EVENT(3) BSD Library Functions Manual EVENT(3)
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, evhttp_new,
evhttp_bind_socket, evhttp_free -- 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, void *data, size_t size);
bufferevent_write_buffer(struct bufferevent *bufev, struct evbuffer *buf);
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 timeout_write);
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);
struct evhttp *
evhttp_new(struct event_base *base);
evhttp_bind_socket(struct evhttp *http, const char *address, u_short port);
evhttp_free(struct evhttp *http);
volatile sig_atomic_t event_gotsig;
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 used.
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 indi-
cating 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. Addition-
ally, an event which has registered interest in more than one of the preceeding 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 called.
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 descrip-
tor. 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 no effect.
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 EV_PERSIST.
In order to avoid races in signal handlers, the event API provides two variables:
event_sigcb and event_gotsig. A signal handler sets event_gotsig to indicate that a signal
has been received. The application sets event_sigcb to a callback function. After the sig-
nal handler sets event_gotsig, event_dispatch will execute the callback function to process
received signals. The callback returns 1 when no events are registered any more. It can
return -1 to indicate an error to the event library, causing event_dispatch() to terminate
with errno set to EINTR.
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 initialized.
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 com-
plete 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 com-
pleted; 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
It is the responsibility of the caller to provide these functions with pre-allocated event
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() func-
tion. 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 explicitly set.
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 pre-
paring 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 associ-
ated 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.
NON-BLOCKING HTTP SUPPORT
libevent provides a very thin HTTP layer that can be used both to host an HTTP server and
also to make HTTP requests. An HTTP server can be created by calling evhttp_new(). It can
be bound to any port and address with the evhttp_bind_socket() function. When the HTTP
server is no longer used, it can be freed via evhttp_free().
To be notified of HTTP requests, a user needs to register callbacks with the HTTP server.
This can be done by calling evhttp_set_cb(). The second argument is the URI for which a
callback is being registered. The corresponding callback will receive an struct
evhttp_request object that contains all information about the request.
This section does not document all the possible function calls; please check event.h for the
It is possible to disable support for epoll, kqueue, devpoll, poll or select by setting the
environment variable EVENT_NOEPOLL, EVENT_NOKQUEUE, EVENT_NODEVPOLL, 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.
kqueue(2), poll(2), select(2), evdns(3), timeout(9)
The event API manpage is based on the timeout(9) manpage by Artur Grabowski. The port of
libevent to Windows is due to Michael A. Davis. Support for real-time signals is due to
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.
BSD August 8, 2000 BSD