of the more advanced event mechanisms: mere annoyances include silently
dropping file descriptors, requiring a system call per change per file
descriptor (and unnecessary guessing of parameters), problems with dup 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
of course I<doesn't>, and epoll just loves to report events for totally
I<different> file descriptors (even already closed ones, so one cannot
even remove them from the set) than registered in the set (especially
on SMP systems). Libev tries to counter these spurious notifications by
employing an additional generation counter and comparing that against the
events to filter out spurious ones, recreating the set when required. Last
not least, it also refuses to work with some file descriptors which work
perfectly fine with C<select> (files, many character devices...).

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
file descriptors might not work very well if you register events for both
file descriptors.

Best performance from this backend is achieved by not unregistering all
watchers for a file descriptor until it has been closed, if possible,
i.e. keep at least one watcher active per fd at all times. Stopping and
starting a watcher (without re-setting it) also usually doesn't cause
extra overhead. A fork can both result in spurious notifications as well
as in libev having to destroy and recreate the epoll object, which can
take considerable time and thus should be avoided.

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.

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
was broken on all BSDs except NetBSD (usually it doesn't work reliably
with anything but sockets and pipes, except on Darwin, where of course
it's completely useless). Unlike epoll, however, whose brokenness
is by design, these kqueue bugs can (and eventually will) be fixed
without API changes to existing programs. For this reason it's not being
"auto-detected" unless you explicitly specify it in the flags (i.e. using
C<EVBACKEND_KQUEUE>) or libev was compiled on a 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 (but
sane, unlike 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
everywhere, so you might need to test for this. And since it is broken
almost everywhere, you should only use it when you have a lot of sockets
(for which it usually works), by embedding it into another event loop
(e.g. C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> (but C<poll> is of course
also broken on OS X)) and, did I mention it, using it only for sockets.

This backend maps C<EV_READ> into an C<EVFILT_READ> kevent with
C<NOTE_EOF>, and C<EV_WRITE> into an C<EVFILT_WRITE> kevent with
C<NOTE_EOF>.

=item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8)

This is not implemented yet (and might never be, unless you send me an
implementation). According to reports, C</dev/poll> only supports sockets
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,
it's really slow, but it still scales very well (O(active_fds)).

Please note that Solaris event ports can deliver a lot of spurious
notifications, so you need to use non-blocking I/O or other means to avoid
blocking when no data (or space) is available.

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.

On the positive side, with the exception of the spurious readiness
notifications, this backend actually performed fully to specification
in all tests and is fully embeddable, which is a rare feat among the
OS-specific backends (I vastly prefer correctness over speed hacks).

This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
C<EVBACKEND_POLL>.

=item C<EVBACKEND_ALL>

Try all backends (even potentially broken ones that wouldn't be tried
with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.

It is definitely not recommended to use this flag.

=back

If one or more of the backend flags are or'ed into the flags value,
then only these backends will be tried (in the reverse order as listed
here). If none are specified, all backends in C<ev_recommended_backends
()> will be tried.

Example: This is the most typical usage.

   if (!ev_default_loop (0))
     fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");

Example: Restrict libev to the select and poll backends, and do not allow
environment settings to be taken into account:

   ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);

Example: Use whatever libev has to offer, but make sure that kqueue is
used if available (warning, breaks stuff, best use only with your own
private event loop and only if you know the OS supports your types of
fds):

   ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);

=item struct ev_loop *ev_loop_new (unsigned int flags)

Similar to C<ev_default_loop>, but always creates a new event loop that is
always distinct from the default loop.

Note that this function I<is> thread-safe, and one common way to use
libev with threads is indeed to create one loop per thread, and using the
default loop in the "main" or "initial" thread.

Example: Try to create a event loop that uses epoll and nothing else.

   struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
   if (!epoller)
     fatal ("no epoll found here, maybe it hides under your chair");

=item ev_default_destroy ()

Destroys the default loop (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 responsibility to
either stop all watchers cleanly yourself I<before> calling this function,
or cope with the fact afterwards (which is usually the easiest thing, you
can just ignore the watchers and/or C<free ()> them for example).

Note that certain global state, such as signal state (and installed signal
handlers), will not be freed by this function, and related watchers (such
as signal and child watchers) would need to be stopped manually.

In general it is not advisable to call this function except in the
rare occasion where you really need to free e.g. the signal handling
pipe fds. If you need dynamically allocated loops it is better to use
C<ev_loop_new> and C<ev_loop_destroy>.

=item ev_loop_destroy (loop)

Like C<ev_default_destroy>, but destroys an event loop created by an
earlier call to C<ev_loop_new>.

=item ev_default_fork ()

This function sets a flag that causes subsequent C<ev_loop> iterations
to reinitialise the kernel state for backends that have one. Despite the
name, you can call it anytime, but it makes most sense after forking, in
the child process (or both child and parent, but that again makes little
sense). You I<must> call it in the child before using any of the libev
functions, and it will only take effect at the next C<ev_loop> iteration.

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
process if and only if you want to use the event loop in the child. If you
just fork+exec or create a new loop in the child, you don't have to call
it at all.

The function itself is quite fast and it's usually not a problem to call
it just in case after a fork. To make this easy, the function will fit in
quite nicely into a call to C<pthread_atfork>:

    pthread_atfork (0, 0, ev_default_fork);

=item ev_loop_fork (loop)

Like C<ev_default_fork>, but acts on an event loop created by
C<ev_loop_new>. Yes, you have to call this on every allocated event loop
after fork that you want to re-use in the child, and how you keep track of 
them is entirely your own problem.

=item int ev_is_default_loop (loop)

Returns true when the given loop is, in fact, the default loop, and false
otherwise.

=item unsigned int ev_iteration (loop)

Returns the current iteration count for the loop, which is identical to
the number of times libev did poll for new events. It starts at C<0> and
happily wraps around with enough iterations.

This value can sometimes be useful as a generation counter of sorts (it
"ticks" the number of loop iterations), as it roughly corresponds with
C<ev_prepare> and C<ev_check> calls - and is incremented between the
prepare and check phases.

=item unsigned int ev_depth (loop)

Returns the number of times C<ev_loop> was entered minus the number of
times C<ev_loop> was exited, in other words, the recursion depth.

Outside C<ev_loop>, this number is zero. In a callback, this number is
C<1>, unless C<ev_loop> was invoked recursively (or from another thread),
in which case it is higher.

Leaving C<ev_loop> abnormally (setjmp/longjmp, cancelling the thread
etc.), doesn't count as "exit" - consider this as a hint to avoid such
ungentleman behaviour unless it's really convenient.

=item unsigned int ev_backend (loop)

Returns one of the C<EVBACKEND_*> flags indicating the event backend in
use.

=item ev_tstamp ev_now (loop)

Returns the current "event loop time", which is the time the event loop
received events and started processing them. This timestamp does not
change as long as callbacks are being processed, and this is also the base
time used for relative timers. You can treat it as the timestamp of the
event occurring (or more correctly, libev finding out about it).

=item ev_now_update (loop)

Establishes the current time by querying the kernel, updating the time
returned by C<ev_now ()> in the progress. This is a costly operation and
is usually done automatically within C<ev_loop ()>.

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.

=item ev_suspend (loop)

=item ev_resume (loop)

These two functions suspend and resume a loop, for use when the loop is
not used for a while and timeouts should not be processed.

A typical use case would be an interactive program such as a game:  When
the user presses C<^Z> to suspend the game and resumes it an hour later it
would be best to handle timeouts as if no time had actually passed while
the program was suspended. This can be achieved by calling C<ev_suspend>
in your C<SIGTSTP> handler, sending yourself a C<SIGSTOP> and calling
C<ev_resume> directly afterwards to resume timer processing.

Effectively, all C<ev_timer> watchers will be delayed by the time spend
between C<ev_suspend> and C<ev_resume>, and all C<ev_periodic> watchers
will be rescheduled (that is, they will lose any events that would have
occurred while suspended).

After calling C<ev_suspend> you B<must not> call I<any> function on the
given loop other than C<ev_resume>, and you B<must not> call C<ev_resume>
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_loop (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.

If the flags argument is specified as C<0>, it will not return until
either no event watchers are active anymore or C<ev_unloop> was called.

Please note that an explicit C<ev_unloop> 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.

A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle
those events and any already outstanding ones, but will not block your
process in case there are no events and will return after one iteration of
the loop.

A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
necessary) and will handle those and any already outstanding ones. It
will block your process until at least one new event arrives (which could
be an event internal to libev itself, so there is no guarantee that a
user-registered callback will be called), and will return after one
iteration of the loop.

This is useful if you are waiting for some external event in conjunction
with something not expressible using other libev watchers (i.e. "roll your
own C<ev_loop>"). However, a pair of C<ev_prepare>/C<ev_check> watchers is
usually a better approach for this kind of thing.

Here are the gory details of what C<ev_loop> does:

   - Before the first iteration, call any pending watchers.
   * If EVFLAG_FORKCHECK was used, check for a fork.
   - If a fork was detected (by any means), queue and call all fork watchers.
   - Queue and call all prepare watchers.
   - If we have been forked, detach and recreate the kernel state
     as to not disturb the other process.
   - Update the kernel state with all outstanding changes.
   - Update the "event loop time" (ev_now ()).
   - Calculate for how long to sleep or block, if at all
     (active idle watchers, EVLOOP_NONBLOCK or not having
     any active watchers at all will result in not sleeping).
   - Sleep if the I/O and timer collect interval say so.
   - Block the process, waiting for any events.
   - Queue all outstanding I/O (fd) events.
   - Update the "event loop time" (ev_now ()), and do time jump adjustments.
   - Queue all expired timers.
   - Queue all expired periodics.
   - Unless any events are pending now, queue all idle watchers.
   - 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
     be handled here by queueing them when their watcher gets executed.
   - If ev_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
     were used, or there are no active watchers, return, otherwise
     continue with step *.

