libev - a high performance full-featured event loop written in C
=head2 EXAMPLE PROGRAM
// a single header file is required
// every watcher type has its own typedef'd struct
// with the name ev_<type>
// all watcher callbacks have a similar signature
// this callback is called when data is readable on stdin
stdin_cb (EV_P_ struct ev_io *w, int revents)
puts ("stdin ready");
// for one-shot events, one must manually stop the watcher
// with its corresponding stop function.
ev_io_stop (EV_A_ w);
// this causes all nested ev_loop's to stop iterating
ev_unloop (EV_A_ EVUNLOOP_ALL);
// another callback, this time for a time-out
timeout_cb (EV_P_ struct ev_timer *w, int revents)
// this causes the innermost ev_loop to stop iterating
ev_unloop (EV_A_ EVUNLOOP_ONE);
// use the default event loop unless you have special needs
struct ev_loop *loop = ev_default_loop (0);
// initialise an io watcher, then start it
// this one will watch for stdin to become readable
ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
ev_io_start (loop, &stdin_watcher);
// initialise a timer watcher, then start it
// simple non-repeating 5.5 second timeout
ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
ev_timer_start (loop, &timeout_watcher);
// now wait for events to arrive
ev_loop (loop, 0);
// unloop was called, so exit
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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 I<event loop> handler, and will then
communicate events via a callback mechanism.
You register interest in certain events by registering so-called I<event
watchers>, which are relatively small C structures you initialise with the
details of the event, and then hand it over to libev by I<starting> the
Libev supports C<select>, C<poll>, the Linux-specific C<epoll>, the
BSD-specific C<kqueue> and the Solaris-specific event port mechanisms
for file descriptor events (C<ev_io>), the Linux C<inotify> interface
(for C<ev_stat>), relative timers (C<ev_timer>), absolute timers
with customised rescheduling (C<ev_periodic>), synchronous signals
(C<ev_signal>), process status change events (C<ev_child>), and event
watchers dealing with the event loop mechanism itself (C<ev_idle>,
C<ev_embed>, C<ev_prepare> and C<ev_check> watchers) as well as
file watchers (C<ev_stat>) and even limited support for fork events
It also is quite fast (see this
L<benchmark|http://libev.schmorp.de/bench.html> comparing it to libevent
Libev is very configurable. In this manual the default (and most common)
configuration will be described, which supports multiple event loops. For
more info about various configuration options please have a look at
B<EMBED> section in this manual. If libev was configured without support
for multiple event loops, then all functions taking an initial argument of
name C<loop> (which is always of type C<struct ev_loop *>) will not have
=head2 TIME REPRESENTATION
Libev represents time as a single floating point number, representing the
(fractional) number of seconds since the (POSIX) epoch (somewhere near
the beginning of 1970, details are complicated, don't ask). This type is
called C<ev_tstamp>, which is what you should use too. It usually aliases
to the C<double> type in C, and when you need to do any calculations on
it, you should treat it as some floating point value. Unlike the name
component C<stamp> might indicate, it is also used for time differences
=head1 ERROR HANDLING
Libev knows three classes of errors: operating system errors, usage errors
and internal errors (bugs).
When libev catches an operating system error it cannot handle (for example
a system call indicating a condition libev cannot fix), it calls the callback
set via C<ev_set_syserr_cb>, which is supposed to fix the problem or
abort. The default is to print a diagnostic message and to call C<abort
When libev detects a usage error such as a negative timer interval, then
it will print a diagnostic message and abort (via the C<assert> mechanism,
so C<NDEBUG> will disable this checking): these are programming errors in
the libev caller and need to be fixed there.
Libev also has a few internal error-checking C<assert>ions, and also has
extensive consistency checking code. These do not trigger under normal
circumstances, as they indicate either a bug in libev or worse.
=head1 GLOBAL FUNCTIONS
These functions can be called anytime, even before initialising the
library in any way.
=item ev_tstamp ev_time ()
Returns the current time as libev would use it. Please note that the
C<ev_now> function is usually faster and also often returns the timestamp
you actually want to know.
=item ev_sleep (ev_tstamp interval)
Sleep for the given interval: The current thread will be blocked until
either it is interrupted or the given time interval has passed. Basically
this is a sub-second-resolution C<sleep ()>.
=item int ev_version_major ()
=item int ev_version_minor ()
You can find out the major and minor ABI version numbers of the library
you linked against by calling the functions C<ev_version_major> and
C<ev_version_minor>. If you want, you can compare against the global
symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
version of the library your program was compiled against.
These version numbers refer to the ABI version of the library, not the
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
assert (("libev version mismatch",
ev_version_major () == EV_VERSION_MAJOR
&& ev_version_minor () >= EV_VERSION_MINOR));
=item unsigned int ev_supported_backends ()
Return the set of all backends (i.e. their corresponding C<EV_BACKEND_*>
value) compiled into this binary of libev (independent of their
availability on the system you are running on). See C<ev_default_loop> for
a description of the set values.
Example: make sure we have the epoll method, because yeah this is cool and
a must have and can we have a torrent of it please!!!11
assert (("sorry, no epoll, no sex",
ev_supported_backends () & EVBACKEND_EPOLL));
=item unsigned int ev_recommended_backends ()
Return the set of all backends compiled into this binary of libev and also
recommended for this platform. This set is often smaller than the one
returned by C<ev_supported_backends>, as for example kqueue is broken on
most BSDs and will not be auto-detected unless you explicitly request it
(assuming you know what you are doing). This is the set of backends that
libev will probe for if you specify no backends explicitly.
=item unsigned int ev_embeddable_backends ()
Returns the set of backends that are embeddable in other event loops. This
is the theoretical, all-platform, value. To find which backends
might be supported on the current system, you would need to look at
C<ev_embeddable_backends () & ev_supported_backends ()>, likewise for
See the description of C<ev_embed> watchers for more info.
