[software/libev.git] / ev.pod

=head1 NAME

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

=head1 SYNOPSIS

  #include <ev.h>

=head1 DESCRIPTION

Libev is an event loop: you register interest in certain events (such as a
file descriptor being readable or a timeout occuring), and it will manage
these event sources and provide your program with events.

To do this, it must take more or less complete control over your process
(or thread) by executing the I<event loop> handler, and will then
communicate events via a callback mechanism.

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

=head1 FEATURES

Libev supports select, poll, the linux-specific epoll and the bsd-specific
kqueue mechanisms for file descriptor events, relative timers, absolute
timers with customised rescheduling, signal events, process status change
events (related to SIGCHLD), and event watchers dealing with the event
loop mechanism itself (idle, prepare and check watchers).

=head1 CONVENTIONS

Libev is very configurable. In this manual the default configuration
will be described, which supports multiple event loops. For more info
about various configuraiton options please have a look at the file
F<README.embed> in the libev distribution. 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 this argument.

=head1 TIME AND OTHER GLOBAL FUNCTIONS

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 double type in C.

=over 4

=item ev_tstamp ev_time ()

Returns the current time as libev would use it.

=item int ev_version_major ()

=item int ev_version_minor ()

You can find out the major and minor 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.

Usually, its 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.

=item ev_set_allocator (void *(*cb)(void *ptr, long size))

Sets the allocation function to use (the prototype is similar to the
realloc function). It is used to allocate and free memory (no surprises
here). If it returns zero when memory needs to be allocated, the library
might abort or take some potentially destructive action. The default is
your system realloc function.

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.

=item ev_set_syserr_cb (void (*cb)(const char *msg));

Set the callback function to call on a retryable syscall 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 sitution, no
matter what, when it returns. That is, libev will geenrally retry the
requested operation, or, if the condition doesn't go away, do bad stuff
(such as abort).

=back

=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.

If you use threads, a common model is to run the default event loop
in your main thread (or in a separate thrad) and for each thread you
create, you also create another event loop. Libev itself does no lockign
whatsoever, so if you mix calls to different event loops, make sure you
lock (this is usually a bad idea, though, even if done right).

=over 4

=item struct ev_loop *ev_default_loop (unsigned int flags)

This will initialise the default event loop if it hasn't been initialised
yet and return it. If the default loop could not be initialised, returns
false. If it already was initialised it simply returns it (and ignores the
flags).

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

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

It supports the following flags:

=over 4

=item EVFLAG_AUTO

The default flags value. Use this if you have no clue (its the right
thing, believe me).

=item EVFLAG_NOENV

If this flag bit is ored into the flag value then libev will I<not> look
at the environment variable C<LIBEV_FLAGS>. Otherwise (the default), this
environment variable will override the flags completely. This is useful
to try out specific backends to tets their performance, or to work around
bugs.

=item EVMETHOD_SELECT  portable select backend

=item EVMETHOD_POLL    poll backend (everywhere except windows)

=item EVMETHOD_EPOLL   linux only

=item EVMETHOD_KQUEUE  some bsds only

=item EVMETHOD_DEVPOLL solaris 8 only

=item EVMETHOD_PORT    solaris 10 only

If one or more of these are ored into the flags value, then only these
backends will be tried (in the reverse order as given here). If one are
specified, any backend will do.

=back

=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).

=item ev_default_destroy ()

Destroys the default loop again (frees all memory and kernel state
etc.). This stops all registered event watchers (by not touching them in
any way whatsoever, although you cnanot rely on this :).

=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 reinitialises the kernel state for backends that have
one. Despite the name, you can call it anytime, but it makes most sense
after forking, in either the parent or child process (or both, but that
again makes little sense).

You I<must> call this function after forking if and only if you want to
use the event library in both processes. If you just fork+exec, you don't
have to call it.

The function itself is quite fast and its 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 unsigned int ev_method (loop)

Returns one of the C<EVMETHOD_*> flags indicating the event backend in
use.

=item ev_tstamp = ev_now (loop)

Returns the current "event loop time", which is the time the event loop
got 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
occuring (or more correctly, the mainloop 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
events.

If the flags argument is specified as 0, it will not return until either
no event watchers are active anymore or C<ev_unloop> was called.

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.