Example: Queue some jobs and then loop until no events are outstanding
anymore.

   ... queue jobs here, make sure they register event watchers as long
   ... as they still have work to do (even an idle watcher will do..)
   ev_loop (my_loop, 0);
   ... jobs done or somebody called unloop. yeah!

=item ev_unloop (loop, how)

Can be used to make a call to C<ev_loop> return early (but only after it
has processed all outstanding events). The C<how> argument must be either
C<EVUNLOOP_ONE>, which will make the innermost C<ev_loop> call return, or
C<EVUNLOOP_ALL>, which will make all nested C<ev_loop> calls return.

This "unloop state" will be cleared when entering C<ev_loop> again.

It is safe to call C<ev_unloop> from outside any C<ev_loop> calls.

=item ev_ref (loop)

=item ev_unref (loop)

Ref/unref can be used to add or remove a reference count on the event
loop: Every watcher keeps one reference, and as long as the reference
count is nonzero, C<ev_loop> will not return on its own.

This is useful when you have a watcher that you never intend to
unregister, but that nevertheless should not keep C<ev_loop> from
returning. In such a case, call C<ev_unref> after starting, and C<ev_ref>
before stopping it.

As an example, libev itself uses this for its internal signal pipe: It
is not visible to the libev user and should not keep C<ev_loop> from
exiting if no event watchers registered by it are active. It is also an
excellent way to do this for generic recurring timers or from within
third-party libraries. Just remember to I<unref after start> and I<ref
before stop> (but only if the watcher wasn't active before, or was active
before, respectively. Note also that libev might stop watchers itself
(e.g. non-repeating timers) in which case you have to C<ev_ref>
in the callback).

Example: Create a signal watcher, but keep it from keeping C<ev_loop>
running when nothing else is active.

   ev_signal exitsig;
   ev_signal_init (&exitsig, sig_cb, SIGINT);
   ev_signal_start (loop, &exitsig);
   evf_unref (loop);

Example: For some weird reason, unregister the above signal handler again.

   ev_ref (loop);
   ev_signal_stop (loop, &exitsig);

=item ev_set_io_collect_interval (loop, ev_tstamp interval)

=item ev_set_timeout_collect_interval (loop, ev_tstamp interval)

These advanced functions influence the time that libev will spend waiting
for events. Both time intervals are by default C<0>, meaning that libev
will try to invoke timer/periodic callbacks and I/O callbacks with minimum
latency.

Setting these to a higher value (the C<interval> I<must> be >= C<0>)
allows libev to delay invocation of I/O and timer/periodic callbacks
to increase efficiency of loop iterations (or to increase power-saving
opportunities).

The idea is that sometimes your program runs just fast enough to handle
one (or very few) event(s) per loop iteration. While this makes the
program responsive, it also wastes a lot of CPU time to poll for new
events, especially with backends like C<select ()> which have a high
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
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.

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
value will not introduce any overhead in libev.

Many (busy) programs can usually benefit by setting the I/O collect
interval to a value near C<0.1> or so, which is often enough for
interactive servers (of course not for games), likewise for timeouts. It
usually doesn't make much sense to set it to a lower value than C<0.01>,
as this approaches the timing granularity of most systems. Note that if
you do transactions with the outside world and you can't increase the
parallelity, then this setting will limit your transaction rate (if you
need to poll once per transaction and the I/O collect interval is 0.01,
then you can't do more than 100 transactions per second).

Setting the I<timeout collect interval> can improve the opportunity for
saving power, as the program will "bundle" timer callback invocations that
are "near" in time together, by delaying some, thus reducing the number of
times the process sleeps and wakes up again. Another useful technique to
reduce iterations/wake-ups is to use C<ev_periodic> watchers and make sure
they fire on, say, one-second boundaries only.

Example: we only need 0.1s timeout granularity, and we wish not to poll
more often than 100 times per second:

   ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
   ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);

=item ev_invoke_pending (loop)

This call will simply invoke all pending watchers while resetting their
pending state. Normally, C<ev_loop> does this automatically when required,
but when overriding the invoke callback this call comes handy.

=item int ev_pending_count (loop)

Returns the number of pending watchers - zero indicates that no watchers
are pending.

=item ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))

This overrides the invoke pending functionality of the loop: Instead of
invoking all pending watchers when there are any, C<ev_loop> will call
this callback instead. This is useful, for example, when you want to
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))

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_loop> 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 loop via
C<ev_unloop> and C<av_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.

Ideally, C<release> will just call your mutex_unlock function, and
C<acquire> will just call the mutex_lock function again.

While event loop modifications are allowed between invocations of
C<release> and C<acquire> (that's their only purpose after all), no
modifications done will affect the event loop, i.e. adding watchers will
have no effect on the set of file descriptors being watched, or the time
waited. Use an C<ev_async> watcher to wake up C<ev_loop> when you want it
to take note of any changes you made.

In theory, threads executing C<ev_loop> will be async-cancel safe between
invocations of C<release> and C<acquire>.

See also the locking example in the C<THREADS> section later in this
document.

=item ev_set_userdata (loop, void *data)

=item ev_userdata (loop)

Set and retrieve a single C<void *> associated with a loop. When
C<ev_set_userdata> has never been called, then C<ev_userdata> returns
C<0.>

These two functions can be used to associate arbitrary data with a loop,
and are intended solely for the C<invoke_pending_cb>, C<release> and
C<acquire> callbacks described above, but of course can be (ab-)used for
any other purpose as well.

=item ev_loop_verify (loop)

This function only does something when C<EV_VERIFY> support has been
compiled in, which is the default for non-minimal builds. It tries to go
through all internal structures and checks them for validity. If anything
is found to be inconsistent, it will print an error message to standard
error and call C<abort ()>.

This can be used to catch bugs inside libev itself: under normal
circumstances, this function will never abort as of course libev keeps its
data structures consistent.

=back


=head1 ANATOMY OF A WATCHER

In the following description, uppercase C<TYPE> in names stands for the
watcher type, e.g. C<ev_TYPE_start> can mean C<ev_timer_start> for timer
watchers and C<ev_io_start> for I/O watchers.

A watcher is a structure that you create and register to record your
interest in some event. For instance, if you want to wait for STDIN to
become readable, you would create an C<ev_io> watcher for that:

   static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
   {
     ev_io_stop (w);
     ev_unloop (loop, EVUNLOOP_ALL);
   }

   struct ev_loop *loop = ev_default_loop (0);

   ev_io stdin_watcher;

   ev_init (&stdin_watcher, my_cb);
   ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
   ev_io_start (loop, &stdin_watcher);

   ev_loop (loop, 0);

As you can see, you are responsible for allocating the memory for your
watcher structures (and it is I<usually> a bad idea to do this on the
stack).

Each watcher has an associated watcher structure (called C<struct ev_TYPE>
or simply C<ev_TYPE>, as typedefs are provided for all watcher structs).

Each watcher structure must be initialised by a call to C<ev_init
(watcher *, callback)>, which expects a callback to be provided. This
callback gets invoked each time the event occurs (or, in the case of I/O
watchers, each time the event loop detects that the file descriptor given
is readable and/or writable).

Each watcher type further has its own C<< ev_TYPE_set (watcher *, ...) >>
macro to configure it, with arguments specific to the watcher type. There
is also a macro to combine initialisation and setting in one call: C<<
ev_TYPE_init (watcher *, callback, ...) >>.

To make the watcher actually watch out for events, you have to start it
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
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.

The received events usually include a single bit per event type received
(you can receive multiple events at the same time). The possible bit masks
are:

=over 4

=item C<EV_READ>

=item C<EV_WRITE>

The file descriptor in the C<ev_io> watcher has become readable and/or
writable.

=item C<EV_TIMER>

The C<ev_timer> watcher has timed out.

=item C<EV_PERIODIC>

The C<ev_periodic> watcher has timed out.

=item C<EV_SIGNAL>

The signal specified in the C<ev_signal> watcher has been received by a thread.

=item C<EV_CHILD>

The pid specified in the C<ev_child> watcher has received a status change.

=item C<EV_STAT>

The path specified in the C<ev_stat> watcher changed its attributes somehow.

=item C<EV_IDLE>

The C<ev_idle> watcher has determined that you have nothing better to do.

=item C<EV_PREPARE>

=item C<EV_CHECK>

All C<ev_prepare> watchers are invoked just I<before> C<ev_loop> starts
to gather new events, and all C<ev_check> watchers are invoked just after
C<ev_loop> has gathered them, but before it invokes any callbacks for any
received events. Callbacks of both watcher types can start and stop as
many watchers as they want, and all of them will be taken into account
(for example, a C<ev_prepare> watcher might start an idle watcher to keep
C<ev_loop> from blocking).

=item C<EV_EMBED>

The embedded event loop specified in the C<ev_embed> watcher needs attention.

=item C<EV_FORK>

The event loop has been resumed in the child process after fork (see
C<ev_fork>).

=item C<EV_ASYNC>

The given async watcher has been asynchronously notified (see C<ev_async>).

=item C<EV_CUSTOM>

Not ever sent (or otherwise used) by libev itself, but can be freely used
by libev users to signal watchers (e.g. via C<ev_feed_event>).

=item C<EV_ERROR>

An unspecified error has occurred, the watcher has been stopped. This might
happen because the watcher could not be properly started because libev
ran out of memory, a file descriptor was found to be closed or any other
problem. Libev considers these application bugs.

You best act on it by reporting the problem and somehow coping with the
watcher being stopped. Note that well-written programs should not receive
an error ever, so when your watcher receives it, this usually indicates a
bug in your program.