=item ev_set_allocator (void *(*cb)(void *ptr, long size))
Sets the allocation function to use (the prototype is similar - the
semantics are identical to the C<realloc> C89/SuS/POSIX function). It is
used to allocate and free memory (no surprises here). If it returns zero
when memory needs to be allocated (C<size != 0>), the library might abort
or take some potentially destructive action.
Since some systems (at least OpenBSD and Darwin) fail to implement
correct C<realloc> semantics, libev will use a wrapper around the system
C<realloc> and C<free> functions by default.
You could override this function in high-availability programs to, say,
free some memory if it cannot allocate memory, to use a special allocator,
or even to sleep a while and retry until some memory is available.
Example: Replace the libev allocator with one that waits a bit and then
retries (example requires a standards-compliant C<realloc>).
static void *
persistent_realloc (void *ptr, size_t size)
void *newptr = realloc (ptr, size);
=item ev_set_syserr_cb (void (*cb)(const char *msg));
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.
fatal_error (const char *msg)
=head1 FUNCTIONS CONTROLLING THE EVENT LOOP
An event loop is described by a C<struct ev_loop *>. The library knows two
types of such loops, the I<default> loop, which supports signals and child
events, and dynamically created loops which do not.
=item struct ev_loop *ev_default_loop (unsigned int flags)
This will initialise the default event loop if it hasn't been initialised
yet and return it. If the default loop could not be initialised, returns
false. If it already was initialised it simply returns it (and ignores the
flags. If that is troubling you, check C<ev_backend ()> afterwards).
If you don't know what event loop to use, use the one returned from this
Note that this function is I<not> thread-safe, so if you want to use it
from multiple threads, you have to lock (note also that this is unlikely,
as loops cannot bes hared easily between threads anyway).
The default loop is the only loop that can handle C<ev_signal> and
C<ev_child> watchers, and to do this, it always registers a handler
for C<SIGCHLD>. If this is a problem for your application you can either
create a dynamic loop with C<ev_loop_new> that doesn't do that, or you
can simply overwrite the C<SIGCHLD> signal handler I<after> calling
The flags argument can be used to specify special behaviour or specific
backends to use, and is usually specified as C<0> (or C<EVFLAG_AUTO>).
The following flags are supported:
The default flags value. Use this if you have no clue (it's the right
thing, believe me).
If this flag bit is or'ed into the flag value (or the program runs setuid
or setgid) then libev will I<not> look at the environment variable
C<LIBEV_FLAGS>. Otherwise (the default), this environment variable will
override the flags completely if it is found in the environment. This is
useful to try out specific backends to test their performance, or to work
Instead of calling C<ev_default_fork> or C<ev_loop_fork> manually after
a fork, you can also make libev check for a fork in each iteration by
enabling this flag.
This works by calling C<getpid ()> on every iteration of the loop,
and thus this might slow down your event loop if you do a lot of loop
iterations and little real work, but is usually not noticeable (on my
GNU/Linux system for example, C<getpid> is actually a simple 5-insn sequence
without a system call and thus I<very> fast, but my GNU/Linux system also has
C<pthread_atfork> which is even faster).
The big advantage of this flag is that you can forget about fork (and
forget about forgetting to tell libev about forking) when you use this
This flag setting cannot be overridden or specified in the C<LIBEV_FLAGS>
=item C<EVBACKEND_SELECT> (value 1, portable select backend)
This is your standard select(2) backend. Not I<completely> standard, as
libev tries to roll its own fd_set with no limits on the number of fds,
but if that fails, expect a fairly low limit on the number of fds when
using this backend. It doesn't scale too well (O(highest_fd)), but its
usually the fastest backend for a low number of (low-numbered :) fds.
To get good performance out of this backend you need a high amount of
parallelism (most of the file descriptors should be busy). If you are
writing a server, you should C<accept ()> in a loop to accept as many
connections as possible during one iteration. You might also want to have
a look at C<ev_set_io_collect_interval ()> to increase the amount of
readiness notifications you get per iteration.
=item C<EVBACKEND_POLL> (value 2, poll backend, available everywhere except on windows)
And this is your standard poll(2) backend. It's more complicated
than select, but handles sparse fds better and has no artificial
limit on the number of fds you can use (except it will slow down
considerably with a lot of inactive fds). It scales similarly to select,
i.e. O(total_fds). See the entry for C<EVBACKEND_SELECT>, above, for
=item C<EVBACKEND_EPOLL> (value 4, Linux)
For few fds, this backend is a bit little slower than poll and select,
but it scales phenomenally better. While poll and select usually scale
like O(total_fds) where n is the total number of fds (or the highest fd),
epoll scales either O(1) or O(active_fds). The epoll design has a number
of shortcomings, such as silently dropping events in some hard-to-detect
cases and requiring a system call per fd change, no fork support and bad
support for dup.
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 fd could point to a different file description now), so its
best to avoid that. Also, C<dup ()>'ed file descriptors might not work
very well if you register events for both fds.
Please note that epoll sometimes generates spurious notifications, so you
need to use non-blocking I/O or other means to avoid blocking when no data
(or space) is available.
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.
While nominally embeddable in other event loops, this feature is broken in
all kernel versions tested so far.
=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)
Kqueue deserves special mention, as at the time of this writing, it
was broken on all BSDs except NetBSD (usually it doesn't work reliably
with anything but sockets and pipes, except on Darwin, where of course
it's completely useless). For this reason it's not being "auto-detected"
unless you explicitly specify it explicitly in the flags (i.e. using
C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
system like NetBSD.
You still can embed kqueue into a normal poll or select backend and use it
only for sockets (after having made sure that sockets work with kqueue on
the target platform). See C<ev_embed> watchers for more info.
It scales in the same way as the epoll backend, but the interface to the
kernel is more efficient (which says nothing about its actual speed, of
course). While stopping, setting and starting an I/O watcher does never
cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to
two event changes per incident, support for C<fork ()> is very bad and it
drops fds silently in similarly hard-to-detect cases.
This backend usually performs well under most conditions.
