+ +

Index

+ +

NAME
SYNOPSIS
DESCRIPTION
FEATURES
CONVENTIONS
TIME AND OTHER GLOBAL FUNCTIONS
FUNCTIONS CONTROLLING THE EVENT LOOP
ANATOMY OF A WATCHER +
- ASSOCIATING CUSTOM DATA WITH A WATCHER
+
WATCHER TYPES +
+
OTHER FUNCTIONS
AUTHOR +

+ + +

NAME

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

+ +

SYNOPSIS

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  #include <ev.h>
+
+

+ +

DESCRIPTION

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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 occuring), and it will manage +these event sources and provide your program events.

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

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

+ +

FEATURES

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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).

+ +

CONVENTIONS

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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 +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 loop (which is always of type struct ev_loop *) +will not have this argument.

+ +

TIME AND OTHER GLOBAL FUNCTIONS

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Libev represents time as a single floating point number. This type is +called ev_tstamp, which is what you should use too. It usually aliases +to the double type in C.

ev_tstamp ev_time ()

Returns the current time as libev would use it.

int ev_version_major ()

int ev_version_minor ()

You can find out the major and minor version numbers of the library +you linked against by calling the functions ev_version_major and +ev_version_minor. If you want, you can compare against the global +symbols EV_VERSION_MAJOR and EV_VERSION_MINOR, which specify the +version of the library your program was compiled against.

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.

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.

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).

+ +

FUNCTIONS CONTROLLING THE EVENT LOOP

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An event loop is described by a struct ev_loop *. The library knows two +types of such loops, the 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).

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:

EVFLAG_AUTO: +
The default flags value. Use this if you have no clue (its the right +thing, believe me).
+
EVFLAG_NOENV: +
If this flag bit is ored into the flag value then libev will not look +at the environment variable 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.
+
EVMETHOD_SELECT portable select backend
EVMETHOD_POLL poll backend (everywhere except windows)
EVMETHOD_EPOLL linux only
EVMETHOD_KQUEUE some bsds only
EVMETHOD_DEVPOLL solaris 8 only
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.
+

struct ev_loop *ev_loop_new (unsigned int flags)

Similar to 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).

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 :).

ev_loop_destroy (loop)

Like ev_default_destroy, but destroys an event loop created by an +earlier call to ev_loop_new.

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 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 pthread_atfork:

    pthread_atfork (0, 0, ev_default_fork);
+
+

ev_loop_fork (loop)

Like ev_default_fork, but acts on an event loop created by +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.

unsigned int ev_method (loop)

Returns one of the EVMETHOD_* flags indicating the event backend in +use.

ev_tstamp = ev_now (loop)

Returns the current "event loop time", which is the time the event loop +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).

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 ev_unloop was called.

A flags value of 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 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 prepare and check watchers provide a better and +more generic mechanism.

ev_unloop (loop, how)

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

ev_ref (loop)

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 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.

+ +

ANATOMY OF A WATCHER

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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 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 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: ev_<type>_init +(watcher *, callback, ...).

To make the watcher actually watch out for events, you have to start it +with a watcher-specific start function (ev_<type>_start (loop, watcher +*)), and you can stop watching for events at any time by calling the +corresponding stop function (ev_<type>_stop (loop, watcher *).

As long as your watcher is active (has been started but not stopped) you +must not touch the values stored in it. Most specifically you must never +reinitialise it or call its set method.

You cna check wether an event is active by calling the ev_is_active +(watcher *) macro. To see wether an event is outstanding (but the +callback for it has not been called yet) you cna use the 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:

EV_READ

EV_WRITE

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

EV_TIMEOUT

The ev_timer watcher has timed out.

EV_PERIODIC

The ev_periodic watcher has timed out.

EV_SIGNAL

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

EV_CHILD

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

EV_IDLE

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

EV_PREPARE

EV_CHECK

All ev_prepare watchers are invoked just before ev_loop starts +to gather new events, and all ev_check watchers are invoked just after +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 +ev_loop from blocking).

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.

+ +

ASSOCIATING CUSTOM DATA WITH A WATCHER

Each watcher has, by default, a member 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....

+ + + + + +

WATCHER TYPES

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

+ +

struct ev_io - is my file descriptor readable or writable

I/O watchers check wether 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).

ev_io_init (ev_io *, callback, int fd, int events)
ev_io_set (ev_io *, int fd, int events): +
Configures an ev_io watcher. The fd is the file descriptor to rceeive +events for and events is either EV_READ, EV_WRITE or EV_READ | +EV_WRITE to receive the given events.
+

+ +

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).

ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)

ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)

Configure the timer to trigger after after seconds. If repeat is +0., then it will automatically be stopped. If it is positive, then the +timer will automatically be configured to trigger again repeat seconds +later, again, and again, until stopped manually.

The timer itself will do a best-effort at avoiding drift, that is, if you +configure a timer to trigger every 10 seconds, then it will 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.

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.

+ +

ev_periodic

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.

ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)

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:

+ + + + +

* absolute timer (interval = reschedule_cb = 0)

In this configuration the watcher triggers an event at the wallclock time +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.

* non-repeating interval timer (interval > 0, reschedule_cb = 0)

In this mode the watcher will always be scheduled to time out at the next +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 time = at (mod interval), regardless of any time jumps.

* manual reschedule mode (reschedule_cb = callback)

In this mode the values for interval and 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: 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 now and return the timestamp value for this. How you do this +is, again, up to you (but it is not trivial).

ev_periodic_again (loop, ev_periodic *)

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

+ +

ev_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).

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

+ +

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).

ev_child_init (ev_child *, callback, int pid)
ev_child_set (ev_child *, int pid): +
Configures the watcher to wait for status changes of process pid (or +any process if pid is specified as 0). The callback can look +at the rstatus member of the ev_child watcher structure to see +the status word (use the macros from sys/wait.h). The rpid member +contains the pid of the process causing the status change.
+

+ +

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.

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

+ +

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.

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

+ +

OTHER FUNCTIONS

Top

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

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 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 fd and events set +will be craeted and started.

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

The callback has the type 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 arg value passed to 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);
+
+

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).

ev_feed_fd_event (loop, int fd, int revents)

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

ev_feed_signal_event (loop, int signum)

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

+ +

AUTHOR

Top

Marc Lehmann <libev@schmorp.de>.

+ +