CSE2410 Fall 2014 Bounce

=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 DESCRIPTION

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>.

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 (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<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 floating point value. Unlike the name

component C<stamp> might indicate, it is also used for time differences

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.

   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_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

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<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)

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 these 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. 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).

Note that this function I<is> thread-safe, and the recommended 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 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 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.

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 that you want to re-use in the child, and how you do this 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_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_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 "The special problem of time updates" in the C<ev_timer> section.

=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, 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.

If you have a watcher you never unregister that should not keep C<ev_loop>

from returning, call ev_unref() after starting, and 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).

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.

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.

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.

=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_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 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 *, [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, 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 clamped to the 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, 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.

=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;