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.\" ========================================================================
.IX Title "LIBEV 3"
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.TH LIBEV 3 "2018-12-21" "libev-4.25" "libev - high performance full featured event loop"
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.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
libev \- a high performance full\-featured event loop written in C
.Vb 1
\&   #include <ev.h>
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.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);
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\&     // this causes all nested ev_run\*(Aqs to stop iterating
\&     ev_break (EV_A_ EVBREAK_ALL);
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\&   }
\&   // another callback, this time for a time\-out
\&   static void
\&   timeout_cb (EV_P_ ev_timer *w, int revents)
\&   {
\&     puts ("timeout");
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\&     // this causes the innermost ev_run to stop iterating
\&     ev_break (EV_A_ EVBREAK_ONE);
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\&   }
\&   int
\&   main (void)
\&   {
\&     // use the default event loop unless you have special needs
\&     struct ev_loop *loop = EV_DEFAULT;
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\&     // 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
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\&     return 0;
\&   }
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.
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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
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reading \*(L"\s-1ANATOMY OF A WATCHER\*(R"\s0, then the \*(L"\s-1EXAMPLE PROGRAM\*(R"\s0 above and
look up the missing functions in \*(L"\s-1GLOBAL FUNCTIONS\*(R"\s0 and the \f(CW\*(C`ev_io\*(C'\fR and
\&\f(CW\*(C`ev_timer\*(C'\fR sections in \*(L"\s-1WATCHER TYPES\*(R"\s0.
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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 \fIevent loop\fR handler, and will then
communicate events via a callback mechanism.
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
.SS "\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), Linux eventfd/signalfd (for faster and cleaner
inter-thread wakeup (\f(CW\*(C`ev_async\*(C'\fR)/signal handling (\f(CW\*(C`ev_signal\*(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).
It also is quite fast (see this
benchmark <http://libev.schmorp.de/bench.html> comparing it to libevent
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for example).
.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`struct ev_loop *\*(C'\fR) will not have
this argument.
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Libev represents time as a single floating point number, representing
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the (fractional) number of seconds since the (\s-1POSIX\s0) epoch (in practice
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. 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 (e.g. delays) throughout libev.
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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 \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
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.
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.
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. Also interesting is the combination of
\&\f(CW\*(C`ev_now_update\*(C'\fR and \f(CW\*(C`ev_now\*(C'\fR.
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.IP "ev_sleep (ev_tstamp interval)" 4
.IX Item "ev_sleep (ev_tstamp interval)"
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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 \f(CW\*(C`interval <= 0\*(C'\fR.
Basically this is a sub-second-resolution \f(CW\*(C`sleep ()\*(C'\fR.
The range of the \f(CW\*(C`interval\*(C'\fR is limited \- libev only guarantees to work
with sleep times of up to one day (\f(CW\*(C`interval <= 86400\*(C'\fR).
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.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 ()"
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.
These version numbers refer to the \s-1ABI\s0 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
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version (note, however, that this will not detect other \s-1ABI\s0 mismatches,
such as \s-1LFS\s0 or reentrancy).
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.Vb 3
\&   assert (("libev version mismatch",
\&            ev_version_major () == EV_VERSION_MAJOR
\&            && ev_version_minor () >= EV_VERSION_MINOR));
.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.
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
.Vb 2
\&   assert (("sorry, no epoll, no sex",
\&            ev_supported_backends () & EVBACKEND_EPOLL));
.IP "unsigned int ev_recommended_backends ()" 4
.IX Item "unsigned int ev_recommended_backends ()"
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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
\&\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.
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.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
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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 \f(CW\*(C`ev_embeddable_backends ()
& ev_supported_backends ()\*(C'\fR, likewise for recommended ones.
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See the description of \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
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.IP "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())" 4
.IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())"
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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.
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.
You could override this function in high-availability programs to, say,
free some memory if it cannot allocate memory, to use a special allocator,
or even to sleep a while and retry until some memory is available.
Example: Replace the libev allocator with one that waits a bit and then
retries (example requires a standards-compliant \f(CW\*(C`realloc\*(C'\fR).
.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);
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.IP "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())" 4
.IX Item "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())"
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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.
.Vb 6
\&   static void
\&   fatal_error (const char *msg)
\&   {
\&     perror (msg);
\&     abort ();
\&   }
\&   ...
\&   ev_set_syserr_cb (fatal_error);
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.IP "ev_feed_signal (int signum)" 4
.IX Item "ev_feed_signal (int signum)"
This function can be used to \*(L"simulate\*(R" 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 \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when
creating any loops), and in one thread, use \f(CW\*(C`sigwait\*(C'\fR or any other
mechanism to wait for signals, then \*(L"deliver\*(R" them to libev by calling
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 unless libev 3 compatibility is disabled, as
libev 3 had an \f(CW\*(C`ev_loop\*(C'\fR function colliding with the struct name).
