EVENT(3) OpenBSD Programmer's Manual EVENT(3)NAME
event_init, event_dispatch, event_loop, event_loopexit, event_loopbreak,
event_set, event_base_dispatch, event_base_loop, event_base_loopexit,
event_base_loopbreak, event_base_set, event_base_free, event_add,
event_del, event_once, event_base_once, event_pending, event_initialized,
event_priority_init, event_priority_set, evtimer_set, evtimer_add,
evtimer_del, evtimer_pending, evtimer_initialized, signal_set,
signal_add, signal_del, signal_pending, signal_initialized,
bufferevent_new, bufferevent_free, bufferevent_write,
bufferevent_write_buffer, bufferevent_read, bufferevent_enable,
bufferevent_disable, bufferevent_settimeout, bufferevent_base_set,
evbuffer_new, evbuffer_free, evbuffer_add, evbuffer_add_buffer,
evbuffer_add_printf, evbuffer_add_vprintf, evbuffer_drain,
evbuffer_write, evbuffer_read, evbuffer_find, evbuffer_readline - execute
a function when a specific event occurs
struct event_base *
event_loopexit(struct timeval *tv);
event_set(struct event *ev, int fd, short event, void (*fn)(int, short,
void *), void *arg);
event_base_dispatch(struct event_base *base);
event_base_loop(struct event_base *base, int flags);
event_base_loopexit(struct event_base *base, struct timeval *tv);
event_base_loopbreak(struct event_base *base);
event_base_set(struct event_base *base, struct event *);
event_base_free(struct event_base *base);
event_add(struct event *ev, struct timeval *tv);
event_del(struct event *ev);
event_once(int fd, short event, void (*fn)(int, short, void *), void
*arg, struct timeval *tv);
event_base_once(struct event_base *base, int fd, short event, void
(*fn)(int, short, void *), void *arg, struct timeval *tv);
event_pending(struct event *ev, short event, struct timeval *tv);
event_initialized(struct event *ev);
event_priority_set(struct event *ev, int priority);
evtimer_set(struct event *ev, void (*fn)(int, short, void *), void *arg);
evtimer_add(struct event *ev, struct timeval *);
evtimer_del(struct event *ev);
evtimer_pending(struct event *ev, struct timeval *tv);
evtimer_initialized(struct event *ev);
signal_set(struct event *ev, int signal, void (*fn)(int, short, void
*), void *arg);
signal_add(struct event *ev, struct timeval *);
signal_del(struct event *ev);
signal_pending(struct event *ev, struct timeval *tv);
signal_initialized(struct event *ev);
struct bufferevent *
bufferevent_new(int fd, evbuffercb readcb, evbuffercb
writecb, everrorcb, void *cbarg);
bufferevent_free(struct bufferevent *bufev);
bufferevent_write(struct bufferevent *bufev, const void *data, size_t
bufferevent_write_buffer(struct bufferevent *bufev, struct evbuffer
bufferevent_read(struct bufferevent *bufev, void *data, size_t size);
bufferevent_enable(struct bufferevent *bufev, short event);
bufferevent_disable(struct bufferevent *bufev, short event);
bufferevent_settimeout(struct bufferevent *bufev, int timeout_read, int
bufferevent_base_set(struct event_base *base, struct bufferevent *bufev);
struct evbuffer *
evbuffer_free(struct evbuffer *buf);
evbuffer_add(struct evbuffer *buf, const void *data, size_t size);
evbuffer_add_buffer(struct evbuffer *dst, struct evbuffer *src);
evbuffer_add_printf(struct evbuffer *buf, const char *fmt, ...);
evbuffer_add_vprintf(struct evbuffer *buf, const char *fmt, va_list ap);
evbuffer_drain(struct evbuffer *buf, size_t size);
evbuffer_write(struct evbuffer *buf, int fd);
evbuffer_read(struct evbuffer *buf, int fd, int size);
evbuffer_find(struct evbuffer *buf, const u_char *data, size_t size);
evbuffer_readline(struct evbuffer *buf);
The event API provides a mechanism to execute a function when a specific
event on a file descriptor occurs or after a given time has passed.
The event API needs to be initialized with event_init() before it can be
In order to process events, an application needs to call
event_dispatch(). This function only returns on error, and should
replace the event core of the application program.
The function event_set() prepares the event structure ev to be used in
future calls to event_add() and event_del(). The event will be prepared
to call the function specified by the fn argument with an int argument
indicating the file descriptor, a short argument indicating the type of
event, and a void * argument given in the arg argument. The fd indicates
the file descriptor that should be monitored for events. The events can
be either EV_READ, EV_WRITE, or both, indicating that an application can
read or write from the file descriptor respectively without blocking.
The function fn will be called with the file descriptor that triggered
the event and the type of event which will be either EV_TIMEOUT,
EV_SIGNAL, EV_READ, or EV_WRITE. Additionally, an event which has
registered interest in more than one of the preceding events, via
bitwise-OR to event_set(), can provide its callback function with a
bitwise-OR of more than one triggered event. The additional flag
EV_PERSIST makes an event_add() persistent until event_del() has been
Once initialized, the ev structure can be used repeatedly with
event_add() and event_del() and does not need to be reinitialized unless
the function called and/or the argument to it are to be changed.
However, when an ev structure has been added to libevent using
event_add() the structure must persist until the event occurs (assuming
EV_PERSIST is not set) or is removed using event_del(). You may not
reuse the same ev structure for multiple monitored descriptors; each
descriptor needs its own ev.
