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LIBEV(3) libev - high performance full featured event loop LIBEV(3)

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

SYNOPSIS
#include <ev.h>

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_run's to stop iterating
ev_break (EV_A_ EVBREAK_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_run to stop iterating
ev_break (EV_A_ EVBREAK_ONE);
}

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

// 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_run (loop, 0);

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

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:
<http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.

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

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

WHAT TO READ WHEN IN A HURRY
This manual tries to be very detailed, but unfortunately, this also
makes it very long. If you just want to know the basics of libev, I
suggest reading "ANATOMY OF A WATCHER", then the "EXAMPLE PROGRAM"
above and look up the missing functions in "GLOBAL FUNCTIONS" and the
"ev_io" and "ev_timer" sections in "WATCHER TYPES".

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 event loop handler, and will then
communicate events via a callback mechanism.

You register interest in certain events by registering so-called event
watchers, which are relatively small C structures you initialise with
the details of the event, and then hand it over to libev by starting
the watcher.

FEATURES
Libev supports "select", "poll", the Linux-specific aio and "epoll"
interfaces, the BSD-specific "kqueue" and the Solaris-specific event
port mechanisms for file descriptor events ("ev_io"), the Linux
"inotify" interface (for "ev_stat"), Linux eventfd/signalfd (for faster
and cleaner inter-thread wakeup ("ev_async")/signal handling
("ev_signal")) relative timers ("ev_timer"), absolute timers with
customised rescheduling ("ev_periodic"), synchronous signals
("ev_signal"), process status change events ("ev_child"), and event
watchers dealing with the event loop mechanism itself ("ev_idle",
"ev_embed", "ev_prepare" and "ev_check" watchers) as well as file
watchers ("ev_stat") and even limited support for fork events
("ev_fork").

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

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 EMBED section in this manual. If libev was configured without
support for multiple event loops, then all functions taking an initial
argument of name "loop" (which is always of type "struct ev_loop *")
will not have this argument.

TIME REPRESENTATION
Libev represents time as a single floating point number, representing
the (fractional) number of seconds since the (POSIX) epoch (in practice
somewhere near the beginning of 1970, details are complicated, don't
ask). This type is called "ev_tstamp", which is what you should use
too. It usually aliases to the "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 "stamp" might indicate, it is also used for
time differences (e.g. delays) throughout libev.

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 "ev_set_syserr_cb", which is supposed to fix
the problem or abort. The default is to print a diagnostic message and
to call "abort ()".

When libev detects a usage error such as a negative timer interval,
then it will print a diagnostic message and abort (via the "assert"
mechanism, so "NDEBUG" will disable this checking): these are
programming errors in the libev caller and need to be fixed there.

Via the "EV_FREQUENT" macro you can compile in and/or enable extensive
consistency checking code inside libev that can be used to check for
internal inconsistencies, suually caused by application bugs.

Libev also has a few internal error-checking "assert"ions. These do not
trigger under normal circumstances, as they indicate either a bug in
libev or worse.

GLOBAL FUNCTIONS
These functions can be called anytime, even before initialising the
library in any way.

ev_tstamp ev_time ()
Returns the current time as libev would use it. Please note that
the "ev_now" function is usually faster and also often returns the
timestamp you actually want to know. Also interesting is the
combination of "ev_now_update" and "ev_now".

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 (approximately - it might return a bit earlier even if not
interrupted). Returns immediately if "interval <= 0".

Basically this is a sub-second-resolution "sleep ()".

The range of the "interval" is limited - libev only guarantees to
work with sleep times of up to one day ("interval <= 86400").

int ev_version_major ()
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
"ev_version_major" and "ev_version_minor". If you want, you can
compare against the global symbols "EV_VERSION_MAJOR" and
"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 other ABI
mismatches, such as LFS or reentrancy).

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

unsigned int ev_supported_backends ()
Return the set of all backends (i.e. their corresponding
"EV_BACKEND_*" value) compiled into this binary of libev
(independent of their availability on the system you are running
on). See "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));

unsigned int ev_recommended_backends ()
Return the set of all backends compiled into this binary of libev
and also recommended for this platform, meaning it will work for
most file descriptor types. This set is often smaller than the one
returned by "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.

unsigned int ev_embeddable_backends ()
Returns the set of backends that are embeddable in other event
loops. This value is platform-specific but can include backends not
available on the current system. To find which embeddable backends
might be supported on the current system, you would need to look at
"ev_embeddable_backends () & ev_supported_backends ()", likewise
for recommended ones.

See the description of "ev_embed" watchers for more info.

ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())
Sets the allocation function to use (the prototype is similar - the
semantics are identical to the "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 ("size != 0"), the
library might abort or take some potentially destructive action.

Since some systems (at least OpenBSD and Darwin) fail to implement
correct "realloc" semantics, libev will use a wrapper around the
system "realloc" and "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: The following is the "realloc" function that libev itself
uses which should work with "realloc" and "free" functions of all
kinds and is probably a good basis for your own implementation.

static void *
ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
{
if (size)
return realloc (ptr, size);

free (ptr);
return 0;
}

Example: Replace the libev allocator with one that waits a bit and
then retries.

static void *
persistent_realloc (void *ptr, size_t size)
{
if (!size)
{
free (ptr);
return 0;
}

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

if (newptr)
return newptr;

sleep (60);
}
}

...
ev_set_allocator (persistent_realloc);

ev_set_syserr_cb (void (*cb)(const char *msg) throw ())
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);

ev_feed_signal (int signum)
This function can be used to "simulate" a signal receive. It is
completely safe to call this function at any time, from any
context, including signal handlers or random threads.

Its main use is to customise signal handling in your process,
especially in the presence of threads. For example, you could block
signals by default in all threads (and specifying
"EVFLAG_NOSIGMASK" when creating any loops), and in one thread, use
"sigwait" or any other mechanism to wait for signals, then
"deliver" them to libev by calling "ev_feed_signal".

FUNCTIONS CONTROLLING EVENT LOOPS
An event loop is described by a "struct ev_loop *" (the "struct" is not
optional in this case unless libev 3 compatibility is disabled, as
libev 3 had an "ev_loop" function colliding with the struct name).

The library knows two types of such loops, the default loop, which
supports child process events, and dynamically created event loops
which do not.

struct ev_loop *ev_default_loop (unsigned int flags)
This returns the "default" event loop object, which is what you
should normally use when you just need "the event loop". Event loop
objects and the "flags" parameter are described in more detail in
the entry for "ev_loop_new".

If the default loop is already initialised then this function
simply returns it (and ignores the flags. If that is troubling you,
check "ev_backend ()" afterwards). Otherwise it will create it with
the given flags, which should almost always be 0, unless the caller
is also the one calling "ev_run" or otherwise qualifies as "the
main program".

If you don't know what event loop to use, use the one returned from
this function (or via the "EV_DEFAULT" macro).

Note that this function is not thread-safe, so if you want to use
it from multiple threads, you have to employ some kind of mutex
(note also that this case is unlikely, as loops cannot be shared
easily between threads anyway).

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

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);

struct ev_loop *ev_loop_new (unsigned int flags)
This will create and initialise a new event loop object. If the
loop could not be initialised, returns false.

This function 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.

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

The following flags are supported:

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

"EVFLAG_NOENV"
If this flag bit is or'ed into the flag value (or the program
runs setuid or setgid) then libev will not look at the
environment variable "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, to work around
bugs, or to make libev threadsafe (accessing environment
variables cannot be done in a threadsafe way, but usually it
works if no other thread modifies them).

"EVFLAG_FORKCHECK"
Instead of calling "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 "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, "getpid" is
actually a simple 5-insn sequence without a system call and
thus very fast, but my GNU/Linux system also has
"pthread_atfork" which is even faster). (Update: glibc versions
2.25 apparently removed the "getpid" optimisation again).

The big advantage of this flag is that you can forget about
fork (and forget about forgetting to tell libev about forking,
although you still have to ignore "SIGPIPE") when you use this
flag.

This flag setting cannot be overridden or specified in the
"LIBEV_FLAGS" environment variable.

"EVFLAG_NOINOTIFY"
When this flag is specified, then libev will not attempt to use
the inotify API for its "ev_stat" watchers. Apart from
debugging and testing, this flag can be useful to conserve
inotify file descriptors, as otherwise each loop using
"ev_stat" watchers consumes one inotify handle.

"EVFLAG_SIGNALFD"
When this flag is specified, then libev will attempt to use the
signalfd API for its "ev_signal" (and "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.

"EVFLAG_NOSIGMASK"
When this flag is specified, then libev will avoid to modify
the signal mask. Specifically, this means you have to make sure
signals are unblocked when you want to receive them.

This behaviour is useful when you want to do your own signal
handling, or want to handle signals only in specific threads
and want to avoid libev unblocking the signals.

It's also required by POSIX in a threaded program, as libev
calls "sigprocmask", whose behaviour is officially unspecified.

"EVFLAG_NOTIMERFD"
When this flag is specified, the libev will avoid using a
"timerfd" to detect time jumps. It will still be able to detect
time jumps, but takes longer and has a lower accuracy in doing
so, but saves a file descriptor per loop.

The current implementation only tries to use a "timerfd" when
the first "ev_periodic" watcher is started and falls back on
other methods if it cannot be created, but this behaviour might
change in the future.

"EVBACKEND_SELECT" (value 1, portable select backend)
This is your standard select(2) backend. Not 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 "accept ()" in a
loop to accept as many connections as possible during one
iteration. You might also want to have a look at
"ev_set_io_collect_interval ()" to increase the amount of
readiness notifications you get per iteration.

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

"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 "EVBACKEND_SELECT", above, for performance tips.

This backend maps "EV_READ" to "POLLIN | POLLERR | POLLHUP",
and "EV_WRITE" to "POLLOUT | POLLERR | POLLHUP".

"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 total_fds 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,
returning before the timeout value, resulting in additional
iterations (and only giving 5ms accuracy while select on the
same platform gives 0.1ms) and so on. The biggest issue is fork
races, however - if a program forks then 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 should
work, but of course doesn't, and epoll just loves to report
events for totally 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. Epoll also erroneously rounds down timeouts, but
gives you no way to know when and by how much, so sometimes you
have to busy-wait because epoll returns immediately despite a
nonzero timeout. And last not least, it also refuses to work
with some file descriptors which work perfectly fine with
"select" (files, many character devices...).

Epoll is truly the train wreck among event poll mechanisms, a
frankenpoll, cobbled together in a hurry, no thought to design
or interaction with others. Oh, the pain, will it ever stop...

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 file descriptor could
point to a different file description now), so its best to
avoid that. Also, "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, "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 a lot of kernel revisions, but probably(!) works
in current versions.

This backend maps "EV_READ" and "EV_WRITE" in the same way as
"EVBACKEND_POLL".

"EVBACKEND_LINUXAIO" (value 64, Linux)
Use the Linux-specific Linux AIO (not aio(7) but io_submit(2))
event interface available in post-4.18 kernels (but libev only
tries to use it in 4.19+).

This is another Linux train wreck of an event interface.

If this backend works for you (as of this writing, it was very
experimental), it is the best event interface available on
Linux and might be well worth enabling it - if it isn't
available in your kernel this will be detected and this backend
will be skipped.

This backend can batch oneshot requests and supports a user-
space ring buffer to receive events. It also doesn't suffer
from most of the design problems of epoll (such as not being
able to remove event sources from the epoll set), and generally
sounds too good to be true. Because, this being the Linux
kernel, of course it suffers from a whole new set of
limitations, forcing you to fall back to epoll, inheriting all
its design issues.

For one, it is not easily embeddable (but probably could be
done using an event fd at some extra overhead). It also is
subject to a system wide limit that can be configured in
/proc/sys/fs/aio-max-nr. If no AIO requests are left, this
backend will be skipped during initialisation, and will switch
to epoll when the loop is active.

Most problematic in practice, however, is that not all file
descriptors work with it. For example, in Linux 5.1, TCP
sockets, pipes, event fds, files, /dev/null and many others are
supported, but ttys do not work properly (a known bug that the
kernel developers don't care about, see
<https://lore.kernel.org/patchwork/patch/1047453/>), so this is
not (yet?) a generic event polling interface.

Overall, it seems the Linux developers just don't want it to
have a generic event handling mechanism other than "select" or
"poll".

To work around all these problem, the current version of libev
uses its epoll backend as a fallback for file descriptor types
that do not work. Or falls back completely to epoll if the
kernel acts up.

This backend maps "EV_READ" and "EV_WRITE" in the same way as
"EVBACKEND_POLL".

"EVBACKEND_KQUEUE" (value 8, most BSD clones)
Kqueue deserves special mention, as at the time this backend
was implemented, 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 be (and mostly have been) fixed without
API changes to existing programs. For this reason it's not
being "auto-detected" on all platforms unless you explicitly
specify it in the flags (i.e. using "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
"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 "EVBACKEND_EPOLL", it still adds up to two event
changes per incident. Support for "fork ()" is very bad (you
might have to leak fds on fork, but it's more sane than epoll)
and it drops fds silently in similarly hard-to-detect cases.

This backend usually performs well under most conditions.

While nominally embeddable in other event loops, this doesn't
work 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. "EVBACKEND_SELECT"
or "EVBACKEND_POLL" (but "poll" is of course also broken on OS
X)) and, did I mention it, using it only for sockets.

This backend maps "EV_READ" into an "EVFILT_READ" kevent with
"NOTE_EOF", and "EV_WRITE" into an "EVFILT_WRITE" kevent with
"NOTE_EOF".

"EVBACKEND_DEVPOLL" (value 16, Solaris 8)
This is not implemented yet (and might never be, unless you
send me an implementation). According to reports, "/dev/poll"
only supports sockets and is not embeddable, which would limit
the usefulness of this backend immensely.

