CSE2410 Fall 2014 Bounce

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.\" ========================================================================

.\"

.IX Title "LIBEV 3"

.TH LIBEV 3 "2009-02-06" "libev-3.53" "libev - high performance full featured event loop"

.\" For nroff, turn off justification.  Always turn off hyphenation; it makes

.\" way too many mistakes in technical documents.

.if n .ad l

.nh

.SH "NAME"

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

.SH "SYNOPSIS"

.IX Header "SYNOPSIS"

.Vb 1

\&   #include <ev.h>

.Ve

.Sh "\s-1EXAMPLE\s0 \s-1PROGRAM\s0"

.IX Subsection "EXAMPLE PROGRAM"

.Vb 2

\&   // a single header file is required

\&   #include <ev.h>

\&

\&   #include <stdio.h> // for puts

\&

\&   // every watcher type has its own typedef\*(Aqd struct

\&   // with the name ev_TYPE

\&   ev_io stdin_watcher;

\&   ev_timer timeout_watcher;

\&

\&   // all watcher callbacks have a similar signature

\&   // this callback is called when data is readable on stdin

\&   static void

\&   stdin_cb (EV_P_ ev_io *w, int revents)

\&   {

\&     puts ("stdin ready");

\&     // for one\-shot events, one must manually stop the watcher

\&     // with its corresponding stop function.

\&     ev_io_stop (EV_A_ w);

\&

\&     // this causes all nested ev_loop\*(Aqs to stop iterating

\&     ev_unloop (EV_A_ EVUNLOOP_ALL);

\&   }

\&

\&   // another callback, this time for a time\-out

\&   static void

\&   timeout_cb (EV_P_ ev_timer *w, int revents)

\&   {

\&     puts ("timeout");

\&     // this causes the innermost ev_loop to stop iterating

\&     ev_unloop (EV_A_ EVUNLOOP_ONE);

\&   }

\&

\&   int

\&   main (void)

\&   {

\&     // use the default event loop unless you have special needs

\&     struct ev_loop *loop = ev_default_loop (0);

\&

\&     // initialise an io watcher, then start it

\&     // this one will watch for stdin to become readable

\&     ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);

\&     ev_io_start (loop, &stdin_watcher);

\&

\&     // initialise a timer watcher, then start it

\&     // simple non\-repeating 5.5 second timeout

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

\&     ev_timer_start (loop, &timeout_watcher);

\&

\&     // now wait for events to arrive

\&     ev_loop (loop, 0);

\&

\&     // unloop was called, so exit

\&     return 0;

\&   }

.Ve

.SH "DESCRIPTION"

.IX Header "DESCRIPTION"

The newest version of this document is also available as an html-formatted

web page you might find easier to navigate when reading it for the first

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

.PP

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.

.PP

To do this, it must take more or less complete control over your process

(or thread) by executing the \fIevent loop\fR handler, and will then

communicate events via a callback mechanism.

.PP

You register interest in certain events by registering so-called \fIevent

watchers\fR, which are relatively small C structures you initialise with the

details of the event, and then hand it over to libev by \fIstarting\fR the

watcher.

.Sh "\s-1FEATURES\s0"

.IX Subsection "FEATURES"

Libev supports \f(CW\*(C`select\*(C'\fR, \f(CW\*(C`poll\*(C'\fR, the Linux-specific \f(CW\*(C`epoll\*(C'\fR, the

BSD-specific \f(CW\*(C`kqueue\*(C'\fR and the Solaris-specific event port mechanisms

for file descriptor events (\f(CW\*(C`ev_io\*(C'\fR), the Linux \f(CW\*(C`inotify\*(C'\fR interface

(for \f(CW\*(C`ev_stat\*(C'\fR), relative timers (\f(CW\*(C`ev_timer\*(C'\fR), absolute timers

with customised rescheduling (\f(CW\*(C`ev_periodic\*(C'\fR), synchronous signals

(\f(CW\*(C`ev_signal\*(C'\fR), process status change events (\f(CW\*(C`ev_child\*(C'\fR), and event

watchers dealing with the event loop mechanism itself (\f(CW\*(C`ev_idle\*(C'\fR,

\&\f(CW\*(C`ev_embed\*(C'\fR, \f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR watchers) as well as

file watchers (\f(CW\*(C`ev_stat\*(C'\fR) and even limited support for fork events

(\f(CW\*(C`ev_fork\*(C'\fR).

.PP

It also is quite fast (see this

benchmark comparing it to libevent

for example).

.Sh "\s-1CONVENTIONS\s0"

.IX Subsection "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

\&\fB\s-1EMBED\s0\fR section in this manual. If libev was configured without support

for multiple event loops, then all functions taking an initial argument of

name \f(CW\*(C`loop\*(C'\fR (which is always of type \f(CW\*(C`ev_loop *\*(C'\fR) will not have

this argument.

.Sh "\s-1TIME\s0 \s-1REPRESENTATION\s0"

.IX Subsection "TIME REPRESENTATION"

Libev represents time as a single floating point number, representing the

(fractional) number of seconds since the (\s-1POSIX\s0) epoch (somewhere near

the beginning of 1970, details are complicated, don't ask). This type is

called \f(CW\*(C`ev_tstamp\*(C'\fR, which is what you should use too. It usually aliases

to the \f(CW\*(C`double\*(C'\fR type in C, and when you need to do any calculations on

it, you should treat it as some floating point value. Unlike the name

component \f(CW\*(C`stamp\*(C'\fR might indicate, it is also used for time differences

throughout libev.

.SH "ERROR HANDLING"

.IX Header "ERROR HANDLING"

Libev knows three classes of errors: operating system errors, usage errors

and internal errors (bugs).

.PP

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 \f(CW\*(C`ev_set_syserr_cb\*(C'\fR, which is supposed to fix the problem or

abort. The default is to print a diagnostic message and to call \f(CW\*(C`abort

()\*(C'\fR.

.PP

When libev detects a usage error such as a negative timer interval, then

it will print a diagnostic message and abort (via the \f(CW\*(C`assert\*(C'\fR mechanism,

so \f(CW\*(C`NDEBUG\*(C'\fR will disable this checking): these are programming errors in

the libev caller and need to be fixed there.

.PP

Libev also has a few internal error-checking \f(CW\*(C`assert\*(C'\fRions, and also has

extensive consistency checking code. These do not trigger under normal

circumstances, as they indicate either a bug in libev or worse.

.SH "GLOBAL FUNCTIONS"

.IX Header "GLOBAL FUNCTIONS"

These functions can be called anytime, even before initialising the

library in any way.

.IP "ev_tstamp ev_time ()" 4

.IX Item "ev_tstamp ev_time ()"

Returns the current time as libev would use it. Please note that the

\&\f(CW\*(C`ev_now\*(C'\fR function is usually faster and also often returns the timestamp

you actually want to know.