While nominally embeddable in other event loops, this doesn't work
everywhere, so you might need to test for this. And since it is broken
almost everywhere, you should only use it when you have a lot of sockets
(for which it usually works), by embedding it into another event loop
(e.g. C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>) and using it only for
=item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8)
This is not implemented yet (and might never be, unless you send me an
implementation). According to reports, C</dev/poll> only supports sockets
and is not embeddable, which would limit the usefulness of this backend
=item C<EVBACKEND_PORT> (value 32, Solaris 10)
This uses the Solaris 10 event port mechanism. As with everything on Solaris,
it's really slow, but it still scales very well (O(active_fds)).
Please note that Solaris event ports can deliver a lot of spurious
notifications, so you need to use non-blocking I/O or other means to avoid
blocking when no data (or space) is available.
While this backend scales well, it requires one system call per active
file descriptor per loop iteration. For small and medium numbers of file
descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
might perform better.
On the positive side, ignoring the spurious readiness notifications, this
backend actually performed to specification in all tests and is fully
embeddable, which is a rare feat among the OS-specific backends.
Try all backends (even potentially broken ones that wouldn't be tried
with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.
It is definitely not recommended to use this flag.
If one or more of these are or'ed into the flags value, then only these
backends will be tried (in the reverse order as listed here). If none are
specified, all backends in C<ev_recommended_backends ()> will be tried.
The most typical usage is like this:
if (!ev_default_loop (0))
fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
Restrict libev to the select and poll backends, and do not allow
environment settings to be taken into account:
ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
Use whatever libev has to offer, but make sure that kqueue is used if
available (warning, breaks stuff, best use only with your own private
event loop and only if you know the OS supports your types of fds):
ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);
=item struct ev_loop *ev_loop_new (unsigned int flags)
Similar to C<ev_default_loop>, but always creates a new event loop that is
always distinct from the default loop. Unlike the default loop, it cannot
handle signal and child watchers, and attempts to do so will be greeted by
undefined behaviour (or a failed assertion if assertions are enabled).
Note that this function I<is> thread-safe, and the recommended way to use
libev with threads is indeed to create one loop per thread, and using the
default loop in the "main" or "initial" thread.
Example: Try to create a event loop that uses epoll and nothing else.
struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
fatal ("no epoll found here, maybe it hides under your chair");
=item ev_default_destroy ()
Destroys the default loop again (frees all memory and kernel state
etc.). None of the active event watchers will be stopped in the normal
sense, so e.g. C<ev_is_active> might still return true. It is your
responsibility to either stop all watchers cleanly yourself I<before>
calling this function, or cope with the fact afterwards (which is usually
the easiest thing, you can just ignore the watchers and/or C<free ()> them
Note that certain global state, such as signal state, will not be freed by
this function, and related watchers (such as signal and child watchers)
would need to be stopped manually.
In general it is not advisable to call this function except in the
rare occasion where you really need to free e.g. the signal handling
pipe fds. If you need dynamically allocated loops it is better to use
C<ev_loop_new> and C<ev_loop_destroy>).
=item ev_loop_destroy (loop)
Like C<ev_default_destroy>, but destroys an event loop created by an
earlier call to C<ev_loop_new>.
=item ev_default_fork ()
This function sets a flag that causes subsequent C<ev_loop> iterations
to reinitialise the kernel state for backends that have one. Despite the
name, you can call it anytime, but it makes most sense after forking, in
the child process (or both child and parent, but that again makes little
sense). You I<must> call it in the child before using any of the libev
functions, and it will only take effect at the next C<ev_loop> iteration.
On the other hand, you only need to call this function in the child
process if and only if you want to use the event library in the child. If
you just fork+exec, you don't have to call it at all.
The function itself is quite fast and it's usually not a problem to call
it just in case after a fork. To make this easy, the function will fit in
quite nicely into a call to C<pthread_atfork>:
pthread_atfork (0, 0, ev_default_fork);
=item ev_loop_fork (loop)
Like C<ev_default_fork>, but acts on an event loop created by
C<ev_loop_new>. Yes, you have to call this on every allocated event loop
after fork, and how you do this is entirely your own problem.
=item int ev_is_default_loop (loop)
Returns true when the given loop actually is the default loop, false otherwise.
=item unsigned int ev_loop_count (loop)
Returns the count of loop iterations for the loop, which is identical to
the number of times libev did poll for new events. It starts at C<0> and
happily wraps around with enough iterations.
This value can sometimes be useful as a generation counter of sorts (it
"ticks" the number of loop iterations), as it roughly corresponds with
C<ev_prepare> and C<ev_check> calls.
=item unsigned int ev_backend (loop)
Returns one of the C<EVBACKEND_*> flags indicating the event backend in
=item ev_tstamp ev_now (loop)
Returns the current "event loop time", which is the time the event loop
received events and started processing them. This timestamp does not
change as long as callbacks are being processed, and this is also the base
time used for relative timers. You can treat it as the timestamp of the
event occurring (or more correctly, libev finding out about it).
=item ev_loop (loop, int flags)
Finally, this is it, the event handler. This function usually is called
after you initialised all your watchers and you want to start handling
If the flags argument is specified as C<0>, it will not return until
either no event watchers are active anymore or C<ev_unloop> was called.
Please note that an explicit C<ev_unloop> is usually better than
relying on all watchers to be stopped when deciding when a program has
finished (especially in interactive programs), but having a program that
automatically loops as long as it has to and no longer by virtue of
relying on its watchers stopping correctly is a thing of beauty.
A flags value of C<EVLOOP_NONBLOCK> will look for new events, will handle
those events and any outstanding ones, but will not block your process in
case there are no events and will return after one iteration of the loop.
A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
necessary) and will handle those and any outstanding ones. It will block
your process until at least one new event arrives, 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. However, a pair of C<ev_prepare>/C<ev_check> watchers is
usually a better approach for this kind of thing.
Here are the gory details of what C<ev_loop> does:
- Before the first iteration, call any pending watchers.
* If EVFLAG_FORKCHECK was used, check for a fork.
- If a fork was detected, queue and call all fork watchers.
- Queue and call all prepare watchers.
- If we have been forked, recreate the kernel state.