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The library knows two types of such loops, the \fIdefault\fR loop, which
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supports child process events, and dynamically created event loops which
do not.
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.IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4
.IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"
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This returns the \*(L"default\*(R" event loop object, which is what you should
normally use when you just need \*(L"the event loop\*(R". Event loop objects and
the \f(CW\*(C`flags\*(C'\fR parameter are described in more detail in the entry for
If the default loop is already initialised then this function simply
returns it (and ignores the flags. If that is troubling you, check
\&\f(CW\*(C`ev_backend ()\*(C'\fR afterwards). Otherwise it will create it with the given
flags, which should almost always be \f(CW0\fR, unless the caller is also the
one calling \f(CW\*(C`ev_run\*(C'\fR or otherwise qualifies as \*(L"the main program\*(R".
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If you don't know what event loop to use, use the one returned from this
function (or via the \f(CW\*(C`EV_DEFAULT\*(C'\fR macro).
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Note that this function is \fInot\fR thread-safe, so if you want to use it
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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 \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 which 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.
Example: This is the most typical usage.
.Vb 2
\&   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:
.Vb 1
.IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4
.IX Item "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.
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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 \*(L"main\*(R" or \*(L"initial\*(R" thread.
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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).
The following flags are supported:
.RS 4
.ie n .IP """EVFLAG_AUTO""" 4
.el .IP "\f(CWEVFLAG_AUTO\fR" 4
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
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
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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).
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.ie n .IP """EVFLAG_FORKCHECK""" 4
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Instead of calling \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.
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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
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Boyuan Yang committed
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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). (Update: glibc
versions 2.25 apparently removed the \f(CW\*(C`getpid\*(C'\fR optimisation again).
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The big advantage of this flag is that you can forget about fork (and
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forget about forgetting to tell libev about forking, although you still
have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR) when you use this flag.
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This flag setting cannot be overridden or specified in the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR
environment variable.
.ie n .IP """EVFLAG_NOINOTIFY""" 4
When this flag is specified, then libev will not attempt to use the
\&\fIinotify\fR \s-1API\s0 for its \f(CW\*(C`ev_stat\*(C'\fR watchers. Apart from debugging and
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testing, this flag can be useful to conserve inotify file descriptors, as
otherwise each loop using \f(CW\*(C`ev_stat\*(C'\fR watchers consumes one inotify handle.
.ie n .IP """EVFLAG_SIGNALFD""" 4
When this flag is specified, then libev will attempt to use the
\&\fIsignalfd\fR \s-1API\s0 for its \f(CW\*(C`ev_signal\*(C'\fR (and \f(CW\*(C`ev_child\*(C'\fR) watchers. This \s-1API\s0
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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.
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.ie n .IP """EVFLAG_NOSIGMASK""" 4
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
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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 \s-1POSIX\s0 in a threaded program, as libev calls
\&\f(CW\*(C`sigprocmask\*(C'\fR, whose behaviour is officially unspecified.
This flag's behaviour will become the default in future versions of libev.
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.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)"
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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.
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.
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)"
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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.
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)"
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Use the linux-specific \fIepoll\fR\|(7) interface (for both pre\- and post\-2.6.9
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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).
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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
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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 \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.
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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. 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
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not least, it also refuses to work with some file descriptors which work
perfectly fine with \f(CW\*(C`select\*(C'\fR (files, many character devices...).
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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...
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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.
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, \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.
While nominally embeddable in other event loops, this feature is broken in
all kernel versions tested so far.
This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
.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)"
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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.
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.
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
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two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you
might have to leak fd's on fork, but it's more sane than epoll) and it
drops fds silently in similarly hard-to-detect cases.
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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. \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 X\s0)) and, did I mention it, using it only for sockets.
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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
.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
.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)"
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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 \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend
might perform better.
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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
On the negative side, the interface is \fIbizarre\fR \- 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
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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.
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Fortunately libev seems to be able to work around these idiocies.
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This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
.ie n .IP """EVBACKEND_ALL""" 4
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
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It is definitely not recommended to use this flag, use whatever
\&\f(CW\*(C`ev_recommended_backends ()\*(C'\fR returns, or simply do not specify a backend
at all.
.ie n .IP """EVBACKEND_MASK""" 4
Not a backend at all, but a mask to select all backend bits from a
\&\f(CW\*(C`flags\*(C'\fR value, in case you want to mask out any backends from a flags
value (e.g. when modifying the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR environment variable).
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.RS 4
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 \f(CW\*(C`ev_recommended_backends
()\*(C'\fR will be tried.
