EVENT(3) UNIX Programmer's Manual EVENT(3) NAME event_init, event_dispatch, event_loop, event_loopexit, event_set, event_base_dispatch, event_base_loop, event_base_loopexit, 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_start, evhttp_free - execute a function when a specific event occurs SYNOPSIS event_init(void) event_dispatch(void) event_loop(int, flags) event_loopexit(struct, timeval, *tv) event_set(struct, event, *ev, int, fd, short, event) 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_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,) event_base_once(struct, event_base, *base, int, fd, short, event) event_pending(struct, event, *ev, short, event, struct, timeval) 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) 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,) 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) bufferevent_new(int, fd, evbuf- fercb, readcb, evbuffercb, writecb, everrorcb) 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, buf- ferevent, *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) evbuffer_new(void) evbuffer_free(struct, evbuffer, *buf) evbuffer_add(struct, evbuffer, *buf, const, void, *data, size_t) 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) 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) evbuffer_readline(struct, evbuffer, *buf) evhttp_start(const, char, *ad- dress, u_short, port) evhttp_free(struct, evhttp*, http) DESCRIPTION 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 re- place the event core of the application program. 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 re- gistered 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 event_loop function provides an interface for single pass execution of pending events. The flags EVLOOP_ONCE and EVLOOP_NONBLOCK are recog- nized. The event_loopexit function allows the loop to be terminated after some amount of time has passed. The parameter indicates the time after which the loop should terminate. It is the responsibility of the caller to provide these functions with pre-allocated event structures. The function event_set() prepares the event structure to be used in fu- ture calls to event_add() and event_del(). The event will be prepared to call the function specified by the argument with an argument indicating the file descriptor, a argument indicating the type of event, and a argu- ment given in the argument. The 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 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. The additional flag EV_PERSIST makes an event_add() persistent until event_del() has been called. Once initialized, the 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 structure has been added to libevent using event_add() the structure must persist until the event occurs (assuming is not set) or is removed using event_del(). You may not reuse the same structure for multiple monitored descriptors; each descriptor needs its own The function event_add() schedules the execution of the event when the event specified in event_set() occurs or in at least the time specified in the If is no timeout occurs and the function will only be called if a matching event occurs on the file descriptor. The event in the 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 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 If the event has already executed or has never been added the call will have no effect. 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 structure. This function supports and The event_pending() function can be used to check if the event specified by is pending to run. If EV_TIMEOUT was specified and is not the expira- tion time of the event will be returned in The event_initialized() macro can be used to check if an event has been initialized. 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. 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 set- ting the environment variable EVENT_SHOW_METHOD, libevent displays the kernel notification method that it uses. EVENT PRIORITIES 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 as- signs the middle priority to all events unless their priority is expli- citly set. THREAD SAFE EVENTS Libevent has experimental support for thread-safe events. When initial- izing 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. BUFFERED EVENTS 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 in- stead is reading from input and writing to output buffers. A new bufferevent is created by bufferevent_new(). The parameter speci- fies 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 parame- ters are callbacks. The read and write callback have the following form: (*cb)(struct, bufferevent, *bufev, void, *arg). The error callback has the following form: (*cb)(struct, bufferevent, *bufev, short, what, void, *arg) The argument is specified by the fourth parameter A pointer is returned on success, NULL on error. Both the read and the write call- back 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 buf- ferevent 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 be- fore 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 creat- ed by calling evhttp_start(). 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 public interfaces. RETURN VALUES Upon successful completion event_add() and event_del() return 0. Other- wise, -1 is returned and the global variable errno is set to indicate the error. SEE ALSO kqueue(2), poll(2), select(2), evdns(3), timeout(9) HISTORY The event API manpage is based on the timeout(9) manpage by Artur Gra- bowski. The port of libevent to Windows is due to Michael A. Davis. Support for real-time signals is due to Taral. AUTHORS The event library was written by Niels Provos. BUGS 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.