.IP "ev_sleep (ev_tstamp interval)" 4

.IX Item "ev_sleep (ev_tstamp interval)"

Sleep for the given interval: The current thread will be blocked until

either it is interrupted or the given time interval has passed. Basically

this is a sub-second-resolution \f(CW\*(C`sleep ()\*(C'\fR.

.IP "int ev_version_major ()" 4

.IX Item "int ev_version_major ()"

.PD 0

.IP "int ev_version_minor ()" 4

.IX Item "int ev_version_minor ()"

.PD

You can find out the major and minor \s-1ABI\s0 version numbers of the library

you linked against by calling the functions \f(CW\*(C`ev_version_major\*(C'\fR and

\&\f(CW\*(C`ev_version_minor\*(C'\fR. If you want, you can compare against the global

symbols \f(CW\*(C`EV_VERSION_MAJOR\*(C'\fR and \f(CW\*(C`EV_VERSION_MINOR\*(C'\fR, which specify the

version of the library your program was compiled against.

.Sp

These version numbers refer to the \s-1ABI\s0 version of the library, not the

release version.

.Sp

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.

.Sp

Example: Make sure we haven't accidentally been linked against the wrong

version.

.Sp

.Vb 3

\&   assert (("libev version mismatch",

\&            ev_version_major () == EV_VERSION_MAJOR

\&            && ev_version_minor () >= EV_VERSION_MINOR));

.Ve

.IP "unsigned int ev_supported_backends ()" 4

.IX Item "unsigned int ev_supported_backends ()"

Return the set of all backends (i.e. their corresponding \f(CW\*(C`EV_BACKEND_*\*(C'\fR

value) compiled into this binary of libev (independent of their

availability on the system you are running on). See \f(CW\*(C`ev_default_loop\*(C'\fR for

a description of the set values.

.Sp

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

.Sp

.Vb 2

\&   assert (("sorry, no epoll, no sex",

\&            ev_supported_backends () & EVBACKEND_EPOLL));

.Ve

.IP "unsigned int ev_recommended_backends ()" 4

.IX Item "unsigned int ev_recommended_backends ()"

Return the set of all backends compiled into this binary of libev and also

recommended for this platform. This set is often smaller than the one

returned by \f(CW\*(C`ev_supported_backends\*(C'\fR, 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.

.IP "unsigned int ev_embeddable_backends ()" 4

.IX Item "unsigned int ev_embeddable_backends ()"

Returns the set of backends that are embeddable in other event loops. This

is the theoretical, all-platform, value. To find which backends

might be supported on the current system, you would need to look at

\&\f(CW\*(C`ev_embeddable_backends () & ev_supported_backends ()\*(C'\fR, likewise for

recommended ones.

.Sp

See the description of \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.

.IP "ev_set_allocator (void *(*cb)(void *ptr, long size)) [\s-1NOT\s0 \s-1REENTRANT\s0]" 4

.IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size)) [NOT REENTRANT]"

Sets the allocation function to use (the prototype is similar \- the

semantics are identical to the \f(CW\*(C`realloc\*(C'\fR 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 (\f(CW\*(C`size != 0\*(C'\fR), the library might abort

or take some potentially destructive action.

.Sp

Since some systems (at least OpenBSD and Darwin) fail to implement

correct \f(CW\*(C`realloc\*(C'\fR semantics, libev will use a wrapper around the system

\&\f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions by default.

.Sp

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.

.Sp

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

retries (example requires a standards-compliant \f(CW\*(C`realloc\*(C'\fR).

.Sp

.Vb 6

\&   static void *

\&   persistent_realloc (void *ptr, size_t size)

\&   {

\&     for (;;)

\&       {

\&         void *newptr = realloc (ptr, size);

\&

\&         if (newptr)

\&           return newptr;

\&

\&         sleep (60);

\&       }

\&   }

\&

\&   ...

\&   ev_set_allocator (persistent_realloc);

.Ve

.IP "ev_set_syserr_cb (void (*cb)(const char *msg)); [\s-1NOT\s0 \s-1REENTRANT\s0]" 4

.IX Item "ev_set_syserr_cb (void (*cb)(const char *msg)); [NOT REENTRANT]"

Set the callback function to call on a retryable system call error (such

as failed select, poll, epoll_wait). The message is a printable string

indicating the system call or subsystem causing the problem. If this

callback is set, then libev will expect it to remedy the situation, no

matter what, when it returns. That is, libev will generally retry the

requested operation, or, if the condition doesn't go away, do bad stuff

(such as abort).

.Sp

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

.Sp

.Vb 6

\&   static void

\&   fatal_error (const char *msg)

\&   {

\&     perror (msg);

\&     abort ();

\&   }

\&

\&   ...

\&   ev_set_syserr_cb (fatal_error);

.Ve

.SH "FUNCTIONS CONTROLLING THE EVENT LOOP"

.IX Header "FUNCTIONS CONTROLLING THE EVENT LOOP"

An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR (the \f(CW\*(C`struct\*(C'\fR

is \fInot\fR optional in this case, as there is also an \f(CW\*(C`ev_loop\*(C'\fR

\&\fIfunction\fR).

.PP

The library knows two types of such loops, the \fIdefault\fR loop, which

supports signals and child events, and dynamically created loops which do

not.

.IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4

.IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"

This will initialise the default event loop if it hasn't been initialised

yet and return it. If the default loop could not be initialised, returns

false. If it already was initialised it simply returns it (and ignores the

flags. If that is troubling you, check \f(CW\*(C`ev_backend ()\*(C'\fR afterwards).

.Sp

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

function.

.Sp

Note that this function is \fInot\fR thread-safe, so if you want to use it

from multiple threads, you have to lock (note also that this is unlikely,

as loops cannot be shared easily between threads anyway).

.Sp

The default loop is the only loop that can handle \f(CW\*(C`ev_signal\*(C'\fR and

\&\f(CW\*(C`ev_child\*(C'\fR watchers, and to do this, it always registers a handler

for \f(CW\*(C`SIGCHLD\*(C'\fR. If this is a problem for your application you can either

create a dynamic loop with \f(CW\*(C`ev_loop_new\*(C'\fR that doesn't do that, or you

can simply overwrite the \f(CW\*(C`SIGCHLD\*(C'\fR signal handler \fIafter\fR calling

\&\f(CW\*(C`ev_default_init\*(C'\fR.

.Sp

The flags argument can be used to specify special behaviour or specific

backends to use, and is usually specified as \f(CW0\fR (or \f(CW\*(C`EVFLAG_AUTO\*(C'\fR).

.Sp

The following flags are supported:

.RS 4

.ie n .IP """EVFLAG_AUTO""" 4

.el .IP "\f(CWEVFLAG_AUTO\fR" 4

.IX Item "EVFLAG_AUTO"

The default flags value. Use this if you have no clue (it's the right

thing, believe me).