Libev will usually signal a few "dummy" events together with an error, for
example it might indicate that a fd is readable or writable, and if your
callbacks is well-written it can just attempt the operation and cope with
the error from read() or write(). This will not work in multi-threaded
programs, though, as the fd could already be closed and reused for another
thing, so beware.

=back

=head2 GENERIC WATCHER FUNCTIONS

=over 4

=item C<ev_init> (ev_TYPE *watcher, callback)

This macro initialises the generic portion of a watcher. The contents
of the watcher object can be arbitrary (so C<malloc> will do). Only
the generic parts of the watcher are initialised, you I<need> to call
the type-specific C<ev_TYPE_set> macro afterwards to initialise the
type-specific parts. For each type there is also a C<ev_TYPE_init> macro
which rolls both calls into one.

You can reinitialise a watcher at any time as long as it has been stopped
(or never started) and there are no pending events outstanding.

The callback is always of type C<void (*)(struct ev_loop *loop, ev_TYPE *watcher,
int revents)>.

Example: Initialise an C<ev_io> watcher in two steps.

   ev_io w;
   ev_init (&w, my_cb);
   ev_io_set (&w, STDIN_FILENO, EV_READ);

=item C<ev_TYPE_set> (ev_TYPE *watcher, [args])

This macro initialises the type-specific parts of a watcher. You need to
call C<ev_init> at least once before you call this macro, but you can
call C<ev_TYPE_set> any number of times. You must not, however, call this
macro on a watcher that is active (it can be pending, however, which is a
difference to the C<ev_init> macro).

Although some watcher types do not have type-specific arguments
(e.g. C<ev_prepare>) you still need to call its C<set> macro.

See C<ev_init>, above, for an example.

=item C<ev_TYPE_init> (ev_TYPE *watcher, callback, [args])

This convenience macro rolls both C<ev_init> and C<ev_TYPE_set> macro
calls into a single call. This is the most convenient method to initialise
a watcher. The same limitations apply, of course.

Example: Initialise and set an C<ev_io> watcher in one step.

   ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);

=item C<ev_TYPE_start> (loop, ev_TYPE *watcher)

Starts (activates) the given watcher. Only active watchers will receive
events. If the watcher is already active nothing will happen.

Example: Start the C<ev_io> watcher that is being abused as example in this
whole section.

   ev_io_start (EV_DEFAULT_UC, &w);

=item C<ev_TYPE_stop> (loop, ev_TYPE *watcher)

Stops the given watcher if active, and clears the pending status (whether
the watcher was active or not).

It is possible that stopped watchers are pending - for example,
non-repeating timers are being stopped when they become pending - but
calling C<ev_TYPE_stop> ensures that the watcher is neither active nor
pending. If you want to free or reuse the memory used by the watcher it is
therefore a good idea to always call its C<ev_TYPE_stop> function.

=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.

=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).

=item callback ev_cb (ev_TYPE *watcher)

Returns the callback currently set on the watcher.

=item ev_cb_set (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)

=item int ev_priority (ev_TYPE *watcher)

Set and query the priority of the watcher. The priority is a small
integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI>
(default: C<-2>). Pending watchers with higher priority will be invoked
before watchers with lower priority, but priority will not keep watchers
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
pending.

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.

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
priorities.

=item ev_invoke (loop, ev_TYPE *watcher, int revents)

Invoke the C<watcher> with the given C<loop> and C<revents>. Neither
C<loop> nor C<revents> need to be valid as long as the watcher callback
can deal with that fact, as both are simply passed through to the
callback.

=item int ev_clear_pending (loop, ev_TYPE *watcher)

If the watcher is pending, this function clears its pending status and
returns its C<revents> bitset (as if its callback was invoked). If the
watcher isn't pending it does nothing and returns C<0>.

Sometimes it can be useful to "poll" a watcher instead of waiting for its
callback to be invoked, which can be accomplished with this function.

=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
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.

See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related
functions that do not need a watcher.

=back


=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER

Each watcher has, by default, a member C<void *data> that you can change
and read at any time: libev will completely ignore it. This can be used
to associate arbitrary data with your watcher. If you need more data and
don't want to allocate memory and store a pointer to it in that data
member, you can also "subclass" the watcher type and provide your own
data:

   struct my_io
   {
     ev_io io;
     int otherfd;
     void *somedata;
     struct whatever *mostinteresting;
   };

   ...
   struct my_io w;
   ev_io_init (&w.io, my_cb, fd, EV_READ);

And since your callback will be called with a pointer to the watcher, you
can cast it back to your own type:

   static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
   {
     struct my_io *w = (struct my_io *)w_;
     ...
   }

More interesting and less C-conformant ways of casting your callback type
instead have been omitted.

Another common scenario is to use some data structure with multiple
embedded watchers:

   struct my_biggy
   {
     int some_data;
     ev_timer t1;
     ev_timer t2;
   }

In this case getting the pointer to C<my_biggy> is a bit more
complicated: Either you store the address of your C<my_biggy> struct
in the C<data> member of the watcher (for woozies), or you need to use
some pointer arithmetic using C<offsetof> inside your watchers (for real
programmers):

   #include <stddef.h>

   static void
   t1_cb (EV_P_ ev_timer *w, int revents)
   {
     struct my_biggy big = (struct my_biggy *)
       (((char *)w) - offsetof (struct my_biggy, t1));
   }

   static void
   t2_cb (EV_P_ ev_timer *w, int revents)
   {
     struct my_biggy big = (struct my_biggy *)
       (((char *)w) - offsetof (struct my_biggy, t2));
   }

=head2 WATCHER PRIORITY MODELS

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
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:

In the more common lock-out model, higher priorities "lock out" invocation
of lower priority watchers, which means as long as higher priority
watchers receive events, lower priority watchers are not being invoked.

The less common only-for-ordering model uses priorities solely to order
callback invocation within a single event loop iteration: Higher priority
watchers are invoked before lower priority ones, but they all get invoked
before polling for new events.

Libev uses the second (only-for-ordering) model for all its watchers
except for idle watchers (which use the lock-out model).

The rationale behind this is that implementing the lock-out model for
watchers is not well supported by most kernel interfaces, and most event
libraries will just poll for the same events again and again as long as
their callbacks have not been executed, which is very inefficient in the
common case of one high-priority watcher locking out a mass of lower
priority ones.

Static (ordering) priorities are most useful when you have two or more
watchers handling the same resource: a typical usage example is having an
C<ev_io> watcher to receive data, and an associated C<ev_timer> to handle
timeouts. Under load, data might be received while the program handles
other jobs, but since timers normally get invoked first, the timeout
handler will be executed before checking for data. In that case, giving
the timer a lower priority than the I/O watcher ensures that I/O will be
handled first even under adverse conditions (which is usually, but not
always, what you want).

Since idle watchers use the "lock-out" model, meaning that idle watchers
will only be executed when no same or higher priority watchers have
received events, they can be used to implement the "lock-out" model when
required.

For example, to emulate how many other event libraries handle priorities,
you can associate an C<ev_idle> watcher to each such watcher, and in
the normal watcher callback, you just start the idle watcher. The real
processing is done in the idle watcher callback. This causes libev to
continuously poll and process kernel event data for the watcher, but when
the lock-out case is known to be rare (which in turn is rare :), this is
workable.

Usually, however, the lock-out model implemented that way will perform
miserably under the type of load it was designed to handle. In that case,
it might be preferable to stop the real watcher before starting the
idle watcher, so the kernel will not have to process the event in case
the actual processing will be delayed for considerable time.

Here is an example of an I/O watcher that should run at a strictly lower
priority than the default, and which should only process data when no
other events are pending:

   ev_idle idle; // actual processing watcher
   ev_io io;     // actual event watcher

   static void
   io_cb (EV_P_ ev_io *w, int revents)
   {
     // stop the I/O watcher, we received the event, but
     // are not yet ready to handle it.
     ev_io_stop (EV_A_ w);

     // start the idle watcher to handle the actual event.
     // it will not be executed as long as other watchers
     // with the default priority are receiving events.
     ev_idle_start (EV_A_ &idle);
   }

   static void
   idle_cb (EV_P_ ev_idle *w, int revents)
   {
     // actual processing
     read (STDIN_FILENO, ...);

     // have to start the I/O watcher again, as
     // we have handled the event
     ev_io_start (EV_P_ &io);
   }

   // initialisation
   ev_idle_init (&idle, idle_cb);
   ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
   ev_io_start (EV_DEFAULT_ &io);

In the "real" world, it might also be beneficial to start a timer, so that
low-priority connections can not be locked out forever under load. This
enables your program to keep a lower latency for important connections
during short periods of high load, while not completely locking out less
important ones.


=head1 WATCHER TYPES

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,
while the watcher is active, you can look at the member and expect some
sensible content, but you must not modify it (you can modify it while 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
is active, but you can also modify it. 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.


=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
would not block the process and writing would at least be able to write
some data. This behaviour is called level-triggering because you keep
receiving events as long as the condition persists. Remember you can stop
the watcher if you don't want to act on the event and neither want to
receive future events.

In general you can register as many read and/or write event watchers per
fd as you want (as long as you don't confuse yourself). Setting all file
descriptors to non-blocking mode is also usually a good idea (but not
required if you know what you are doing).

If you cannot use non-blocking mode, then force the use of a
known-to-be-good backend (at the time of this writing, this includes only
C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). The same applies to file
descriptors for which non-blocking operation makes no sense (such as
files) - libev doesn't guarantee any specific behaviour in that case.

