libev/ev.pod

=head1 NAME

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

=head1 SYNOPSIS

  #include <ev.h>

=head2 EXAMPLE PROGRAM

  #include <ev.h>

  ev_io stdin_watcher;
  ev_timer timeout_watcher;

  /* called when data readable on stdin */
  static void
  stdin_cb (EV_P_ struct ev_io *w, int revents)
  {
    /* puts ("stdin ready"); */
    ev_io_stop (EV_A_ w); /* just a syntax example */
    ev_unloop (EV_A_ EVUNLOOP_ALL); /* leave all loop calls */
  }

  static void
  timeout_cb (EV_P_ struct ev_timer *w, int revents)
  {
    /* puts ("timeout"); */
    ev_unloop (EV_A_ EVUNLOOP_ONE); /* leave one loop call */
  }

  int
  main (void)
  {
    struct ev_loop *loop = ev_default_loop (0);

    /* initialise an io watcher, then start it */
    ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
    ev_io_start (loop, &stdin_watcher);

    /* simple non-repeating 5.5 second timeout */
    ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
    ev_timer_start (loop, &timeout_watcher);

    /* loop till timeout or data ready */
    ev_loop (loop, 0);

    return 0;
  }

=head1 DESCRIPTION

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

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>), 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 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 (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, and when you need to do any calculations on
it, you should treat it as some floatingpoint value. Unlike the name
component C<stamp> might indicate, it is also used for time differences
throughout libev.

=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 subsecond-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.

  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 autodetected 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))

Sets the allocation function to use (the prototype is similar - the
semantics is identical - to the realloc C function). It is used to
allocate and free memory (no surprises here). If it returns zero when
memory needs to be allocated, the library might abort or take some
potentially destructive action. The default is your system realloc
function.

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).

   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));

Set the callback function to call on a retryable syscall 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 sitution, 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 library knows two
types of such loops, the I<default> loop, which supports signals and child
events, and dynamically created loops which do not.

If you use threads, a common model is to run the default event loop
in your main thread (or in a separate thread) and for each thread you
create, you also create another event loop. Libev itself does no locking
whatsoever, so if you mix calls to the same event loop in different
threads, make sure you lock (this is usually a bad idea, though, even if
done correctly, because it's hideous and inefficient).

=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.

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 app 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 ored 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_default_fork> or 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
Linux system for example, C<getpid> is actually a simple 5-insn sequence
without a syscall and thus I<very> fast, but my 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 overriden or specified in the C<LIBEV_FLAGS>
environment variable.

=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
parallelity (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
readyness notifications you get per iteration.

=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.

=item C<EVBACKEND_EPOLL>   (value 4, Linux)

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 design has a number
of shortcomings, such as silently dropping events in some hard-to-detect
cases and rewiring a syscall per fd change, no fork support and bad
support for dup.

While stopping, setting and starting an I/O watcher in the same iteration
will result in some caching, there is still a syscall per such incident
(because the fd could point to a different 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 fds.

Please note that epoll sometimes generates spurious notifications, so you
need to use non-blocking I/O or other means to avoid blocking when no data
(or space) is available.

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.

While nominally embeddeble in other event loops, this feature is broken in
all kernel versions tested so far.

=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). For this reason it's not being "autodetected"
unless you explicitly specify it explicitly 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 syscall as with C<EVBACKEND_EPOLL>, it still adds up to
two event changes per incident, support for C<fork ()> is very bad 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>) and using it only for
sockets.

=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, ignoring the spurious readyness notifications, this
backend actually performed to specification in all tests and is fully
embeddable, which is a rare feat among the OS-specific backends.

=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 these are ored 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.

The most typical usage is like this:

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

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);

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. Unlike the default loop, it cannot
handle signal and child watchers, and attempts to do so will be greeted by
undefined behaviour (or a failed assertion if assertions are enabled).

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 again (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 yoursef 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, 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.

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 library in the child. If
you just fork+exec, 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, and how you do this is entirely your own problem.

=item int ev_is_default_loop (loop)

Returns true when the given loop actually is the default loop, false otherwise.

=item unsigned int ev_loop_count (loop)

Returns the count of loop iterations 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.

=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_loop (loop, int flags)

Finally, this is it, the event handler. This function usually is called
after you 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 is a thing of beauty.

A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle
those events and any 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
neccessary) and will handle those and any outstanding ones. It will block
your process until at least one new event arrives, 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. 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, queue and call all fork watchers.
   - Queue and call all prepare watchers.
   - If we have been forked, recreate the kernel state.
   - Update the kernel state with all outstanding changes.
   - Update the "event loop time".
   - 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" and do time jump handling.
   - Queue all outstanding timers.
   - Queue all outstanding periodics.
   - If no 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. 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.

=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. If you have
a watcher you never unregister that should not keep C<ev_loop> from
returning, ev_unref() after starting, and ev_ref() before stopping it. For
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).

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

  struct 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 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.

The background 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.

Likewise, by setting a higher I<timeout collect interval> you allow libev
to spend more time collecting timeouts, at the expense of increased
latency (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 io 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 approsaches the timing granularity of most systems.

=back


=head1 ANATOMY OF A WATCHER

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

  struct ev_loop *loop = ev_default_loop (0);
  struct 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 usually a bad idea to do this on the stack,
although this can sometimes be quite valid).

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 io
watchers, each time the event loop detects that the file descriptor given
is readable and/or writable).

Each watcher type has its own C<< ev_<type>_set (watcher *, ...) >> macro
with arguments specific to this 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<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_TIMEOUT>

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_ERROR>

An unspecified error has occured, 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. You best act on it by reporting the problem and somehow coping
with the watcher being stopped.

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 multithreaded
programs, though, so beware.

=back

=head2 GENERIC WATCHER FUNCTIONS

In the following description, C<TYPE> stands for the watcher type,
e.g. C<timer> for C<ev_timer> watchers and C<io> for C<ev_io> watchers.

=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 (*)(ev_loop *loop, ev_TYPE *watcher,
int revents)>.

=item C<ev_TYPE_set> (ev_TYPE *, [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.

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

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

=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.

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

Stops the given watcher again (if active) and clears the pending
status. It is possible that stopped watchers are pending (for example,
non-repeating timers are being stopped when they become pending), but
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, 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).

This means that priorities are I<only> used for ordering callback
invocation after new events have been received. This is useful, for
example, to reduce latency after idling, or more often, to bind two
watchers on the same event and make sure one is called first.

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.

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 :).

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 adjusted to be within valid range.

=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.

=item int ev_clear_pending (loop, ev_TYPE *watcher)

If the watcher is pending, this function returns 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>.

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

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, struct 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 having some data structure with multiple
watchers:

  struct my_biggy
  {