"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)).

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" "EVBACKEND_SELECT" or
"EVBACKEND_POLL" backend might perform better.

On the positive side, 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).

On the negative side, the interface is bizarre - so bizarre
that even sun itself gets it wrong in their code examples: The
event polling function sometimes returns events to the caller
even though an error occurred, but with no indication whether
it has done so or not (yes, it's even documented that way) -
deadly for edge-triggered interfaces where you absolutely have
to know whether an event occurred or not because you have to
re-arm the watcher.

Fortunately libev seems to be able to work around these
idiocies.

This backend maps "EV_READ" and "EV_WRITE" in the same way as
"EVBACKEND_POLL".

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

It is definitely not recommended to use this flag, use whatever
"ev_recommended_backends ()" returns, or simply do not specify
a backend at all.

"EVBACKEND_MASK"
Not a backend at all, but a mask to select all backend bits
from a "flags" value, in case you want to mask out any backends
from a flags value (e.g. when modifying the "LIBEV_FLAGS"
environment variable).

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
"ev_recommended_backends ()" will be tried.

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");

Example: Use whatever libev has to offer, but make sure that kqueue
is used if available.

struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);

Example: Similarly, on linux, you mgiht want to take advantage of
the linux aio backend if possible, but fall back to something else
if that isn't available.

struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);

ev_loop_destroy (loop)
Destroys an event loop object (frees all memory and kernel state
etc.). None of the active event watchers will be stopped in the
normal sense, so e.g. "ev_is_active" might still return true. It is
your responsibility to either stop all watchers cleanly yourself
before calling this function, or cope with the fact afterwards
(which is usually the easiest thing, you can just ignore the
watchers and/or "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.

This function is normally used on loop objects allocated by
"ev_loop_new", but it can also be used on the default loop returned
by "ev_default_loop", in which case it is not thread-safe.

Note that it is not advisable to call this function on the default
loop except in the rare occasion where you really need to free its
resources. If you need dynamically allocated loops it is better to
use "ev_loop_new" and "ev_loop_destroy".

ev_loop_fork (loop)
This function sets a flag that causes subsequent "ev_run"
iterations to reinitialise the kernel state for backends that have
one. Despite the name, you can call it anytime you are allowed to
start or stop watchers (except inside an "ev_prepare" callback),
but it makes most sense after forking, in the child process. You
must call it (or use "EVFLAG_FORKCHECK") in the child before
resuming or calling "ev_run".

In addition, if you want to reuse a loop (via this function or
"EVFLAG_FORKCHECK"), you also have to ignore "SIGPIPE".

Again, you have to call it on any loop that you want to re-use
after a fork, even if you do not plan to use the loop in the
parent. This is because some kernel interfaces *cough* 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 (in fact, "epoll" is so badly broken that it
makes a difference, but libev will usually detect this case on its
own and do a costly reset of the backend).

The function itself is quite fast and it's usually not a problem to
call it just in case after a fork.

Example: Automate calling "ev_loop_fork" on the default loop when
using pthreads.

static void
post_fork_child (void)
{
ev_loop_fork (EV_DEFAULT);
}

...
pthread_atfork (0, 0, post_fork_child);

int ev_is_default_loop (loop)
Returns true when the given loop is, in fact, the default loop, and
false otherwise.

unsigned int ev_iteration (loop)
Returns the current iteration count for the event loop, which is
identical to the number of times libev did poll for new events. It
starts at 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 "ev_prepare" and "ev_check" calls - and is
incremented between the prepare and check phases.

unsigned int ev_depth (loop)
Returns the number of times "ev_run" was entered minus the number
of times "ev_run" was exited normally, in other words, the
recursion depth.

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

Leaving "ev_run" abnormally (setjmp/longjmp, cancelling the thread,
throwing an exception etc.), doesn't count as "exit" - consider
this as a hint to avoid such ungentleman-like behaviour unless it's
really convenient, in which case it is fully supported.

unsigned int ev_backend (loop)
Returns one of the "EVBACKEND_*" flags indicating the event backend
in use.

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).

ev_now_update (loop)
Establishes the current time by querying the kernel, updating the
time returned by "ev_now ()" in the progress. This is a costly
operation and is usually done automatically within "ev_run ()".

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 "ev_timer"
section.

ev_suspend (loop)
ev_resume (loop)
These two functions suspend and resume an event 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 "^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 "ev_suspend" in your "SIGTSTP" handler, sending
yourself a "SIGSTOP" and calling "ev_resume" directly afterwards to
resume timer processing.

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

After calling "ev_suspend" you must not call any function on the
given loop other than "ev_resume", and you must not call
"ev_resume" without a previous call to "ev_suspend".

Calling "ev_suspend"/"ev_resume" has the side effect of updating
the event loop time (see "ev_now_update").

bool ev_run (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. It will ask the operating system for any new
events, call the watcher callbacks, and then repeat the whole
process indefinitely: This is why event loops are called loops.

If the flags argument is specified as 0, it will keep handling
events until either no event watchers are active anymore or
"ev_break" was called.

The return value is false if there are no more active watchers
(which usually means "all jobs done" or "deadlock"), and true in
all other cases (which usually means " you should call "ev_run"
again").

Please note that an explicit "ev_break" 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.

This function is mostly exception-safe - you can break out of a
"ev_run" call by calling "longjmp" in a callback, throwing a C++
exception and so on. This does not decrement the "ev_depth" value,
nor will it clear any outstanding "EVBREAK_ONE" breaks.

A flags value of "EVRUN_NOWAIT" will look for new events, will
handle those events and any already outstanding ones, but will not
wait and block your process in case there are no events and will
return after one iteration of the loop. This is sometimes useful to
poll and handle new events while doing lengthy calculations, to
keep the program responsive.

A flags value of "EVRUN_ONCE" 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 "ev_run""). However, a pair of
"ev_prepare"/"ev_check" watchers is usually a better approach for
this kind of thing.

Here are the gory details of what "ev_run" does (this is for your
understanding, not a guarantee that things will work exactly like
this in future versions):

- Increment loop depth.
- Reset the ev_break status.
- Before the first iteration, call any pending watchers.
LOOP:
- 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 ev_break was called, goto FINISH.
- 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, EVRUN_NOWAIT or not having
any active watchers at all will result in not sleeping).
- Sleep if the I/O and timer collect interval say so.
- Increment loop iteration counter.
- 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.
- Queue all idle watchers with priority higher than that of pending events.
- Queue all check watchers.
- Call all queued watchers in reverse order (i.e. check watchers first).
Signals and child watchers are implemented as I/O watchers, and will
be handled here by queueing them when their watcher gets executed.
- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
were used, or there are no active watchers, goto FINISH, otherwise
continue with step LOOP.
FINISH:
- Reset the ev_break status iff it was EVBREAK_ONE.
- Decrement the loop depth.
- Return.

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_run (my_loop, 0);
... jobs done or somebody called break. yeah!

ev_break (loop, how)
Can be used to make a call to "ev_run" return early (but only after
it has processed all outstanding events). The "how" argument must
be either "EVBREAK_ONE", which will make the innermost "ev_run"
call return, or "EVBREAK_ALL", which will make all nested "ev_run"
calls return.

This "break state" will be cleared on the next call to "ev_run".

It is safe to call "ev_break" from outside any "ev_run" calls, too,
in which case it will have no effect.

ev_ref (loop)
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, "ev_run" 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 "ev_run" from
returning. In such a case, 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 "ev_run"
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 unref after
start and 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 "ev_ref" in the callback).

Example: Create a signal watcher, but keep it from keeping "ev_run"
running when nothing else is active.

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

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

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

ev_set_io_collect_interval (loop, ev_tstamp interval)
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 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 "interval" must be >= 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 "select ()"
which have a high overhead for the actual polling but can deliver
many events at once.

By setting a higher 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
"ev_periodic" and "ev_timer") will not be affected. Setting this to
a non-null value will introduce an additional "ev_sleep ()" call
into most loop iterations. The sleep time ensures that libev will
not poll for I/O events more often then once per this interval, on
average (as long as the host time resolution is good enough).

Likewise, by setting a higher 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). "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 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 0.01, as this approaches the timing granularity of most
systems. Note that if you do transactions with the outside world
and you can't increase the parallelity, then this setting will
limit your transaction rate (if you need to poll once per
transaction and the I/O collect interval is 0.01, then you can't do
more than 100 transactions per second).

Setting the 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 "ev_periodic" watchers and make sure they fire on, say, one-
second boundaries only.

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

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

ev_invoke_pending (loop)
This call will simply invoke all pending watchers while resetting
their pending state. Normally, "ev_run" does this automatically
when required, but when overriding the invoke callback this call
comes handy. This function can be invoked from a watcher - this can
be useful for example when you want to do some lengthy calculation
and want to pass further event handling to another thread (you
still have to make sure only one thread executes within
"ev_invoke_pending" or "ev_run" of course).

int ev_pending_count (loop)
Returns the number of pending watchers - zero indicates that no
watchers are pending.

ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))
This overrides the invoke pending functionality of the loop:
Instead of invoking all pending watchers when there are any,
"ev_run" will call this callback instead. This is useful, for
example, when you want to invoke the actual watchers inside another
context (another thread etc.).

If you want to reset the callback, use "ev_invoke_pending" as new
callback.

ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void
(*acquire)(EV_P) throw ())
Sometimes you want to share the same loop between multiple threads.
This can be done relatively simply by putting mutex_lock/unlock
calls around each call to a libev function.

However, "ev_run" can run an indefinite time, so it is not feasible
to wait for it to return. One way around this is to wake up the
event loop via "ev_break" and "ev_async_send", another way is to
set these release and acquire callbacks on the loop.

When set, then "release" will be called just before the thread is
suspended waiting for new events, and "acquire" is called just
afterwards.

Ideally, "release" will just call your mutex_unlock function, and
"acquire" will just call the mutex_lock function again.

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

In theory, threads executing "ev_run" will be async-cancel safe
between invocations of "release" and "acquire".

See also the locking example in the "THREADS" section later in this
document.

ev_set_userdata (loop, void *data)
void *ev_userdata (loop)
Set and retrieve a single "void *" associated with a loop. When
"ev_set_userdata" has never been called, then "ev_userdata" returns
0.

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

ev_verify (loop)
This function only does something when "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 "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.

ANATOMY OF A WATCHER
In the following description, uppercase "TYPE" in names stands for the
watcher type, e.g. "ev_TYPE_start" can mean "ev_timer_start" for timer
watchers and "ev_io_start" for I/O watchers.

A watcher is an opaque structure that you allocate and register to
record your interest in some event. To make a concrete example, imagine
you want to wait for STDIN to become readable, you would create an
"ev_io" watcher for that:

static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
{
ev_io_stop (w);
ev_break (loop, EVBREAK_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_run (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).

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

Each watcher structure must be initialised by a call to "ev_init
(watcher *, callback)", which expects a callback to be provided. This
callback is 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 "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: "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 ("ev_TYPE_start (loop, watcher
*)"), and you can stop watching for events at any time by calling the
corresponding stop function ("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 except when explicitly
documented otherwise. Most specifically you must never reinitialise it
or call its "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:

"EV_READ"
"EV_WRITE"
The file descriptor in the "ev_io" watcher has become readable
and/or writable.

"EV_TIMER"
The "ev_timer" watcher has timed out.

"EV_PERIODIC"
The "ev_periodic" watcher has timed out.

"EV_SIGNAL"
The signal specified in the "ev_signal" watcher has been received
by a thread.

"EV_CHILD"
The pid specified in the "ev_child" watcher has received a status
change.

"EV_STAT"
The path specified in the "ev_stat" watcher changed its attributes
somehow.

"EV_IDLE"
The "ev_idle" watcher has determined that you have nothing better
to do.

"EV_PREPARE"
"EV_CHECK"
All "ev_prepare" watchers are invoked just before "ev_run" starts
to gather new events, and all "ev_check" watchers are queued (not
invoked) just after "ev_run" has gathered them, but before it
queues any callbacks for any received events. That means
"ev_prepare" watchers are the last watchers invoked before the
event loop sleeps or polls for new events, and "ev_check" watchers
will be invoked before any other watchers of the same or lower
priority within an event loop iteration.

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 "ev_prepare" watcher might start an idle watcher to keep
"ev_run" from blocking).

"EV_EMBED"
The embedded event loop specified in the "ev_embed" watcher needs
attention.

"EV_FORK"
The event loop has been resumed in the child process after fork
(see "ev_fork").

"EV_CLEANUP"
The event loop is about to be destroyed (see "ev_cleanup").

"EV_ASYNC"
The given async watcher has been asynchronously notified (see
"ev_async").

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

"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.

GENERIC WATCHER FUNCTIONS
"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 "malloc" will
do). Only the generic parts of the watcher are initialised, you
need to call the type-specific "ev_TYPE_set" macro afterwards to
initialise the type-specific parts. For each type there is also a
"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 "void (*)(struct ev_loop *loop,
ev_TYPE *watcher, int revents)".

Example: Initialise an "ev_io" watcher in two steps.

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

"ev_TYPE_set" (ev_TYPE *watcher, [args])
This macro initialises the type-specific parts of a watcher. You
need to call "ev_init" at least once before you call this macro,
but you can call "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 "ev_init" macro).

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

See "ev_init", above, for an example.

"ev_TYPE_init" (ev_TYPE *watcher, callback, [args])
This convenience macro rolls both "ev_init" and "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 "ev_io" watcher in one step.

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

"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 "ev_io" watcher that is being abused as example
in this whole section.

ev_io_start (EV_DEFAULT_UC, &w);

"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 "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
"ev_TYPE_stop" function.

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.