Another thing you have to watch out for is that it is quite easy to
receive "spurious" readiness notifications, that is your callback might
be called with C<EV_READ> but a subsequent C<read>(2) will actually block
because there is no data. Not only are some backends known to create a
lot of those (for example Solaris ports), it is very easy to get into
this situation even with a relatively standard program structure. Thus
it is best to always use non-blocking I/O: An extra C<read>(2) returning
C<EAGAIN> is far preferable to a program hanging until some data arrives.

If you cannot run the fd in non-blocking mode (for example you should
not play around with an Xlib connection), then you have to separately
re-test whether a file descriptor is really ready with a known-to-be good
interface such as poll (fortunately in our Xlib example, Xlib already
does this on its own, so its quite safe to use). Some people additionally
use C<SIGALRM> and an interval timer, just to be sure you won't block
indefinitely.

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
descriptor (either due to calling C<close> explicitly or any other means,
such as C<dup2>). The reason is that you register interest in some file
descriptor, but when it goes away, the operating system will silently drop
this interest. If another file descriptor with the same number then is
registered with libev, there is no efficient way to see that this is, 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
you I<have> to call C<ev_io_set> (or C<ev_io_init>) when you change the
descriptor even if the file descriptor number itself did not change.

This is how one would do it normally anyway, the important point is that
the libev application should not optimise around libev but should leave
optimisations to libev.

=head3 The special problem of dup'ed file descriptors

Some backends (e.g. epoll), cannot register events for file descriptors,
but only events for the underlying file descriptions. That means when you
have C<dup ()>'ed file descriptors or weirder constellations, and register
events for them, only one file descriptor might actually receive events.

There is no workaround possible except not registering events
for potentially C<dup ()>'ed file descriptors, or to resort to
C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.

=head3 The special problem of fork

Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
useless behaviour. Libev fully supports fork, but needs to be told about
it in the child.

To support fork in your programs, you either have to call
C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,
enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or
C<EVBACKEND_POLL>.

=head3 The special problem of SIGPIPE

While not really specific to libev, it is easy to forget about C<SIGPIPE>:
when writing to a pipe whose other end has been closed, your program gets
sent a SIGPIPE, which, by default, aborts your program. For most programs
this is sensible behaviour, for daemons, this is usually undesirable.

So when you encounter spurious, unexplained daemon exits, make sure you
ignore SIGPIPE (and maybe make sure you log the exit status of your daemon
somewhere, as that would have given you a big clue).

=head3 The special problem of accept()ing when you can't

Many implementations of the POSIX C<accept> function (for example,
found in post-2004 Linux) have the peculiar behaviour of not removing a
connection from the pending queue in all error cases.

For example, larger servers often run out of file descriptors (because
of resource limits), causing C<accept> to fail with C<ENFILE> but not
rejecting the connection, leading to libev signalling readiness on
the next iteration again (the connection still exists after all), and
typically causing the program to loop at 100% CPU usage.

Unfortunately, the set of errors that cause this issue differs between
operating systems, there is usually little the app can do to remedy the
situation, and no known thread-safe method of removing the connection to
cope with overload is known (to me).

One of the easiest ways to handle this situation is to just ignore it
- when the program encounters an overload, it will just loop until the
situation is over. While this is a form of busy waiting, no OS offers an
event-based way to handle this situation, so it's the best one can do.

A better way to handle the situation is to log any errors other than
C<EAGAIN> and C<EWOULDBLOCK>, making sure not to flood the log with such
messages, and continue as usual, which at least gives the user an idea of
what could be wrong ("raise the ulimit!"). For extra points one could stop
the C<ev_io> watcher on the listening fd "for a while", which reduces CPU
usage.

If your program is single-threaded, then you could also keep a dummy file
descriptor for overload situations (e.g. by opening F</dev/null>), and
when you run into C<ENFILE> or C<EMFILE>, close it, run C<accept>,
close that fd, and create a new dummy fd. This will gracefully refuse
clients under typical overload conditions.

The last way to handle it is to simply log the error and C<exit>, as
is often done with C<malloc> failures, but this results in an easy
opportunity for a DoS attack.

=head3 Watcher-Specific Functions

=over 4

=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
C<EV_READ | EV_WRITE>, to express the desire to receive the given events.

=item int fd [read-only]

The file descriptor being watched.

=item int events [read-only]

The events being watched.

=back

=head3 Examples

Example: Call C<stdin_readable_cb> when STDIN_FILENO has become, well
readable, but only once. Since it is likely line-buffered, you could
attempt to read a whole line in the callback.

   static void
   stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
   {
      ev_io_stop (loop, w);
     .. read from stdin here (or from w->fd) and handle any I/O errors
   }

   ...
   struct ev_loop *loop = ev_default_init (0);
   ev_io stdin_readable;
   ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
   ev_io_start (loop, &stdin_readable);
   ev_loop (loop, 0);


=head2 C<ev_timer> - relative and optionally repeating timeouts

Timer watchers are simple relative timers that generate an event after a
given time, and optionally repeating in regular intervals after that.

The timers are based on real time, that is, if you register an event that
times out after an hour and you reset your system clock to January last
year, it will still time out after (roughly) one hour. "Roughly" because
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
same loop iteration then the ones with earlier time-out values are invoked
before ones of the same priority with later time-out values (but this is
no longer true when a callback calls C<ev_loop> 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,
you want to raise some error after a while.

What follows are some ways to handle this problem, from obvious and
inefficient to smart and efficient.

In the following, a 60 second activity timeout is assumed - a timeout that
gets reset to 60 seconds each time there is activity (e.g. each time some
data or other life sign was received).

=over 4

=item 1. Use a timer and stop, reinitialise and start it on activity.

This is the most obvious, but not the most simple way: In the beginning,
start the watcher:

   ev_timer_init (timer, callback, 60., 0.);
   ev_timer_start (loop, timer);

Then, each time there is some activity, C<ev_timer_stop> it, initialise it
and start it again:

   ev_timer_stop (loop, timer);
   ev_timer_set (timer, 60., 0.);
   ev_timer_start (loop, timer);

This is relatively simple to implement, but means that each time there is
some activity, libev will first have to remove the timer from its internal
data structure and then add it again. Libev tries to be fast, but it's
still not a constant-time operation.

=item 2. Use a timer and re-start it with C<ev_timer_again> inactivity.

This is the easiest way, and involves using C<ev_timer_again> instead of
C<ev_timer_start>.

To implement this, configure an C<ev_timer> with a C<repeat> value
of C<60> and then call C<ev_timer_again> at start and each time you
successfully read or write some data. If you go into an idle state where
you do not expect data to travel on the socket, you can C<ev_timer_stop>
the timer, and C<ev_timer_again> will automatically restart it if need be.

That means you can ignore both the C<ev_timer_start> function and the
C<after> argument to C<ev_timer_set>, and only ever use the C<repeat>
member and C<ev_timer_again>.

At start:

   ev_init (timer, callback);
   timer->repeat = 60.;
   ev_timer_again (loop, timer);

Each time there is some activity:

   ev_timer_again (loop, timer);

It is even possible to change the time-out on the fly, regardless of
whether the watcher is active or not:

   timer->repeat = 30.;
   ev_timer_again (loop, timer);

This is slightly more efficient then stopping/starting the timer each time
you want to modify its timeout value, as libev does not have to completely
remove and re-insert the timer from/into its internal data structure.

It is, however, even simpler than the "obvious" way to do it.

=item 3. Let the timer time out, but then re-arm it as required.

This method is more tricky, but usually most efficient: Most timeouts are
relatively long compared to the intervals between other activity - in
our example, within 60 seconds, there are usually many I/O events with
associated activity resets.

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 last_activity; // time of last activity

   static void
   callback (EV_P_ ev_timer *w, int revents)
   {
     ev_tstamp now     = ev_now (EV_A);
     ev_tstamp timeout = last_activity + 60.;

     // if last_activity + 60. is older than now, we did time out
     if (timeout < now)
       {
         // timeout occurred, take action
       }
     else
       {
         // callback was invoked, but there was some activity, re-arm
         // the watcher to fire in last_activity + 60, which is
         // guaranteed to be in the future, so "again" is positive:
         w->repeat = timeout - now;
         ev_timer_again (EV_A_ w);
       }
   }

To summarise the callback: first calculate the real timeout (defined
as "60 seconds after the last activity"), then check if that time has
been reached, which means something I<did>, in fact, time out. Otherwise
the callback was invoked too early (C<timeout> is in the future), so
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
C<ev_timer_again> optimisation when the timer is already running.

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 the current time (meaning we just have some activity :), then call the
callback, which will "do the right thing" and start the timer:

   ev_init (timer, callback);
   last_activity = ev_now (loop);
   callback (loop, timer, EV_TIMER);

And when there is some activity, simply store the current time in
C<last_activity>, no libev calls at all:

   last_activity = ev_now (loop);

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
do even better:

When starting the timeout, calculate the timeout value and put the timeout
at the I<end> of the list.

Then use an C<ev_timer> to fire when the timeout at the I<beginning> of
the list is expected to fire (for example, using the technique #3).

When there is some activity, remove the timer from the list, recalculate
the timeout, append it to the end of the list again, and make sure to
update the C<ev_timer> if it was taken from the beginning of the list.

This way, one can manage an unlimited number of timeouts in O(1) time for
starting, stopping and updating the timers, at the expense of a major
complication, and having to use a constant timeout. The constant timeout
ensures that the list stays sorted.

=back

So which method the best?

Method #2 is a simple no-brain-required solution that is adequate in most
situations. Method #3 requires a bit more thinking, but handles many cases
better, and isn't very complicated either. In most case, choosing either
one is fine, with #3 being better in typical situations.

