NAME |SYNOPSIS |DESCRIPTION |EVENT PRIORITIES |THREAD SAFE EVENTS |BUFFERED EVENTS |NON-BLOCKING HTTP SUPPORT |ADDITIONAL NOTES |RETURN VALUES |SEE ALSO |HISTORY |AUTHORS |BUGS |COLOPHON |
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
#include <sys/time.h>#include <event.h>struct event_base *event_init(void);intevent_dispatch(void);intevent_loop(int flags);intevent_loopexit(struct timeval *tv);intevent_loopbreak(void);voidevent_set(struct event *ev,int fd,short event,void (*fn)(int, short, void *),void *arg);intevent_base_dispatch(struct event_base *base);intevent_base_loop(struct event_base *base,int flags);intevent_base_loopexit(struct event_base *base,struct timeval *tv);intevent_base_loopbreak(struct event_base *base);intevent_base_set(struct event_base *base,struct event *);voidevent_base_free(struct event_base *base);intevent_add(struct event *ev,struct timeval *tv);intevent_del(struct event *ev);intevent_once(int fd,short event,void (*fn)(int, short, void *),void *arg,struct timeval *tv);intevent_base_once(struct event_base *base,int fd,short event,void (*fn)(int, short, void *),void *arg,struct timeval *tv);intevent_pending(struct event *ev,short event,struct timeval *tv);intevent_initialized(struct event *ev);intevent_priority_init(int npriorities);intevent_priority_set(struct event *ev,int priority);voidevtimer_set(struct event *ev,void (*fn)(int, short, void *),void *arg);voidevtimer_add(struct event *ev,struct timeval *);voidevtimer_del(struct event *ev);intevtimer_pending(struct event *ev,struct timeval *tv);intevtimer_initialized(struct event *ev);voidsignal_set(struct event *ev,int signal,void (*fn)(int, short, void *),void *arg);voidsignal_add(struct event *ev,struct timeval *);voidsignal_del(struct event *ev);intsignal_pending(struct event *ev,struct timeval *tv);intsignal_initialized(struct event *ev);struct bufferevent *bufferevent_new(int fd,evbuffercb readcb,evbuffercb writecb,everrorcb,void *cbarg);voidbufferevent_free(struct bufferevent *bufev);intbufferevent_write(struct bufferevent *bufev,void *data,size_t size);intbufferevent_write_buffer(struct bufferevent *bufev,struct evbuffer *buf);size_tbufferevent_read(struct bufferevent *bufev,void *data,size_t size);intbufferevent_enable(struct bufferevent *bufev,short event);intbufferevent_disable(struct bufferevent *bufev,short event);voidbufferevent_settimeout(struct bufferevent *bufev,int timeout_read,int timeout_write);intbufferevent_base_set(struct event_base *base,struct bufferevent *bufev);struct evbuffer *evbuffer_new(void);voidevbuffer_free(struct evbuffer *buf);intevbuffer_add(struct evbuffer *buf,const void *data,size_t size);intevbuffer_add_buffer(struct evbuffer *dst,struct evbuffer *src);intevbuffer_add_printf(struct evbuffer *buf,const char *fmt,...);intevbuffer_add_vprintf(struct evbuffer *buf,const char *fmt,va_list ap);voidevbuffer_drain(struct evbuffer *buf,size_t size);intevbuffer_write(struct evbuffer *buf,int fd);intevbuffer_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);intevhttp_bind_socket(struct evhttp *http,const char *address,unsigned short port);voidevhttp_free(struct evhttp *http);int (*event_sigcb)(void);volatile sig_atomic_t event_gotsig;
TheeventAPI provides a mechanism to execute a function when a spe‐ cific event on a file descriptor occurs or after a given time has passed. TheeventAPI needs to be initialized withevent_init() before it can be used. In order to process events, an application needs to callevent_dispatch(). This function only returns on error, and should replace the event core of the application program. The functionevent_set() prepares the event structureev to be used in future calls toevent_add() andevent_del(). The event will be pre‐ pared to call the function specified by thefn argument with anint argument indicating the file descriptor, ashort argument indicating the type of event, and avoid * argument given in thearg argument. Thefd indicates the file descriptor that should be monitored for events. The events can be eitherEV_READ,EV_WRITE, or both, indicat‐ ing that an application can read or write from the file descriptor respectively without blocking. The functionfn will be called with the file descriptor that triggered the event and the type of event which will be eitherEV_TIMEOUT,EV_SIGNAL,EV_READ, orEV_WRITE. Additionally, an event which has reg‐ istered interest in more than one of the preceeding events, via bit‐ wise-OR toevent_set(), can provide its callback function with a bit‐ wise-OR of more than one triggered event. The additional flagEV_PERSIST makes anevent_add() persistent untilevent_del() has been called. Once initialized, theev structure can be used repeatedly withevent_add() andevent_del() and does not need to be reinitialized unless the function called and/or the argument to it are to be changed. However, when anev structure has been added to libevent usingevent_add() the structure must persist until the event occurs (assumingEV_PERSIST is not set) or is removed usingevent_del(). You may not reuse the sameev structure for multiple monitored descriptors; each descriptor needs its ownev. The functionevent_add() schedules the execution of theev event when the event specified inevent_set() occurs or in at least the time spec‐ ified in thetv. Iftv is NULL, no timeout occurs and the function will only be called if a matching event occurs on the file descriptor. The event in theev argument must be already initialized byevent_set() and may not be used in calls toevent_set() until it has timed out or been removed withevent_del(). If the event in theev argument already has a scheduled timeout, the old timeout will be replaced by the new one. The functionevent_del() will cancel the event in the argumentev. If the event has already executed or has