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 "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 "free ()" it).

callback ev_cb (ev_TYPE *watcher)
Returns the callback currently set on the watcher.

ev_set_cb (ev_TYPE *watcher, callback)
Change the callback. You can change the callback at virtually any
time (modulo threads).

ev_set_priority (ev_TYPE *watcher, int priority)
int ev_priority (ev_TYPE *watcher)
Set and query the priority of the watcher. The priority is a small
integer between "EV_MAXPRI" (default: 2) and "EV_MINPRI" (default:
"-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 "ev_idle" watchers).

If you need to suppress invocation when higher priority events are
pending you need to look at "ev_idle" watchers, which provide this
functionality.

You must not change the priority of a watcher as long as it is
active or pending.

Setting a priority outside the range of "EV_MINPRI" to "EV_MAXPRI"
is fine, as long as you do not mind that the priority value you
query might or might not have been clamped to the valid range.

The default priority used by watchers when no priority has been set
is always 0, which is supposed to not be too high and not be too
low :).

See "WATCHER PRIORITY MODELS", below, for a more thorough treatment
of priorities.

ev_invoke (loop, ev_TYPE *watcher, int revents)
Invoke the "watcher" with the given "loop" and "revents". Neither
"loop" nor "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.

int ev_clear_pending (loop, ev_TYPE *watcher)
If the watcher is pending, this function clears its pending status
and returns its "revents" bitset (as if its callback was invoked).
If the watcher isn't pending it does nothing and returns 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.

ev_feed_event (loop, ev_TYPE *watcher, int revents)
Feeds the given event set into the event loop, as if the specified
event had happened for the specified watcher (which must be a
pointer to an initialised but not necessarily started event
watcher). Obviously you must not free the watcher as long as it has
pending events.

Stopping the watcher, letting libev invoke it, or calling
"ev_clear_pending" will clear the pending event, even if the
watcher was not started in the first place.

See also "ev_feed_fd_event" and "ev_feed_signal_event" for related
functions that do not need a watcher.

See also the "ASSOCIATING CUSTOM DATA WITH A WATCHER" and "BUILDING
YOUR OWN COMPOSITE WATCHERS" idioms.

WATCHER STATES
There are various watcher states mentioned throughout this manual -
active, pending and so on. In this section these states and the rules
to transition between them will be described in more detail - and while
these rules might look complicated, they usually do "the right thing".

initialised
Before a watcher can be registered with the event loop it has to be
initialised. This can be done with a call to "ev_TYPE_init", or
calls to "ev_init" followed by the watcher-specific "ev_TYPE_set"
function.

In this state it is simply some block of memory that is suitable
for use in an event loop. It can be moved around, freed, reused
etc. at will - as long as you either keep the memory contents
intact, or call "ev_TYPE_init" again.

started/running/active
Once a watcher has been started with a call to "ev_TYPE_start" it
becomes property of the event loop, and is actively waiting for
events. While in this state it cannot be accessed (except in a few
documented ways), moved, freed or anything else - the only legal
thing is to keep a pointer to it, and call libev functions on it
that are documented to work on active watchers.

pending
If a watcher is active and libev determines that an event it is
interested in has occurred (such as a timer expiring), it will
become pending. It will stay in this pending state until either it
is stopped or its callback is about to be invoked, so it is not
normally pending inside the watcher callback.

The watcher might or might not be active while it is pending (for
example, an expired non-repeating timer can be pending but no
longer active). If it is stopped, it can be freely accessed (e.g.
by calling "ev_TYPE_set"), but it is still property of the event
loop at this time, so cannot be moved, freed or reused. And if it
is active the rules described in the previous item still apply.

It is also possible to feed an event on a watcher that is not
active (e.g. via "ev_feed_event"), in which case it becomes
pending without being active.

stopped
A watcher can be stopped implicitly by libev (in which case it
might still be pending), or explicitly by calling its
"ev_TYPE_stop" function. The latter will clear any pending state
the watcher might be in, regardless of whether it was active or
not, so stopping a watcher explicitly before freeing it is often a
good idea.

While stopped (and not pending) the watcher is essentially in the
initialised state, that is, it can be reused, moved, modified in
any way you wish (but when you trash the memory block, you need to
"ev_TYPE_init" it again).

WATCHER PRIORITY MODELS
Many event loops support watcher priorities, which are usually small
integers that influence the ordering of event callback invocation
between watchers in some way, all else being equal.

In libev, watcher priorities can be set using "ev_set_priority". See
its description for the more technical details such as the actual
priority range.

There are two common ways how these these priorities are being
interpreted by event loops:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

WATCHER TYPES
This section describes each watcher in detail, but will not repeat
information given in the last section. Any initialisation/set macros,
functions and members specific to the watcher type are explained.

Most members are additionally marked with either [read-only], meaning
that, while the watcher is active, you can look at the member and
expect some sensible content, but you must not modify it (you can
modify it while the watcher is stopped to your hearts content), or
[read-write], which means you can expect it to have some sensible
content while the watcher is active, but you can also modify it (within
the same thread as the event loop, i.e. without creating data races).
Modifying it may not do something sensible or take immediate effect (or
do anything at all), but libev will not crash or malfunction in any
way.

In any case, the documentation for each member will explain what the
effects are, and if there are any additional access restrictions.

"ev_io" - is this file descriptor readable or writable?
I/O watchers check whether a file descriptor is readable or writable in
each iteration of the event loop, or, more precisely, when reading
would not block the process and writing would at least be able to write
some data. This behaviour is called level-triggering because you keep
receiving events as long as the condition persists. Remember you can
stop the watcher if you don't want to act on the event and neither want
to receive future events.

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

Another thing you have to watch out for is that it is quite easy to
receive "spurious" readiness notifications, that is, your callback
might be called with "EV_READ" but a subsequent "read"(2) will actually
block because there is no data. It is very easy to get into this
situation even with a relatively standard program structure. Thus it is
best to always use non-blocking I/O: An extra "read"(2) returning
"EAGAIN" is far preferable to a program hanging until some data
arrives.

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

But really, best use non-blocking mode.

The special problem of disappearing file descriptors

Some backends (e.g. kqueue, epoll, linuxaio) need to be told about
closing a file descriptor (either due to calling "close" explicitly or
any other means, such as "dup2"). The reason is that you register
interest in some file descriptor, but when it goes away, the operating
system will silently drop this interest. If another file descriptor
with the same number then is registered with libev, there is no
efficient way to see that this is, in fact, a different file
descriptor.

To avoid having to explicitly tell libev about such cases, libev
follows the following policy: Each time "ev_io_set" is being called,
libev will assume that this is potentially a new file descriptor,
otherwise it is assumed that the file descriptor stays the same. That
means that you have to call "ev_io_set" (or "ev_io_init") when you
change the descriptor even if the file descriptor number itself did not
change.

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

The special problem of dup'ed file descriptors

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

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

The special problem of files

Many people try to use "select" (or libev) on file descriptors
representing files, and expect it to become ready when their program
doesn't block on disk accesses (which can take a long time on their
own).

However, this cannot ever work in the "expected" way - you get a
readiness notification as soon as the kernel knows whether and how much
data is there, and in the case of open files, that's always the case,
so you always get a readiness notification instantly, and your read (or
possibly write) will still block on the disk I/O.

Another way to view it is that in the case of sockets, pipes, character
devices and so on, there is another party (the sender) that delivers
data on its own, but in the case of files, there is no such thing: the
disk will not send data on its own, simply because it doesn't know what
you wish to read - you would first have to request some data.

Since files are typically not-so-well supported by advanced
notification mechanism, libev tries hard to emulate POSIX behaviour
with respect to files, even though you should not use it. The reason
for this is convenience: sometimes you want to watch STDIN or STDOUT,
which is usually a tty, often a pipe, but also sometimes files or
special devices (for example, "epoll" on Linux works with /dev/random
but not with /dev/urandom), and even though the file might better be
served with asynchronous I/O instead of with non-blocking I/O, it is
still useful when it "just works" instead of freezing.

So avoid file descriptors pointing to files when you know it (e.g. use
libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or
when you rarely read from a file instead of from a socket, and want to
reuse the same code path.

The special problem of fork

Some backends (epoll, kqueue, linuxaio, iouring) do not support "fork
()" at all or exhibit useless behaviour. Libev fully supports fork, but
needs to be told about it in the child if you want to continue to use
it in the child.

To support fork in your child processes, you have to call "ev_loop_fork
()" after a fork in the child, enable "EVFLAG_FORKCHECK", or resort to
"EVBACKEND_SELECT" or "EVBACKEND_POLL".

The special problem of SIGPIPE

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

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

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

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

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

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

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

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

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

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

Watcher-Specific Functions

ev_io_init (ev_io *, callback, int fd, int events)
ev_io_set (ev_io *, int fd, int events)
Configures an "ev_io" watcher. The "fd" is the file descriptor to
receive events for and "events" is either "EV_READ", "EV_WRITE",
both "EV_READ | EV_WRITE" or 0, to express the desire to receive
the given events.

Note that setting the "events" to 0 and starting the watcher is
supported, but not specially optimized - if your program sometimes
happens to generate this combination this is fine, but if it is
easy to avoid starting an io watcher watching for no events you
should do so.

ev_io_modify (ev_io *, int events)
Similar to "ev_io_set", but only changes the requested events.
Using this might be faster with some backends, as libev can assume
that the "fd" still refers to the same underlying file description,
something it cannot do when using "ev_io_set".

int fd [no-modify]
The file descriptor being watched. While it can be read at any
time, you must not modify this member even when the watcher is
stopped - always use "ev_io_set" for that.

int events [no-modify]
The set of events the fd is being watched for, among other flags.
Remember that this is a bit set - to test for "EV_READ", use
"w->events & EV_READ", and similarly for "EV_WRITE".

As with "fd", you must not modify this member even when the watcher
is stopped, always use "ev_io_set" or "ev_io_modify" for that.

Examples

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

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

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

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

The timers are based on real time, that is, if you register an event
that times out after an hour and you reset your system clock to January
last year, it will still time out after (roughly) one hour. "Roughly"
because detecting time jumps is hard, and some inaccuracies are
unavoidable (the monotonic clock option helps a lot here).

The callback is guaranteed to be invoked only after its timeout has
passed (not at, so on systems with very low-resolution clocks this
might introduce a small delay, see "the special problem of being too
early", below). If multiple timers become ready during the same loop
iteration then the ones with earlier time-out values are invoked before
ones of the same priority with later time-out values (but this is no
longer true when a callback calls "ev_run" recursively).

Be smart about timeouts

Many real-world problems involve some kind of timeout, usually for
error recovery. A typical example is an HTTP request - if the other
side hangs, you want to raise some error after a while.

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

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

1. Use a timer and stop, reinitialise and start it on activity.
This is the most obvious, but not the most simple way: In the
beginning, start the watcher:

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

Then, each time there is some activity, "ev_timer_stop" it,
initialise it and start it again:

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

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

2. Use a timer and re-start it with "ev_timer_again" inactivity.
This is the easiest way, and involves using "ev_timer_again"
instead of "ev_timer_start".

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

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

At start:

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

Each time there is some activity:

ev_timer_again (loop, timer);

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

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

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

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

3. Let the timer time out, but then re-arm it as required.
This method is more tricky, but usually most efficient: Most
timeouts are relatively long compared to the intervals between
other activity - in our example, within 60 seconds, there are
usually many I/O events with associated activity resets.

In this case, it would be more efficient to leave the "ev_timer"
alone, but remember the time of last activity, and check for a real
timeout only within the callback:

ev_tstamp timeout = 60.;
ev_tstamp last_activity; // time of last activity
ev_timer timer;

static void
callback (EV_P_ ev_timer *w, int revents)
{
// calculate when the timeout would happen
ev_tstamp after = last_activity - ev_now (EV_A) + timeout;

// if negative, it means we the timeout already occurred
if (after < 0.)
{
// timeout occurred, take action
}
else
{
// callback was invoked, but there was some recent
// activity. simply restart the timer to time out
// after "after" seconds, which is the earliest time
// the timeout can occur.
ev_timer_set (w, after, 0.);
ev_timer_start (EV_A_ w);
}
}

To summarise the callback: first calculate in how many seconds the
timeout will occur (by calculating the absolute time when it would
occur, "last_activity + timeout", and subtracting the current time,
"ev_now (EV_A)" from that).

If this value is negative, then we are already past the timeout,
i.e. we timed out, and need to do whatever is needed in this case.

Otherwise, we now the earliest time at which the timeout would
trigger, and simply start the timer with this timeout value.

In other words, each time the callback is invoked it will check
whether the timeout occurred. If not, it will simply reschedule
itself to check again at the earliest time it could time out.
Rinse. Repeat.

This scheme causes more callback invocations (about one every 60
seconds minus half the average time between activity), but
virtually no calls to libev to change the timeout.

To start the machinery, simply initialise the watcher and set
"last_activity" to the current time (meaning there was some
activity just now), then call the callback, which will "do the
right thing" and start the timer:

last_activity = ev_now (EV_A);
ev_init (&timer, callback);
callback (EV_A_ &timer, 0);

When there is some activity, simply store the current time in
"last_activity", no libev calls at all:

if (activity detected)
last_activity = ev_now (EV_A);

When your timeout value changes, then the timeout can be changed by
simply providing a new value, stopping the timer and calling the
callback, which will again do the right thing (for example, time
out immediately :).

timeout = new_value;
ev_timer_stop (EV_A_ &timer);
callback (EV_A_ &timer, 0);

This technique is slightly more complex, but in most cases where
the time-out is unlikely to be triggered, much more efficient.

4. Wee, just use a double-linked list for your timeouts.
If there is not one request, but many thousands (millions...), all
employing some kind of timeout with the same timeout value, then
one can do even better:

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

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

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

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

So which method the best?