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 time updates

Establishing the current time is a costly operation (it usually takes at
least two system calls): EV therefore updates its idea of the current
time only before and after C<ev_loop> 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:

   ev_timer_set (&timer, after + ev_now () - ev_time (), 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
()>.

=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?

Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
all processes, while the clocks (C<times>, C<CLOCK_MONOTONIC>) continue
to run until the system is suspended, but they will not advance while the
system is suspended. That means, on resume, it will be as if the program
was frozen for a few seconds, but the suspend time will not be counted
towards C<ev_timer> when a monotonic clock source is used. The real time
clock advanced as expected, but if it is used as sole clocksource, then a
long suspend would be detected as a time jump by libev, and timers would
be adjusted accordingly.

I would not be surprised to see different behaviour in different between
operating systems, OS versions or even different hardware.

The other form of suspend (job control, or sending a SIGSTOP) will see a
time jump in the monotonic clocks and the realtime clock. If the program
is suspended for a very long time, and monotonic clock sources are in use,
then you can expect C<ev_timer>s to expire as the full suspension time
will be counted towards the timers. When no monotonic clock source is in
use, then libev will again assume a timejump and adjust accordingly.

It might be beneficial for this latter case to call C<ev_suspend>
and C<ev_resume> in code that handles C<SIGTSTP>, to at least get
deterministic behaviour in this case (you can do nothing against
C<SIGSTOP>).

=head3 Watcher-Specific Functions and Data Members

=over 4

=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>
is C<0.>, then it will automatically be stopped once the timeout is
reached. If it is positive, then the timer will automatically be
configured to trigger again C<repeat> 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
repeating. The exact semantics are:

If the timer is pending, its pending status is cleared.

If the timer is started but non-repeating, stop it (as if it timed out).

If the timer is repeating, either start it if necessary (with the
C<repeat> value), or reset the running timer to the C<repeat> value.

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,
then this time is relative to the current event loop time, otherwise it's
the timeout value currently configured.

That is, after an C<ev_timer_set (w, 5, 7)>, C<ev_timer_remaining> returns
C<5>. When the timer is started and one second passes, C<ev_timer_remaining>
will return C<4>. When the timer expires and is restarted, it will return
roughly C<7> (likely slightly less as callback invocation takes some time,
too), and so on.

=item ev_tstamp repeat [read-write]

The current C<repeat> value. Will be used each time the watcher times out
or C<ev_timer_again> is called, and determines the next timeout (if any),
which is also when any modifications are taken into account.

=back

=head3 Examples

Example: Create a timer that fires after 60 seconds.

   static void
   one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
   {
     .. one minute over, w is actually stopped right here
   }

   ev_timer mytimer;
   ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
   ev_timer_start (loop, &mytimer);

Example: Create a timeout timer that times out after 10 seconds of
inactivity.

   static void
   timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
   {
     .. ten seconds without any activity
   }

   ev_timer mytimer;
   ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
   ev_timer_again (&mytimer); /* start timer */
   ev_loop (loop, 0);

   // and in some piece of code that gets executed on any "activity":
   // reset the timeout to start ticking again at 10 seconds
   ev_timer_again (&mytimer);


=head2 C<ev_periodic> - to cron or not to cron?

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
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
in time: for example, if you tell a periodic watcher to trigger "in 10
seconds" (by specifying e.g. C<ev_now () + 10.>, that is, an absolute time
not a delay) and then reset your system clock to January of the previous
year, then it will take a year or more to trigger the event (unlike an
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
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
(but this is no longer true when a callback calls C<ev_loop> recursively).

=head3 Watcher-Specific Functions and Data Members

=over 4

=item ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)

=item ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)

Lots of arguments, let's sort it out... There are basically three modes of
operation, and we will explain them from simplest to most complex:

=over 4

=item * absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)

In this configuration the watcher triggers an event after the wall clock
time C<offset> has passed. It will not repeat and will not adjust when a
time jump occurs, that is, if it is to be run at January 1st 2011 then it
will be stopped and invoked when the system clock reaches or surpasses
this point in time.

=item * repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)

In this mode the watcher will always be scheduled to time out at the next
C<offset + N * interval> time (for some integer N, which can also be
negative) and then repeat, regardless of any time jumps. The C<offset>
argument is merely an offset into the C<interval> periods.

This can be used to create timers that do not drift with respect to the
system clock, for example, here is an C<ev_periodic> that triggers each
hour, on the hour (with respect to UTC):

   ev_periodic_set (&periodic, 0., 3600., 0);

This doesn't mean there will always be 3600 seconds in between triggers,
but only that the callback will be called when the system time shows a
full hour (UTC), or more correctly, when the system time is evenly divisible
by 3600.

Another way to think about it (for the mathematically inclined) is that
C<ev_periodic> will try to run the callback in this mode at the next possible
time where C<time = offset (mod interval)>, regardless of any time jumps.

For numerical stability it is preferable that the C<offset> value is near
C<ev_now ()> (the current time), but there is no range requirement for
this value, and in fact is often specified as zero.

Note also that there is an upper limit to how often a timer can fire (CPU
speed for example), so if C<interval> is very small then timing stability
will of course deteriorate. Libev itself tries to be exact to be about one
millisecond (if the OS supports it and the machine is fast enough).

=item * manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)

In this mode the values for C<interval> and C<offset> are both being
ignored. Instead, each time the periodic watcher gets scheduled, the
reschedule callback will be called with the watcher as first, and the
current time as second argument.

NOTE: I<This callback MUST NOT stop or destroy any periodic watcher, ever,
or make ANY other event loop modifications whatsoever, unless explicitly
allowed by documentation here>.

If you need to stop it, return C<now + 1e30> (or so, fudge fudge) and stop
it afterwards (e.g. by starting an C<ev_prepare> watcher, which is the
only event loop modification you are allowed to do).

The callback prototype is C<ev_tstamp (*reschedule_cb)(ev_periodic
*w, ev_tstamp now)>, e.g.:

   static ev_tstamp
   my_rescheduler (ev_periodic *w, ev_tstamp now)
   {
     return now + 60.;
   }

It must return the next time to trigger, based on the passed time value
(that is, the lowest time value larger than to the second argument). It
will usually be called just before the callback will be triggered, but
might be called at other times, too.

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
next midnight after C<now> and return the timestamp value for this. How
you do this is, again, up to you (but it is not trivial, which is the main
reason I omitted it as an example).

=back

=item ev_periodic_again (loop, ev_periodic *)

Simply stops and restarts the periodic watcher again. This is only useful
when you changed some parameters or the reschedule callback would return
a different time than the last time it was called (e.g. in a crond like
program when the crontabs have changed).

=item ev_tstamp ev_periodic_at (ev_periodic *)

When active, returns the absolute time that the watcher is supposed
to trigger next. This is not the same as the C<offset> argument to
C<ev_periodic_set>, but indeed works even in interval and manual
rescheduling modes.

=item ev_tstamp offset [read-write]

When repeating, this contains the offset value, otherwise this is the
absolute point in time (the C<offset> value passed to C<ev_periodic_set>,
although libev might modify this value for better numerical stability).

Can be modified any time, but changes only take effect when the periodic
timer fires or C<ev_periodic_again> is being called.

=item ev_tstamp interval [read-write]

The current interval value. Can be modified any time, but changes only
take effect when the periodic timer fires or C<ev_periodic_again> is being
called.

=item ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]

The current reschedule callback, or C<0>, if this functionality is
switched off. Can be changed any time, but changes only take effect when
the periodic timer fires or C<ev_periodic_again> is being called.

=back

=head3 Examples

Example: Call a callback every hour, or, more precisely, whenever the
system time is divisible by 3600. The callback invocation times have
potentially a lot of jitter, but good long-term stability.

   static void
   clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
   {
     ... its now a full hour (UTC, or TAI or whatever your clock follows)
   }

   ev_periodic hourly_tick;
   ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
   ev_periodic_start (loop, &hourly_tick);

Example: The same as above, but use a reschedule callback to do it:

   #include <math.h>

   static ev_tstamp
   my_scheduler_cb (ev_periodic *w, ev_tstamp now)
   {
     return now + (3600. - fmod (now, 3600.));
   }

   ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);

Example: Call a callback every hour, starting now:

   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
will try it's best to deliver signals synchronously, i.e. as part of the
normal event processing, like any other event.

If you want signals to be delivered truly asynchronously, just use
C<sigaction> as you would do without libev and forget about sharing
the signal. You can even use C<ev_async> from a signal handler to
synchronously wake up an event loop.

You can configure as many watchers as you like for the same signal, but
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
with the kernel (thus it coexists with your own 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
and unblock them in an C<ev_prepare> watcher.

=head3 The special problem of inheritance over fork/execve/pthread_create

Both the signal mask (C<sigprocmask>) and the signal disposition
(C<sigaction>) are unspecified after starting a signal watcher (and after
stopping it again), that is, libev might or might not block the signal,
and might or might not set or restore the installed signal handler.

While this does not matter for the signal disposition (libev never
sets signals to C<SIG_IGN>, so handlers will be reset to C<SIG_DFL> on
C<execve>), this matters for the signal mask: many programs do not expect
certain signals to be blocked.

This means that before calling C<exec> (from the child) you should reset
the signal mask to whatever "default" you expect (all clear is a good
choice usually).

The simplest way to ensure that the signal mask is reset in the child is
to install a fork handler with C<pthread_atfork> that resets it. That will
catch fork calls done by libraries (such as the libc) as well.

In current versions of libev, the signal will not be blocked indefinitely
unless you use the C<signalfd> API (C<EV_SIGNALFD>). While this reduces
the window of opportunity for problems, it will not go away, as libev
I<has> to modify the signal mask, at least temporarily.

So I can't stress this enough: I<If you do not reset your signal mask when
you expect it to be empty, you have a race condition in your code>. This
is not a libev-specific thing, this is true for most event libraries.

