EVENT_INIT(3) | Library Functions Manual | EVENT_INIT(3)

# NAME

`event_init`, `event_dispatch`, `event_set`, `event_add`, `event_del`, `event_pending`, `event_initialized`, `evtimer_set`, `evtimer_add`, `evtimer_del`, `evtimer_pending`, `evtimer_initialized`, `signal_set`, `signal_add`, `signal_del`, `signal_pending`, `signal_initialized`, `event_once`, `event_loop`, `event_loopexit`, `event_loopbreak`, `event_asr_run`, `event_asr_abort`, `event_priority_init`, `event_priority_set`, `event_base_dispatch`, `event_base_loop`, `event_base_loopexit`, `event_base_loopbreak`, `event_base_set`, `event_base_once`, `event_base_free`, `bufferevent_base_set`, `bufferevent_new`, `bufferevent_free`, `bufferevent_write`, `bufferevent_write_buffer`, `bufferevent_read`, `bufferevent_enable`, `bufferevent_disable`, `bufferevent_settimeout`, `bufferevent_setwatermark`, `EVBUFFER_INPUT`, `EVBUFFER_OUTPUT` — execute a function when a specific event occurs

# SYNOPSIS

Event Notification Library (libevent, -levent)
`#include <sys/time.h>`
`#include <event.h>`

struct event_base *
`event_init`(void);

int
`event_dispatch`(void);

void
`event_set`(struct event *ev, int fd, short event, void (*fn)(int, short, void *), void *arg);

int
`event_add`(struct event *ev, const struct timeval *tv);

int
`event_del`(struct event *ev);

int
`event_pending`(struct event *ev, short event, struct timeval *tv);

int
`event_initialized`(struct event *ev);

void
`evtimer_set`(struct event *ev, void (*fn)(int, short, void *), void *arg);

void
`evtimer_add`(struct event *ev, const struct timeval *tv);

void
`evtimer_del`(struct event *ev);

int
`evtimer_pending`(struct event *ev, struct timeval *tv);

int
`evtimer_initialized`(struct event *ev);

void
`signal_set`(struct event *ev, int signal, void (*fn)(int, short, void *), void *arg);

void
`signal_add`(struct event *ev, const struct timeval *tv);

void
`signal_del`(struct event *ev);

int
`signal_pending`(struct event *ev, struct timeval *tv);

int
`signal_initialized`(struct event *ev);

int
`event_once`(int fd, short event, void (*fn)(int, short, void *), void *arg, const struct timeval *tv);

int
`event_loop`(int flags);

int
`event_loopexit`(const struct timeval *tv);

int
`event_loopbreak`(void);

struct event_asr *
`event_asr_run`(struct asr_query *aq, void (*fn)(struct asr_result *, void *), void *arg);

void
`event_asr_abort`(struct event_asr *eva);

int
`event_priority_init`(int npriorities);

int
`event_priority_set`(struct event *ev, int priority);

int
`event_base_dispatch`(struct event_base *base);

int
`event_base_loop`(struct event_base *base, int flags);

int
`event_base_loopexit`(struct event_base *base, const struct timeval *tv);

int
`event_base_loopbreak`(struct event_base *base);

int
`event_base_set`(struct event_base *base, struct event *ev);

int
`event_base_once`(struct event_base *base, int fd, short event, void (*fn)(int, short, void *), void *arg, const struct timeval *tv);

void
`event_base_free`(struct event_base *base);

int
`bufferevent_base_set`(struct event_base *base, struct bufferevent *bufev);

struct bufferevent *
`bufferevent_new`(int fd, evbuffercb readcb, evbuffercb writecb, everrorcb errorcb, void *cbarg);

void
`bufferevent_free`(struct bufferevent *bufev);

int
`bufferevent_write`(struct bufferevent *bufev, const void *data, size_t size);

int
`bufferevent_write_buffer`(struct bufferevent *bufev, struct evbuffer *buf);

size_t
`bufferevent_read`(struct bufferevent *bufev, void *data, size_t size);

int
`bufferevent_enable`(struct bufferevent *bufev, short event);

int
`bufferevent_disable`(struct bufferevent *bufev, short event);

void
`bufferevent_settimeout`(struct bufferevent *bufev, int timeout_read, int timeout_write);

void
`bufferevent_setwatermark`(struct bufferevent *bufev, short events, size_t lowmark, size_t highmark);

struct evbuffer *
`EVBUFFER_INPUT`(struct bufferevent *bufev);

struct evbuffer *
`EVBUFFER_OUTPUT`(struct bufferevent *bufev);

# 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 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 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 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 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.

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 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.

The `event_asr_run`() function is used to schedule the asynchronous resolver query aq to run within a libevent event loop, and call the fn callback when the result is available. The extra arg parameter is passed to the callback. The user does not need to set up an event structure for using this function. It returns an opaque handle representing the running query. This handle becomes invalid before the callback is run. It can be cancelled by calling the `event_asr_abort`() function. See asr_run(3) for details on constructing asynchronous resolver queries.

# 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` assigns the middle priority to all events unless their priority is explicitly set.

# THREAD SAFE EVENTS

The `event` API 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.

# BUFFERED EVENTS

The `event` API 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_setwatermark`() function can set the low and high watermarks for read and write events. The events can be either EV_READ, EV_WRITE or both. When used with EV_READ, a bufferevent does not invoke the user read callback unless there is at least lowmark data in the buffer. If the read buffer is beyond highmark, the bufferevent stops reading from the file descriptor. When used with EV_WRITE, the user write callback is invoked whenever the buffered data falls below lowmark.

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.

The `EVBUFFER_INPUT`() and `EVBUFFER_OUTPUT`() macros return a pointer to evbuffer input and output respectively for the specified bufferevent bufev.

# ADDITIONAL NOTES

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.

# RETURN VALUES

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 ALSO

kqueue(2), poll(2), select(2), asr_run(3), evbuffer_new(3), 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.

# AUTHORS

The `event` library was written by Niels Provos.

June 13, 2025 | Debian