- Update the kernel state with all outstanding changes.
- Update the "event loop time".
- Calculate for how long to sleep or block, if at all
(active idle watchers, EVLOOP_NONBLOCK or not having
any active watchers at all will result in not sleeping).
- Sleep if the I/O and timer collect interval say so.
- Block the process, waiting for any events.
- Queue all outstanding I/O (fd) events.
- Update the "event loop time" and do time jump handling.
- Queue all outstanding timers.
- Queue all outstanding periodics.
- If no events are pending now, queue all idle watchers.
- Queue all check watchers.
- Call all queued watchers in reverse order (i.e. check watchers first).
Signals and child watchers are implemented as I/O watchers, and will
be handled here by queueing them when their watcher gets executed.
- If ev_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
were used, or there are no active watchers, return, otherwise
continue with step *.
Example: Queue some jobs and then loop until no events are outstanding
... queue jobs here, make sure they register event watchers as long
... as they still have work to do (even an idle watcher will do..)
ev_loop (my_loop, 0);
... jobs done. yeah!
=item ev_unloop (loop, how)
Can be used to make a call to C<ev_loop> return early (but only after it
has processed all outstanding events). The C<how> argument must be either
C<EVUNLOOP_ONE>, which will make the innermost C<ev_loop> call return, or
C<EVUNLOOP_ALL>, which will make all nested C<ev_loop> calls return.
This "unloop state" will be cleared when entering C<ev_loop> again.
=item ev_ref (loop)
=item ev_unref (loop)
Ref/unref can be used to add or remove a reference count on the event
loop: Every watcher keeps one reference, and as long as the reference
count is nonzero, C<ev_loop> will not return on its own. If you have
a watcher you never unregister that should not keep C<ev_loop> from
returning, ev_unref() after starting, and ev_ref() before stopping it. For
example, libev itself uses this for its internal signal pipe: It is not
visible to the libev user and should not keep C<ev_loop> from exiting if
no event watchers registered by it are active. It is also an excellent
way to do this for generic recurring timers or from within third-party
libraries. Just remember to I<unref after start> and I<ref before stop>
(but only if the watcher wasn't active before, or was active before,
Example: Create a signal watcher, but keep it from keeping C<ev_loop>
running when nothing else is active.
struct ev_signal exitsig;
ev_signal_init (&exitsig, sig_cb, SIGINT);
ev_signal_start (loop, &exitsig);
Example: For some weird reason, unregister the above signal handler again.
ev_signal_stop (loop, &exitsig);
=item ev_set_io_collect_interval (loop, ev_tstamp interval)
=item ev_set_timeout_collect_interval (loop, ev_tstamp interval)
These advanced functions influence the time that libev will spend waiting
for events. Both are by default C<0>, meaning that libev will try to
invoke timer/periodic callbacks and I/O callbacks with minimum latency.
Setting these to a higher value (the C<interval> I<must> be >= C<0>)
allows libev to delay invocation of I/O and timer/periodic callbacks to
increase efficiency of loop iterations.
The background is that sometimes your program runs just fast enough to
handle one (or very few) event(s) per loop iteration. While this makes
the program responsive, it also wastes a lot of CPU time to poll for new
events, especially with backends like C<select ()> which have a high
overhead for the actual polling but can deliver many events at once.
By setting a higher I<io collect interval> you allow libev to spend more
time collecting I/O events, so you can handle more events per iteration,
at the cost of increasing latency. Timeouts (both C<ev_periodic> and
C<ev_timer>) will be not affected. Setting this to a non-null value will
introduce an additional C<ev_sleep ()> call into most loop iterations.
Likewise, by setting a higher I<timeout collect interval> you allow libev
to spend more time collecting timeouts, at the expense of increased
latency (the watcher callback will be called later). C<ev_io> watchers
will not be affected. Setting this to a non-null value will not introduce
any overhead in libev.
Many (busy) programs can usually benefit by setting the I/O collect
interval to a value near C<0.1> or so, which is often enough for
interactive servers (of course not for games), likewise for timeouts. It
usually doesn't make much sense to set it to a lower value than C<0.01>,
as this approaches the timing granularity of most systems.
=item ev_loop_verify (loop)
This function only does something when C<EV_VERIFY> support has been
compiled in. It tries to go through all internal structures and checks
them for validity. If anything is found to be inconsistent, it will print
an error message to standard error and call C<abort ()>.
This can be used to catch bugs inside libev itself: under normal
circumstances, this function will never abort as of course libev keeps its
data structures consistent.
=head1 ANATOMY OF A WATCHER
A watcher is a structure that you create and register to record your
interest in some event. For instance, if you want to wait for STDIN to
become readable, you would create an C<ev_io> watcher for that:
static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
ev_unloop (loop, EVUNLOOP_ALL);
struct ev_loop *loop = ev_default_loop (0);
struct ev_io stdin_watcher;
ev_init (&stdin_watcher, my_cb);
ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
ev_io_start (loop, &stdin_watcher);
ev_loop (loop, 0);
As you can see, you are responsible for allocating the memory for your
watcher structures (and it is usually a bad idea to do this on the stack,
although this can sometimes be quite valid).
Each watcher structure must be initialised by a call to C<ev_init
(watcher *, callback)>, which expects a callback to be provided. This
callback gets invoked each time the event occurs (or, in the case of I/O
watchers, each time the event loop detects that the file descriptor given
is readable and/or writable).
Each watcher type has its own C<< ev_<type>_set (watcher *, ...) >> macro
with arguments specific to this watcher type. There is also a macro
to combine initialisation and setting in one call: C<< ev_<type>_init
(watcher *, callback, ...) >>.
To make the watcher actually watch out for events, you have to start it
with a watcher-specific start function (C<< ev_<type>_start (loop, watcher
*) >>), and you can stop watching for events at any time by calling the
corresponding stop function (C<< ev_<type>_stop (loop, watcher *) >>.
As long as your watcher is active (has been started but not stopped) you
must not touch the values stored in it. Most specifically you must never
reinitialise it or call its C<set> macro.