Example: Try to create a event loop that uses epoll and nothing else.
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.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");
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Example: Use whatever libev has to offer, but make sure that kqueue is
used if available.
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.Vb 1
\&   struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
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.IP "ev_loop_destroy (loop)" 4
.IX Item "ev_loop_destroy (loop)"
Destroys an event loop object (frees all memory and kernel state
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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).
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.
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This function is normally used on loop objects allocated by
\&\f(CW\*(C`ev_loop_new\*(C'\fR, but it can also be used on the default loop returned by
\&\f(CW\*(C`ev_default_loop\*(C'\fR, 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 \f(CW\*(C`ev_loop_new\*(C'\fR
and \f(CW\*(C`ev_loop_destroy\*(C'\fR.
.IP "ev_loop_fork (loop)" 4
.IX Item "ev_loop_fork (loop)"
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This function sets a flag that causes subsequent \f(CW\*(C`ev_run\*(C'\fR 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 \f(CW\*(C`ev_prepare\*(C'\fR callback), but it makes most
sense after forking, in the child process. You \fImust\fR call it (or use
\&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR) in the child before resuming or calling \f(CW\*(C`ev_run\*(C'\fR.
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In addition, if you want to reuse a loop (via this function or
\&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR), you \fIalso\fR have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR.
Again, you \fIhave\fR to call it on \fIany\fR loop that you want to re-use after
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a fork, \fIeven if you do not plan to use the loop in the parent\fR. This is
because some kernel interfaces *cough* \fIkqueue\fR *cough* do funny things
during fork.
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On the other hand, you only need to call this function in the child
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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, \f(CW\*(C`epoll\*(C'\fR 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).
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The function itself is quite fast and it's usually not a problem to call
it just in case after a fork.
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Example: Automate calling \f(CW\*(C`ev_loop_fork\*(C'\fR on the default loop when
using pthreads.
.Vb 5
\&   static void
\&   post_fork_child (void)
\&   {
\&     ev_loop_fork (EV_DEFAULT);
\&   }
\&   ...
\&   pthread_atfork (0, 0, post_fork_child);
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.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
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.IP "unsigned int ev_iteration (loop)" 4
.IX Item "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 \f(CW0\fR
and happily wraps around with enough iterations.
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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
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\&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls \- and is incremented between the
prepare and check phases.
.IP "unsigned int ev_depth (loop)" 4
.IX Item "unsigned int ev_depth (loop)"
Returns the number of times \f(CW\*(C`ev_run\*(C'\fR was entered minus the number of
times \f(CW\*(C`ev_run\*(C'\fR was exited normally, in other words, the recursion depth.
Outside \f(CW\*(C`ev_run\*(C'\fR, this number is zero. In a callback, this number is
\&\f(CW1\fR, unless \f(CW\*(C`ev_run\*(C'\fR was invoked recursively (or from another thread),
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in which case it is higher.
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Leaving \f(CW\*(C`ev_run\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread,
throwing an exception etc.), doesn't count as \*(L"exit\*(R" \- consider this
as a hint to avoid such ungentleman-like behaviour unless it's really
convenient, in which case it is fully supported.
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.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
.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_run ()\*(C'\fR.
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This function is rarely useful, but when some event callback runs for a
very long time without entering the event loop, updating libev's idea of
the current time is a good idea.
See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.
.IP "ev_suspend (loop)" 4
.IX Item "ev_suspend (loop)"
.PD 0
.IP "ev_resume (loop)" 4
.IX Item "ev_resume (loop)"
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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.
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A typical use case would be an interactive program such as a game:  When
the user presses \f(CW\*(C`^Z\*(C'\fR 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 \f(CW\*(C`ev_suspend\*(C'\fR
in your \f(CW\*(C`SIGTSTP\*(C'\fR handler, sending yourself a \f(CW\*(C`SIGSTOP\*(C'\fR and calling
\&\f(CW\*(C`ev_resume\*(C'\fR directly afterwards to resume timer processing.
Effectively, all \f(CW\*(C`ev_timer\*(C'\fR watchers will be delayed by the time spend
between \f(CW\*(C`ev_suspend\*(C'\fR and \f(CW\*(C`ev_resume\*(C'\fR, and all \f(CW\*(C`ev_periodic\*(C'\fR watchers
will be rescheduled (that is, they will lose any events that would have
occurred while suspended).
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After calling \f(CW\*(C`ev_suspend\*(C'\fR you \fBmust not\fR call \fIany\fR function on the
given loop other than \f(CW\*(C`ev_resume\*(C'\fR, and you \fBmust not\fR call \f(CW\*(C`ev_resume\*(C'\fR
without a previous call to \f(CW\*(C`ev_suspend\*(C'\fR.