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

Method #1 is almost always a bad idea, and buys you nothing. Method #4
is rather complicated, but extremely efficient, something that really
pays off after the first million or so of active timers, i.e. it's
usually overkill :)

The special problem of being too early

If you ask a timer to call your callback after three seconds, then you
expect it to be invoked after three seconds - but of course, this
cannot be guaranteed to infinite precision. Less obviously, it cannot
be guaranteed to any precision by libev - imagine somebody suspending
the process with a STOP signal for a few hours for example.

So, libev tries to invoke your callback as soon as possible after the
delay has occurred, but cannot guarantee this.

A less obvious failure mode is calling your callback too early: many
event loops compare timestamps with a "elapsed delay >= requested
delay", but this can cause your callback to be invoked much earlier
than you would expect.

To see why, imagine a system with a clock that only offers full second
resolution (think windows if you can't come up with a broken enough OS
yourself). If you schedule a one-second timer at the time 500.9, then
the event loop will schedule your timeout to elapse at a system time of
500 (500.9 truncated to the resolution) + 1, or 501.

If an event library looks at the timeout 0.1s later, it will see "501
>= 501" and invoke the callback 0.1s after it was started, even though
a one-second delay was requested - this is being "too early", despite
best intentions.

This is the reason why libev will never invoke the callback if the
elapsed delay equals the requested delay, but only when the elapsed
delay is larger than the requested delay. In the example above, libev
would only invoke the callback at system time 502, or 1.1s after the
timer was started.

So, while libev cannot guarantee that your callback will be invoked
exactly when requested, it can and does guarantee that the requested
delay has actually elapsed, or in other words, it always errs on the
"too late" side of things.

The special problem of time updates

Establishing the current time is a costly operation (it usually takes
at least one system call): EV therefore updates its idea of the current
time only before and after "ev_run" collects new events, which causes a
growing difference between "ev_now ()" and "ev_time ()" when handling
lots of events in one iteration.

The relative timeouts are calculated relative to the "ev_now ()" time.
This is usually the right thing as this timestamp refers to the time of
the event triggering whatever timeout you are modifying/starting. If
you suspect event processing to be delayed and you need to base the
timeout on the current time, use something like the following to adjust
for it:

ev_timer_set (&timer, after + (ev_time () - ev_now ()), 0.);

If the event loop is suspended for a long time, you can also force an
update of the time returned by "ev_now ()" by calling "ev_now_update
()", although that will push the event time of all outstanding events
further into the future.

The special problem of unsynchronised clocks

Modern systems have a variety of clocks - libev itself uses the normal
"wall clock" clock and, if available, the monotonic clock (to avoid
time jumps).

Neither of these clocks is synchronised with each other or any other
clock on the system, so "ev_time ()" might return a considerably
different time than "gettimeofday ()" or "time ()". On a GNU/Linux
system, for example, a call to "gettimeofday" might return a second
count that is one higher than a directly following call to "time".

The moral of this is to only compare libev-related timestamps with
"ev_time ()" and "ev_now ()", at least if you want better precision
than a second or so.

One more problem arises due to this lack of synchronisation: if libev
uses the system monotonic clock and you compare timestamps from
"ev_time" or "ev_now" from when you started your timer and when your
callback is invoked, you will find that sometimes the callback is a bit
"early".

This is because "ev_timer"s work in real time, not wall clock time, so
libev makes sure your callback is not invoked before the delay
happened, measured according to the real time, not the system clock.

If your timeouts are based on a physical timescale (e.g. "time out this
connection after 100 seconds") then this shouldn't bother you as it is
exactly the right behaviour.

If you want to compare wall clock/system timestamps to your timers,
then you need to use "ev_periodic"s, as these are based on the wall
clock time, where your comparisons will always generate correct
results.

The special problems of suspended animation

When you leave the server world it is quite customary to hit machines
that can suspend/hibernate - what happens to the clocks during such a
suspend?

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

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

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

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

Watcher-Specific Functions and Data Members

ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
Configure the timer to trigger after "after" seconds (fractional
and negative values are supported). If "repeat" is 0., then it will
automatically be stopped once the timeout is reached. If it is
positive, then the timer will automatically be configured to
trigger again "repeat" seconds later, again, and again, until
stopped manually.

The timer itself will do a best-effort at avoiding drift, that is,
if you configure a timer to trigger every 10 seconds, then it will
normally trigger at exactly 10 second intervals. If, however, your
program cannot keep up with the timer (because it takes longer than
those 10 seconds to do stuff) the timer will not fire more than
once per event loop iteration.

ev_timer_again (loop, ev_timer *)
This will act as if the timer timed out, and restarts it again if
it is repeating. It basically works like calling "ev_timer_stop",
updating the timeout to the "repeat" value and calling
"ev_timer_start".

The exact semantics are as in the following rules, all of which
will be applied to the watcher:

If the timer is pending, the pending status is always cleared.
If the timer is started but non-repeating, stop it (as if it timed
out, without invoking it).
If the timer is repeating, make the "repeat" value the new timeout
and start the timer, if necessary.

This sounds a bit complicated, see "Be smart about timeouts",
above, for a usage example.

ev_tstamp ev_timer_remaining (loop, ev_timer *)
Returns the remaining time until a timer fires. If the timer is
active, then this time is relative to the current event loop time,
otherwise it's the timeout value currently configured.

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

ev_tstamp repeat [read-write]
The current "repeat" value. Will be used each time the watcher
times out or "ev_timer_again" is called, and determines the next
timeout (if any), which is also when any modifications are taken
into account.

Examples

Example: Create a timer that fires after 60 seconds.

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

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

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

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

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

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

"ev_periodic" - to cron or not to cron?
Periodic watchers are also timers of a kind, but they are very
versatile (and unfortunately a bit complex).

Unlike "ev_timer", periodic watchers are not based on real time (or
relative time, the physical time that passes) but on wall clock time
(absolute time, the thing you can read on your calendar or clock). The
difference is that wall clock time can run faster or slower than real
time, and time jumps are not uncommon (e.g. when you adjust your wrist-
watch).

You can tell a periodic watcher to trigger after some specific point in
time: for example, if you tell a periodic watcher to trigger "in 10
seconds" (by specifying e.g. "ev_now () + 10.", that is, an absolute
time not a delay) and then reset your system clock to January of the
previous year, then it will take a year or more to trigger the event
(unlike an "ev_timer", which would still trigger roughly 10 seconds
after starting it, as it uses a relative timeout).

"ev_periodic" watchers can also be used to implement vastly more
complex timers, such as triggering an event on each "midnight, local
time", or other complicated rules. This cannot easily be done with
"ev_timer" watchers, as those cannot react to time jumps.

As with timers, the callback is guaranteed to be invoked only when the
point in time where it is supposed to trigger has passed. If multiple
timers become ready during the same loop iteration then the ones with
earlier time-out values are invoked before ones with later time-out
values (but this is no longer true when a callback calls "ev_run"
recursively).

Watcher-Specific Functions and Data Members

ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp
interval, reschedule_cb)
ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval,
reschedule_cb)
Lots of arguments, let's sort it out... There are basically three
modes of operation, and we will explain them from simplest to most
complex:

• absolute timer (offset = absolute time, interval = 0,
reschedule_cb = 0)

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

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

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

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

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

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

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

The "interval" MUST be positive, and for numerical stability,
the interval value should be higher than "1/8192" (which is
around 100 microseconds) and "offset" should be higher than 0
and should have at most a similar magnitude as the current time
(say, within a factor of ten). Typical values for offset are,
in fact, 0 or something between 0 and "interval", which is also
the recommended range.

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

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

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

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

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

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

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

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

NOTE: This callback must always return a time that is higher
than or equal to the passed "now" value.

This can be used to create very complex timers, such as a timer
that triggers on "next midnight, local time". To do this, you
would calculate the next midnight after "now" and return the
timestamp value for this. Here is a (completely untested, no
error checking) example on how to do this:

#include <time.h>

static ev_tstamp
my_rescheduler (ev_periodic *w, ev_tstamp now)
{
time_t tnow = (time_t)now;
struct tm tm;
localtime_r (&tnow, &tm);

tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
++tm.tm_mday; // midnight next day

return mktime (&tm);
}

Note: this code might run into trouble on days that have more
then two midnights (beginning and end).

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

ev_tstamp ev_periodic_at (ev_periodic *)
When active, returns the absolute time that the watcher is supposed
to trigger next. This is not the same as the "offset" argument to
"ev_periodic_set", but indeed works even in interval and manual
rescheduling modes.

ev_tstamp offset [read-write]
When repeating, this contains the offset value, otherwise this is
the absolute point in time (the "offset" value passed to
"ev_periodic_set", although libev might modify this value for
better numerical stability).

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

ev_tstamp interval [read-write]
The current interval value. Can be modified any time, but changes
only take effect when the periodic timer fires or
"ev_periodic_again" is being called.

ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]
The current reschedule callback, or 0, if this functionality is
switched off. Can be changed any time, but changes only take effect
when the periodic timer fires or "ev_periodic_again" is being
called.

Examples

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

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

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

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

#include <math.h>

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

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

Example: Call a callback every hour, starting now:

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

"ev_signal" - signal me when a signal gets signalled!
Signal watchers will trigger an event when the process receives a
specific signal one or more times. Even though signals are very
asynchronous, libev will try its best to deliver signals synchronously,
i.e. as part of the normal event processing, like any other event.

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

You can configure as many watchers as you like for the same signal, but
only within the same loop, i.e. you can watch for "SIGINT" in your
default loop and for "SIGIO" in another loop, but you cannot watch for
"SIGINT" in both the default loop and another loop at the same time. At
the moment, "SIGCHLD" is permanently tied to the default loop.

Only after the first watcher for a signal is started will libev
actually register something with the kernel. It thus coexists with your
own signal handlers as long as you don't register any with libev for
the same signal.

If possible and supported, libev will install its handlers with
"SA_RESTART" (or equivalent) behaviour enabled, so system calls should
not be unduly interrupted. If you have a problem with system calls
getting interrupted by signals you can block all signals in an
"ev_check" watcher and unblock them in an "ev_prepare" watcher.

The special problem of inheritance over fork/execve/pthread_create

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

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

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

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

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

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

The special problem of threads signal handling

POSIX threads has problematic signal handling semantics, specifically,
a lot of functionality (sigfd, sigwait etc.) only really works if all
threads in a process block signals, which is hard to achieve.

When you want to use sigwait (or mix libev signal handling with your
own for the same signals), you can tackle this problem by globally
blocking all signals before creating any threads (or creating them with
a fully set sigprocmask) and also specifying the "EVFLAG_NOSIGMASK"
when creating loops. Then designate one thread as "signal receiver
thread" which handles these signals. You can pass on any signals that
libev might be interested in by calling "ev_feed_signal".

Watcher-Specific Functions and Data Members

ev_signal_init (ev_signal *, callback, int signum)
ev_signal_set (ev_signal *, int signum)
Configures the watcher to trigger on the given signal number
(usually one of the "SIGxxx" constants).

int signum [read-only]
The signal the watcher watches out for.

Examples

Example: Try to exit cleanly on SIGINT.

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

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

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

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

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

Process Interaction

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

Overriding the Built-In Processing

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

Stopping the Child Watcher

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

Watcher-Specific Functions and Data Members

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

int pid [read-only]
The process id this watcher watches out for, or 0, meaning any
process id.

int rpid [read-write]
The process id that detected a status change.

int rstatus [read-write]
The process exit/trace status caused by "rpid" (see your systems
"waitpid" and "sys/wait.h" documentation for details).

Examples

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

ev_child cw;

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

pid_t pid = fork ();

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

"ev_stat" - did the file attributes just change?
This watches a file system path for attribute changes. That is, it
calls "stat" on that path in regular intervals (or when the OS says it
changed) and sees if it changed compared to the last time, invoking the
callback if it did. Starting the watcher "stat"'s the file, so only
changes that happen after the watcher has been started will be
reported.

The path does not need to exist: changing from "path exists" to "path
does not exist" is a status change like any other. The condition "path
does not exist" (or more correctly "path cannot be stat'ed") is
signified by the "st_nlink" field being zero (which is otherwise always
forced to be at least one) and all the other fields of the stat buffer
having unspecified contents.

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

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

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

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

ABI Issues (Largefile Support)

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

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

Inotify and Kqueue

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

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

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

"stat ()" is a synchronous operation

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

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

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

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

The special problem of stat time resolution

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

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

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

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

Watcher-Specific Functions and Data Members

ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp
interval)
ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)
Configures the watcher to wait for status changes of the given
"path". The "interval" is a hint on how quickly a change is
expected to be detected and should normally be specified as 0 to
let libev choose a suitable value. The memory pointed to by "path"
must point to the same path for as long as the watcher is active.

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

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

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

ev_statdata prev [read-only]
The previous attributes of the file. The callback gets invoked
whenever "prev" != "attr", or, more precisely, one or more of these
members differ: "st_dev", "st_ino", "st_mode", "st_nlink",
"st_uid", "st_gid", "st_rdev", "st_size", "st_atime", "st_mtime",
"st_ctime".

ev_tstamp interval [read-only]
The specified interval.

const char *path [read-only]
The file system path that is being watched.

Examples

Example: Watch "/etc/passwd" for attribute changes.

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

...
ev_stat passwd;

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

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

static ev_stat passwd;
static ev_timer timer;

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

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

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

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

"ev_idle" - when you've got nothing better to do...
Idle watchers trigger events when no other events of the same or higher
priority are pending (prepare, check and other idle watchers do not
count as receiving "events").