.ie n .IP """EVFLAG_NOENV""" 4

.el .IP "\f(CWEVFLAG_NOENV\fR" 4

.IX Item "EVFLAG_NOENV"

If this flag bit is or'ed into the flag value (or the program runs setuid

or setgid) then libev will \fInot\fR look at the environment variable

\&\f(CW\*(C`LIBEV_FLAGS\*(C'\fR. Otherwise (the default), this environment variable will

override the flags completely if it is found in the environment. This is

useful to try out specific backends to test their performance, or to work

around bugs.

.ie n .IP """EVFLAG_FORKCHECK""" 4

.el .IP "\f(CWEVFLAG_FORKCHECK\fR" 4

.IX Item "EVFLAG_FORKCHECK"

Instead of calling \f(CW\*(C`ev_default_fork\*(C'\fR or \f(CW\*(C`ev_loop_fork\*(C'\fR manually after

a fork, you can also make libev check for a fork in each iteration by

enabling this flag.

.Sp

This works by calling \f(CW\*(C`getpid ()\*(C'\fR 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, \f(CW\*(C`getpid\*(C'\fR is actually a simple 5\-insn sequence

without a system call and thus \fIvery\fR fast, but my GNU/Linux system also has

\&\f(CW\*(C`pthread_atfork\*(C'\fR which is even faster).

.Sp

The big advantage of this flag is that you can forget about fork (and

forget about forgetting to tell libev about forking) when you use this

flag.

.Sp

This flag setting cannot be overridden or specified in the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR

environment variable.

.ie n .IP """EVBACKEND_SELECT""  (value 1, portable select backend)" 4

.el .IP "\f(CWEVBACKEND_SELECT\fR  (value 1, portable select backend)" 4

.IX Item "EVBACKEND_SELECT  (value 1, portable select backend)"

This is your standard \fIselect\fR\|(2) backend. Not \fIcompletely\fR 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.

.Sp

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 \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many

connections as possible during one iteration. You might also want to have

a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of

readiness notifications you get per iteration.

.Sp

This backend maps \f(CW\*(C`EV_READ\*(C'\fR to the \f(CW\*(C`readfds\*(C'\fR set and \f(CW\*(C`EV_WRITE\*(C'\fR to the

\&\f(CW\*(C`writefds\*(C'\fR set (and to work around Microsoft Windows bugs, also onto the

\&\f(CW\*(C`exceptfds\*(C'\fR set on that platform).

.ie n .IP """EVBACKEND_POLL""    (value 2, poll backend, available everywhere except on windows)" 4

.el .IP "\f(CWEVBACKEND_POLL\fR    (value 2, poll backend, available everywhere except on windows)" 4

.IX Item "EVBACKEND_POLL    (value 2, poll backend, available everywhere except on windows)"

And this is your standard \fIpoll\fR\|(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 \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for

performance tips.

.Sp

This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLLHUP\*(C'\fR, and

\&\f(CW\*(C`EV_WRITE\*(C'\fR to \f(CW\*(C`POLLOUT | POLLERR | POLLHUP\*(C'\fR.

.ie n .IP """EVBACKEND_EPOLL""   (value 4, Linux)" 4

.el .IP "\f(CWEVBACKEND_EPOLL\fR   (value 4, Linux)" 4

.IX Item "EVBACKEND_EPOLL   (value 4, Linux)"

For few fds, this backend is a bit little slower than poll and select,

but it scales phenomenally better. While poll and select usually scale

like O(total_fds) where n is the total number of fds (or the highest fd),

epoll scales either O(1) or O(active_fds).

.Sp

The epoll mechanism deserves honorable mention as the most misdesigned

of the more advanced event mechanisms: mere annoyances include silently

dropping file descriptors, requiring a system call per change per file

descriptor (and unnecessary guessing of parameters), problems with dup and

so on. The biggest issue is fork races, however \- if a program forks then

\&\fIboth\fR 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.

.Sp

Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work, but

of course \fIdoesn't\fR, and epoll just loves to report events for totally

\&\fIdifferent\fR file descriptors (even already closed ones, so one cannot

even remove them from the set) than registered in the set (especially

on \s-1SMP\s0 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.

.Sp

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 \fIfile descriptor\fR could point to a different

\&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed

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

file descriptors.

.Sp

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.

.Sp

All this means that, in practice, \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR can be as fast or

faster than epoll for maybe up to a hundred file descriptors, depending on

the usage. So sad.

.Sp

While nominally embeddable in other event loops, this feature is broken in

all kernel versions tested so far.

.Sp

This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as

\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.

.ie n .IP """EVBACKEND_KQUEUE""  (value 8, most \s-1BSD\s0 clones)" 4

.el .IP "\f(CWEVBACKEND_KQUEUE\fR  (value 8, most \s-1BSD\s0 clones)" 4

.IX Item "EVBACKEND_KQUEUE  (value 8, most BSD clones)"

Kqueue deserves special mention, as at the time of this writing, it

was broken on all BSDs except NetBSD (usually it doesn't work reliably

with anything but sockets and pipes, except on Darwin, where of course

it's completely useless). Unlike epoll, however, whose brokenness

is by design, these kqueue bugs can (and eventually will) be fixed

without \s-1API\s0 changes to existing programs. For this reason it's not being

\&\*(L"auto-detected\*(R" unless you explicitly specify it in the flags (i.e. using

\&\f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a known-to-be-good (\-enough)

system like NetBSD.

.Sp

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 \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.

.Sp

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 \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to

two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (but

sane, unlike epoll) and it drops fds silently in similarly hard-to-detect

cases

.Sp

This backend usually performs well under most conditions.

.Sp

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. \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR (but \f(CW\*(C`poll\*(C'\fR is of course

also broken on \s-1OS\s0 X)) and, did I mention it, using it only for sockets.

.Sp

This backend maps \f(CW\*(C`EV_READ\*(C'\fR into an \f(CW\*(C`EVFILT_READ\*(C'\fR kevent with

\&\f(CW\*(C`NOTE_EOF\*(C'\fR, and \f(CW\*(C`EV_WRITE\*(C'\fR into an \f(CW\*(C`EVFILT_WRITE\*(C'\fR kevent with

\&\f(CW\*(C`NOTE_EOF\*(C'\fR.

.ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4

.el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4

.IX Item "EVBACKEND_DEVPOLL (value 16, Solaris 8)"

This is not implemented yet (and might never be, unless you send me an

implementation). According to reports, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets

and is not embeddable, which would limit the usefulness of this backend

immensely.

.ie n .IP """EVBACKEND_PORT""    (value 32, Solaris 10)" 4

.el .IP "\f(CWEVBACKEND_PORT\fR    (value 32, Solaris 10)" 4

.IX Item "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)).

.Sp

Please note that Solaris event ports can deliver a lot of spurious

notifications, so you need to use non-blocking I/O or other means to avoid

blocking when no data (or space) is available.

.Sp

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 \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend

might perform better.

.Sp

On the positive side, with the exception of the spurious readiness

notifications, this backend actually performed fully to specification

in all tests and is fully embeddable, which is a rare feat among the

OS-specific backends (I vastly prefer correctness over speed hacks).