Each and every callback receives the event loop pointer as first, the
registered watcher structure as second, and a bitset of received events as
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
The file descriptor in the C<ev_io> watcher has become readable and/or
The C<ev_timer> watcher has timed out.
The C<ev_periodic> watcher has timed out.
The signal specified in the C<ev_signal> watcher has been received by a thread.
The pid specified in the C<ev_child> watcher has received a status change.
The path specified in the C<ev_stat> watcher changed its attributes somehow.
The C<ev_idle> watcher has determined that you have nothing better to do.
All C<ev_prepare> watchers are invoked just I<before> C<ev_loop> starts
to gather new events, and all C<ev_check> watchers are invoked just after
C<ev_loop> has gathered them, but before it invokes any callbacks for any
received events. Callbacks of both watcher types can start and stop as
many watchers as they want, and all of them will be taken into account
(for example, a C<ev_prepare> watcher might start an idle watcher to keep
C<ev_loop> from blocking).
The embedded event loop specified in the C<ev_embed> watcher needs attention.
The event loop has been resumed in the child process after fork (see
The given async watcher has been asynchronously notified (see C<ev_async>).
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. You best act on it by reporting the problem and somehow coping
with the watcher being stopped.
Libev will usually signal a few "dummy" events together with an error,
for example it might indicate that a fd is readable or writable, and if
your callbacks is well-written it can just attempt the operation and cope
with the error from read() or write(). This will not work in multi-threaded
programs, though, so beware.
=head2 GENERIC WATCHER FUNCTIONS
In the following description, C<TYPE> stands for the watcher type,
e.g. C<timer> for C<ev_timer> watchers and C<io> for C<ev_io> watchers.
=item C<ev_init> (ev_TYPE *watcher, callback)
This macro initialises the generic portion of a watcher. The contents
of the watcher object can be arbitrary (so C<malloc> will do). Only
the generic parts of the watcher are initialised, you I<need> to call
the type-specific C<ev_TYPE_set> macro afterwards to initialise the
type-specific parts. For each type there is also a C<ev_TYPE_init> macro
which rolls both calls into one.
You can reinitialise a watcher at any time as long as it has been stopped
(or never started) and there are no pending events outstanding.
The callback is always of type C<void (*)(ev_loop *loop, ev_TYPE *watcher,
=item C<ev_TYPE_set> (ev_TYPE *, [args])
This macro initialises the type-specific parts of a watcher. You need to
call C<ev_init> at least once before you call this macro, but you can
call C<ev_TYPE_set> any number of times. You must not, however, call this
macro on a watcher that is active (it can be pending, however, which is a
difference to the C<ev_init> macro).
Although some watcher types do not have type-specific arguments
(e.g. C<ev_prepare>) you still need to call its C<set> macro.
=item C<ev_TYPE_init> (ev_TYPE *watcher, callback, [args])
This convenience macro rolls both C<ev_init> and C<ev_TYPE_set> macro
calls into a single call. This is the most convenient method to initialise
a watcher. The same limitations apply, of course.
=item C<ev_TYPE_start> (loop *, ev_TYPE *watcher)
Starts (activates) the given watcher. Only active watchers will receive
events. If the watcher is already active nothing will happen.
=item C<ev_TYPE_stop> (loop *, ev_TYPE *watcher)
Stops the given watcher again (if active) and clears the pending
status. It is possible that stopped watchers are pending (for example,
non-repeating timers are being stopped when they become pending), but
C<ev_TYPE_stop> ensures that the watcher is neither active nor pending. If
you want to free or reuse the memory used by the watcher it is therefore a
good idea to always call its C<ev_TYPE_stop> function.
=item bool ev_is_active (ev_TYPE *watcher)
Returns a true value iff the watcher is active (i.e. it has been started
and not yet been stopped). As long as a watcher is active you must not modify
=item bool ev_is_pending (ev_TYPE *watcher)
Returns a true value iff the watcher is pending, (i.e. it has outstanding
events but its callback has not yet been invoked). As long as a watcher
is pending (but not active) you must not call an init function on it (but
C<ev_TYPE_set> is safe), you must not change its priority, and you must
make sure the watcher is available to libev (e.g. you cannot C<free ()>
=item callback ev_cb (ev_TYPE *watcher)
Returns the callback currently set on the watcher.
=item ev_cb_set (ev_TYPE *watcher, callback)
Change the callback. You can change the callback at virtually any time
=item ev_set_priority (ev_TYPE *watcher, priority)
=item int ev_priority (ev_TYPE *watcher)
Set and query the priority of the watcher. The priority is a small
integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI>
(default: C<-2>). Pending watchers with higher priority will be invoked
before watchers with lower priority, but priority will not keep watchers
from being executed (except for C<ev_idle> watchers).
This means that priorities are I<only> used for ordering callback
invocation after new events have been received. This is useful, for
example, to reduce latency after idling, or more often, to bind two
watchers on the same event and make sure one is called first.
If you need to suppress invocation when higher priority events are pending
you need to look at C<ev_idle> watchers, which provide this functionality.
You I<must not> change the priority of a watcher as long as it is active or
The default priority used by watchers when no priority has been set is
always C<0>, which is supposed to not be too high and not be too low :).
Setting a priority outside the range of C<EV_MINPRI> to C<EV_MAXPRI> is
fine, as long as you do not mind that the priority value you query might
or might not have been adjusted to be within valid range.
=item ev_invoke (loop, ev_TYPE *watcher, int revents)
Invoke the C<watcher> with the given C<loop> and C<revents>. Neither
C<loop> nor C<revents> need to be valid as long as the watcher callback
can deal with that fact.
=item int ev_clear_pending (loop, ev_TYPE *watcher)
If the watcher is pending, this function returns clears its pending status
and returns its C<revents> bitset (as if its callback was invoked). If the
watcher isn't pending it does nothing and returns C<0>.