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

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

Apart from keeping your process non-blocking (which is a useful effect
on its own sometimes), idle watchers are a good place to do "pseudo-
background processing", or delay processing stuff to after the event
loop has handled all outstanding events.

Abusing an "ev_idle" watcher for its side-effect

As long as there is at least one active idle watcher, libev will never
sleep unnecessarily. Or in other words, it will loop as fast as
possible. For this to work, the idle watcher doesn't need to be
invoked at all - the lowest priority will do.

This mode of operation can be useful together with an "ev_check"
watcher, to do something on each event loop iteration - for example to
balance load between different connections.

See "Abusing an ev_check watcher for its side-effect" for a longer
example.

Watcher-Specific Functions and Data Members

ev_idle_init (ev_idle *, callback)
Initialises and configures the idle watcher - it has no parameters
of any kind. There is a "ev_idle_set" macro, but using it is
utterly pointless, believe me.

Examples

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

static void
idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
{
// stop the watcher
ev_idle_stop (loop, w);

// now we can free it
free (w);

// now do something you wanted to do when the program has
// no longer anything immediate to do.
}

ev_idle *idle_watcher = malloc (sizeof (ev_idle));
ev_idle_init (idle_watcher, idle_cb);
ev_idle_start (loop, idle_watcher);

"ev_prepare" and "ev_check" - customise your event loop!
Prepare and check watchers are often (but not always) used in pairs:
prepare watchers get invoked before the process blocks and check
watchers afterwards.

You must not call "ev_run" (or similar functions that enter the current
event loop) or "ev_loop_fork" from either "ev_prepare" or "ev_check"
watchers. Other loops than the current one are fine, however. The
rationale behind this is that you do not need to check for recursion in
those watchers, i.e. the sequence will always be "ev_prepare",
blocking, "ev_check" so if you have one watcher of each kind they will
always be called in pairs bracketing the blocking call.

Their main purpose is to integrate other event mechanisms into libev
and their use is somewhat advanced. They could be used, for example, to
track variable changes, implement your own watchers, integrate net-snmp
or a coroutine library and lots more. They are also occasionally useful
if you cache some data and want to flush it before blocking (for
example, in X programs you might want to do an "XFlush ()" in an
"ev_prepare" watcher).

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

As another example, the Perl Coro module uses these hooks to integrate
coroutines into libev programs, by yielding to other active coroutines
during each prepare and only letting the process block if no coroutines
are ready to run (it's actually more complicated: it only runs
coroutines with priority higher than or equal to the event loop and one
coroutine of lower priority, but only once, using idle watchers to keep
the event loop from blocking if lower-priority coroutines are active,
thus mapping low-priority coroutines to idle/background tasks).

When used for this purpose, it is recommended to give "ev_check"
watchers highest ("EV_MAXPRI") priority, to ensure that they are being
run before any other watchers after the poll (this doesn't matter for
"ev_prepare" watchers).

Also, "ev_check" watchers (and "ev_prepare" watchers, too) should not
activate ("feed") events into libev. While libev fully supports this,
they might get executed before other "ev_check" watchers did their job.
As "ev_check" watchers are often used to embed other (non-libev) event
loops those other event loops might be in an unusable state until their
"ev_check" watcher ran (always remind yourself to coexist peacefully
with others).

Abusing an "ev_check" watcher for its side-effect

"ev_check" (and less often also "ev_prepare") watchers can also be
useful because they are called once per event loop iteration. For
example, if you want to handle a large number of connections fairly,
you normally only do a bit of work for each active connection, and if
there is more work to do, you wait for the next event loop iteration,
so other connections have a chance of making progress.

Using an "ev_check" watcher is almost enough: it will be called on the
next event loop iteration. However, that isn't as soon as possible -
without external events, your "ev_check" watcher will not be invoked.

This is where "ev_idle" watchers come in handy - all you need is a
single global idle watcher that is active as long as you have one
active "ev_check" watcher. The "ev_idle" watcher makes sure the event
loop will not sleep, and the "ev_check" watcher makes sure a callback
gets invoked. Neither watcher alone can do that.

Watcher-Specific Functions and Data Members

ev_prepare_init (ev_prepare *, callback)
ev_check_init (ev_check *, callback)
Initialises and configures the prepare or check watcher - they have
no parameters of any kind. There are "ev_prepare_set" and
"ev_check_set" macros, but using them is utterly, utterly, utterly
and completely pointless.

Examples

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

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

static ev_io iow [nfd];
static ev_timer tw;

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

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

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

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

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

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

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

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

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

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

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

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

adns_processtimeouts (ads, &tv_now);
}

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

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

// do not ever call adns_afterpoll

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

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

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

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

// poll
ev_run (EV_A_ 0);

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

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

return got_events;
}

"ev_embed" - when one backend isn't enough...
This is a rather advanced watcher type that lets you embed one event
loop into another (currently only "ev_io" events are supported in the
embedded loop, other types of watchers might be handled in a delayed or
incorrect fashion and must not be used).

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

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

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

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

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

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

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

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

"ev_embed" and fork

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

Watcher-Specific Functions and Data Members

ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
ev_embed_set (ev_embed *, struct ev_loop *embedded_loop)
Configures the watcher to embed the given loop, which must be
embeddable. If the callback is 0, then "ev_embed_sweep" will be
invoked automatically, otherwise it is the responsibility of the
callback to invoke it (it will continue to be called until the
sweep has been done, if you do not want that, you need to
temporarily stop the embed watcher).

ev_embed_sweep (loop, ev_embed *)
Make a single, non-blocking sweep over the embedded loop. This
works similarly to "ev_run (embedded_loop, EVRUN_NOWAIT)", but in
the most appropriate way for embedded loops.

struct ev_loop *other [read-only]
The embedded event loop.

Examples

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

struct ev_loop *loop_hi = ev_default_init (0);
struct ev_loop *loop_lo = 0;
ev_embed embed;

// see if there is a chance of getting one that works
// (remember that a flags value of 0 means autodetection)
loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
: 0;

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

Example: Check if kqueue is available but not recommended and create a
kqueue backend for use with sockets (which usually work with any kqueue
implementation). Store the kqueue/socket-only event loop in
"loop_socket". (One might optionally use "EVFLAG_NOENV", too).

struct ev_loop *loop = ev_default_init (0);
struct ev_loop *loop_socket = 0;
ev_embed embed;

if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
{
ev_embed_init (&embed, 0, loop_socket);
ev_embed_start (loop, &embed);
}

if (!loop_socket)
loop_socket = loop;

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

"ev_fork" - the audacity to resume the event loop after a fork
Fork watchers are called when a "fork ()" was detected (usually because
whoever is a good citizen cared to tell libev about it by calling
"ev_loop_fork"). The invocation is done before the event loop blocks
next and before "ev_check" watchers are being called, and only in the
child after the fork. If whoever good citizen calling "ev_default_fork"
cheats and calls it in the wrong process, the fork handlers will be
invoked, too, of course.

The special problem of life after fork - how is it possible?

Most uses of "fork ()" consist of forking, then some simple calls to
set up/change the process environment, followed by a call to "exec()".
This sequence should be handled by libev without any problems.

This changes when the application actually wants to do event handling
in the child, or both parent in child, in effect "continuing" after the
fork.

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

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

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

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

Watcher-Specific Functions and Data Members

ev_fork_init (ev_fork *, callback)
Initialises and configures the fork watcher - it has no parameters
of any kind. There is a "ev_fork_set" macro, but using it is
utterly pointless, really.

"ev_cleanup" - even the best things end
Cleanup watchers are called just before the event loop is being
destroyed by a call to "ev_loop_destroy".

While there is no guarantee that the event loop gets destroyed, cleanup
watchers provide a convenient method to install cleanup hooks for your
program, worker threads and so on - you just to make sure to destroy
the loop when you want them to be invoked.

Cleanup watchers are invoked in the same way as any other watcher.
Unlike all other watchers, they do not keep a reference to the event
loop (which makes a lot of sense if you think about it). Like all other
watchers, you can call libev functions in the callback, except
"ev_cleanup_start".

Watcher-Specific Functions and Data Members

ev_cleanup_init (ev_cleanup *, callback)
Initialises and configures the cleanup watcher - it has no
parameters of any kind. There is a "ev_cleanup_set" macro, but
using it is utterly pointless, I assure you.

Example: Register an atexit handler to destroy the default loop, so any
cleanup functions are called.

static void
program_exits (void)
{
ev_loop_destroy (EV_DEFAULT_UC);
}

...
atexit (program_exits);

"ev_async" - how to wake up an event loop
In general, you cannot use an "ev_loop" from multiple threads or other
asynchronous sources such as signal handlers (as opposed to multiple
event loops - those are of course safe to use in different threads).

Sometimes, however, you need to wake up an event loop you do not
control, for example because it belongs to another thread. This is what
"ev_async" watchers do: as long as the "ev_async" watcher is active,
you can signal it by calling "ev_async_send", which is thread- and
signal safe.

This functionality is very similar to "ev_signal" watchers, as signals,
too, are asynchronous in nature, and signals, too, will be compressed
(i.e. the number of callback invocations may be less than the number of
"ev_async_send" calls). In fact, you could use signal watchers as a
kind of "global async watchers" by using a watcher on an otherwise
unused signal, and "ev_feed_signal" to signal this watcher from another
thread, even without knowing which loop owns the signal.

Queueing

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

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

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

static ev_async mysig;

static void
sigusr1_handler (void)
{
sometype data;

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

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

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

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

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

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

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

static ev_async mysig;
static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

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

ev_async_send (EV_DEFAULT_ &mysig);
}

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

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

pthread_mutex_unlock (&mymutex);
}

Watcher-Specific Functions and Data Members

ev_async_init (ev_async *, callback)
Initialises and configures the async watcher - it has no parameters
of any kind. There is a "ev_async_set" macro, but using it is
utterly pointless, trust me.

ev_async_send (loop, ev_async *)
Sends/signals/activates the given "ev_async" watcher, that is,
feeds an "EV_ASYNC" event on the watcher into the event loop, and
instantly returns.

Unlike "ev_feed_event", this call is safe to do from other threads,
signal or similar contexts (see the discussion of "EV_ATOMIC_T" in
the embedding section below on what exactly this means).

Note that, as with other watchers in libev, multiple events might
get compressed into a single callback invocation (another way to
look at this is that "ev_async" watchers are level-triggered: they
are set on "ev_async_send", reset when the event loop detects
that).

This call incurs the overhead of at most one extra system call per
event loop iteration, if the event loop is blocked, and no syscall
at all if the event loop (or your program) is processing events.
That means that repeated calls are basically free (there is no need
to avoid calls for performance reasons) and that the overhead
becomes smaller (typically zero) under load.

bool = ev_async_pending (ev_async *)
Returns a non-zero value when "ev_async_send" has been called on
the watcher but the event has not yet been processed (or even
noted) by the event loop.

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

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

OTHER FUNCTIONS
There are some other functions of possible interest. Described. Here.
Now.

ev_once (loop, int fd, int events, ev_tstamp timeout, callback, arg)
This function combines a simple timer and an I/O watcher, calls
your callback on whichever event happens first and automatically
stops both watchers. This is useful if you want to wait for a
single event on an fd or timeout without having to
allocate/configure/start/stop/free one or more watchers yourself.

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

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

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

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

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

ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);

ev_feed_fd_event (loop, int fd, int revents)
Feed an event on the given fd, as if a file descriptor backend
detected the given events.

ev_feed_signal_event (loop, int signum)
Feed an event as if the given signal occurred. See also
"ev_feed_signal", which is async-safe.

COMMON OR USEFUL IDIOMS (OR BOTH)
This section explains some common idioms that are not immediately
obvious. Note that examples are sprinkled over the whole manual, and
this section only contains stuff that wouldn't fit anywhere else.

ASSOCIATING CUSTOM DATA WITH A WATCHER
Each watcher has, by default, a "void *data" member that you can read
or modify 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 separately 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
function type instead have been omitted.

BUILDING YOUR OWN COMPOSITE WATCHERS
Another common scenario is to use some data structure with multiple
embedded watchers, in effect creating your own watcher that combines
multiple libev event sources into one "super-watcher":

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

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

#include <stddef.h>

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

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

AVOIDING FINISHING BEFORE RETURNING
Often you have structures like this in event-based programs:

callback ()
{
free (request);
}

request = start_new_request (..., callback);

The intent is to start some "lengthy" operation. The "request" could be
used to cancel the operation, or do other things with it.

It's not uncommon to have code paths in "start_new_request" that
immediately invoke the callback, for example, to report errors. Or you
add some caching layer that finds that it can skip the lengthy aspects
of the operation and simply invoke the callback with the result.

The problem here is that this will happen before "start_new_request"
has returned, so "request" is not set.

Even if you pass the request by some safer means to the callback, you
might want to do something to the request after starting it, such as
canceling it, which probably isn't working so well when the callback
has already been invoked.

A common way around all these issues is to make sure that
"start_new_request" always returns before the callback is invoked. If
"start_new_request" immediately knows the result, it can artificially
delay invoking the callback by using a "prepare" or "idle" watcher for
example, or more sneakily, by reusing an existing (stopped) watcher and
pushing it into the pending queue:

ev_set_cb (watcher, callback);
ev_feed_event (EV_A_ watcher, 0);

This way, "start_new_request" can safely return before the callback is
invoked, while not delaying callback invocation too much.

MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS
Often (especially in GUI toolkits) there are places where you have
modal interaction, which is most easily implemented by recursively
invoking "ev_run".

This brings the problem of exiting - a callback might want to finish
the main "ev_run" call, but not the nested one (e.g. user clicked
"Quit", but a modal "Are you sure?" dialog is still waiting), or just
the nested one and not the main one (e.g. user clocked "Ok" in a modal
dialog), or some other combination: In these cases, a simple "ev_break"
will not work.

