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.

Familarity 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.

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
occured 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 otuside 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.