Calling \f(CW\*(C`ev_suspend\*(C'\fR/\f(CW\*(C`ev_resume\*(C'\fR has the side effect of updating the
event loop time (see \f(CW\*(C`ev_now_update\*(C'\fR).
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.IP "bool ev_run (loop, int flags)" 4
.IX Item "bool ev_run (loop, int flags)"
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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 \fIloops\fR.
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If the flags argument is specified as \f(CW0\fR, it will keep handling events
until either no event watchers are active anymore or \f(CW\*(C`ev_break\*(C'\fR was
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The return value is false if there are no more active watchers (which
usually means \*(L"all jobs done\*(R" or \*(L"deadlock\*(R"), and true in all other cases
(which usually means " you should call \f(CW\*(C`ev_run\*(C'\fR again").
Please note that an explicit \f(CW\*(C`ev_break\*(C'\fR is usually better than
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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
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This function is \fImostly\fR exception-safe \- you can break out of a
\&\f(CW\*(C`ev_run\*(C'\fR call by calling \f(CW\*(C`longjmp\*(C'\fR in a callback, throwing a \*(C+
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exception and so on. This does not decrement the \f(CW\*(C`ev_depth\*(C'\fR value, nor
will it clear any outstanding \f(CW\*(C`EVBREAK_ONE\*(C'\fR breaks.
A flags value of \f(CW\*(C`EVRUN_NOWAIT\*(C'\fR 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 \f(CW\*(C`EVRUN_ONCE\*(C'\fR will look for new events (waiting if
965 966 967 968 969 970 971 972
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 \f(CW\*(C`ev_run\*(C'\fR"). However, a pair of \f(CW\*(C`ev_prepare\*(C'\fR/\f(CW\*(C`ev_check\*(C'\fR watchers is
974 975
usually a better approach for this kind of thing.
976 977 978
Here are the gory details of what \f(CW\*(C`ev_run\*(C'\fR does (this is for your
understanding, not a guarantee that things will work exactly like this in
future versions):
979 980
.Vb 10
981 982
\&   \- Increment loop depth.
\&   \- Reset the ev_break status.
\&   \- Before the first iteration, call any pending watchers.
984 985
\&   LOOP:
\&   \- If EVFLAG_FORKCHECK was used, check for a fork.
986 987
\&   \- 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.
989 990 991 992 993
\&   \- 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
995 996
\&     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.
998 999 1000 1001 1002
\&   \- 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.
1004 1005 1006 1007
\&   \- 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.
1008 1009 1010 1011 1012 1013 1014
\&   \- 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.
1015 1016 1017 1018 1019 1020 1021 1022
Example: Queue some jobs and then loop until no events are outstanding
.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..)
1023 1024
\&   ev_run (my_loop, 0);
\&   ... jobs done or somebody called break. yeah!
1026 1027 1028
.IP "ev_break (loop, how)" 4
.IX Item "ev_break (loop, how)"
Can be used to make a call to \f(CW\*(C`ev_run\*(C'\fR return early (but only after it
has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either
1030 1031
\&\f(CW\*(C`EVBREAK_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_run\*(C'\fR call return, or
\&\f(CW\*(C`EVBREAK_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_run\*(C'\fR calls return.
This \*(L"break state\*(R" will be cleared on the next call to \f(CW\*(C`ev_run\*(C'\fR.
1035 1036
It is safe to call \f(CW\*(C`ev_break\*(C'\fR from outside any \f(CW\*(C`ev_run\*(C'\fR calls, too, in
which case it will have no effect.
1037 1038 1039 1040 1041 1042 1043 1044
.IP "ev_ref (loop)" 4
.IX Item "ev_ref (loop)"
.PD 0
.IP "ev_unref (loop)" 4
.IX Item "ev_unref (loop)"
Ref/unref can be used to add or remove a reference count on the event
loop: Every watcher keeps one reference, and as long as the reference
count is nonzero, \f(CW\*(C`ev_run\*(C'\fR will not return on its own.
1046 1047
This is useful when you have a watcher that you never intend to
unregister, but that nevertheless should not keep \f(CW\*(C`ev_run\*(C'\fR from
1049 1050 1051 1052
returning. In such a case, call \f(CW\*(C`ev_unref\*(C'\fR after starting, and \f(CW\*(C`ev_ref\*(C'\fR
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 \f(CW\*(C`ev_run\*(C'\fR from
1054 1055 1056 1057 1058 1059 1060 1061
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. Note also that libev might stop watchers itself
(e.g. non-repeating timers) in which case you have to \f(CW\*(C`ev_ref\*(C'\fR
in the callback).
Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_run\*(C'\fR
1063 1064 1065 1066 1067 1068
running when nothing else is active.