A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
neccessary) and will handle those and any outstanding ones. It will block
your process until at least one new event arrives.

This flags value could be used to implement alternative looping
constructs, but the C<prepare> and C<check> watchers provide a better and
more generic mechanism.

=item ev_unloop (loop, how)

Can be used to make a call to C<ev_loop> return early. The C<how> argument
must be either C<EVUNLOOP_ONCE>, which will make the innermost C<ev_loop>
call return, or C<EVUNLOOP_ALL>, which will make all nested C<ev_loop>
calls return.

=item ev_ref (loop)

=item ev_unref (loop)

Ref/unref can be used to add or remove a refcount on the event loop: Every
watcher keeps one reference. If you have a long-runing watcher you never
unregister that should not keep ev_loop from running, ev_unref() after
starting, and ev_ref() before stopping it. Libev itself uses this for
example for its internal signal pipe: It is not visible to you as a user
and should not keep C<ev_loop> from exiting if the work is done. It is
also an excellent way to do this for generic recurring timers or from
within third-party libraries. Just remember to unref after start and ref
before stop.

=back

=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 ev_io watcher for that:

  static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
  {
    ev_io_stop (w);
    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 io
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 set method.

You cna check whether an event is active by calling the C<ev_is_active
(watcher *)> macro. To see whether an event is outstanding (but the
callback for it has not been called yet) you cna use the C<ev_is_pending
(watcher *)> 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 rceeived events usually include a single bit per event type received
(you can receive multiple events at the same time). The possible bit masks
are:

=over 4

=item EV_READ

=item EV_WRITE

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

=item EV_TIMEOUT

The ev_timer watcher has timed out.

=item EV_PERIODIC

The ev_periodic watcher has timed out.

=item EV_SIGNAL

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

=item EV_CHILD

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

=item EV_IDLE

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

=item EV_PREPARE

=item EV_CHECK

All ev_prepare watchers are invoked just I<before> C<ev_loop> starts
to gather new events, and all 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 ev_prepare watcher might start an idle watcher to keep
C<ev_loop> from blocking).

=item EV_ERROR

An unspecified error has occured, 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 multithreaded
programs, though, so beware.

=back

=head2 ASSOCIATING CUSTOM DATA WITH A WATCHER

Each watcher has, by default, a member C<void *data> that you can change
and read at any time, libev will completely ignore it. This cna be used
to associate arbitrary data with your watcher. If you need more data and
don't want to allocate memory and store a pointer to it in that data
member, you can also "subclass" the watcher type and provide your own
data:

  struct my_io
  {
    struct ev_io io;
    int otherfd;
    void *somedata;
    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 catsing your callback type
have been omitted....


=head1 WATCHER TYPES

This section describes each watcher in detail, but will not repeat
information given in the last section.

=head2 struct ev_io - is my file descriptor readable or writable

I/O watchers check whether a file descriptor is readable or writable
in each iteration of the event loop (This behaviour is called
level-triggering because you keep receiving events as long as the
condition persists. Remember you cna stop the watcher if you don't want to
act on the event and neither want to receive future events).

=over 4

=item ev_io_init (ev_io *, callback, int fd, int events)

=item ev_io_set (ev_io *, int fd, int events)

Configures an ev_io watcher. The fd is the file descriptor to rceeive
events for and events is either C<EV_READ>, C<EV_WRITE> or C<EV_READ |
EV_WRITE> to receive the given events.

=back

=head2 struct ev_timer - relative and optionally recurring 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 youreset your system clock to last years
time, it will still time out after (roughly) and hour. "Roughly" because
detecting time jumps is hard, and soem inaccuracies are unavoidable (the
monotonic clock option helps a lot here).

=over 4

=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. 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 trigger at
exactly 10 second intervals. If, however, your program cannot keep up with
the timer (ecause 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)

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 started but nonrepeating, stop it.

If the timer is repeating, either start it if necessary (with the repeat
value), or reset the running timer to the 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 ev_timer with after=repeat=60 and calling 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 stop
the timer, and again will automatically restart it if need be.

=back

=head2 ev_periodic - to cron or not to cron it

Periodic watchers are also timers of a kind, but they are very versatile
(and unfortunately a bit complex).