never been added the call will have no effect. The functionsevtimer_set(),evtimer_add(),evtimer_del(),evtimer_initialized(), andevtimer_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 beEV_TIMEOUT. The functionssignal_set(),signal_add(),signal_del(),signal_initialized(), andsignal_pending() are abbreviations. The event type will be a persistentEV_SIGNAL. That meanssignal_set() addsEV_PERSIST. In order to avoid races in signal handlers, theeventAPI provides two variables:event_sigcb andevent_gotsig. A signal handler setsevent_gotsig to indicate that a signal has been received. The applica‐ tion setsevent_sigcb to a callback function. After the signal handler setsevent_gotsig,event_dispatchwill 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 theeventlibrary, causingevent_dispatch() to terminate witherrno set to EINTR. The functionevent_once() is similar toevent_set(). However, it schedules a callback to be called exactly once and does not require the caller to prepare anevent structure. This function supportsEV_TIMEOUT,EV_READ, andEV_WRITE. Theevent_pending() function can be used to check if the event speci‐ fied byevent is pending to run. IfEV_TIMEOUT was specified andtv is not NULL, the expiration time of the event will be returned intv. Theevent_initialized() macro can be used to check if an event has been initialized. Theevent_loopfunction provides an interface for single pass execution of pending events. The flagsEVLOOP_ONCE andEVLOOP_NONBLOCK are rec‐ ognized. Theevent_loopexitfunction exits from the event loop. The nextevent_loop() iteration after the given timer expires will complete normally (handling all queued events) then exit without blocking for events again. Subsequent invocations ofevent_loop() will proceed nor‐ mally. Theevent_loopbreakfunction 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 ofevent_loop() will proceed normally. It is the responsibility of the caller to provide these functions with pre-allocated event structures.
By defaultlibeventschedules 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,libeventsupports 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 theevent_priority_init() function. This function should be called before the first call toevent_dispatch(). Theevent_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.
Libeventhas experimental support for thread-safe events. When ini‐ tializing the library viaevent_init(), an event base is returned. This event base can be used in conjunction with calls toevent_base_set(),event_base_dispatch(),event_base_loop(),event_base_loopexit(),bufferevent_base_set() andevent_base_free().event_base_set() should be called after preparing an event withevent_set(), asevent_set() assigns the provided event to the most recently created event base.bufferevent_base_set() should be called after preparing a bufferevent withbufferevent_new().event_base_free() should be used to free memory associated with the event base when it is no longer needed.
libeventprovides an abstraction on top of the regular event callbacks. This abstraction is called abuffered 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 bybufferevent_new(). The parameterfd 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‐ etercbarg. Abufferevent 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 ofEV_READ andEV_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 is0 by default. Thebufferevent_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. Thebufferevent_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.
libeventprovides 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 callingevhttp_new(). It can be bound to any port and address with theevhttp_bind_socket() function. When the HTTP server is no longer used, it can be freed viaevhttp_free(). To be notified of HTTP requests, a user needs to register callbacks with the HTTP server. This can be done by callingevhttp_set_cb(). The second argument is the URI for which a callback is being regis‐ tered. The corresponding callback will receive anstructevhttp_request object that contains all information about the request. This section does not document all the possible function calls; please checkevent.h for the public interfaces.
It is possible to disable support forepoll,kqueue,devpoll,poll orselect by setting the environment variableEVENT_NOEPOLL,EVENT_NOKQUEUE,EVENT_NODEVPOLL,EVENT_NOPOLL orEVENT_NOSELECT, respectively. By setting the environment variableEVENT_SHOW_METHOD,libeventdisplays the kernel notification method that it uses.
Upon successful completionevent_add() andevent_del() return 0. Oth‐ erwise, -1 is returned and the global variable errno is set to indicate the error.
kqueue(2), poll(2), select(2), evdns(3), timeout(9)
TheeventAPI manpage is based on the timeout(9) manpage by Artur Grabowski. The port oflibeventto Windows is due to Michael A. Davis. Support for real-time signals is due to Taral.
Theeventlibrary 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.
This page is part of thelibevent (an event notification library) project. Information about the project can be found athttp://libevent.org/. If you have a bug report for this manual page, see ⟨http://sourceforge.net/p/levent/bugs/⟩. This page was obtained from the project's upstream Git repository ⟨https://github.com/libevent/libevent.git⟩ on 2020-08-13. (At that time, the date of the most recent commit that was found in the reposi‐ tory was 2020-08-11.) 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 isnot part of the original manual page), send a mail to man-pages@man7.orgBSD August 8, 2000 BSDHTML rendering created 2020-08-13 byMichael Kerrisk, author ofThe Linux Programming Interface, maintainer of theLinuxman-pages project. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. |  |