.Vb 4
\&   ev_signal exitsig;
\&   ev_signal_init (&exitsig, sig_cb, SIGINT);
\&   ev_signal_start (loop, &exitsig);
\&   ev_unref (loop);
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Example: For some weird reason, unregister the above signal handler again.
.Vb 2
\&   ev_ref (loop);
\&   ev_signal_stop (loop, &exitsig);
.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)"
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
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
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.
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 not be affected. Setting this to a non-null value will
1104 1105
introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. The
sleep time ensures that libev will not poll for I/O events more often then
1106 1107
once per this interval, on average (as long as the host time resolution is
good enough).
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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.
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. 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).
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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.
Example: we only need 0.1s timeout granularity, and we wish not to poll
more often than 100 times per second:
.Vb 2
\&   ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
\&   ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
.IP "ev_invoke_pending (loop)" 4
.IX Item "ev_invoke_pending (loop)"
This call will simply invoke all pending watchers while resetting their
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pending state. Normally, \f(CW\*(C`ev_run\*(C'\fR 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 \f(CW\*(C`ev_invoke_pending\*(C'\fR or \f(CW\*(C`ev_run\*(C'\fR of course).
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.IP "int ev_pending_count (loop)" 4
.IX Item "int ev_pending_count (loop)"
Returns the number of pending watchers \- zero indicates that no watchers
are pending.
.IP "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(\s-1EV_P\s0))" 4
.IX Item "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, \f(CW\*(C`ev_run\*(C'\fR will call
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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 \f(CW\*(C`ev_invoke_pending\*(C'\fR as new
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.IP "ev_set_loop_release_cb (loop, void (*release)(\s-1EV_P\s0) throw (), void (*acquire)(\s-1EV_P\s0) throw ())" 4
.IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())"
1163 1164 1165 1166
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.
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However, \f(CW\*(C`ev_run\*(C'\fR 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 \f(CW\*(C`ev_break\*(C'\fR and \f(CW\*(C`ev_async_send\*(C'\fR, another way is to set these
\&\fIrelease\fR and \fIacquire\fR callbacks on the loop.
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When set, then \f(CW\*(C`release\*(C'\fR will be called just before the thread is
suspended waiting for new events, and \f(CW\*(C`acquire\*(C'\fR is called just
Ideally, \f(CW\*(C`release\*(C'\fR will just call your mutex_unlock function, and
\&\f(CW\*(C`acquire\*(C'\fR will just call the mutex_lock function again.
While event loop modifications are allowed between invocations of
\&\f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR (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 \f(CW\*(C`ev_async\*(C'\fR watcher to wake up \f(CW\*(C`ev_run\*(C'\fR when you want it
1184 1185
to take note of any changes you made.
In theory, threads executing \f(CW\*(C`ev_run\*(C'\fR will be async-cancel safe between
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invocations of \f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR.
See also the locking example in the \f(CW\*(C`THREADS\*(C'\fR section later in this
.IP "ev_set_userdata (loop, void *data)" 4
.IX Item "ev_set_userdata (loop, void *data)"
.PD 0
1194 1195
.IP "void *ev_userdata (loop)" 4
.IX Item "void *ev_userdata (loop)"
1196 1197 1198
Set and retrieve a single \f(CW\*(C`void *\*(C'\fR associated with a loop. When
\&\f(CW\*(C`ev_set_userdata\*(C'\fR has never been called, then \f(CW\*(C`ev_userdata\*(C'\fR returns
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These two functions can be used to associate arbitrary data with a loop,
and are intended solely for the \f(CW\*(C`invoke_pending_cb\*(C'\fR, \f(CW\*(C`release\*(C'\fR and
\&\f(CW\*(C`acquire\*(C'\fR callbacks described above, but of course can be (ab\-)used for
any other purpose as well.
1205 1206
.IP "ev_verify (loop)" 4
.IX Item "ev_verify (loop)"
1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221
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.
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.
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.
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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 \s-1STDIN\s0 to become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher
for that:
1226 1227 1228 1229 1230
.Vb 5
\&   static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
\&   {
\&     ev_io_stop (w);
\&     ev_break (loop, EVBREAK_ALL);
1232 1233 1234 1235 1236 1237 1238 1239 1240 1241
\&   }
\&   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);
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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
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).
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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 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).
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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.
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.
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.