.Sp

This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as

\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.

.ie n .IP """EVBACKEND_ALL""" 4

.el .IP "\f(CWEVBACKEND_ALL\fR" 4

.IX Item "EVBACKEND_ALL"

Try all backends (even potentially broken ones that wouldn't be tried

with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as

\&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR.

.Sp

It is definitely not recommended to use this flag.

.RE

.RS 4

.Sp

If one or more of these are or'ed into the flags value, then only these

backends will be tried (in the reverse order as listed here). If none are

specified, all backends in \f(CW\*(C`ev_recommended_backends ()\*(C'\fR will be tried.

.Sp

Example: This is the most typical usage.

.Sp

.Vb 2

\&   if (!ev_default_loop (0))

\&     fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");

.Ve

.Sp

Example: Restrict libev to the select and poll backends, and do not allow

environment settings to be taken into account:

.Sp

.Vb 1

\&   ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);

.Ve

.Sp

Example: Use whatever libev has to offer, but make sure that kqueue is

used if available (warning, breaks stuff, best use only with your own

private event loop and only if you know the \s-1OS\s0 supports your types of

fds):

.Sp

.Vb 1

\&   ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);

.Ve

.RE

.IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4

.IX Item "struct ev_loop *ev_loop_new (unsigned int flags)"

Similar to \f(CW\*(C`ev_default_loop\*(C'\fR, but always creates a new event loop that is

always distinct from the default loop. Unlike the default loop, it cannot

handle signal and child watchers, and attempts to do so will be greeted by

undefined behaviour (or a failed assertion if assertions are enabled).

.Sp

Note that this function \fIis\fR thread-safe, and the recommended way to use

libev with threads is indeed to create one loop per thread, and using the

default loop in the \*(L"main\*(R" or \*(L"initial\*(R" thread.

.Sp

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

.Sp

.Vb 3

\&   struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);

\&   if (!epoller)

\&     fatal ("no epoll found here, maybe it hides under your chair");

.Ve

.IP "ev_default_destroy ()" 4

.IX Item "ev_default_destroy ()"

Destroys the default loop again (frees all memory and kernel state

etc.). None of the active event watchers will be stopped in the normal

sense, so e.g. \f(CW\*(C`ev_is_active\*(C'\fR might still return true. It is your

responsibility to either stop all watchers cleanly yourself \fIbefore\fR

calling this function, or cope with the fact afterwards (which is usually

the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them

for example).

.Sp

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.

.Sp

In general it is not advisable to call this function except in the

rare occasion where you really need to free e.g. the signal handling

pipe fds. If you need dynamically allocated loops it is better to use

\&\f(CW\*(C`ev_loop_new\*(C'\fR and \f(CW\*(C`ev_loop_destroy\*(C'\fR).

.IP "ev_loop_destroy (loop)" 4

.IX Item "ev_loop_destroy (loop)"

Like \f(CW\*(C`ev_default_destroy\*(C'\fR, but destroys an event loop created by an

earlier call to \f(CW\*(C`ev_loop_new\*(C'\fR.

.IP "ev_default_fork ()" 4

.IX Item "ev_default_fork ()"

This function sets a flag that causes subsequent \f(CW\*(C`ev_loop\*(C'\fR iterations

to reinitialise the kernel state for backends that have one. Despite the

name, you can call it anytime, but it makes most sense after forking, in

the child process (or both child and parent, but that again makes little

sense). You \fImust\fR call it in the child before using any of the libev

functions, and it will only take effect at the next \f(CW\*(C`ev_loop\*(C'\fR iteration.

.Sp

On the other hand, you only need to call this function in the child

process if and only if you want to use the event library in the child. If

you just fork+exec, you don't have to call it at all.

.Sp

The function itself is quite fast and it's usually not a problem to call

it just in case after a fork. To make this easy, the function will fit in

quite nicely into a call to \f(CW\*(C`pthread_atfork\*(C'\fR:

.Sp

.Vb 1

\&    pthread_atfork (0, 0, ev_default_fork);

.Ve

.IP "ev_loop_fork (loop)" 4

.IX Item "ev_loop_fork (loop)"

Like \f(CW\*(C`ev_default_fork\*(C'\fR, but acts on an event loop created by

\&\f(CW\*(C`ev_loop_new\*(C'\fR. Yes, you have to call this on every allocated event loop

after fork that you want to re-use in the child, and how you do this is

entirely your own problem.

.IP "int ev_is_default_loop (loop)" 4

.IX Item "int ev_is_default_loop (loop)"

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

otherwise.

.IP "unsigned int ev_loop_count (loop)" 4

.IX Item "unsigned int ev_loop_count (loop)"

Returns the count of loop iterations for the loop, which is identical to

the number of times libev did poll for new events. It starts at \f(CW0\fR and

happily wraps around with enough iterations.

.Sp

This value can sometimes be useful as a generation counter of sorts (it

\&\*(L"ticks\*(R" the number of loop iterations), as it roughly corresponds with

\&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls.

.IP "unsigned int ev_backend (loop)" 4

.IX Item "unsigned int ev_backend (loop)"

Returns one of the \f(CW\*(C`EVBACKEND_*\*(C'\fR flags indicating the event backend in

use.

.IP "ev_tstamp ev_now (loop)" 4

.IX Item "ev_tstamp ev_now (loop)"

Returns the current \*(L"event loop time\*(R", 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).

.IP "ev_now_update (loop)" 4

.IX Item "ev_now_update (loop)"

Establishes the current time by querying the kernel, updating the time

returned by \f(CW\*(C`ev_now ()\*(C'\fR in the progress. This is a costly operation and

is usually done automatically within \f(CW\*(C`ev_loop ()\*(C'\fR.

.Sp

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.

.Sp

See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.

.IP "ev_loop (loop, int flags)" 4

.IX Item "ev_loop (loop, int flags)"

Finally, this is it, the event handler. This function usually is called

after you initialised all your watchers and you want to start handling

events.

.Sp

If the flags argument is specified as \f(CW0\fR, it will not return until

either no event watchers are active anymore or \f(CW\*(C`ev_unloop\*(C'\fR was called.

.Sp

Please note that an explicit \f(CW\*(C`ev_unloop\*(C'\fR 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.

.Sp

A flags value of \f(CW\*(C`EVLOOP_NONBLOCK\*(C'\fR will look for new events, will handle

those events and any already outstanding ones, but will not block your

process in case there are no events and will return after one iteration of

the loop.

.Sp

A flags value of \f(CW\*(C`EVLOOP_ONESHOT\*(C'\fR 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.

.Sp

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 \f(CW\*(C`ev_loop\*(C'\fR"). However, a pair of \f(CW\*(C`ev_prepare\*(C'\fR/\f(CW\*(C`ev_check\*(C'\fR watchers is

usually a better approach for this kind of thing.

.Sp

Here are the gory details of what \f(CW\*(C`ev_loop\*(C'\fR does:

.Sp

.Vb 10

\&   \- Before the first iteration, call any pending watchers.