The solution is to maintain "break this loop" variable for each
"ev_run" invocation, and use a loop around "ev_run" until the condition
is triggered, using "EVRUN_ONCE":

// main loop
int exit_main_loop = 0;

while (!exit_main_loop)
ev_run (EV_DEFAULT_ EVRUN_ONCE);

// in a modal watcher
int exit_nested_loop = 0;

while (!exit_nested_loop)
ev_run (EV_A_ EVRUN_ONCE);

To exit from any of these loops, just set the corresponding exit
variable:

// exit modal loop
exit_nested_loop = 1;

// exit main program, after modal loop is finished
exit_main_loop = 1;

// exit both
exit_main_loop = exit_nested_loop = 1;

THREAD LOCKING EXAMPLE
Here is a fictitious example of how to run an event loop in a different
thread from where callbacks are being invoked and watchers are
created/added/removed.

For a real-world example, see the "EV::Loop::Async" perl module, which
uses exactly this technique (which is suited for many high-level
languages).

The example uses a pthread mutex to protect the loop data, a condition
variable to wait for callback invocations, an async watcher to notify
the event loop thread and an unspecified mechanism to wake up the main
thread.

First, you need to associate some data with the event loop:

typedef struct {
mutex_t lock; /* global loop lock */
ev_async async_w;
thread_t tid;
cond_t invoke_cv;
} userdata;

void prepare_loop (EV_P)
{
// for simplicity, we use a static userdata struct.
static userdata u;

ev_async_init (&u->async_w, async_cb);
ev_async_start (EV_A_ &u->async_w);

pthread_mutex_init (&u->lock, 0);
pthread_cond_init (&u->invoke_cv, 0);

// now associate this with the loop
ev_set_userdata (EV_A_ u);
ev_set_invoke_pending_cb (EV_A_ l_invoke);
ev_set_loop_release_cb (EV_A_ l_release, l_acquire);

// then create the thread running ev_run
pthread_create (&u->tid, 0, l_run, EV_A);
}

The callback for the "ev_async" watcher does nothing: the watcher is
used solely to wake up the event loop so it takes notice of any new
watchers that might have been added:

static void
async_cb (EV_P_ ev_async *w, int revents)
{
// just used for the side effects
}

The "l_release" and "l_acquire" callbacks simply unlock/lock the mutex
protecting the loop data, respectively.

static void
l_release (EV_P)
{
userdata *u = ev_userdata (EV_A);
pthread_mutex_unlock (&u->lock);
}

static void
l_acquire (EV_P)
{
userdata *u = ev_userdata (EV_A);
pthread_mutex_lock (&u->lock);
}

The event loop thread first acquires the mutex, and then jumps straight
into "ev_run":

void *
l_run (void *thr_arg)
{
struct ev_loop *loop = (struct ev_loop *)thr_arg;

l_acquire (EV_A);
pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
ev_run (EV_A_ 0);
l_release (EV_A);

return 0;
}

Instead of invoking all pending watchers, the "l_invoke" callback will
signal the main thread via some unspecified mechanism (signals? pipe
writes? "Async::Interrupt"?) and then waits until all pending watchers
have been called (in a while loop because a) spurious wakeups are
possible and b) skipping inter-thread-communication when there are no
pending watchers is very beneficial):

static void
l_invoke (EV_P)
{
userdata *u = ev_userdata (EV_A);

while (ev_pending_count (EV_A))
{
wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
pthread_cond_wait (&u->invoke_cv, &u->lock);
}
}

Now, whenever the main thread gets told to invoke pending watchers, it
will grab the lock, call "ev_invoke_pending" and then signal the loop
thread to continue:

static void
real_invoke_pending (EV_P)
{
userdata *u = ev_userdata (EV_A);

pthread_mutex_lock (&u->lock);
ev_invoke_pending (EV_A);
pthread_cond_signal (&u->invoke_cv);
pthread_mutex_unlock (&u->lock);
}

Whenever you want to start/stop a watcher or do other modifications to
an event loop, you will now have to lock:

ev_timer timeout_watcher;
userdata *u = ev_userdata (EV_A);

ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);

pthread_mutex_lock (&u->lock);
ev_timer_start (EV_A_ &timeout_watcher);
ev_async_send (EV_A_ &u->async_w);
pthread_mutex_unlock (&u->lock);

Note that sending the "ev_async" watcher is required because otherwise
an event loop currently blocking in the kernel will have no knowledge
about the newly added timer. By waking up the loop it will pick up any
new watchers in the next event loop iteration.

THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS
While the overhead of a callback that e.g. schedules a thread is small,
it is still an overhead. If you embed libev, and your main usage is
with some kind of threads or coroutines, you might want to customise
libev so that doesn't need callbacks anymore.

Imagine you have coroutines that you can switch to using a function
"switch_to (coro)", that libev runs in a coroutine called "libev_coro"
and that due to some magic, the currently active coroutine is stored in
a global called "current_coro". Then you can build your own "wait for
libev event" primitive by changing "EV_CB_DECLARE" and "EV_CB_INVOKE"
(note the differing ";" conventions):

#define EV_CB_DECLARE(type) struct my_coro *cb;
#define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)

That means instead of having a C callback function, you store the
coroutine to switch to in each watcher, and instead of having libev
call your callback, you instead have it switch to that coroutine.

A coroutine might now wait for an event with a function called
"wait_for_event". (the watcher needs to be started, as always, but it
doesn't matter when, or whether the watcher is active or not when this
function is called):

void
wait_for_event (ev_watcher *w)
{
ev_set_cb (w, current_coro);
switch_to (libev_coro);
}

That basically suspends the coroutine inside "wait_for_event" and
continues the libev coroutine, which, when appropriate, switches back
to this or any other coroutine.

You can do similar tricks if you have, say, threads with an event queue
- instead of storing a coroutine, you store the queue object and
instead of switching to a coroutine, you push the watcher onto the
queue and notify any waiters.

To embed libev, see "EMBEDDING", but in short, it's easiest to create
two files, my_ev.h and my_ev.c that include the respective libev files:

// my_ev.h
#define EV_CB_DECLARE(type) struct my_coro *cb;
#define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
#include "../libev/ev.h"

// my_ev.c
#define EV_H "my_ev.h"
#include "../libev/ev.c"

And then use my_ev.h when you would normally use ev.h, and compile
my_ev.c into your project. When properly specifying include paths, you
can even use ev.h as header file name directly.

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

• Only the libevent-1.4.1-beta API is being emulated.

This was the newest libevent version available when libev was
implemented, and is still mostly unchanged in 2010.

• Use it by including <event.h>, as usual.

• The following members are fully supported: ev_base, ev_callback,
ev_arg, ev_fd, ev_res, ev_events.

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

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

• In libevent, the last base created gets the signals, in libev, the
base that registered the signal gets the signals.

• Other members are not supported.

• The libev emulation is not ABI compatible to libevent, you need to
use the libev header file and library.

C++ SUPPORT
C API
The normal C API should work fine when used from C++: both ev.h and the
libev sources can be compiled as C++. Therefore, code that uses the C
API will work fine.

Proper exception specifications might have to be added to callbacks
passed to libev: exceptions may be thrown only from watcher callbacks,
all other callbacks (allocator, syserr, loop acquire/release and
periodic reschedule callbacks) must not throw exceptions, and might
need a "noexcept" specification. If you have code that needs to be
compiled as both C and C++ you can use the "EV_NOEXCEPT" macro for
this:

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

...
ev_set_syserr_cb (fatal_error);

The only API functions that can currently throw exceptions are
"ev_run", "ev_invoke", "ev_invoke_pending" and "ev_loop_destroy" (the
latter because it runs cleanup watchers).

Throwing exceptions in watcher callbacks is only supported if libev
itself is compiled with a C++ compiler or your C and C++ environments
allow throwing exceptions through C libraries (most do).

C++ API
Libev comes with some simplistic wrapper classes for C++ that mainly
allow you to use some convenience methods to start/stop watchers and
also change the callback model to a model using method callbacks on
objects.

To use it,

#include <ev++.h>

This automatically includes ev.h and puts all of its definitions (many
of them macros) into the global namespace. All C++ specific things are
put into the "ev" namespace. It should support all the same embedding
options as ev.h, most notably "EV_MULTIPLICITY".

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

Currently, functions, static and non-static member functions and
classes with "operator ()" can be used as callbacks. Other types should
be easy to add as long as they only need one additional pointer for
context. If you need support for other types of functors please contact
the author (preferably after implementing it).

For all this to work, your C++ compiler either has to use the same
calling conventions as your C compiler (for static member functions),
or you have to embed libev and compile libev itself as C++.

Here is a list of things available in the "ev" namespace:

"ev::READ", "ev::WRITE" etc.
These are just enum values with the same values as the "EV_READ"
etc. macros from ev.h.

"ev::tstamp", "ev::now"
Aliases to the same types/functions as with the "ev_" prefix.

"ev::io", "ev::timer", "ev::periodic", "ev::idle", "ev::sig" etc.
For each "ev_TYPE" watcher in ev.h there is a corresponding class
of the same name in the "ev" namespace, with the exception of
"ev_signal" which is called "ev::sig" to avoid clashes with the
"signal" macro defined by many implementations.

All of those classes have these methods:

ev::TYPE::TYPE ()
ev::TYPE::TYPE (loop)
ev::TYPE::~TYPE
The constructor (optionally) takes an event loop to associate
the watcher with. If it is omitted, it will use "EV_DEFAULT".

The constructor calls "ev_init" for you, which means you have
to call the "set" method before starting it.

It will not set a callback, however: You have to call the
templated "set" method to set a callback before you can start
the watcher.

(The reason why you have to use a method is a limitation in C++
which does not allow explicit template arguments for
constructors).

The destructor automatically stops the watcher if it is active.

w->set<class, &class::method> (object *)
This method sets the callback method to call. The method has to
have a signature of "void (*)(ev_TYPE &, int)", it receives the
watcher as first argument and the "revents" as second. The
object must be given as parameter and is stored in the "data"
member of the watcher.

This method synthesizes efficient thunking code to call your
method from the C callback that libev requires. If your
compiler can inline your callback (i.e. it is visible to it at
the place of the "set" call and your compiler is good :), then
the method will be fully inlined into the thunking function,
making it as fast as a direct C callback.

Example: simple class declaration and watcher initialisation

struct myclass
{
void io_cb (ev::io &w, int revents) { }
}

myclass obj;
ev::io iow;
iow.set <myclass, &myclass::io_cb> (&obj);

w->set (object *)
This is a variation of a method callback - leaving out the
method to call will default the method to "operator ()", which
makes it possible to use functor objects without having to
manually specify the "operator ()" all the time. Incidentally,
you can then also leave out the template argument list.

The "operator ()" method prototype must be "void operator
()(watcher &w, int revents)".

See the method-"set" above for more details.

Example: use a functor object as callback.

struct myfunctor
{
void operator() (ev::io &w, int revents)
{
...
}
}

myfunctor f;

ev::io w;
w.set (&f);

w->set<function> (void *data = 0)
Also sets a callback, but uses a static method or plain
function as callback. The optional "data" argument will be
stored in the watcher's "data" member and is free for you to
use.

The prototype of the "function" must be "void (*)(ev::TYPE &w,
int)".

See the method-"set" above for more details.

Example: Use a plain function as callback.

static void io_cb (ev::io &w, int revents) { }
iow.set <io_cb> ();

w->set (loop)
Associates a different "struct ev_loop" with this watcher. You
can only do this when the watcher is inactive (and not pending
either).

w->set ([arguments])
Basically the same as "ev_TYPE_set" (except for "ev::embed"
watchers>), with the same arguments. Either this method or a
suitable start method must be called at least once. Unlike the
C counterpart, an active watcher gets automatically stopped and
restarted when reconfiguring it with this method.

For "ev::embed" watchers this method is called "set_embed", to
avoid clashing with the "set (loop)" method.

For "ev::io" watchers there is an additional "set" method that
acepts a new event mask only, and internally calls
"ev_io_modfify".

w->start ()
Starts the watcher. Note that there is no "loop" argument, as
the constructor already stores the event loop.

w->start ([arguments])
Instead of calling "set" and "start" methods separately, it is
often convenient to wrap them in one call. Uses the same type
of arguments as the configure "set" method of the watcher.

w->stop ()
Stops the watcher if it is active. Again, no "loop" argument.

w->again () ("ev::timer", "ev::periodic" only)
For "ev::timer" and "ev::periodic", this invokes the
corresponding "ev_TYPE_again" function.

w->sweep () ("ev::embed" only)
Invokes "ev_embed_sweep".

w->update () ("ev::stat" only)
Invokes "ev_stat_stat".

Example: Define a class with two I/O and idle watchers, start the I/O
watchers in the constructor.

class myclass
{
ev::io io ; void io_cb (ev::io &w, int revents);
ev::io io2 ; void io2_cb (ev::io &w, int revents);
ev::idle idle; void idle_cb (ev::idle &w, int revents);

myclass (int fd)
{
io .set <myclass, &myclass::io_cb > (this);
io2 .set <myclass, &myclass::io2_cb > (this);
idle.set <myclass, &myclass::idle_cb> (this);

io.set (fd, ev::WRITE); // configure the watcher
io.start (); // start it whenever convenient

io2.start (fd, ev::READ); // set + start in one call
}
};

OTHER LANGUAGE BINDINGS
Libev does not offer other language bindings itself, but bindings for a
number of languages exist in the form of third-party packages. If you
know any interesting language binding in addition to the ones listed
here, drop me a note.