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
.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
The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or
1288 1289 1290
.ie n .IP """EV_TIMER""" 4
.el .IP "\f(CWEV_TIMER\fR" 4
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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
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
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
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
.PD 0
.ie n .IP """EV_CHECK""" 4
.el .IP "\f(CWEV_CHECK\fR" 4
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All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_run\*(C'\fR starts to
gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are queued (not invoked)
just after \f(CW\*(C`ev_run\*(C'\fR has gathered them, but before it queues any callbacks
for any received events. That means \f(CW\*(C`ev_prepare\*(C'\fR watchers are the last
watchers invoked before the event loop sleeps or polls for new events, and
\&\f(CW\*(C`ev_check\*(C'\fR 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
\&\f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep \f(CW\*(C`ev_run\*(C'\fR from
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.ie n .IP """EV_EMBED""" 4
.el .IP "\f(CWEV_EMBED\fR" 4
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
1341 1342 1343 1344
.ie n .IP """EV_CLEANUP""" 4
.el .IP "\f(CWEV_CLEANUP\fR" 4
The event loop is about to be destroyed (see \f(CW\*(C`ev_cleanup\*(C'\fR).
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.ie n .IP """EV_ASYNC""" 4
.el .IP "\f(CWEV_ASYNC\fR" 4
The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).
.ie n .IP """EV_CUSTOM""" 4
.el .IP "\f(CWEV_CUSTOM\fR" 4
Not ever sent (or otherwise used) by libev itself, but can be freely used
by libev users to signal watchers (e.g. via \f(CW\*(C`ev_feed_event\*(C'\fR).
.ie n .IP """EV_ERROR""" 4
.el .IP "\f(CWEV_ERROR\fR" 4
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 \*(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.
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.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.
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 \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
int revents)\*(C'\fR.
Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
.Vb 3
\&   ev_io w;
\&   ev_init (&w, my_cb);
\&   ev_io_set (&w, STDIN_FILENO, EV_READ);
.ie n .IP """ev_TYPE_set"" (ev_TYPE *watcher, [args])" 4
.el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *watcher, [args])" 4
.IX Item "ev_TYPE_set (ev_TYPE *watcher, [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).
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.
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.
Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.
.Vb 1
\&   ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
.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.
Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this
whole section.
.Vb 1
\&   ev_io_start (EV_DEFAULT_UC, &w);
.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).
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
.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
.IP "callback ev_cb (ev_TYPE *watcher)" 4
.IX Item "callback ev_cb (ev_TYPE *watcher)"
Returns the callback currently set on the watcher.
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.IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
.IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
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Change the callback. You can change the callback at virtually any time
(modulo threads).
.IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
.IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
.PD 0
.IP "int ev_priority (ev_TYPE *watcher)" 4
.IX Item "int ev_priority (ev_TYPE *watcher)"
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).
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.
You \fImust not\fR change the priority of a watcher as long as it is active or
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.
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 :).
See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
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.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
.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.
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.
.IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
.IX Item "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
\&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
not started in the first place.
See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
functions that do not need a watcher.
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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 \*(L"the right thing\*(R".
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.IP "initialised" 4
.IX Item "initialised"
Before a watcher can be registered with the event loop it has to be
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initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
\&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR 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
\&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
.IP "started/running/active" 4
.IX Item "started/running/active"
Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR 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.
.IP "pending" 4
.IX Item "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
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 \f(CW\*(C`ev_TYPE_set\*(C'\fR),
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 \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
.IP "stopped" 4
.IX Item "stopped"
A watcher can be stopped implicitly by libev (in which case it might still
be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR 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 \f(CW\*(C`ev_TYPE_init\*(C'\fR
it again).
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Many event loops support \fIwatcher priorities\fR, 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 \f(CW\*(C`ev_set_priority\*(C'\fR. See its
description for the more technical details such as the actual priority
There are two common ways how these these priorities are being interpreted
by event loops:
In the more common lock-out model, higher priorities \*(L"lock out\*(R" 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
\&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR 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 \*(L"lock-out\*(R" 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 \*(L"lock-out\*(R" model when
For example, to emulate how many other event libraries handle priorities,
you can associate an \f(CW\*(C`ev_idle\*(C'\fR 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
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the lock-out case is known to be rare (which in turn is rare :), this is
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:
.Vb 2
\&   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.
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\&     // 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 \*(L"real\*(R" 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.
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.
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 .SS """ev_io"" \- is this file descriptor readable or writable?"
.el .SS "\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.
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 \*(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
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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 \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.
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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
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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
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use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
But really, best use non-blocking mode.
\fIThe special problem of disappearing file descriptors\fR
.IX Subsection "The special problem of disappearing file descriptors"
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.
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.
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.
\fIThe special problem of dup'ed file descriptors\fR
.IX Subsection "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 \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.