\&   * If EVFLAG_FORKCHECK was used, check for a fork.

\&   \- If a fork was detected (by any means), queue and call all fork watchers.

\&   \- Queue and call all prepare watchers.

\&   \- If we have been forked, detach and recreate the kernel state

\&     as to not disturb the other process.

\&   \- Update the kernel state with all outstanding changes.

\&   \- Update the "event loop time" (ev_now ()).

\&   \- Calculate for how long to sleep or block, if at all

\&     (active idle watchers, EVLOOP_NONBLOCK or not having

\&     any active watchers at all will result in not sleeping).

\&   \- Sleep if the I/O and timer collect interval say so.

\&   \- Block the process, waiting for any events.

\&   \- Queue all outstanding I/O (fd) events.

\&   \- Update the "event loop time" (ev_now ()), and do time jump adjustments.

\&   \- Queue all expired timers.

\&   \- Queue all expired periodics.

\&   \- Unless any events are pending now, queue all idle watchers.

\&   \- Queue all check watchers.

\&   \- Call all queued watchers in reverse order (i.e. check watchers first).

\&     Signals and child watchers are implemented as I/O watchers, and will

\&     be handled here by queueing them when their watcher gets executed.

\&   \- If ev_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK

\&     were used, or there are no active watchers, return, otherwise

\&     continue with step *.

.Ve

.Sp

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

anymore.

.Sp

.Vb 4

\&   ... queue jobs here, make sure they register event watchers as long

\&   ... as they still have work to do (even an idle watcher will do..)

\&   ev_loop (my_loop, 0);

\&   ... jobs done or somebody called unloop. yeah!

.Ve

.IP "ev_unloop (loop, how)" 4

.IX Item "ev_unloop (loop, how)"

Can be used to make a call to \f(CW\*(C`ev_loop\*(C'\fR return early (but only after it

has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either

\&\f(CW\*(C`EVUNLOOP_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_loop\*(C'\fR call return, or

\&\f(CW\*(C`EVUNLOOP_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_loop\*(C'\fR calls return.

.Sp

This \*(L"unloop state\*(R" will be cleared when entering \f(CW\*(C`ev_loop\*(C'\fR again.

.Sp

It is safe to call \f(CW\*(C`ev_unloop\*(C'\fR from otuside any \f(CW\*(C`ev_loop\*(C'\fR calls.

.IP "ev_ref (loop)" 4

.IX Item "ev_ref (loop)"

.PD 0

.IP "ev_unref (loop)" 4

.IX Item "ev_unref (loop)"

.PD

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, \f(CW\*(C`ev_loop\*(C'\fR will not return on its own.

.Sp

If you have a watcher you never unregister that should not keep \f(CW\*(C`ev_loop\*(C'\fR

from returning, call \fIev_unref()\fR after starting, and \fIev_ref()\fR before

stopping it.

.Sp

As an example, libev itself uses this for its internal signal pipe: It is

not visible to the libev user and should not keep \f(CW\*(C`ev_loop\*(C'\fR 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 \fIunref after start\fR and \fIref before stop\fR

(but only if the watcher wasn't active before, or was active before,

respectively).

.Sp

Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_loop\*(C'\fR

running when nothing else is active.

.Sp

.Vb 4

\&   ev_signal exitsig;

\&   ev_signal_init (&exitsig, sig_cb, SIGINT);

\&   ev_signal_start (loop, &exitsig);

\&   evf_unref (loop);

.Ve

.Sp

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

.Sp

.Vb 2

\&   ev_ref (loop);

\&   ev_signal_stop (loop, &exitsig);

.Ve

.IP "ev_set_io_collect_interval (loop, ev_tstamp interval)" 4

.IX Item "ev_set_io_collect_interval (loop, ev_tstamp interval)"

.PD 0

.IP "ev_set_timeout_collect_interval (loop, ev_tstamp interval)" 4

.IX Item "ev_set_timeout_collect_interval (loop, ev_tstamp interval)"

.PD

These advanced functions influence the time that libev will spend waiting

for events. Both time intervals are by default \f(CW0\fR, meaning that libev

will try to invoke timer/periodic callbacks and I/O callbacks with minimum

latency.

.Sp

Setting these to a higher value (the \f(CW\*(C`interval\*(C'\fR \fImust\fR be >= \f(CW0\fR)

allows libev to delay invocation of I/O and timer/periodic callbacks

to increase efficiency of loop iterations (or to increase power-saving

opportunities).

.Sp

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 \s-1CPU\s0 time to poll for new

events, especially with backends like \f(CW\*(C`select ()\*(C'\fR which have a high

overhead for the actual polling but can deliver many events at once.

.Sp

By setting a higher \fIio collect interval\fR 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 \f(CW\*(C`ev_periodic\*(C'\fR and

\&\f(CW\*(C`ev_timer\*(C'\fR) will be not affected. Setting this to a non-null value will

introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations.

.Sp

Likewise, by setting a higher \fItimeout collect interval\fR you allow libev

to spend more time collecting timeouts, at the expense of increased

latency/jitter/inexactness (the watcher callback will be called

later). \f(CW\*(C`ev_io\*(C'\fR watchers will not be affected. Setting this to a non-null

value will not introduce any overhead in libev.

.Sp

Many (busy) programs can usually benefit by setting the I/O collect

interval to a value near \f(CW0.1\fR 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 \f(CW0.01\fR,

as this approaches the timing granularity of most systems.

.Sp

Setting the \fItimeout collect interval\fR can improve the opportunity for

saving power, as the program will \*(L"bundle\*(R" timer callback invocations that

are \*(L"near\*(R" 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 \f(CW\*(C`ev_periodic\*(C'\fR watchers and make sure

they fire on, say, one-second boundaries only.

.IP "ev_loop_verify (loop)" 4

.IX Item "ev_loop_verify (loop)"

This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR 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 \f(CW\*(C`abort ()\*(C'\fR.

.Sp

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.

.SH "ANATOMY OF A WATCHER"

.IX Header "ANATOMY OF A WATCHER"

In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the

watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer

watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.

.PP

A watcher is a structure that you create and register to record your

interest in some event. For instance, if you want to wait for \s-1STDIN\s0 to

become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher for that:

.PP

.Vb 5

\&   static void my_cb (struct ev_loop *loop, ev_io *w, int revents)

\&   {

\&     ev_io_stop (w);

\&     ev_unloop (loop, EVUNLOOP_ALL);

\&   }

\&

\&   struct ev_loop *loop = ev_default_loop (0);

\&

\&   ev_io stdin_watcher;

\&

\&   ev_init (&stdin_watcher, my_cb);

\&   ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);

\&   ev_io_start (loop, &stdin_watcher);

\&

\&   ev_loop (loop, 0);

.Ve

.PP

As you can see, you are responsible for allocating the memory for your

watcher structures (and it is \fIusually\fR a bad idea to do this on the

stack).

.PP

Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR

or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).

