EVENT(3)                BSD Library Functions Manual                EVENT(3)

NAME         top

     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

SYNOPSIS         top

     #include <sys/time.h>
     #include <event.h>

     struct event_base *


     event_loop(int flags);

     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_init(int npriorities);

     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);

     unsigned char *
     evbuffer_find(struct evbuffer *buf, const unsigned char *data,
         size_t size);

     char *
     evbuffer_readline(struct evbuffer *buf);

     struct evhttp *
     evhttp_new(struct event_base *base);

     evhttp_bind_socket(struct evhttp *http, const char *address,
         unsigned short port);

     evhttp_free(struct evhttp *http);

     int (*event_sigcb)(void);

     volatile sig_atomic_t event_gotsig;

DESCRIPTION         top

     The event API provides a mechanism to execute a function when a spe‐
     cific event on a file descriptor occurs or after a given time has

     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 pre‐
     pared 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, indicat‐
     ing 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 reg‐
     istered interest in more than one of the preceeding events, via bit‐
     wise-OR to event_set(), can provide its callback function with a bit‐
     wise-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 spec‐
     ified 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

     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 com‐
     mon 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 applica‐
     tion sets event_sigcb to a callback function.  After the signal 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

     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

     The event_pending() function can be used to check if the event speci‐
     fied 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 rec‐
     ognized.  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 nor‐
     mally.  The event_loopbreak function exits from the event loop immedi‐
     ately.  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 prior‐
     ity 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
     explicitly set.


     Libevent has experimental support for thread-safe events.  When ini‐
     tializing 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 follow‐
     ing 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 param‐
     eter 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.


     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 regis‐
     tered.  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 public interfaces.


     It is possible to disable support for epoll, kqueue, devpoll, poll or
     select by setting the environment variable EVENT_NOEPOLL,
     respectively.  By setting the environment variable EVENT_SHOW_METHOD,
     libevent displays the kernel notification method that it uses.

RETURN VALUES         top

     Upon successful completion event_add() and event_del() return 0.  Oth‐
     erwise, -1 is returned and the global variable errno is set to indicate
     the error.

SEE ALSO         top

     kqueue(2), poll(2), select(2), evdns(3), timeout(9)

HISTORY         top

     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 Taral.

AUTHORS         top

     The event library was written by Niels Provos.

BUGS         top

     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.

COLOPHON         top

     This page is part of the libevent (an event notification library)
     project.  Information about the project can be found at  If you have a bug report for this manual page,
     see ⟨⟩.  This page was obtained
     from the project's upstream Git repository
     ⟨⟩ on 2018-02-02.  (At that
     time, the date of the most recent commit that was found in the reposi‐
     tory was 2018-01-15.)  If you discover any rendering problems in this
     HTML version of the page, or you believe there is a better or more up-
     to-date source for the page, or you have corrections or improvements to
     the information in this COLOPHON (which is not part of the original
     manual page), send a mail to

BSD                            August 8, 2000                            BSD