There is no workaround possible except not registering events
for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
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\fIThe special problem of files\fR
.IX Subsection "The special problem of files"
Many people try to use \f(CW\*(C`select\*(C'\fR (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 \*(L"expected\*(R" 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 \s-1POSIX\s0 behaviour with respect
to files, even though you should not use it. The reason for this is
convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
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usually a tty, often a pipe, but also sometimes files or special devices
(for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
\&\fI/dev/urandom\fR), 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 \*(L"just works\*(R" 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 \s-1STDIN/STDOUT,\s0 or
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when you rarely read from a file instead of from a socket, and want to
reuse the same code path.
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\fIThe special problem of fork\fR
.IX Subsection "The special problem of fork"
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 if you want to continue to use it in the child.
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To support fork in your child processes, you have to call \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
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\fIThe special problem of \s-1SIGPIPE\s0\fR
.IX Subsection "The special problem of SIGPIPE"
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
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this is sensible behaviour, for daemons, this is usually undesirable.
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
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somewhere, as that would have given you a big clue).
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\fIThe special problem of \fIaccept()\fIing when you can't\fR
.IX Subsection "The special problem of accept()ing when you can't"
Many implementations of the \s-1POSIX \s0\f(CW\*(C`accept\*(C'\fR function (for example,
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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 \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR 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% \s-1CPU\s0 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 \s-1OS\s0 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
\&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, 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 (\*(L"raise the ulimit!\*(R"). For extra points one could stop
the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
If your program is single-threaded, then you could also keep a dummy file
descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
when you run into \f(CW\*(C`ENFILE\*(C'\fR or \f(CW\*(C`EMFILE\*(C'\fR, close it, run \f(CW\*(C`accept\*(C'\fR,
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 \f(CW\*(C`exit\*(C'\fR, as
is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
opportunity for a DoS attack.
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\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)"
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.
.IX Subsection "Examples"
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.
.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_run (loop, 0);
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.ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
.el .SS "\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.
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).
The callback is guaranteed to be invoked only \fIafter\fR its timeout has
passed (not \fIat\fR, so on systems with very low-resolution clocks this
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might introduce a small delay, see \*(L"the special problem of being too
early\*(R", 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 \f(CW\*(C`ev_run\*(C'\fR recursively).
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\fIBe smart about timeouts\fR
.IX Subsection "Be smart about timeouts"
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.
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).
.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:
.Vb 2
\&   ev_timer_init (timer, callback, 60., 0.);
\&   ev_timer_start (loop, timer);
Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
and start it again:
.Vb 3
\&   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.
.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
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.
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.
At start:
.Vb 3
\&   ev_init (timer, callback);
\&   timer\->repeat = 60.;
\&   ev_timer_again (loop, timer);
Each time there is some activity:
.Vb 1
\&   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:
.Vb 2
\&   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 \*(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.
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:
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.Vb 3
\&   ev_tstamp timeout = 60.;
\&   ev_tstamp last_activity; // time of last activity
\&   ev_timer timer;
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\&   static void
\&   callback (EV_P_ ev_timer *w, int revents)
\&   {
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\&     // 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
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\&       }
\&     else
\&       {
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\&         // 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);
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\&       }
\&   }
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To summarise the callback: first calculate in how many seconds the
timeout will occur (by calculating the absolute time when it would occur,
\&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
(EV_A)\*(C'\fR from that).
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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.
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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.
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To start the machinery, simply initialise the watcher and set
\&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
now), then call the callback, which will \*(L"do the right thing\*(R" and start
the timer:
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.Vb 3
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\&   last_activity = ev_now (EV_A);
\&   ev_init (&timer, callback);
\&   callback (EV_A_ &timer, 0);
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When there is some activity, simply store the current time in
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\&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
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.Vb 2
\&   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 :).
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.Vb 3
\&   timeout = new_value;
\&   ev_timer_stop (EV_A_ &timer);
\&   callback (EV_A_ &timer, 0);
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This technique is slightly more complex, but in most cases where the
time-out is unlikely to be triggered, much more efficient.
.IP "4. Wee, just use a double-linked list for your timeouts." 4
.IX Item "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 \fIend\fR of the list.
Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR 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 \f(CW\*(C`ev_timer\*(C'\fR 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 :)
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\fIThe special problem of being too early\fR
.IX Subsection "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 \s-1STOP\s0 signal for a few hours for example.
So, libev tries to invoke your callback as soon as possible \fIafter\fR 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 \*(L"elapsed delay >= requested delay\*(R", but
this can cause your callback to be invoked much earlier than you would
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 \s-1OS\s0
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 \*(L"501 >=
501\*(R" and invoke the callback 0.1s after it was started, even though a
one-second delay was requested \- this is being \*(L"too early\*(R", despite best
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 \fIcan\fR and \fIdoes\fR guarantee that the requested
delay has actually elapsed, or in other words, it always errs on the \*(L"too
late\*(R" side of things.