.PP

Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init

(watcher *, callback)\*(C'\fR, which expects a callback to be provided. This

callback gets invoked each time the event occurs (or, in the case of I/O

watchers, each time the event loop detects that the file descriptor given

is readable and/or writable).

.PP

Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR

macro to configure it, with arguments specific to the watcher type. There

is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.

.PP

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

with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher

*)\*(C'\fR), and you can stop watching for events at any time by calling the

corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.

.PP

As long as your watcher is active (has been started but not stopped) you

must not touch the values stored in it. Most specifically you must never

reinitialise it or call its \f(CW\*(C`ev_TYPE_set\*(C'\fR macro.

.PP

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.

.PP

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:

.ie n .IP """EV_READ""" 4

.el .IP "\f(CWEV_READ\fR" 4

.IX Item "EV_READ"

.PD 0

.ie n .IP """EV_WRITE""" 4

.el .IP "\f(CWEV_WRITE\fR" 4

.IX Item "EV_WRITE"

.PD

The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or

writable.

.ie n .IP """EV_TIMEOUT""" 4

.el .IP "\f(CWEV_TIMEOUT\fR" 4

.IX Item "EV_TIMEOUT"

The \f(CW\*(C`ev_timer\*(C'\fR watcher has timed out.

.ie n .IP """EV_PERIODIC""" 4

.el .IP "\f(CWEV_PERIODIC\fR" 4

.IX Item "EV_PERIODIC"

The \f(CW\*(C`ev_periodic\*(C'\fR watcher has timed out.

.ie n .IP """EV_SIGNAL""" 4

.el .IP "\f(CWEV_SIGNAL\fR" 4

.IX Item "EV_SIGNAL"

The signal specified in the \f(CW\*(C`ev_signal\*(C'\fR watcher has been received by a thread.

.ie n .IP """EV_CHILD""" 4

.el .IP "\f(CWEV_CHILD\fR" 4

.IX Item "EV_CHILD"

The pid specified in the \f(CW\*(C`ev_child\*(C'\fR watcher has received a status change.

.ie n .IP """EV_STAT""" 4

.el .IP "\f(CWEV_STAT\fR" 4

.IX Item "EV_STAT"

The path specified in the \f(CW\*(C`ev_stat\*(C'\fR watcher changed its attributes somehow.

.ie n .IP """EV_IDLE""" 4

.el .IP "\f(CWEV_IDLE\fR" 4

.IX Item "EV_IDLE"

The \f(CW\*(C`ev_idle\*(C'\fR watcher has determined that you have nothing better to do.

.ie n .IP """EV_PREPARE""" 4

.el .IP "\f(CWEV_PREPARE\fR" 4

.IX Item "EV_PREPARE"

.PD 0

.ie n .IP """EV_CHECK""" 4

.el .IP "\f(CWEV_CHECK\fR" 4

.IX Item "EV_CHECK"

.PD

All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_loop\*(C'\fR starts

to gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are invoked just after

\&\f(CW\*(C`ev_loop\*(C'\fR has gathered them, but before it invokes any callbacks for any

received events. Callbacks of both watcher types can start and stop as

many watchers as they want, and all of them will be taken into account

(for example, a \f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep

\&\f(CW\*(C`ev_loop\*(C'\fR from blocking).

.ie n .IP """EV_EMBED""" 4

.el .IP "\f(CWEV_EMBED\fR" 4

.IX Item "EV_EMBED"

The embedded event loop specified in the \f(CW\*(C`ev_embed\*(C'\fR watcher needs attention.

.ie n .IP """EV_FORK""" 4

.el .IP "\f(CWEV_FORK\fR" 4

.IX Item "EV_FORK"

The event loop has been resumed in the child process after fork (see

\&\f(CW\*(C`ev_fork\*(C'\fR).

.ie n .IP """EV_ASYNC""" 4

.el .IP "\f(CWEV_ASYNC\fR" 4

.IX Item "EV_ASYNC"

The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).

.ie n .IP """EV_ERROR""" 4

.el .IP "\f(CWEV_ERROR\fR" 4

.IX Item "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.

.Sp

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.

.Sp

Libev will usually signal a few \*(L"dummy\*(R" 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 \fIread()\fR or \fIwrite()\fR. This will not work in multi-threaded

programs, though, as the fd could already be closed and reused for another

thing, so beware.

.Sh "\s-1GENERIC\s0 \s-1WATCHER\s0 \s-1FUNCTIONS\s0"

.IX Subsection "GENERIC WATCHER FUNCTIONS"

.ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4

.el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4

.IX Item "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 \f(CW\*(C`malloc\*(C'\fR will do). Only

the generic parts of the watcher are initialised, you \fIneed\fR to call

the type-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR macro afterwards to initialise the

type-specific parts. For each type there is also a \f(CW\*(C`ev_TYPE_init\*(C'\fR macro

which rolls both calls into one.

.Sp

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.

.Sp

The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,

int revents)\*(C'\fR.

.Sp

Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.

.Sp

.Vb 3

\&   ev_io w;

\&   ev_init (&w, my_cb);

\&   ev_io_set (&w, STDIN_FILENO, EV_READ);

.Ve

.ie n .IP """ev_TYPE_set"" (ev_TYPE *, [args])" 4

.el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *, [args])" 4

.IX Item "ev_TYPE_set (ev_TYPE *, [args])"

This macro initialises the type-specific parts of a watcher. You need to

call \f(CW\*(C`ev_init\*(C'\fR at least once before you call this macro, but you can

call \f(CW\*(C`ev_TYPE_set\*(C'\fR 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 \f(CW\*(C`ev_init\*(C'\fR macro).

.Sp

Although some watcher types do not have type-specific arguments

(e.g. \f(CW\*(C`ev_prepare\*(C'\fR) you still need to call its \f(CW\*(C`set\*(C'\fR macro.

.Sp

See \f(CW\*(C`ev_init\*(C'\fR, above, for an example.

.ie n .IP """ev_TYPE_init"" (ev_TYPE *watcher, callback, [args])" 4

.el .IP "\f(CWev_TYPE_init\fR (ev_TYPE *watcher, callback, [args])" 4

.IX Item "ev_TYPE_init (ev_TYPE *watcher, callback, [args])"

This convenience macro rolls both \f(CW\*(C`ev_init\*(C'\fR and \f(CW\*(C`ev_TYPE_set\*(C'\fR macro

calls into a single call. This is the most convenient method to initialise

a watcher. The same limitations apply, of course.

.Sp

Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.

.Sp

.Vb 1

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

.Ve

.ie n .IP """ev_TYPE_start"" (loop *, ev_TYPE *watcher)" 4

.el .IP "\f(CWev_TYPE_start\fR (loop *, ev_TYPE *watcher)" 4

.IX Item "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.

.Sp

Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this

whole section.

.Sp

.Vb 1

\&   ev_io_start (EV_DEFAULT_UC, &w);

.Ve

.ie n .IP """ev_TYPE_stop"" (loop *, ev_TYPE *watcher)" 4

.el .IP "\f(CWev_TYPE_stop\fR (loop *, ev_TYPE *watcher)" 4

.IX Item "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).