=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
Each watcher has, by default, a member C<void *data> that you can change
and read at any time, libev will completely ignore it. This can be used
to associate arbitrary data with your watcher. If you need more data and
don't want to allocate memory and store a pointer to it in that data
member, you can also "subclass" the watcher type and provide your own
struct ev_io io;
struct whatever *mostinteresting;
And since your callback will be called with a pointer to the watcher, you
can cast it back to your own type:
static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
struct my_io *w = (struct my_io *)w_;
More interesting and less C-conformant ways of casting your callback type
instead have been omitted.
Another common scenario is having some data structure with multiple
In this case getting the pointer to C<my_biggy> is a bit more complicated,
you need to use C<offsetof>:
t1_cb (EV_P_ struct ev_timer *w, int revents)
struct my_biggy big = (struct my_biggy *
(((char *)w) - offsetof (struct my_biggy, t1));
t2_cb (EV_P_ struct ev_timer *w, int revents)
struct my_biggy big = (struct my_biggy *
(((char *)w) - offsetof (struct my_biggy, t2));
=head1 WATCHER TYPES
This section describes each watcher in detail, but will not repeat
information given in the last section. Any initialisation/set macros,
functions and members specific to the watcher type are explained.
Members are additionally marked with either I<[read-only]>, meaning that,
while the watcher is active, you can look at the member and expect some
sensible content, but you must not modify it (you can modify it while the
watcher is stopped to your hearts content), or I<[read-write]>, 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.
=head2 C<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).
If you must do this, then force the use of a known-to-be-good backend
(at the time of this writing, this includes only C<EVBACKEND_SELECT> and
Another thing you have to watch out for is that it is quite easy to
receive "spurious" readiness notifications, that is your callback might
be called with C<EV_READ> but a subsequent C<read>(2) will actually block
because there is no data. Not only are some backends known to create a
lot of those (for example Solaris ports), it is very easy to get into
this situation even with a relatively standard program structure. Thus
it is best to always use non-blocking I/O: An extra C<read>(2) returning
C<EAGAIN> is far preferable to a program hanging until some data arrives.
If you cannot run the fd in non-blocking mode (for example you should not
play around with an Xlib connection), then you have to separately re-test
whether a file descriptor is really ready with a known-to-be good interface
such as poll (fortunately in our Xlib example, Xlib already does this on
its own, so its quite safe to use).
=head3 The special problem of disappearing file descriptors
Some backends (e.g. kqueue, epoll) need to be told about closing a file
descriptor (either by calling C<close> explicitly or by any other means,
such as C<dup>). 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 C<ev_io_set> is being called, libev
will assume that this is potentially a new file descriptor, otherwise
it is assumed that the file descriptor stays the same. That means that
you I<have> to call C<ev_io_set> (or C<ev_io_init>) when you change the
descriptor even if the file descriptor number itself did not change.
This is how one would do it normally anyway, the important point is that
the libev application should not optimise around libev but should leave
optimisations to libev.
=head3 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 C<dup ()>'ed file descriptors or weirder constellations, and register
events for them, only one file descriptor might actually receive events.
There is no workaround possible except not registering events
for potentially C<dup ()>'ed file descriptors, or to resort to
C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
=head3 The special problem of fork
Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
useless behaviour. Libev fully supports fork, but needs to be told about
it in the child.
To support fork in your programs, you either have to call
C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,
enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or
=head3 The special problem of SIGPIPE
While not really specific to libev, it is easy to forget about SIGPIPE:
when reading from a pipe whose other end has been closed, your program
gets send a SIGPIPE, which, by default, aborts your program. For most
programs this is sensible behaviour, for daemons, this is usually
So when you encounter spurious, unexplained daemon exits, make sure you
ignore SIGPIPE (and maybe make sure you log the exit status of your daemon
somewhere, as that would have given you a big clue).
=head3 Watcher-Specific Functions
=item ev_io_init (ev_io *, callback, int fd, int events)
=item ev_io_set (ev_io *, int fd, int events)
Configures an C<ev_io> watcher. The C<fd> is the file descriptor to
receive events for and events is either C<EV_READ>, C<EV_WRITE> or
C<EV_READ | EV_WRITE> to receive the given events.
=item int fd [read-only]
The file descriptor being watched.
=item int events [read-only]
The events being watched.
Example: Call C<stdin_readable_cb> when STDIN_FILENO has become, well
readable, but only once. Since it is likely line-buffered, you could
attempt to read a whole line in the callback.
stdin_readable_cb (struct ev_loop *loop, struct ev_io *w, int revents)
ev_io_stop (loop, w);
.. read from stdin here (or from w->fd) and haqndle any I/O errors
struct ev_loop *loop = ev_default_init (0);
struct ev_io stdin_readable;
ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
ev_io_start (loop, &stdin_readable);
ev_loop (loop, 0);
=head2 C<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) and hour. "Roughly" because
detecting time jumps is hard, and some inaccuracies are unavoidable (the
monotonic clock option helps a lot here).
The relative timeouts are calculated relative to the C<ev_now ()>
time. This is usually the right thing as this timestamp refers to the time
of the event triggering whatever timeout you are modifying/starting. If
you suspect event processing to be delayed and you I<need> to base the timeout
on the current time, use something like this to adjust for this:
ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
The callback is guaranteed to be invoked only after its timeout has passed,
but if multiple timers become ready during the same loop iteration then
order of execution is undefined.
=head3 Watcher-Specific Functions and Data Members
=item ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)
=item ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)
Configure the timer to trigger after C<after> seconds. If C<repeat>
is C<0.>, then it will automatically be stopped once the timeout is
reached. If it is positive, then the timer will automatically be
configured to trigger again C<repeat> seconds later, again, and again,
until stopped manually.
The timer itself will do a best-effort at avoiding drift, that is, if
you configure a timer to trigger every 10 seconds, then it will normally
trigger at exactly 10 second intervals. If, however, your program cannot
keep up with the timer (because it takes longer than those 10 seconds to
do stuff) the timer will not fire more than once per event loop iteration.
=item ev_timer_again (loop, ev_timer *)
This will act as if the timer timed out and restart it again if it is
repeating. The exact semantics are:
If the timer is pending, its pending status is cleared.