The function event_add() schedules the execution of the ev event when the
event specified in event_set() occurs or in at least the time specified
in the tv. If tv is NULL, no timeout occurs and the function will only
be called if a matching event occurs on the file descriptor. The event
in the ev argument must be already initialized by event_set() and may not
be used in calls to event_set() until it has timed out or been removed
with event_del(). If the event in the ev argument already has a
scheduled timeout, the old timeout will be replaced by the new one.
The function event_del() will cancel the event in the argument ev. If
the event has already executed or has never been added the call will have
The functions evtimer_set(), evtimer_add(), evtimer_del(),
evtimer_initialized(), and evtimer_pending() are abbreviations for common
situations where only a timeout is required. The file descriptor passed
will be -1, and the event type will be EV_TIMEOUT.
The functions signal_set(), signal_add(), signal_del(),
signal_initialized(), and signal_pending() are abbreviations. The event
type will be a persistent EV_SIGNAL. That means signal_set() adds
The function event_once() is similar to event_set(). However, it
schedules a callback to be called exactly once and does not require the
caller to prepare an event structure. This function supports EV_TIMEOUT,
EV_READ, and EV_WRITE.
The event_pending() function can be used to check if the event specified
by event is pending to run. If EV_TIMEOUT was specified and tv is not
NULL, the expiration time of the event will be returned in tv.
The event_initialized() macro can be used to check if an event has been
The event_loop function provides an interface for single pass execution
of pending events. The flags EVLOOP_ONCE and EVLOOP_NONBLOCK are
recognized. The event_loopexit function exits from the event loop. The
next event_loop() iteration after the given timer expires will complete
normally (handling all queued events) then exit without blocking for
events again. Subsequent invocations of event_loop() will proceed
normally. The event_loopbreak function exits from the event loop
immediately. event_loop() will abort after the next event is completed;
event_loopbreak() is typically invoked from this event's callback. This
behavior is analogous to the "break;" statement. Subsequent invocations
of event_loop() will proceed normally.
It is the responsibility of the caller to provide these functions with
pre-allocated event structures.
By default libevent schedules all active events with the same priority.
However, sometimes it is desirable to process some events with a higher
priority than others. For that reason, libevent supports strict priority
queues. Active events with a lower priority are always processed before
events with a higher priority.
The number of different priorities can be set initially with the
event_priority_init() function. This function should be called before
the first call to event_dispatch(). The event_priority_set() function
can be used to assign a priority to an event. By default, libevent
assigns the middle priority to all events unless their priority is
THREAD SAFE EVENTS
Libevent has experimental support for thread-safe events. When
initializing the library via event_init(), an event base is returned.
This event base can be used in conjunction with calls to
event_base_set(), event_base_dispatch(), event_base_loop(),
event_base_loopexit(), bufferevent_base_set() and event_base_free().
event_base_set() should be called after preparing an event with
event_set(), as event_set() assigns the provided event to the most
recently created event base. bufferevent_base_set() should be called
after preparing a bufferevent with bufferevent_new(). event_base_free()
should be used to free memory associated with the event base when it is
no longer needed.
libevent provides an abstraction on top of the regular event callbacks.
This abstraction is called a buffered event. A buffered event provides
input and output buffers that get filled and drained automatically. The
user of a buffered event no longer deals directly with the IO, but
instead is reading from input and writing to output buffers.
A new bufferevent is created by bufferevent_new(). The parameter fd
specifies the file descriptor from which data is read and written to.
This file descriptor is not allowed to be a pipe(2). The next three
parameters are callbacks. The read and write callback have the following
form: void (*cb)(struct bufferevent *bufev, void *arg). The error
callback has the following form: void (*cb)(struct bufferevent *bufev,
short what, void *arg). The argument is specified by the fourth
parameter cbarg. A bufferevent struct pointer is returned on success,
NULL on error. Both the read and the write callback may be NULL. The
error callback has to be always provided.
Once initialized, the bufferevent structure can be used repeatedly with
bufferevent_enable() and bufferevent_disable(). The flags parameter can
be a combination of EV_READ and EV_WRITE. When read enabled the
bufferevent will try to read from the file descriptor and call the read
callback. The write callback is executed whenever the output buffer is
drained below the write low watermark, which is 0 by default.
The bufferevent_write() function can be used to write data to the file
descriptor. The data is appended to the output buffer and written to the
descriptor automatically as it becomes available for writing.
bufferevent_write() returns 0 on success or -1 on failure. The
bufferevent_read() function is used to read data from the input buffer,
returning the amount of data read.
If multiple bases are in use, bufferevent_base_set() must be called
before enabling the bufferevent for the first time.
It is possible to disable support for kqueue, poll or select by setting
the environment variable EVENT_NOKQUEUE, EVENT_NOPOLL or EVENT_NOSELECT,
respectively. By setting the environment variable EVENT_SHOW_METHOD,
libevent displays the kernel notification method that it uses.
Upon successful completion event_add() and event_del() return 0.
Otherwise, -1 is returned and the global variable errno is set to
indicate the error.
SEE ALSOkqueue(2), poll(2), select(2), timeout(9)HISTORY
The event API manpage is based on the timeout(9) manpage by Artur
Grabowski. Support for real-time signals was added by Taral.
The event library was written by Niels Provos.
This documentation is neither complete nor authoritative. If you are in
doubt about the usage of this API then check the source code to find out
how it works, write up the missing piece of documentation and send it to
me for inclusion in this man page.
OpenBSD 4.9 April 22, 2010 OpenBSD 4.9