Unlike ev_timer's, they are not based on real time (or relative time)
but on wallclock time (absolute time). You can tell a periodic watcher
to trigger "at" some specific point in time. For example, if you tell a
periodic watcher to trigger in 10 seconds (by specifiying e.g. c<ev_now ()
+ 10.>) and then reset your system clock to the last year, then it will
take a year to trigger the event (unlike an ev_timer, which would trigger
roughly 10 seconds later and of course not if you reset your system time
again).

They can also be used to implement vastly more complex timers, such as
triggering an event on eahc midnight, local time.

=over 4

=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:


=over 4

=item * absolute timer (interval = reschedule_cb = 0)

In this configuration the watcher triggers an event at the wallclock time
C<at> 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 * non-repeating interval timer (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) and then repeat, regardless
of any time jumps.

This can be used to create timers that do not drift with respect to system
time:

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

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

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

=item * manual reschedule mode (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 the periodic or any other
periodic watcher, ever, or make any event loop modificstions>. If you need
to stop it, return 1e30 (or so, fudge fudge) and stop it afterwards.

Its 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.

This can be used to create very complex timers, such as a timer that
triggers on each 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).

=back

=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).

=back

=head2 ev_signal - signal me when a signal gets signalled

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

You cna 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).

=over 4

=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).

=back

=head2 ev_child - wait for pid 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).

=over 4

=item ev_child_init (ev_child *, callback, int pid)

=item ev_child_set (ev_child *, int pid)

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>). The C<rpid> member
contains the pid of the process causing the status change.

=back

=head2 ev_idle - when you've got nothing better to do

Idle watchers trigger events when there are no other I/O or timer (or
periodic) events pending. That is, as long as your process is busy
handling sockets or timeouts it will not be called. But when your process
is idle all idle watchers are being called again and again - until
stopped, that is, or your process receives more events.

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

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

=over 4

=item ev_idle_init (ev_signal *, callback)

Initialises and configures the idle watcher - it has no parameters of any
kind. There is a C<ev_idle_set> macro, but using it is utterly pointless,
believe me.

=back

=head2 prepare and check - your hooks into the event loop

Prepare and check watchers usually (but not always) are used in
tandom. Prepare watchers get invoked before the process blocks and check
watchers afterwards.

Their main purpose is to integrate other event mechanisms into libev. This
could be used, for example, to track variable changes, implement your own
watchers, integrate net-snmp or a coroutine library and lots more.

This is done by examining in each prepare call which file descriptors need
to be watched by the other library, registering ev_io watchers for them
and starting an ev_timer watcher for any timeouts (many libraries provide
just this functionality). Then, in the check watcher you check for any
events that occured (by making your callbacks set soem flags for example)
and call back into the library.

As another example, the perl Coro module uses these hooks to integrate
coroutines into libev programs, by yielding to other active coroutines
during each prepare and only letting the process block if no coroutines
are ready to run.

=over 4

=item ev_prepare_init (ev_prepare *, callback)

=item ev_check_init (ev_check *, callback)

Initialises and configures the prepare or check watcher - they have no
parameters of any kind. There are C<ev_prepare_set> and C<ev_check_set>
macros, but using them is utterly, utterly pointless.

=back

=head1 OTHER FUNCTIONS

There are some other fucntions of possible interest. Described. Here. Now.

=over 4

=item ev_once (loop, int fd, int events, ev_tstamp timeout, callback)

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

If C<fd> is less than 0, then no I/O watcher will be started and events is
ignored. Otherwise, an ev_io watcher for the given C<fd> and C<events> set
will be craeted and started.

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

The callback has the type C<void (*cb)(int revents, void *arg)> and
gets passed an events set (normally a combination of EV_ERROR, EV_READ,
EV_WRITE or EV_TIMEOUT) and the C<arg> value passed to C<ev_once>:

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

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

=item ev_feed_event (loop, watcher, int events)

Feeds the given event set into the event loop, as if the specified event
has happened for the specified watcher (which must be a pointer to an
initialised but not necessarily active event watcher).

=item ev_feed_fd_event (loop, int fd, int revents)

Feed an event on the given fd, as if a file descriptor backend detected it.

=item ev_feed_signal_event (loop, int signum)

Feed an event as if the given signal occured (loop must be the default loop!).

=back

=head1 AUTHOR

Marc Lehmann <libev@schmorp.de>.