Perl
The EV module implements the full libev API and is actually used to
test libev. EV is developed together with libev. Apart from the EV
core module, there are additional modules that implement libev-
compatible interfaces to "libadns" ("EV::ADNS", but "AnyEvent::DNS"
is preferred nowadays), "Net::SNMP" ("Net::SNMP::EV") and the
"libglib" event core ("Glib::EV" and "EV::Glib").

It can be found and installed via CPAN, its homepage is at
<http://software.schmorp.de/pkg/EV>.

Python
Python bindings can be found at <http://code.google.com/p/pyev/>.
It seems to be quite complete and well-documented.

Ruby
Tony Arcieri has written a ruby extension that offers access to a
subset of the libev API and adds file handle abstractions,
asynchronous DNS and more on top of it. It can be found via gem
servers. Its homepage is at <http://rev.rubyforge.org/>.

Roger Pack reports that using the link order "-lws2_32
-lmsvcrt-ruby-190" makes rev work even on mingw.

Haskell
A haskell binding to libev is available at
<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>.

D Leandro Lucarella has written a D language binding (ev.d) for
libev, to be found at
<http://www.llucax.com.ar/proj/ev.d/index.html>.

Ocaml
Erkki Seppala has written Ocaml bindings for libev, to be found at
<http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.

Lua Brian Maher has written a partial interface to libev for lua (at
the time of this writing, only "ev_io" and "ev_timer"), to be found
at <http://github.com/brimworks/lua-ev>.

Javascript
Node.js (<http://nodejs.org>) uses libev as the underlying event
library.

Others
There are others, and I stopped counting.

MACRO MAGIC
Libev can be compiled with a variety of options, the most fundamental
of which is "EV_MULTIPLICITY". This option determines whether (most)
functions and callbacks have an initial "struct ev_loop *" argument.

To make it easier to write programs that cope with either variant, the
following macros are defined:

"EV_A", "EV_A_"
This provides the loop argument for functions, if one is required
("ev loop argument"). The "EV_A" form is used when this is the sole
argument, "EV_A_" is used when other arguments are following.
Example:

ev_unref (EV_A);
ev_timer_add (EV_A_ watcher);
ev_run (EV_A_ 0);

It assumes the variable "loop" of type "struct ev_loop *" is in
scope, which is often provided by the following macro.

"EV_P", "EV_P_"
This provides the loop parameter for functions, if one is required
("ev loop parameter"). The "EV_P" form is used when this is the
sole parameter, "EV_P_" is used when other parameters are
following. Example:

// this is how ev_unref is being declared
static void ev_unref (EV_P);

// this is how you can declare your typical callback
static void cb (EV_P_ ev_timer *w, int revents)

It declares a parameter "loop" of type "struct ev_loop *", quite
suitable for use with "EV_A".

"EV_DEFAULT", "EV_DEFAULT_"
Similar to the other two macros, this gives you the value of the
default loop, if multiple loops are supported ("ev loop default").
The default loop will be initialised if it isn't already
initialised.

For non-multiplicity builds, these macros do nothing, so you always
have to initialise the loop somewhere.

"EV_DEFAULT_UC", "EV_DEFAULT_UC_"
Usage identical to "EV_DEFAULT" and "EV_DEFAULT_", but requires
that the default loop has been initialised ("UC" == unchecked).
Their behaviour is undefined when the default loop has not been
initialised by a previous execution of "EV_DEFAULT", "EV_DEFAULT_"
or "ev_default_init (...)".

It is often prudent to use "EV_DEFAULT" when initialising the first
watcher in a function but use "EV_DEFAULT_UC" afterwards.

Example: Declare and initialise a check watcher, utilising the above
macros so it will work regardless of whether multiple loops are
supported or not.

static void
check_cb (EV_P_ ev_timer *w, int revents)
{
ev_check_stop (EV_A_ w);
}

ev_check check;
ev_check_init (&check, check_cb);
ev_check_start (EV_DEFAULT_ &check);
ev_run (EV_DEFAULT_ 0);

EMBEDDING
Libev can (and often is) directly embedded into host applications.
Examples of applications that embed it include the Deliantra Game
Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe) and
rxvt-unicode.

The goal is to enable you to just copy the necessary files into your
source directory without having to change even a single line in them,
so you can easily upgrade by simply copying (or having a checked-out
copy of libev somewhere in your source tree).

FILESETS
Depending on what features you need you need to include one or more
sets of files in your application.

CORE EVENT LOOP

To include only the libev core (all the "ev_*" functions), with manual
configuration (no autoconf):

#define EV_STANDALONE 1
#include "ev.c"

This will automatically include ev.h, too, and should be done in a
single C source file only to provide the function implementations. To
use it, do the same for ev.h in all files wishing to use this API (best
done by writing a wrapper around ev.h that you can include instead and
where you can put other configuration options):

#define EV_STANDALONE 1
#include "ev.h"

Both header files and implementation files can be compiled with a C++
compiler (at least, that's a stated goal, and breakage will be treated
as a bug).

You need the following files in your source tree, or in a directory in
your include path (e.g. in libev/ when using -Ilibev):

ev.h
ev.c
ev_vars.h
ev_wrap.h

ev_win32.c required on win32 platforms only

ev_select.c only when select backend is enabled
ev_poll.c only when poll backend is enabled
ev_epoll.c only when the epoll backend is enabled
ev_linuxaio.c only when the linux aio backend is enabled
ev_iouring.c only when the linux io_uring backend is enabled
ev_kqueue.c only when the kqueue backend is enabled
ev_port.c only when the solaris port backend is enabled

ev.c includes the backend files directly when enabled, so you only need
to compile this single file.

LIBEVENT COMPATIBILITY API

To include the libevent compatibility API, also include:

#include "event.c"

in the file including ev.c, and:

#include "event.h"

in the files that want to use the libevent API. This also includes
ev.h.

You need the following additional files for this:

event.h
event.c

AUTOCONF SUPPORT

Instead of using "EV_STANDALONE=1" and providing your configuration in
whatever way you want, you can also "m4_include([libev.m4])" in your
configure.ac and leave "EV_STANDALONE" undefined. ev.c will then
include config.h and configure itself accordingly.

For this of course you need the m4 file:

libev.m4

PREPROCESSOR SYMBOLS/MACROS
Libev can be configured via a variety of preprocessor symbols you have
to define before including (or compiling) any of its files. The default
in the absence of autoconf is documented for every option.

Symbols marked with "(h)" do not change the ABI, and can have different
values when compiling libev vs. including ev.h, so it is permissible to
redefine them before including ev.h without breaking compatibility to a
compiled library. All other symbols change the ABI, which means all
users of libev and the libev code itself must be compiled with
compatible settings.

EV_COMPAT3 (h)
Backwards compatibility is a major concern for libev. This is why
this release of libev comes with wrappers for the functions and
symbols that have been renamed between libev version 3 and 4.

You can disable these wrappers (to test compatibility with future
versions) by defining "EV_COMPAT3" to 0 when compiling your
sources. This has the additional advantage that you can drop the
"struct" from "struct ev_loop" declarations, as libev will provide
an "ev_loop" typedef in that case.

In some future version, the default for "EV_COMPAT3" will become 0,
and in some even more future version the compatibility code will be
removed completely.

EV_STANDALONE (h)
Must always be 1 if you do not use autoconf configuration, which
keeps libev from including config.h, and it also defines dummy
implementations for some libevent functions (such as logging, which
is not supported). It will also not define any of the structs
usually found in event.h that are not directly supported by the
libev core alone.

In standalone mode, libev will still try to automatically deduce
the configuration, but has to be more conservative.

EV_USE_FLOOR
If defined to be 1, libev will use the "floor ()" function for its
periodic reschedule calculations, otherwise libev will fall back on
a portable (slower) implementation. If you enable this, you usually
have to link against libm or something equivalent. Enabling this
when the "floor" function is not available will fail, so the safe
default is to not enable this.

EV_USE_MONOTONIC
If defined to be 1, libev will try to detect the availability of
the monotonic clock option at both compile time and runtime.
Otherwise no use of the monotonic clock option will be attempted.
If you enable this, you usually have to link against librt or
something similar. Enabling it when the functionality isn't
available is safe, though, although you have to make sure you link
against any libraries where the "clock_gettime" function is hiding
in (often -lrt). See also "EV_USE_CLOCK_SYSCALL".

EV_USE_REALTIME
If defined to be 1, libev will try to detect the availability of
the real-time clock option at compile time (and assume its
availability at runtime if successful). Otherwise no use of the
real-time clock option will be attempted. This effectively replaces
"gettimeofday" by "clock_get (CLOCK_REALTIME, ...)" and will not
normally affect correctness. See the note about libraries in the
description of "EV_USE_MONOTONIC", though. Defaults to the opposite
value of "EV_USE_CLOCK_SYSCALL".

EV_USE_CLOCK_SYSCALL
If defined to be 1, libev will try to use a direct syscall instead
of calling the system-provided "clock_gettime" function. This
option exists because on GNU/Linux, "clock_gettime" is in "librt",
but "librt" unconditionally pulls in "libpthread", slowing down
single-threaded programs needlessly. Using a direct syscall is
slightly slower (in theory), because no optimised vdso
implementation can be used, but avoids the pthread dependency.
Defaults to 1 on GNU/Linux with glibc 2.x or higher, as it
simplifies linking (no need for "-lrt").

EV_USE_NANOSLEEP
If defined to be 1, libev will assume that "nanosleep ()" is
available and will use it for delays. Otherwise it will use "select
()".

EV_USE_EVENTFD
If defined to be 1, then libev will assume that "eventfd ()" is
available and will probe for kernel support at runtime. This will
improve "ev_signal" and "ev_async" performance and reduce resource
consumption. If undefined, it will be enabled if the headers
indicate GNU/Linux + Glibc 2.7 or newer, otherwise disabled.

EV_USE_SIGNALFD
If defined to be 1, then libev will assume that "signalfd ()" is
available and will probe for kernel support at runtime. This
enables the use of EVFLAG_SIGNALFD for faster and simpler signal
handling. If undefined, it will be enabled if the headers indicate
GNU/Linux + Glibc 2.7 or newer, otherwise disabled.

EV_USE_TIMERFD
If defined to be 1, then libev will assume that "timerfd ()" is
available and will probe for kernel support at runtime. This allows
libev to detect time jumps accurately. If undefined, it will be
enabled if the headers indicate GNU/Linux + Glibc 2.8 or newer and
define "TFD_TIMER_CANCEL_ON_SET", otherwise disabled.

EV_USE_SELECT
If undefined or defined to be 1, libev will compile in support for
the "select"(2) backend. No attempt at auto-detection will be done:
if no other method takes over, select will be it. Otherwise the
select backend will not be compiled in.

EV_SELECT_USE_FD_SET
If defined to 1, then the select backend will use the system
"fd_set" structure. This is useful if libev doesn't compile due to
a missing "NFDBITS" or "fd_mask" definition or it mis-guesses the
bitset layout on exotic systems. This usually limits the range of
file descriptors to some low limit such as 1024 or might have other
limitations (winsocket only allows 64 sockets). The "FD_SETSIZE"
macro, set before compilation, configures the maximum size of the
"fd_set".

EV_SELECT_IS_WINSOCKET
When defined to 1, the select backend will assume that
select/socket/connect etc. don't understand file descriptors but
wants osf handles on win32 (this is the case when the select to be
used is the winsock select). This means that it will call
"_get_osfhandle" on the fd to convert it to an OS handle.
Otherwise, it is assumed that all these functions actually work on
fds, even on win32. Should not be defined on non-win32 platforms.

EV_FD_TO_WIN32_HANDLE(fd)
If "EV_SELECT_IS_WINSOCKET" is enabled, then libev needs a way to
map file descriptors to socket handles. When not defining this
symbol (the default), then libev will call "_get_osfhandle", which
is usually correct. In some cases, programs use their own file
descriptor management, in which case they can provide this function
to map fds to socket handles.

EV_WIN32_HANDLE_TO_FD(handle)
If "EV_SELECT_IS_WINSOCKET" then libev maps handles to file
descriptors using the standard "_open_osfhandle" function. For
programs implementing their own fd to handle mapping, overwriting
this function makes it easier to do so. This can be done by
defining this macro to an appropriate value.

EV_WIN32_CLOSE_FD(fd)
If programs implement their own fd to handle mapping on win32, then
this macro can be used to override the "close" function, useful to
unregister file descriptors again. Note that the replacement
function has to close the underlying OS handle.

EV_USE_WSASOCKET
If defined to be 1, libev will use "WSASocket" to create its
internal communication socket, which works better in some
environments. Otherwise, the normal "socket" function will be used,
which works better in other environments.

EV_USE_POLL
If defined to be 1, libev will compile in support for the "poll"(2)
backend. Otherwise it will be enabled on non-win32 platforms. It
takes precedence over select.

EV_USE_EPOLL
If defined to be 1, libev will compile in support for the Linux
"epoll"(7) backend. Its availability will be detected at runtime,
otherwise another method will be used as fallback. This is the
preferred backend for GNU/Linux systems. If undefined, it will be
enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer,
otherwise disabled.

EV_USE_LINUXAIO
If defined to be 1, libev will compile in support for the Linux aio
backend ("EV_USE_EPOLL" must also be enabled). If undefined, it
will be enabled on linux, otherwise disabled.

EV_USE_IOURING
If defined to be 1, libev will compile in support for the Linux
io_uring backend ("EV_USE_EPOLL" must also be enabled). Due to it's
current limitations it has to be requested explicitly. If
undefined, it will be enabled on linux, otherwise disabled.