.Sp

It is possible that stopped watchers are pending \- for example,

non-repeating timers are being stopped when they become pending \- but

calling \f(CW\*(C`ev_TYPE_stop\*(C'\fR 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 \f(CW\*(C`ev_TYPE_stop\*(C'\fR function.

.IP "bool ev_is_active (ev_TYPE *watcher)" 4

.IX Item "bool ev_is_active (ev_TYPE *watcher)"

Returns a true value iff the watcher is active (i.e. it has been started

and not yet been stopped). As long as a watcher is active you must not modify

it.

.IP "bool ev_is_pending (ev_TYPE *watcher)" 4

.IX Item "bool ev_is_pending (ev_TYPE *watcher)"

Returns a true value iff the watcher is pending, (i.e. it has outstanding

events but its callback has not yet been invoked). As long as a watcher

is pending (but not active) you must not call an init function on it (but

\&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must

make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR

it).

.IP "callback ev_cb (ev_TYPE *watcher)" 4

.IX Item "callback ev_cb (ev_TYPE *watcher)"

Returns the callback currently set on the watcher.

.IP "ev_cb_set (ev_TYPE *watcher, callback)" 4

.IX Item "ev_cb_set (ev_TYPE *watcher, callback)"

Change the callback. You can change the callback at virtually any time

(modulo threads).

.IP "ev_set_priority (ev_TYPE *watcher, priority)" 4

.IX Item "ev_set_priority (ev_TYPE *watcher, priority)"

.PD 0

.IP "int ev_priority (ev_TYPE *watcher)" 4

.IX Item "int ev_priority (ev_TYPE *watcher)"

.PD

Set and query the priority of the watcher. The priority is a small

integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR

(default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked

before watchers with lower priority, but priority will not keep watchers

from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).

.Sp

This means that priorities are \fIonly\fR used for ordering callback

invocation after new events have been received. This is useful, for

example, to reduce latency after idling, or more often, to bind two

watchers on the same event and make sure one is called first.

.Sp

If you need to suppress invocation when higher priority events are pending

you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.

.Sp

You \fImust not\fR change the priority of a watcher as long as it is active or

pending.

.Sp

The default priority used by watchers when no priority has been set is

always \f(CW0\fR, which is supposed to not be too high and not be too low :).

.Sp

Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR 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.

.IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4

.IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"

Invoke the \f(CW\*(C`watcher\*(C'\fR with the given \f(CW\*(C`loop\*(C'\fR and \f(CW\*(C`revents\*(C'\fR. Neither

\&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback

can deal with that fact, as both are simply passed through to the

callback.

.IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4

.IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"

If the watcher is pending, this function clears its pending status and

returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the

watcher isn't pending it does nothing and returns \f(CW0\fR.

.Sp

Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its

callback to be invoked, which can be accomplished with this function.

.Sh "\s-1ASSOCIATING\s0 \s-1CUSTOM\s0 \s-1DATA\s0 \s-1WITH\s0 A \s-1WATCHER\s0"

.IX Subsection "ASSOCIATING CUSTOM DATA WITH A WATCHER"

Each watcher has, by default, a member \f(CW\*(C`void *data\*(C'\fR that you can change

and read at any time: libev will completely ignore it. This can be used

to associate arbitrary data with your watcher. If you need more data and

don't want to allocate memory and store a pointer to it in that data

member, you can also \*(L"subclass\*(R" the watcher type and provide your own

data:

.PP

.Vb 7

\&   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);

.Ve

.PP

And since your callback will be called with a pointer to the watcher, you

can cast it back to your own type:

.PP

.Vb 5

\&   static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)

\&   {

\&     struct my_io *w = (struct my_io *)w_;

\&     ...

\&   }

.Ve

.PP

More interesting and less C\-conformant ways of casting your callback type

instead have been omitted.

.PP

Another common scenario is to use some data structure with multiple

embedded watchers:

.PP

.Vb 6

\&   struct my_biggy

\&   {

\&     int some_data;

\&     ev_timer t1;

\&     ev_timer t2;

\&   }

.Ve

.PP

In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more

complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct

in the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies), or you need to use

some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for real

programmers):

.PP

.Vb 1

\&   #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));

\&   }

.Ve

.SH "WATCHER TYPES"

.IX Header "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.

.PP

Members are additionally marked with either \fI[read\-only]\fR, 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 \fI[read\-write]\fR, which

means you can expect it to have some sensible content while the watcher

is active, but you can also modify it. Modifying it may not do something

sensible or take immediate effect (or do anything at all), but libev will

not crash or malfunction in any way.

.ie n .Sh """ev_io"" \- is this file descriptor readable or writable?"

.el .Sh "\f(CWev_io\fP \- is this file descriptor readable or writable?"

.IX Subsection "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.

.PP

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).

.PP

If you cannot use non-blocking mode, then force the use of a

known-to-be-good backend (at the time of this writing, this includes only

\&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR and \f(CW\*(C`EVBACKEND_POLL\*(C'\fR).

.PP

Another thing you have to watch out for is that it is quite easy to

receive \*(L"spurious\*(R" readiness notifications, that is your callback might

be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block

because there is no data. Not only are some backends known to create a

lot of those (for example Solaris ports), it is very easy to get into

this situation even with a relatively standard program structure. Thus

it is best to always use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning

\&\f(CW\*(C`EAGAIN\*(C'\fR is far preferable to a program hanging until some data arrives.

.PP

If you cannot run the fd in non-blocking mode (for example you should

not play around with an Xlib connection), then you have to separately

re-test whether a file descriptor is really ready with a known-to-be good

interface such as poll (fortunately in our Xlib example, Xlib already

does this on its own, so its quite safe to use). Some people additionally

use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block

indefinitely.

.PP

But really, best use non-blocking mode.

.PP

\fIThe special problem of disappearing file descriptors\fR

.IX Subsection "The special problem of disappearing file descriptors"

.PP

Some backends (e.g. kqueue, epoll) need to be told about closing a file

descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other means,

such as \f(CW\*(C`dup2\*(C'\fR). 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.

.PP

To avoid having to explicitly tell libev about such cases, libev follows

the following policy:  Each time \f(CW\*(C`ev_io_set\*(C'\fR 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 \fIhave\fR to call \f(CW\*(C`ev_io_set\*(C'\fR (or \f(CW\*(C`ev_io_init\*(C'\fR) when you change the

descriptor even if the file descriptor number itself did not change.

.PP

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.

.PP

\fIThe special problem of dup'ed file descriptors\fR

.IX Subsection "The special problem of dup'ed file descriptors"

.PP

Some backends (e.g. epoll), cannot register events for file descriptors,

but only events for the underlying file descriptions. That means when you

have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register

events for them, only one file descriptor might actually receive events.

.PP

There is no workaround possible except not registering events

for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to

\&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.