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\fIThe special problem of time updates\fR
.IX Subsection "The special problem of time updates"
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Establishing the current time is a costly operation (it usually takes
at least one system call): \s-1EV\s0 therefore updates its idea of the current
time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
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growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
lots of events in one iteration.
The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
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 \fIneed\fR to base the
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timeout on the current time, use something like the following to adjust
for it:
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.Vb 1
\&   ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
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If the event loop is suspended for a long time, you can also force an
update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
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()\*(C'\fR, although that will push the event time of all outstanding events
further into the future.
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\fIThe special problem of unsynchronised clocks\fR
.IX Subsection "The special problem of unsynchronised clocks"
Modern systems have a variety of clocks \- libev itself uses the normal
\&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
Neither of these clocks is synchronised with each other or any other clock
on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
than a directly following call to \f(CW\*(C`time\*(C'\fR.
The moral of this is to only compare libev-related timestamps with
\&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, 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 \f(CW\*(C`ev_time\*(C'\fR
or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
libev makes sure your callback is not invoked before the delay happened,
\&\fImeasured according to the real time\fR, not the system clock.
If your timeouts are based on a physical timescale (e.g. \*(L"time out this
connection after 100 seconds\*(R") 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 \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
time, where your comparisons will always generate correct results.
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\fIThe special problems of suspended animation\fR
.IX Subsection "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 (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) 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 \f(CW\*(C`ev_timer\*(C'\fR 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, \s-1OS\s0 versions or even different hardware.
The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) 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 \f(CW\*(C`ev_timer\*(C'\fRs 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 \f(CW\*(C`ev_suspend\*(C'\fR
and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
deterministic behaviour in this case (you can do nothing against
\fIWatcher-Specific Functions and Data Members\fR
.IX Subsection "Watcher-Specific Functions and Data Members"
.IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
.IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
.PD 0
.IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
.IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
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Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, 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 \f(CW\*(C`repeat\*(C'\fR
seconds later, again, and again, until stopped manually.
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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.
.IP "ev_timer_again (loop, ev_timer *)" 4
.IX Item "ev_timer_again (loop, ev_timer *)"
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This will act as if the timer timed out, and restarts it again if it is
repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
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The exact semantics are as in the following rules, all of which will be
applied to the watcher:
.RS 4
.IP "If the timer is pending, the pending status is always cleared." 4
.IX Item "If the timer is pending, the pending status is always cleared."
.PD 0
.IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
.IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
.ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
.el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
.IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
.RS 4
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This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
usage example.
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.IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
.IX Item "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 \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
\&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
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will return \f(CW4\fR. When the timer expires and is restarted, it will return
roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
too), and so on.
.IP "ev_tstamp repeat [read\-write]" 4
.IX Item "ev_tstamp repeat [read-write]"
The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
which is also when any modifications are taken into account.
.IX Subsection "Examples"
Example: Create a timer that fires after 60 seconds.
.Vb 5
\&   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
.Vb 5
\&   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);
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\&   // 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);
.ie n .SS """ev_periodic"" \- to cron or not to cron?"
.el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
.IX Subsection "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 \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
relative time, the physical time that passes) but on wall clock time
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(absolute time, the thing you can read on your calendar or clock). The
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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
You can tell a periodic watcher to trigger after some specific point
in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, 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
\&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
it, as it uses a relative timeout).
\&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
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other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
watchers, as those cannot react to time jumps.
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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 \f(CW\*(C`ev_run\*(C'\fR recursively).
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\fIWatcher-Specific Functions and Data Members\fR
.IX Subsection "Watcher-Specific Functions and Data Members"
.IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
.IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
.PD 0
.IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
.IX Item "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:
.RS 4
.IP "\(bu" 4
absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
In this configuration the watcher triggers an event after the wall clock
time \f(CW\*(C`offset\*(C'\fR 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.
.IP "\(bu" 4
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
\&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
This can be used to create timers that do not drift with respect to the
system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
hour, on the hour (with respect to \s-1UTC\s0):
.Vb 1
\&   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 (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
by 3600.
Another way to think about it (for the mathematically inclined) is that
\&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
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The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR 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, \f(CW0\fR or something between
\&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
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Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
will of course deteriorate. Libev itself tries to be exact to be about one
millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
.IP "\(bu" 4
manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR 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.
\&\s-1NOTE: \s0\fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
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or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
allowed by documentation here\fR.
If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
only event loop modification you are allowed to do).
The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
*w, ev_tstamp now)\*(C'\fR, e.g.:
.Vb 5
\&   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.
\&\s-1NOTE: \s0\fIThis callback must always return a time that is higher than or
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equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
This can be used to create very complex timers, such as a timer that
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triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
this. Here is a (completely untested, no error checking) example on how to
do this:
.Vb 1
\&   #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).
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