=head3 Watcher-Specific Functions and Data Members

=over 4

=item ev_signal_init (ev_signal *, callback, int signum)

=item ev_signal_set (ev_signal *, int signum)

Configures the watcher to trigger on the given signal number (usually one
of the C<SIGxxx> constants).

=item int signum [read-only]

The signal the watcher watches out for.

=back

=head3 Examples

Example: Try to exit cleanly on SIGINT.

   static void
   sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
   {
     ev_unloop (loop, EVUNLOOP_ALL);
   }

   ev_signal signal_watcher;
   ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
   ev_signal_start (loop, &signal_watcher);


=head2 C<ev_child> - watch out for process status changes

Child watchers trigger when your process receives a SIGCHLD in response to
some child status changes (most typically when a child of yours dies or
exits). It is permissible to install a child watcher I<after> the child
has been forked (which implies it might have already exited), as long
as the event loop isn't entered (or is continued from a watcher), i.e.,
forking and then immediately registering a watcher for the child is fine,
but forking and registering a watcher a few event loop iterations later or
in the next callback invocation is not.

Only the default event loop is capable of handling signals, and therefore
you can only register child watchers in the default event loop.

Due to some design glitches inside libev, child watchers will always be
handled at maximum priority (their priority is set to C<EV_MAXPRI> by
libev)

=head3 Process Interaction

Libev grabs C<SIGCHLD> as soon as the default event loop is
initialised. This is necessary to guarantee proper behaviour even if the
first child watcher is started after the child exits. The occurrence
of C<SIGCHLD> is recorded asynchronously, but child reaping is done
synchronously as part of the event loop processing. Libev always reaps all
children, even ones not watched.

=head3 Overriding the Built-In Processing

Libev offers no special support for overriding the built-in child
processing, but if your application collides with libev's default child
handler, you can override it easily by installing your own handler for
C<SIGCHLD> after initialising the default loop, and making sure the
default loop never gets destroyed. You are encouraged, however, to use an
event-based approach to child reaping and thus use libev's support for
that, so other libev users can use C<ev_child> watchers freely.

=head3 Stopping the Child Watcher

Currently, the child watcher never gets stopped, even when the
child terminates, so normally one needs to stop the watcher in the
callback. Future versions of libev might stop the watcher automatically
when a child exit is detected (calling C<ev_child_stop> twice is not a
problem).

=head3 Watcher-Specific Functions and Data Members

=over 4

=item ev_child_init (ev_child *, callback, int pid, int trace)

=item ev_child_set (ev_child *, int pid, int trace)

Configures the watcher to wait for status changes of process C<pid> (or
I<any> process if C<pid> is specified as C<0>). The callback can look
at the C<rstatus> member of the C<ev_child> watcher structure to see
the status word (use the macros from C<sys/wait.h> and see your systems
C<waitpid> documentation). The C<rpid> member contains the pid of the
process causing the status change. C<trace> must be either C<0> (only
activate the watcher when the process terminates) or C<1> (additionally
activate the watcher when the process is stopped or continued).

=item int pid [read-only]

The process id this watcher watches out for, or C<0>, meaning any process id.

=item int rpid [read-write]

The process id that detected a status change.

=item int rstatus [read-write]

The process exit/trace status caused by C<rpid> (see your systems
C<waitpid> and C<sys/wait.h> documentation for details).

=back

=head3 Examples

Example: C<fork()> a new process and install a child handler to wait for
its completion.

   ev_child cw;

   static void
   child_cb (EV_P_ ev_child *w, int revents)
   {
     ev_child_stop (EV_A_ w);
     printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
   }

   pid_t pid = fork ();

   if (pid < 0)
     // error
   else if (pid == 0)
     {
       // the forked child executes here
       exit (1);
     }
   else
     {
       ev_child_init (&cw, child_cb, pid, 0);
       ev_child_start (EV_DEFAULT_ &cw);
     }


=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
it did.

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
least one) and all the other fields of the stat buffer having unspecified
contents.

The path I<must not> end in a slash or contain special components such as
C<.> or C<..>. The path I<should> be absolute: If it is relative and
your working directory changes, then the behaviour is undefined.

Since there is no portable change notification interface available, the
portable implementation simply calls C<stat(2)> regularly on the path
to see if it changed somehow. You can specify a recommended polling
interval for this case. If you specify a polling interval of C<0> (highly
recommended!) then a I<suitable, unspecified default> value will be used
(which you can expect to be around five seconds, although this might
change dynamically). Libev will also impose a minimum interval which is
currently around C<0.1>, but that's usually overkill.

This watcher type is not meant for massive numbers of stat watchers,
as even with OS-supported change notifications, this can be
resource-intensive.

At the time of this writing, the only OS-specific interface implemented
is the Linux inotify interface (implementing kqueue support is left as an
exercise for the reader. Note, however, that the author sees no way of
implementing C<ev_stat> semantics with kqueue, except as a hint).

=head3 ABI Issues (Largefile Support)

Libev by default (unless the user overrides this) uses the default
compilation environment, which means that on systems with large file
support disabled by default, you get the 32 bit version of the stat
structure. When using the library from programs that change the ABI to
use 64 bit file offsets the programs will fail. In that case you have to
compile libev with the same flags to get binary compatibility. This is
obviously the case with any flags that change the ABI, but the problem is
most noticeably displayed with ev_stat and large file support.

The solution for this is to lobby your distribution maker to make large
file interfaces available by default (as e.g. FreeBSD does) and not
optional. Libev cannot simply switch on large file support because it has
to exchange stat structures with application programs compiled using the
default compilation environment.

=head3 Inotify and Kqueue

When C<inotify (7)> support has been compiled into libev and present at
runtime, it will be used to speed up change detection where possible. The
inotify descriptor will be created lazily when the first C<ev_stat>
watcher is being started.

Inotify presence does not change the semantics of C<ev_stat> watchers
except that changes might be detected earlier, and in some cases, to avoid
making regular C<stat> calls. Even in the presence of inotify support
there are many cases where libev has to resort to regular C<stat> polling,
but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
many bugs), the path exists (i.e. stat succeeds), and the path resides on
a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
xfs are fully working) libev usually gets away without polling.

There is no support for kqueue, as apparently it cannot be used to
implement this functionality, due to the requirement of having a file
descriptor open on the object at all times, and detecting renames, unlinks
etc. is difficult.

=head3 C<stat ()> is a synchronous operation

Libev doesn't normally do any kind of I/O itself, and so is not blocking
the process. The exception are C<ev_stat> watchers - those call C<stat
()>, which is a synchronous operation.

For local paths, this usually doesn't matter: unless the system is very
busy or the intervals between stat's are large, a stat call will be fast,
as the path data is usually in memory already (except when starting the
watcher).

For networked file systems, calling C<stat ()> can block an indefinite
time due to network issues, and even under good conditions, a stat call
often takes multiple milliseconds.

Therefore, it is best to avoid using C<ev_stat> watchers on networked
paths, although this is fully supported by libev.

=head3 The special problem of stat time resolution

The C<stat ()> system call only supports full-second resolution portably,
and even on systems where the resolution is higher, most file systems
still only support whole seconds.

That means that, if the time is the only thing that changes, you can
easily miss updates: on the first update, C<ev_stat> detects a change and
calls your callback, which does something. When there is another update
within the same second, C<ev_stat> will be unable to detect unless the
stat data does change in other ways (e.g. file size).

The solution to this is to delay acting on a change for slightly more
than a second (or till slightly after the next full second boundary), using
a roughly one-second-delay C<ev_timer> (e.g. C<ev_timer_set (w, 0., 1.02);
ev_timer_again (loop, w)>).

The C<.02> offset is added to work around small timing inconsistencies
of some operating systems (where the second counter of the current time
might be be delayed. One such system is the Linux kernel, where a call to
C<gettimeofday> might return a timestamp with a full second later than
a subsequent C<time> call - if the equivalent of C<time ()> is used to
update file times then there will be a small window where the kernel uses
the previous second to update file times but libev might already execute
the timer callback).

=head3 Watcher-Specific Functions and Data Members

=over 4

=item ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)

=item ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)

Configures the watcher to wait for status changes of the given
C<path>. The C<interval> is a hint on how quickly a change is expected to
be detected and should normally be specified as C<0> to let libev choose
a suitable value. The memory pointed to by C<path> must point to the same
path for as long as the watcher is active.

The callback will receive an C<EV_STAT> event when a change was detected,
relative to the attributes at the time the watcher was started (or the
last change was detected).

=item ev_stat_stat (loop, ev_stat *)

Updates the stat buffer immediately with new values. If you change the
watched path in your callback, you could call this function to avoid
detecting this change (while introducing a race condition if you are not
the only one changing the path). Can also be useful simply to find out the
new values.

=item ev_statdata attr [read-only]

The most-recently detected attributes of the file. Although the type is
C<ev_statdata>, this is usually the (or one of the) C<struct stat> types
suitable for your system, but you can only rely on the POSIX-standardised
members to be present. If the C<st_nlink> member is C<0>, then there was
some error while C<stat>ing the file.

=item ev_statdata prev [read-only]

The previous attributes of the file. The callback gets invoked whenever
C<prev> != C<attr>, or, more precisely, one or more of these members
differ: C<st_dev>, C<st_ino>, C<st_mode>, C<st_nlink>, C<st_uid>,
C<st_gid>, C<st_rdev>, C<st_size>, C<st_atime>, C<st_mtime>, C<st_ctime>.

=item ev_tstamp interval [read-only]

The specified interval.

=item const char *path [read-only]

The file system path that is being watched.