EV_USE_KQUEUE
If defined to be 1, libev will compile in support for the BSD style
"kqueue"(2) backend. Its actual availability will be detected at
runtime, otherwise another method will be used as fallback. This is
the preferred backend for BSD and BSD-like systems, although on
most BSDs kqueue only supports some types of fds correctly (the
only platform we found that supports ptys for example was NetBSD),
so kqueue might be compiled in, but not be used unless explicitly
requested. The best way to use it is to find out whether kqueue
supports your type of fd properly and use an embedded kqueue loop.

EV_USE_PORT
If defined to be 1, libev will compile in support for the Solaris
10 port style backend. Its availability will be detected at
runtime, otherwise another method will be used as fallback. This is
the preferred backend for Solaris 10 systems.

EV_USE_DEVPOLL
Reserved for future expansion, works like the USE symbols above.

EV_USE_INOTIFY
If defined to be 1, libev will compile in support for the Linux
inotify interface to speed up "ev_stat" watchers. Its actual
availability will be detected at runtime. If undefined, it will be
enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer,
otherwise disabled.

EV_NO_SMP
If defined to be 1, libev will assume that memory is always
coherent between threads, that is, threads can be used, but threads
never run on different cpus (or different cpu cores). This reduces
dependencies and makes libev faster.

EV_NO_THREADS
If defined to be 1, libev will assume that it will never be called
from different threads (that includes signal handlers), which is a
stronger assumption than "EV_NO_SMP", above. This reduces
dependencies and makes libev faster.

EV_ATOMIC_T
Libev requires an integer type (suitable for storing 0 or 1) whose
access is atomic with respect to other threads or signal contexts.
No such type is easily found in the C language, so you can provide
your own type that you know is safe for your purposes. It is used
both for signal handler "locking" as well as for signal and thread
safety in "ev_async" watchers.

In the absence of this define, libev will use "sig_atomic_t
volatile" (from signal.h), which is usually good enough on most
platforms.

EV_H (h)
The name of the ev.h header file used to include it. The default if
undefined is "ev.h" in event.h, ev.c and ev++.h. This can be used
to virtually rename the ev.h header file in case of conflicts.

EV_CONFIG_H (h)
If "EV_STANDALONE" isn't 1, this variable can be used to override
ev.c's idea of where to find the config.h file, similarly to
"EV_H", above.

EV_EVENT_H (h)
Similarly to "EV_H", this macro can be used to override event.c's
idea of how the event.h header can be found, the default is
"event.h".

EV_PROTOTYPES (h)
If defined to be 0, then ev.h will not define any function
prototypes, but still define all the structs and other symbols.
This is occasionally useful if you want to provide your own wrapper
functions around libev functions.

EV_MULTIPLICITY
If undefined or defined to 1, then all event-loop-specific
functions will have the "struct ev_loop *" as first argument, and
you can create additional independent event loops. Otherwise there
will be no support for multiple event loops and there is no first
event loop pointer argument. Instead, all functions act on the
single default loop.

Note that "EV_DEFAULT" and "EV_DEFAULT_" will no longer provide a
default loop when multiplicity is switched off - you always have to
initialise the loop manually in this case.

EV_MINPRI
EV_MAXPRI
The range of allowed priorities. "EV_MINPRI" must be smaller or
equal to "EV_MAXPRI", but otherwise there are no non-obvious
limitations. You can provide for more priorities by overriding
those symbols (usually defined to be "-2" and 2, respectively).

When doing priority-based operations, libev usually has to linearly
search all the priorities, so having many of them (hundreds) uses a
lot of space and time, so using the defaults of five priorities (-2
.. +2) is usually fine.

If your embedding application does not need any priorities,
defining these both to 0 will save some memory and CPU.

EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE,
EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE,
EV_ASYNC_ENABLE, EV_CHILD_ENABLE.
If undefined or defined to be 1 (and the platform supports it),
then the respective watcher type is supported. If defined to be 0,
then it is not. Disabling watcher types mainly saves code size.

EV_FEATURES
If you need to shave off some kilobytes of code at the expense of
some speed (but with the full API), you can define this symbol to
request certain subsets of functionality. The default is to enable
all features that can be enabled on the platform.

A typical way to use this symbol is to define it to 0 (or to a
bitset with some broad features you want) and then selectively re-
enable additional parts you want, for example if you want
everything minimal, but multiple event loop support, async and
child watchers and the poll backend, use this:

#define EV_FEATURES 0
#define EV_MULTIPLICITY 1
#define EV_USE_POLL 1
#define EV_CHILD_ENABLE 1
#define EV_ASYNC_ENABLE 1

The actual value is a bitset, it can be a combination of the
following values (by default, all of these are enabled):

1 - faster/larger code
Use larger code to speed up some operations.

Currently this is used to override some inlining decisions
(enlarging the code size by roughly 30% on amd64).

When optimising for size, use of compiler flags such as "-Os"
with gcc is recommended, as well as "-DNDEBUG", as libev
contains a number of assertions.

The default is off when "__OPTIMIZE_SIZE__" is defined by your
compiler (e.g. gcc with "-Os").

2 - faster/larger data structures
Replaces the small 2-heap for timer management by a faster
4-heap, larger hash table sizes and so on. This will usually
further increase code size and can additionally have an effect
on the size of data structures at runtime.

The default is off when "__OPTIMIZE_SIZE__" is defined by your
compiler (e.g. gcc with "-Os").

4 - full API configuration
This enables priorities (sets "EV_MAXPRI"=2 and
"EV_MINPRI"=-2), and enables multiplicity
("EV_MULTIPLICITY"=1).

8 - full API
This enables a lot of the "lesser used" API functions. See
"ev.h" for details on which parts of the API are still
available without this feature, and do not complain if this
subset changes over time.

16 - enable all optional watcher types
Enables all optional watcher types. If you want to selectively
enable only some watcher types other than I/O and timers (e.g.
prepare, embed, async, child...) you can enable them manually
by defining "EV_watchertype_ENABLE" to 1 instead.

32 - enable all backends
This enables all backends - without this feature, you need to
enable at least one backend manually ("EV_USE_SELECT" is a good
choice).

64 - enable OS-specific "helper" APIs
Enable inotify, eventfd, signalfd and similar OS-specific
helper APIs by default.

Compiling with "gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1
-DEV_FEATURES=0" reduces the compiled size of libev from 24.7Kb
code/2.8Kb data to 6.5Kb code/0.3Kb data on my GNU/Linux amd64
system, while still giving you I/O watchers, timers and monotonic
clock support.

With an intelligent-enough linker (gcc+binutils are intelligent
enough when you use "-Wl,--gc-sections -ffunction-sections")
functions unused by your program might be left out as well - a
binary starting a timer and an I/O watcher then might come out at
only 5Kb.

EV_API_STATIC
If this symbol is defined (by default it is not), then all
identifiers will have static linkage. This means that libev will
not export any identifiers, and you cannot link against libev
anymore. This can be useful when you embed libev, only want to use
libev functions in a single file, and do not want its identifiers
to be visible.

To use this, define "EV_API_STATIC" and include ev.c in the file
that wants to use libev.

This option only works when libev is compiled with a C compiler, as
C++ doesn't support the required declaration syntax.

EV_AVOID_STDIO
If this is set to 1 at compiletime, then libev will avoid using
stdio functions (printf, scanf, perror etc.). This will increase
the code size somewhat, but if your program doesn't otherwise
depend on stdio and your libc allows it, this avoids linking in the
stdio library which is quite big.

Note that error messages might become less precise when this option
is enabled.

EV_NSIG
The highest supported signal number, +1 (or, the number of
signals): Normally, libev tries to deduce the maximum number of
signals automatically, but sometimes this fails, in which case it
can be specified. Also, using a lower number than detected (32
should be good for about any system in existence) can save some
memory, as libev statically allocates some 12-24 bytes per signal
number.

EV_PID_HASHSIZE
"ev_child" watchers use a small hash table to distribute workload
by pid. The default size is 16 (or 1 with "EV_FEATURES" disabled),
usually more than enough. If you need to manage thousands of
children you might want to increase this value (must be a power of
two).

EV_INOTIFY_HASHSIZE
"ev_stat" watchers use a small hash table to distribute workload by
inotify watch id. The default size is 16 (or 1 with "EV_FEATURES"
disabled), usually more than enough. If you need to manage
thousands of "ev_stat" watchers you might want to increase this
value (must be a power of two).

EV_USE_4HEAP
Heaps are not very cache-efficient. To improve the cache-efficiency
of the timer and periodics heaps, libev uses a 4-heap when this
symbol is defined to 1. The 4-heap uses more complicated (longer)
code but has noticeably faster performance with many (thousands) of
watchers.

The default is 1, unless "EV_FEATURES" overrides it, in which case
it will be 0.

EV_HEAP_CACHE_AT
Heaps are not very cache-efficient. To improve the cache-efficiency
of the timer and periodics heaps, libev can cache the timestamp
(at) within the heap structure (selected by defining
"EV_HEAP_CACHE_AT" to 1), which uses 8-12 bytes more per watcher
and a few hundred bytes more code, but avoids random read accesses
on heap changes. This improves performance noticeably with many
(hundreds) of watchers.

The default is 1, unless "EV_FEATURES" overrides it, in which case
it will be 0.

EV_VERIFY
Controls how much internal verification (see "ev_verify ()") will
be done: If set to 0, no internal verification code will be
compiled in. If set to 1, then verification code will be compiled
in, but not called. If set to 2, then the internal verification
code will be called once per loop, which can slow down libev. If
set to 3, then the verification code will be called very
frequently, which will slow down libev considerably.

Verification errors are reported via C's "assert" mechanism, so if
you disable that (e.g. by defining "NDEBUG") then no errors will be
reported.

The default is 1, unless "EV_FEATURES" overrides it, in which case
it will be 0.

EV_COMMON
By default, all watchers have a "void *data" member. By redefining
this macro to something else you can include more and other types
of members. You have to define it each time you include one of the
files, though, and it must be identical each time.

For example, the perl EV module uses something like this:

#define EV_COMMON \
SV *self; /* contains this struct */ \
SV *cb_sv, *fh /* note no trailing ";" */

EV_CB_DECLARE (type)
EV_CB_INVOKE (watcher, revents)
ev_set_cb (ev, cb)
Can be used to change the callback member declaration in each
watcher, and the way callbacks are invoked and set. Must expand to
a struct member definition and a statement, respectively. See the
ev.h header file for their default definitions. One possible use
for overriding these is to avoid the "struct ev_loop *" as first
argument in all cases, or to use method calls instead of plain
function calls in C++.

EXPORTED API SYMBOLS
If you need to re-export the API (e.g. via a DLL) and you need a list
of exported symbols, you can use the provided Symbol.* files which list
all public symbols, one per line:

Symbols.ev for libev proper
Symbols.event for the libevent emulation

This can also be used to rename all public symbols to avoid clashes
with multiple versions of libev linked together (which is obviously bad
in itself, but sometimes it is inconvenient to avoid this).

A sed command like this will create wrapper "#define"'s that you need
to include before including ev.h:

<Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h

This would create a file wrap.h which essentially looks like this:

#define ev_backend myprefix_ev_backend
#define ev_check_start myprefix_ev_check_start
#define ev_check_stop myprefix_ev_check_stop
...

EXAMPLES
For a real-world example of a program the includes libev verbatim, you
can have a look at the EV perl module
(<http://software.schmorp.de/pkg/EV.html>). It has the libev files in
the libev/ subdirectory and includes them in the EV/EVAPI.h (public
interface) and EV.xs (implementation) files. Only the EV.xs file will
be compiled. It is pretty complex because it provides its own header
file.

The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file
that everybody includes and which overrides some configure choices:

#define EV_FEATURES 8
#define EV_USE_SELECT 1
#define EV_PREPARE_ENABLE 1
#define EV_IDLE_ENABLE 1
#define EV_SIGNAL_ENABLE 1
#define EV_CHILD_ENABLE 1
#define EV_USE_STDEXCEPT 0
#define EV_CONFIG_H <config.h>

#include "ev++.h"

And a ev_cpp.C implementation file that contains libev proper and is
compiled:

#include "ev_cpp.h"
#include "ev.c"

INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT
THREADS AND COROUTINES
THREADS

All libev functions are reentrant and thread-safe unless explicitly
documented otherwise, but libev implements no locking itself. This
means that you can use as many loops as you want in parallel, as long
as there are no concurrent calls into any libev function with the same
loop parameter ("ev_default_*" calls have an implicit default loop
parameter, of course): libev guarantees that different event loops
share no data structures that need any locking.

Or to put it differently: calls with different loop parameters can be
done concurrently from multiple threads, calls with the same loop
parameter must be done serially (but can be done from different
threads, as long as only one thread ever is inside a call at any point
in time, e.g. by using a mutex per loop).

Specifically to support threads (and signal handlers), libev implements
so-called "ev_async" watchers, which allow some limited form of
concurrency on the same event loop, namely waking it up "from the
outside".

If you want to know which design (one loop, locking, or multiple loops
without or something else still) is best for your problem, then I
cannot help you, but here is some generic advice:

• most applications have a main thread: use the default libev loop in
that thread, or create a separate thread running only the default
loop.

This helps integrating other libraries or software modules that use
libev themselves and don't care/know about threading.

• one loop per thread is usually a good model.

Doing this is almost never wrong, sometimes a better-performance
model exists, but it is always a good start.

• other models exist, such as the leader/follower pattern, where one
loop is handed through multiple threads in a kind of round-robin
fashion.

Choosing a model is hard - look around, learn, know that usually
you can do better than you currently do :-)

• often you need to talk to some other thread which blocks in the
event loop.

"ev_async" watchers can be used to wake them up from other threads
safely (or from signal contexts...).

An example use would be to communicate signals or other events that
only work in the default loop by registering the signal watcher
with the default loop and triggering an "ev_async" watcher from the
default loop watcher callback into the event loop interested in the
signal.

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