If the timer is started but non-repeating, stop it (as if it timed out).
If the timer is repeating, either start it if necessary (with the
C<repeat> value), or reset the running timer to the C<repeat> value.
This sounds a bit complicated, but here is a useful and typical
example: Imagine you have a TCP connection and you want a so-called idle
timeout, that is, you want to be called when there have been, say, 60
seconds of inactivity on the socket. The easiest way to do this is to
configure an C<ev_timer> with a C<repeat> value of C<60> and then call
C<ev_timer_again> 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 C<ev_timer_stop> the timer, and C<ev_timer_again> will
automatically restart it if need be.
That means you can ignore the C<after> value and C<ev_timer_start>
altogether and only ever use the C<repeat> value and C<ev_timer_again>:
ev_timer_init (timer, callback, 0., 5.);
ev_timer_again (loop, timer);
timer->again = 17.;
ev_timer_again (loop, timer);
timer->again = 10.;
ev_timer_again (loop, timer);
This is more slightly efficient then stopping/starting the timer each time
you want to modify its timeout value.
=item ev_tstamp repeat [read-write]
The current C<repeat> value. Will be used each time the watcher times out
or C<ev_timer_again> is called and determines the next timeout (if any),
which is also when any modifications are taken into account.
Example: Create a timer that fires after 60 seconds.
one_minute_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
.. one minute over, w is actually stopped right here
struct 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
timeout_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
.. ten seconds without any activity
struct ev_timer mytimer;
ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
ev_timer_again (&mytimer); /* start timer */
ev_loop (loop, 0);
// and in some piece of code that gets executed on any "activity":
// reset the timeout to start ticking again at 10 seconds
=head2 C<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 C<ev_timer>'s, they are not based on real time (or relative time)
but on wall clock time (absolute time). You can tell a periodic watcher
to trigger after some specific point in time. For example, if you tell a
periodic watcher to trigger in 10 seconds (by specifying e.g. C<ev_now ()
+ 10.>, that is, an absolute time not a delay) and then reset your system
clock to January of the previous year, then it will take more than year
to trigger the event (unlike an C<ev_timer>, which would still trigger
roughly 10 seconds later as it uses a relative timeout).
C<ev_periodic>s can also be used to implement vastly more complex timers,
such as triggering an event on each "midnight, local time", or other
As with timers, the callback is guaranteed to be invoked only when the
time (C<at>) has passed, but if multiple periodic timers become ready
during the same loop iteration then order of execution is undefined.
=head3 Watcher-Specific Functions and Data Members
=item ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)
=item ev_periodic_set (ev_periodic *, ev_tstamp after, ev_tstamp repeat, reschedule_cb)
Lots of arguments, lets sort it out... There are basically three modes of
operation, and we will explain them from simplest to complex:
=item * absolute timer (at = time, interval = reschedule_cb = 0)
In this configuration the watcher triggers an event after the wall clock
time C<at> has passed and doesn't repeat. It will not adjust when a time
jump occurs, that is, if it is to be run at January 1st 2011 then it will
run when the system time reaches or surpasses this time.
=item * repeating interval timer (at = offset, interval > 0, reschedule_cb = 0)
In this mode the watcher will always be scheduled to time out at the next
C<at + N * interval> time (for some integer N, which can also be negative)
and then repeat, regardless of any time jumps.
This can be used to create timers that do not drift with respect to system
time, for example, here is a C<ev_periodic> that triggers each hour, on
ev_periodic_set (&periodic, 0., 3600., 0);
This doesn't mean there will always be 3600 seconds in between triggers,
but only that the callback will be called when the system time shows a
full hour (UTC), or more correctly, when the system time is evenly divisible
Another way to think about it (for the mathematically inclined) is that
C<ev_periodic> will try to run the callback in this mode at the next possible
time where C<time = at (mod interval)>, regardless of any time jumps.
For numerical stability it is preferable that the C<at> value is near
C<ev_now ()> (the current time), but there is no range requirement for
this value, and in fact is often specified as zero.
Note also that there is an upper limit to how often a timer can fire (CPU
speed for example), so if C<interval> is very small then timing stability
will of course deteriorate. Libev itself tries to be exact to be about one
millisecond (if the OS supports it and the machine is fast enough).
=item * manual reschedule mode (at and interval ignored, reschedule_cb = callback)
In this mode the values for C<interval> and C<at> are both being
ignored. Instead, each time the periodic watcher gets scheduled, the
reschedule callback will be called with the watcher as first, and the
current time as second argument.
NOTE: I<This callback MUST NOT stop or destroy any periodic watcher,
ever, or make ANY event loop modifications whatsoever>.
If you need to stop it, return C<now + 1e30> (or so, fudge fudge) and stop
it afterwards (e.g. by starting an C<ev_prepare> watcher, which is the
only event loop modification you are allowed to do).
The callback prototype is C<ev_tstamp (*reschedule_cb)(struct ev_periodic
*w, ev_tstamp now)>, e.g.:
static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
return now + 60.;
It must return the next time to trigger, based on the passed time value
(that is, the lowest time value larger than to the second argument). It
will usually be called just before the callback will be triggered, but
might be called at other times, too.
NOTE: I<< This callback must always return a time that is higher than or
equal to the passed C<now> value >>.
This can be used to create very complex timers, such as a timer that
triggers on "next midnight, local time". To do this, you would calculate the
next midnight after C<now> and return the timestamp value for this. How
you do this is, again, up to you (but it is not trivial, which is the main
reason I omitted it as an example).
=item ev_periodic_again (loop, ev_periodic *)
Simply stops and restarts the periodic watcher again. This is only useful
when you changed some parameters or the reschedule callback would return
a different time than the last time it was called (e.g. in a crond like
program when the crontabs have changed).
=item ev_tstamp ev_periodic_at (ev_periodic *)
When active, returns the absolute time that the watcher is supposed to
=item ev_tstamp offset [read-write]
When repeating, this contains the offset value, otherwise this is the
absolute point in time (the C<at> value passed to C<ev_periodic_set>).