=back

=head3 Examples

Example: Watch C</etc/passwd> for attribute changes.

   static void
   passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
   {
     /* /etc/passwd changed in some way */
     if (w->attr.st_nlink)
       {
         printf ("passwd current size  %ld\n", (long)w->attr.st_size);
         printf ("passwd current atime %ld\n", (long)w->attr.st_mtime);
         printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime);
       }
     else
       /* you shalt not abuse printf for puts */
       puts ("wow, /etc/passwd is not there, expect problems. "
             "if this is windows, they already arrived\n");
   }

   ...
   ev_stat passwd;

   ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
   ev_stat_start (loop, &passwd);

Example: Like above, but additionally use a one-second delay so we do not
miss updates (however, frequent updates will delay processing, too, so
one might do the work both on C<ev_stat> callback invocation I<and> on
C<ev_timer> callback invocation).

   static ev_stat passwd;
   static ev_timer timer;

   static void
   timer_cb (EV_P_ ev_timer *w, int revents)
   {
     ev_timer_stop (EV_A_ w);

     /* now it's one second after the most recent passwd change */
   }

   static void
   stat_cb (EV_P_ ev_stat *w, int revents)
   {
     /* reset the one-second timer */
     ev_timer_again (EV_A_ &timer);
   }

   ...
   ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
   ev_stat_start (loop, &passwd);
   ev_timer_init (&timer, timer_cb, 0., 1.02);


=head2 C<ev_idle> - when you've got nothing better to do...

Idle watchers trigger events when no other events of the same or higher
priority are pending (prepare, check and other idle watchers do not count
as receiving "events").

That is, as long as your process is busy handling sockets or timeouts
(or even signals, imagine) of the same or higher priority it will not be
triggered. But when your process is idle (or only lower-priority watchers
are pending), the idle watchers are being called once per event loop
iteration - until stopped, that is, or your process receives more events
and becomes busy again with higher priority stuff.

The most noteworthy effect is that as long as any idle watchers are
active, the process will not block when waiting for new events.

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 Watcher-Specific Functions and Data Members

=over 4

=item ev_idle_init (ev_idle *, callback)

Initialises and configures the idle watcher - it has no parameters of any
kind. There is a C<ev_idle_set> macro, but using it is utterly pointless,
believe me.

=back

=head3 Examples

Example: Dynamically allocate an C<ev_idle> watcher, start it, and in the
callback, free it. Also, use no error checking, as usual.

   static void
   idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
   {
     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_init (idle_watcher, idle_cb);
   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 watchers get invoked before the process blocks and check watchers
afterwards.

You I<must not> call C<ev_loop> or similar functions that enter
the current event loop from either C<ev_prepare> or C<ev_check>
watchers. Other loops than the current one are fine, however. The
rationale behind this is that you do not need to check for recursion in
those watchers, i.e. the sequence will always be C<ev_prepare>, blocking,
C<ev_check> so if you have one watcher of each 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
you cache some data and want to flush it before blocking (for example,
in X programs you might want to do an C<XFlush ()> in an C<ev_prepare>
watcher).

This is done by examining in each prepare call which file descriptors
need to be watched by the other library, registering C<ev_io> watchers
for them and starting an C<ev_timer> watcher for any timeouts (many
libraries provide exactly this functionality). Then, in the check watcher,
you check for any events that occurred (by checking the pending status
of all watchers and stopping them) and call back into the library. The
I/O and timer callbacks will never actually be called (but must be valid
nevertheless, because you never know, you know?).

As another example, the Perl Coro module uses these hooks to integrate
coroutines into libev programs, by yielding to other active coroutines
during each prepare and only letting the process block if no coroutines
are ready to run (it's actually more complicated: it only runs coroutines
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>)
priority, to ensure that they are being run before 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 Watcher-Specific Functions and Data Members

=over 4

=item ev_prepare_init (ev_prepare *, callback)

=item ev_check_init (ev_check *, callback)

Initialises and configures the prepare or check watcher - they have no
parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set>
macros, but using them is utterly, utterly, utterly and completely
pointless.

=back

=head3 Examples

There are a number of principal ways to embed other event loops or modules
into libev. Here are some ideas on how to include libadns into libev
(there is a Perl module named C<EV::ADNS> that does this, which you could
use as a working example. Another Perl module named C<EV::Glib> embeds a
Glib main context into libev, and finally, C<Glib::EV> embeds EV into the
Glib event loop).

Method 1: Add IO watchers and a timeout watcher in a prepare handler,
and in a check watcher, destroy them and call into libadns. What follows
is pseudo-code only of course. This requires you to either use a low
priority for the check watcher or use C<ev_clear_pending> explicitly, as
the callbacks for the IO/timeout watchers might not have been called yet.

   static ev_io iow [nfd];
   static ev_timer tw;

   static void
   io_cb (struct ev_loop *loop, ev_io *w, int revents)
   {
   }

   // create io watchers for each fd and a timer before blocking
   static void
   adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
   {
     int timeout = 3600000;
     struct pollfd fds [nfd];
     // actual code will need to loop here and realloc etc.
     adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));

     /* the callback is illegal, but won't be called as we stop during check */
     ev_timer_init (&tw, 0, timeout * 1e-3, 0.);
     ev_timer_start (loop, &tw);

     // create one ev_io per pollfd
     for (int i = 0; i < nfd; ++i)
       {
         ev_io_init (iow + i, io_cb, fds [i].fd,
           ((fds [i].events & POLLIN ? EV_READ : 0)
            | (fds [i].events & POLLOUT ? EV_WRITE : 0)));

         fds [i].revents = 0;
         ev_io_start (loop, iow + i);
       }
   }

   // stop all watchers after blocking
   static void
   adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
   {
     ev_timer_stop (loop, &tw);

     for (int i = 0; i < nfd; ++i)
       {
         // set the relevant poll flags
         // could also call adns_processreadable etc. here
         struct pollfd *fd = fds + i;
         int revents = ev_clear_pending (iow + i);
         if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
         if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;

         // now stop the watcher
         ev_io_stop (loop, iow + i);
       }

     adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
   }

Method 2: This would be just like method 1, but you run C<adns_afterpoll>
in the prepare watcher and would dispose of the check watcher.

Method 3: If the module to be embedded supports explicit event
notification (libadns does), you can also make use of the actual watcher
callbacks, and only destroy/create the watchers in the prepare watcher.

   static void
   timer_cb (EV_P_ ev_timer *w, int revents)
   {
     adns_state ads = (adns_state)w->data;
     update_now (EV_A);

     adns_processtimeouts (ads, &tv_now);
   }

   static void
   io_cb (EV_P_ ev_io *w, int revents)
   {
     adns_state ads = (adns_state)w->data;
     update_now (EV_A);

     if (revents & EV_READ ) adns_processreadable  (ads, w->fd, &tv_now);
     if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
   }

   // do not ever call adns_afterpoll

Method 4: Do not use a prepare or check watcher because the module you
want to embed is not flexible enough to support it. Instead, you can
override their poll function. The drawback with this solution is that the
main loop is now no longer controllable by EV. The C<Glib::EV> module uses
this approach, effectively embedding EV as a client into the horrible
libglib event loop.

   static gint
   event_poll_func (GPollFD *fds, guint nfds, gint timeout)
   {
     int got_events = 0;

     for (n = 0; n < nfds; ++n)
       // create/start io watcher that sets the relevant bits in fds[n] and increment got_events

     if (timeout >= 0)
       // create/start timer

     // poll
     ev_loop (EV_A_ 0);

     // stop timer again
     if (timeout >= 0)
       ev_timer_stop (EV_A_ &to);

     // stop io watchers again - their callbacks should have set
     for (n = 0; n < nfds; ++n)
       ev_io_stop (EV_A_ iow [n]);

     return got_events;
   }


=head2 C<ev_embed> - when one backend isn't enough...

This is a rather advanced watcher type that lets you embed one event loop
into another (currently only C<ev_io> events are supported in the embedded
loop, other types of watchers might be handled in a delayed or incorrect
fashion and must not be used).

There are primarily two reasons you would want that: work around bugs and
prioritise I/O.

As an example for a bug workaround, the kqueue backend might only support
sockets on some platform, so it is unusable as generic backend, but you
still want to make use of it because you have many sockets and it scales
so nicely. In this case, you would create a kqueue-based loop and embed
it into your default loop (which might use e.g. poll). Overall operation
will be a bit slower because first libev has to call C<poll> and then
C<kevent>, but at least you can use both mechanisms for what they are
best: C<kqueue> for scalable sockets and C<poll> if you want it to work :)

As for prioritising I/O: under rare circumstances you have the case where
some fds have to be watched and handled very quickly (with low latency),
and even priorities and idle watchers might have too much overhead. In
this case you would put all the high priority stuff in one loop and all
the rest in a second one, and embed the second one in the first.

As long as the watcher is active, the callback will be invoked every
time there might be events pending in the embedded loop. The callback
must then call C<ev_embed_sweep (mainloop, watcher)> to make a single
sweep and invoke their callbacks (the callback doesn't need to invoke the
C<ev_embed_sweep> function directly, it could also start an idle watcher
to give the embedded loop strictly lower priority for example).

You can also set the callback to C<0>, in which case the embed watcher
will automatically execute the embedded loop sweep whenever necessary.

Fork detection will be handled transparently while the C<ev_embed> watcher
is active, i.e., the embedded loop will automatically be forked when the
embedding loop forks. In other cases, the user is responsible for calling
C<ev_loop_fork> on the embedded loop.

Unfortunately, not all backends are embeddable: only the ones returned by
C<ev_embeddable_backends> are, which, unfortunately, does not include any
portable one.