.PP

\fIThe special problem of fork\fR

.IX Subsection "The special problem of fork"

.PP

Some backends (epoll, kqueue) do not support \f(CW\*(C`fork ()\*(C'\fR at all or exhibit

useless behaviour. Libev fully supports fork, but needs to be told about

it in the child.

.PP

To support fork in your programs, you either have to call

\&\f(CW\*(C`ev_default_fork ()\*(C'\fR or \f(CW\*(C`ev_loop_fork ()\*(C'\fR after a fork in the child,

enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or

\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.

.PP

\fIThe special problem of \s-1SIGPIPE\s0\fR

.IX Subsection "The special problem of SIGPIPE"

.PP

While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:

when writing to a pipe whose other end has been closed, your program gets

sent a \s-1SIGPIPE\s0, which, by default, aborts your program. For most programs

this is sensible behaviour, for daemons, this is usually undesirable.

.PP

So when you encounter spurious, unexplained daemon exits, make sure you

ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon

somewhere, as that would have given you a big clue).

.PP

\fIWatcher-Specific Functions\fR

.IX Subsection "Watcher-Specific Functions"

.IP "ev_io_init (ev_io *, callback, int fd, int events)" 4

.IX Item "ev_io_init (ev_io *, callback, int fd, int events)"

.PD 0

.IP "ev_io_set (ev_io *, int fd, int events)" 4

.IX Item "ev_io_set (ev_io *, int fd, int events)"

.PD

Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to

receive events for and \f(CW\*(C`events\*(C'\fR is either \f(CW\*(C`EV_READ\*(C'\fR, \f(CW\*(C`EV_WRITE\*(C'\fR or

\&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR, to express the desire to receive the given events.

.IP "int fd [read\-only]" 4

.IX Item "int fd [read-only]"

The file descriptor being watched.

.IP "int events [read\-only]" 4

.IX Item "int events [read-only]"

The events being watched.

.PP

\fIExamples\fR

.IX Subsection "Examples"

.PP

Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well

readable, but only once. Since it is likely line-buffered, you could

attempt to read a whole line in the callback.

.PP

.Vb 6

\&   static void

\&   stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)

\&   {

\&      ev_io_stop (loop, w);

\&     .. read from stdin here (or from w\->fd) and handle any I/O errors

\&   }

\&

\&   ...

\&   struct ev_loop *loop = ev_default_init (0);

\&   ev_io stdin_readable;

\&   ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);

\&   ev_io_start (loop, &stdin_readable);

\&   ev_loop (loop, 0);

.Ve

.ie n .Sh """ev_timer"" \- relative and optionally repeating timeouts"

.el .Sh "\f(CWev_timer\fP \- relative and optionally repeating timeouts"

.IX Subsection "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.

.PP

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. \*(L"Roughly\*(R" because

detecting time jumps is hard, and some inaccuracies are unavoidable (the

monotonic clock option helps a lot here).

.PP

The callback is guaranteed to be invoked only \fIafter\fR its timeout has

passed, but if multiple timers become ready during the same loop iteration

then order of execution is undefined.

.PP

\fIBe smart about timeouts\fR

.IX Subsection "Be smart about timeouts"

.PP

Many real-world problems involve some kind of timeout, usually for error

recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,

you want to raise some error after a while.

.PP

What follows are some ways to handle this problem, from obvious and

inefficient to smart and efficient.

.PP

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).

.IP "1. Use a timer and stop, reinitialise and start it on activity." 4

.IX Item "1. Use a timer and stop, reinitialise and start it on activity."

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

start the watcher:

.Sp

.Vb 2

\&   ev_timer_init (timer, callback, 60., 0.);

\&   ev_timer_start (loop, timer);

.Ve

.Sp

Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it

and start it again:

.Sp

.Vb 3

\&   ev_timer_stop (loop, timer);

\&   ev_timer_set (timer, 60., 0.);

\&   ev_timer_start (loop, timer);

.Ve

.Sp

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.

.ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4

.el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4

.IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."

This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of

\&\f(CW\*(C`ev_timer_start\*(C'\fR.

.Sp

To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value

of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR 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 \f(CW\*(C`ev_timer_stop\*(C'\fR

the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.

.Sp

That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the

\&\f(CW\*(C`after\*(C'\fR argument to \f(CW\*(C`ev_timer_set\*(C'\fR, and only ever use the \f(CW\*(C`repeat\*(C'\fR

member and \f(CW\*(C`ev_timer_again\*(C'\fR.

.Sp

At start:

.Sp

.Vb 3

\&   ev_timer_init (timer, callback);

\&   timer\->repeat = 60.;

\&   ev_timer_again (loop, timer);

.Ve

.Sp

Each time there is some activity:

.Sp

.Vb 1

\&   ev_timer_again (loop, timer);

.Ve

.Sp

It is even possible to change the time-out on the fly, regardless of

whether the watcher is active or not:

.Sp

.Vb 2

\&   timer\->repeat = 30.;

\&   ev_timer_again (loop, timer);

.Ve

.Sp

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.

.Sp

It is, however, even simpler than the \*(L"obvious\*(R" way to do it.

.IP "3. Let the timer time out, but then re-arm it as required." 4

.IX Item "3. Let the timer time out, but then re-arm it as required."

This method is more tricky, but usually most efficient: Most timeouts are

relatively long compared to the intervals between other activity \- in

our example, within 60 seconds, there are usually many I/O events with

associated activity resets.

.Sp

In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,

but remember the time of last activity, and check for a real timeout only

within the callback:

.Sp

.Vb 1

\&   ev_tstamp last_activity; // time of last activity

\&

\&   static void

\&   callback (EV_P_ ev_timer *w, int revents)

\&   {

\&     ev_tstamp now     = ev_now (EV_A);

\&     ev_tstamp timeout = last_activity + 60.;

\&

\&     // if last_activity + 60. is older than now, we did time out

\&     if (timeout < now)

\&       {

\&         // timeout occured, take action

\&       }

\&     else

\&       {

\&         // callback was invoked, but there was some activity, re\-arm

\&         // the watcher to fire in last_activity + 60, which is

\&         // guaranteed to be in the future, so "again" is positive:

\&         w\->repeat = timeout \- now;

\&         ev_timer_again (EV_A_ w);

\&       }

\&   }

.Ve

.Sp

To summarise the callback: first calculate the real timeout (defined

as \*(L"60 seconds after the last activity\*(R"), then check if that time has

been reached, which means something \fIdid\fR, in fact, time out. Otherwise

the callback was invoked too early (\f(CW\*(C`timeout\*(C'\fR is in the future), so

re-schedule the timer to fire at that future time, to see if maybe we have

a timeout then.

.Sp

Note how \f(CW\*(C`ev_timer_again\*(C'\fR is used, taking advantage of the

\&\f(CW\*(C`ev_timer_again\*(C'\fR optimisation when the timer is already running.

.Sp

This scheme causes more callback invocations (about one every 60 seconds