Can be modified any time, but changes only take effect when the periodic
timer fires or C<ev_periodic_again> is being called.
=item ev_tstamp interval [read-write]
The current interval value. Can be modified any time, but changes only
take effect when the periodic timer fires or C<ev_periodic_again> is being
=item ev_tstamp (*reschedule_cb)(struct ev_periodic *w, ev_tstamp now) [read-write]
The current reschedule callback, or C<0>, if this functionality is
switched off. Can be changed any time, but changes only take effect when
the periodic timer fires or C<ev_periodic_again> is being called.
Example: Call a callback every hour, or, more precisely, whenever the
system clock is divisible by 3600. The callback invocation times have
potentially a lot of jitter, but good long-term stability.
clock_cb (struct ev_loop *loop, struct ev_io *w, int revents)
... its now a full hour (UTC, or TAI or whatever your clock follows)
struct ev_periodic hourly_tick;
ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
ev_periodic_start (loop, &hourly_tick);
Example: The same as above, but use a reschedule callback to do it:
my_scheduler_cb (struct ev_periodic *w, ev_tstamp now)
return fmod (now, 3600.) + 3600.;
ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
Example: Call a callback every hour, starting now:
struct ev_periodic hourly_tick;
ev_periodic_init (&hourly_tick, clock_cb,
fmod (ev_now (loop), 3600.), 3600., 0);
ev_periodic_start (loop, &hourly_tick);
=head2 C<ev_signal> - signal me when a signal gets signalled!
Signal watchers will trigger an event when the process receives a specific
signal one or more times. Even though signals are very asynchronous, libev
will try it's best to deliver signals synchronously, i.e. as part of the
normal event processing, like any other event.
You can configure as many watchers as you like per signal. Only when the
first watcher gets started will libev actually register a signal watcher
with the kernel (thus it coexists with your own signal handlers as long
as you don't register any with libev). Similarly, when the last signal
watcher for a signal is stopped libev will reset the signal handler to
SIG_DFL (regardless of what it was set to before).
If possible and supported, libev will install its handlers with
C<SA_RESTART> behaviour enabled, so system calls should not be unduly
interrupted. If you have a problem with system calls getting interrupted by
signals you can block all signals in an C<ev_check> watcher and unblock
them in an C<ev_prepare> watcher.
=head3 Watcher-Specific Functions and Data Members
=item ev_signal_init (ev_signal *, callback, int signum)
=item ev_signal_set (ev_signal *, int signum)
Configures the watcher to trigger on the given signal number (usually one
of the C<SIGxxx> constants).
=item int signum [read-only]
The signal the watcher watches out for.
Example: Try to exit cleanly on SIGINT and SIGTERM.
sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents)
ev_unloop (loop, EVUNLOOP_ALL);
struct ev_signal signal_watcher;
ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
ev_signal_start (loop, &sigint_cb);
=head2 C<ev_child> - watch out for process status changes
Child watchers trigger when your process receives a SIGCHLD in response to
some child status changes (most typically when a child of yours dies). It
is permissible to install a child watcher I<after> the child has been
forked (which implies it might have already exited), as long as the event
loop isn't entered (or is continued from a watcher).
Only the default event loop is capable of handling signals, and therefore
you can only register child watchers in the default event loop.
=head3 Process Interaction
Libev grabs C<SIGCHLD> as soon as the default event loop is
initialised. This is necessary to guarantee proper behaviour even if
the first child watcher is started after the child exits. The occurrence
of C<SIGCHLD> is recorded asynchronously, but child reaping is done
synchronously as part of the event loop processing. Libev always reaps all
children, even ones not watched.
=head3 Overriding the Built-In Processing
Libev offers no special support for overriding the built-in child
processing, but if your application collides with libev's default child
handler, you can override it easily by installing your own handler for
C<SIGCHLD> after initialising the default loop, and making sure the
default loop never gets destroyed. You are encouraged, however, to use an
event-based approach to child reaping and thus use libev's support for
that, so other libev users can use C<ev_child> watchers freely.
=head3 Watcher-Specific Functions and Data Members
=item ev_child_init (ev_child *, callback, int pid, int trace)
=item ev_child_set (ev_child *, int pid, int trace)
Configures the watcher to wait for status changes of process C<pid> (or
I<any> process if C<pid> is specified as C<0>). The callback can look
at the C<rstatus> member of the C<ev_child> watcher structure to see
the status word (use the macros from C<sys/wait.h> and see your systems
C<waitpid> documentation). The C<rpid> member contains the pid of the
process causing the status change. C<trace> must be either C<0> (only
activate the watcher when the process terminates) or C<1> (additionally
activate the watcher when the process is stopped or continued).
=item int pid [read-only]
The process id this watcher watches out for, or C<0>, meaning any process id.
=item int rpid [read-write]
The process id that detected a status change.
=item int rstatus [read-write]
The process exit/trace status caused by C<rpid> (see your systems
C<waitpid> and C<sys/wait.h> documentation for details).
Example: C<fork()> a new process and install a child handler to wait for
child_cb (EV_P_ struct ev_child *w, int revents)
ev_child_stop (EV_A_ w);
printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
pid_t pid = fork ();
if (pid < 0)
else if (pid == 0)
// the forked child executes here
ev_child_init (&cw, child_cb, pid, 0);
ev_child_start (EV_DEFAULT_ &cw);
=head2 C<ev_stat> - did the file attributes just change?
This watches a file system path for attribute changes. That is, it calls
C<stat> regularly (or when the OS says it changed) and sees if it changed
compared to the last time, invoking the callback if it did.
The path does not need to exist: changing from "path exists" to "path does
not exist" is a status change like any other. The condition "path does
not exist" is signified by the C<st_nlink> field being zero (which is
otherwise always forced to be at least one) and all the other fields of
the stat buffer having unspecified contents.
The path I<should> be absolute and I<must not> end in a slash. If it is
relative and your working directory changes, the behaviour is undefined.
Since there is no standard to do this, the portable implementation simply