So when you want to use this feature you will always have to be prepared
that you cannot get an embeddable loop. The recommended way to get around
this is to have a separate variables for your embeddable loop, try to
create it, and if that fails, use the normal loop for everything.

=head3 C<ev_embed> and fork

While the C<ev_embed> watcher is running, forks in the embedding loop will
automatically be applied to the embedded loop as well, so no special
fork handling is required in that case. When the watcher is not running,
however, it is still the task of the libev user to call C<ev_loop_fork ()>
as applicable.

=head3 Watcher-Specific Functions and Data Members

=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)

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,
if you do not want that, you need to temporarily stop the embed watcher).

=item ev_embed_sweep (loop, ev_embed *)

Make a single, non-blocking sweep over the embedded loop. This works
similarly to C<ev_loop (embedded_loop, EVLOOP_NONBLOCK)>, but in the most
appropriate way for embedded loops.

=item struct ev_loop *other [read-only]

The embedded event loop.

=back

=head3 Examples

Example: Try to get an embeddable event loop and embed it into the default
event loop. If that is not possible, use the default loop. The default
loop is stored in C<loop_hi>, while the embeddable loop is stored in
C<loop_lo> (which is C<loop_hi> in the case no embeddable loop can be
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;

   // if we got one, then embed it, otherwise default to loop_hi
   if (loop_lo)
     {
       ev_embed_init (&embed, 0, loop_lo);
       ev_embed_start (loop_hi, &embed);
     }
   else
     loop_lo = loop_hi;

Example: Check if kqueue is available but not recommended and create
a kqueue backend for use with sockets (which usually work with any
kqueue implementation). Store the kqueue/socket-only event loop in
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);
       }

   if (!loop_socket)
     loop_socket = loop;

   // now use loop_socket for all sockets, and loop for everything else


=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
event loop blocks next and before C<ev_check> watchers are being called,
and only in the child after the fork. If whoever good citizen calling
C<ev_default_fork> cheats 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
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
fork.

The default mode of operation (for libev, with application help to detect
forks) is to duplicate all the state in the child, as would be expected
when I<either> the parent I<or> the child process continues.

When both processes want to continue using libev, then this is usually the
wrong result. In that case, usually one process (typically the parent) is
supposed to continue with all watchers in place as before, while the other
process typically wants to start fresh, i.e. without any active watchers.

The cleanest and most efficient way to achieve that with libev is to
simply create a new event loop, which of course will be "empty", and
use that for new watchers. This has the advantage of not touching more
memory than necessary, and thus avoiding the copy-on-write, and the
disadvantage of having to use multiple event loops (which do not support
signal watchers).

When this is not possible, or you want to use the default loop for
other reasons, then in the process that wants to start "fresh", call
C<ev_default_destroy ()> followed by C<ev_default_loop (...)>. Destroying
the default loop will "orphan" (not stop) all registered watchers, so you
have to be careful not to execute code that modifies those watchers. Note
also that in that case, you have to re-register any signal watchers.

=head3 Watcher-Specific Functions and Data Members

=over 4

=item ev_fork_init (ev_signal *, callback)

Initialises and configures the fork watcher - it has no parameters of any
kind. There is a C<ev_fork_set> macro, but using it is utterly pointless,
believe me.

=back


=head2 C<ev_async> - how to wake up an event loop

In general, you cannot use an C<ev_loop> from multiple threads or other
asynchronous sources such as signal handlers (as opposed to multiple event
loops - those are of course safe to use in different threads).

Sometimes, however, you need to wake up an event loop you do not control,
for example because it belongs to another thread. This is what C<ev_async>
watchers do: as long as the C<ev_async> watcher is active, you can signal
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).

Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not
just the default loop.

=head3 Queueing

C<ev_async> does not support queueing of data in any way. The reason
is that the author does not know of a simple (or any) algorithm for a
multiple-writer-single-reader queue that works in all cases and doesn't
need elaborate support such as pthreads or unportable memory access
semantics.

That means that if you want to queue data, you have to provide your own
queue. But at least I can tell you how to implement locking around your
queue:

=over 4

=item queueing from a signal handler context

To implement race-free queueing, you simply add to the queue in the signal
handler but you block the signal handler in the watcher callback. Here is
an example that does that for some fictitious SIGUSR1 handler:

   static ev_async mysig;

   static void
   sigusr1_handler (void)
   {
     sometype data;

     // no locking etc.
     queue_put (data);
     ev_async_send (EV_DEFAULT_ &mysig);
   }

   static void
   mysig_cb (EV_P_ ev_async *w, int revents)
   {
     sometype data;
     sigset_t block, prev;

     sigemptyset (&block);
     sigaddset (&block, SIGUSR1);
     sigprocmask (SIG_BLOCK, &block, &prev);

     while (queue_get (&data))
       process (data);

     if (sigismember (&prev, SIGUSR1)
       sigprocmask (SIG_UNBLOCK, &block, 0);
   }

(Note: pthreads in theory requires you to use C<pthread_setmask>
instead of C<sigprocmask> when you use threads, but libev doesn't do it
either...).

=item queueing from a thread context

The strategy for threads is different, as you cannot (easily) block
threads but you can easily preempt them, so to queue safely you need to
employ a traditional mutex lock, such as in this pthread example:

   static ev_async mysig;
   static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

   static void
   otherthread (void)
   {
     // only need to lock the actual queueing operation
     pthread_mutex_lock (&mymutex);
     queue_put (data);
     pthread_mutex_unlock (&mymutex);

     ev_async_send (EV_DEFAULT_ &mysig);
   }

   static void
   mysig_cb (EV_P_ ev_async *w, int revents)
   {
     pthread_mutex_lock (&mymutex);

     while (queue_get (&data))
       process (data);

     pthread_mutex_unlock (&mymutex);
   }

=back


=head3 Watcher-Specific Functions and Data Members

=over 4

=item ev_async_init (ev_async *, callback)

Initialises and configures the async watcher - it has no parameters of any
kind. There is a C<ev_async_set> macro, but using it is utterly pointless,
trust me.

=item ev_async_send (loop, ev_async *)

Sends/signals/activates the given C<ev_async> watcher, that is, feeds
an C<EV_ASYNC> event on the watcher into the event loop. Unlike
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
is that C<ev_async> watchers are level-triggered, set on 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
iteration, so while the overhead might be noticeable, it doesn't apply to
repeated calls to C<ev_async_send> for the same event loop.

=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
event loop.

C<ev_async_send> sets a flag in the watcher and wakes up the loop. When
the loop iterates next and checks for the watcher to have become active,
it will reset the flag again. C<ev_async_pending> can be used to very
quickly check whether invoking the loop might be a good idea.

Not that this does I<not> check whether the watcher itself is pending,
only whether it has been requested to make this watcher pending: there
is a time window between the event loop checking and resetting the async
notification, and the callback being invoked.

=back


=head1 OTHER FUNCTIONS

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)

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
more watchers yourself.

If C<fd> is less than 0, then no I/O watcher will be started and the
C<events> argument is being ignored. Otherwise, an C<ev_io> watcher for
the given C<fd> and C<events> set will be created and started.

If C<timeout> is less than 0, then no timeout watcher will be
started. Otherwise an C<ev_timer> watcher with after = C<timeout> (and
repeat = 0) will be started. C<0> is a valid timeout.

The callback has the type C<void (*cb)(int revents, void *arg)> and is
passed an C<revents> set like normal event callbacks (a combination of
C<EV_ERROR>, C<EV_READ>, C<EV_WRITE> or C<EV_TIMER>) and the C<arg>
value passed to C<ev_once>. Note that it is possible to receive I<both>
a timeout and an io event at the same time - you probably should give io
events precedence.

Example: wait up to ten seconds for data to appear on STDIN_FILENO.

   static void stdin_ready (int revents, void *arg)
   {
     if (revents & EV_READ)
       /* stdin might have data for us, joy! */;
     else if (revents & EV_TIMER)
       /* doh, nothing entered */;
   }

   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.

=item ev_feed_signal_event (loop, int signum)

Feed an event as if the given signal occurred (C<loop> must be the default
loop!).

=back


=head1 LIBEVENT EMULATION

Libev offers a compatibility emulation layer for libevent. It cannot
emulate the internals of libevent, so here are some usage hints:

=over 4

=item * Use it by including <event.h>, as usual.

=item * The following members are fully supported: ev_base, ev_callback,
ev_arg, ev_fd, ev_res, ev_events.

=item * Avoid using ev_flags and the EVLIST_*-macros, while it is
maintained by libev, it does not work exactly the same way as in libevent (consider
it a private API).

=item * Priorities are not currently supported. Initialising priorities
will fail and all watchers will have the same priority, even though there
is an ev_pri field.

=item * In libevent, the last base created gets the signals, in libev, the
first base created (== the default loop) gets the signals.

=item * Other members are not supported.

=item * The libev emulation is I<not> ABI compatible to libevent, you need
to use the libev header file and library.

=back

=head1 C++ SUPPORT

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
options as F<ev.h>, most notably C<EV_MULTIPLICITY>.

Care has been taken to keep the overhead low. The only data member the C++
classes add (compared to plain C-style watchers) is the event loop pointer
that the watcher is associated with (or no additional members at all if
you disable C<EV_MULTIPLICITY> when embedding libev).

Currently, functions, and static and non-static member functions 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).

Here is a list of things available in the C<ev> namespace:

=over 4

=item C<ev::READ>, C<ev::WRITE> etc.

These are just enum values with the same values as the C<EV_READ> etc.
macros from F<ev.h>.

=item C<ev::tstamp>, C<ev::now>

Aliases to the same types/functions as with the C<ev_> prefix.

=item C<ev::io>, C<ev::timer>, C<ev::periodic>, C<ev::idle>, C<ev::sig> etc.