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EVENT

Section: C Library Functions (3)
Index Return to Main Contents

BSD mandoc

NAME

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

Fd #include <sys/time.h> Fd #include <event.h> Ft struct event_base * Fn event_init void Ft int Fn event_dispatch void Ft int Fn event_loop int flags Ft int Fn event_loopexit struct timeval *tv Ft int Fn event_loopbreak void Ft void Fn event_set struct event *ev int fd short event void (*fn)(int, short, void *) void *arg Ft int Fn event_base_dispatch struct event_base *base Ft int Fn event_base_loop struct event_base *base int flags Ft int Fn event_base_loopexit struct event_base *base struct timeval *tv Ft int Fn event_base_loopbreak struct event_base *base Ft int Fn event_base_set struct event_base *base struct event * Ft void Fn event_base_free struct event_base *base Ft int Fn event_add struct event *ev struct timeval *tv Ft int Fn event_del struct event *ev Ft int Fn event_once int fd short event void (*fn)(int, short, void *) void *arg struct timeval *tv Ft int Fn event_base_once struct event_base *base int fd short event void (*fn)(int, short, void *) void *arg struct timeval *tv Ft int Fn event_pending struct event *ev short event struct timeval *tv Ft int Fn event_initialized struct event *ev Ft int Fn event_priority_init int npriorities Ft int Fn event_priority_set struct event *ev int priority Ft void Fn evtimer_set struct event *ev void (*fn)(int, short, void *) void *arg Ft void Fn evtimer_add struct event *ev struct timeval * Ft void Fn evtimer_del struct event *ev Ft int Fn evtimer_pending struct event *ev struct timeval *tv Ft int Fn evtimer_initialized struct event *ev Ft void Fn signal_set struct event *ev int signal void (*fn)(int, short, void *) void *arg Ft void Fn signal_add struct event *ev struct timeval * Ft void Fn signal_del struct event *ev Ft int Fn signal_pending struct event *ev struct timeval *tv Ft int Fn signal_initialized struct event *ev Ft struct bufferevent * Fn bufferevent_new int fd evbuffercb readcb evbuffercb writecb everrorcb void *cbarg Ft void Fn bufferevent_free struct bufferevent *bufev Ft int Fn bufferevent_write struct bufferevent *bufev void *data size_t size Ft int Fn bufferevent_write_buffer struct bufferevent *bufev struct evbuffer *buf Ft size_t Fn bufferevent_read struct bufferevent *bufev void *data size_t size Ft int Fn bufferevent_enable struct bufferevent *bufev short event Ft int Fn bufferevent_disable struct bufferevent *bufev short event Ft void Fn bufferevent_settimeout struct bufferevent *bufev int timeout_read int timeout_write Ft int Fn bufferevent_base_set struct event_base *base struct bufferevent *bufev Ft struct evbuffer * Fn evbuffer_new void Ft void Fn evbuffer_free struct evbuffer *buf Ft int Fn evbuffer_add struct evbuffer *buf const void *data size_t size Ft int Fn evbuffer_add_buffer struct evbuffer *dst struct evbuffer *src Ft int Fn evbuffer_add_printf struct evbuffer *buf const char *fmt ... Ft int Fn evbuffer_add_vprintf struct evbuffer *buf const char *fmt va_list ap Ft void Fn evbuffer_drain struct evbuffer *buf size_t size Ft int Fn evbuffer_write struct evbuffer *buf int fd Ft int Fn evbuffer_read struct evbuffer *buf int fd int size Ft u_char * Fn evbuffer_find struct evbuffer *buf const u_char *data size_t size Ft char * Fn evbuffer_readline struct evbuffer *buf Ft struct evhttp * Fn evhttp_new struct event_base *base Ft int Fn evhttp_bind_socket struct evhttp *http const char *address u_short port Ft void Fn evhttp_free struct evhttp *http Ft int Fa (*event_sigcb)(void) ; Ft volatile sig_atomic_t Fa event_gotsig ;

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 Fn event_init before it can be used.

In order to process events, an application needs to call Fn event_dispatch . This function only returns on error, and should replace the event core of the application program.

The function Fn event_set prepares the event structure Fa ev to be used in future calls to Fn event_add and Fn event_del . The event will be prepared to call the function specified by the Fa fn argument with an Fa int argument indicating the file descriptor, a Fa short argument indicating the type of event, and a Fa void * argument given in the Fa arg argument. The Fa 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 Fa 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 preceeding events, via bitwise-OR to Fn event_set , can provide its callback function with a bitwise-OR of more than one triggered event. The additional flag EV_PERSIST makes an Fn event_add persistent until Fn event_del has been called.

Once initialized, the Fa ev structure can be used repeatedly with Fn event_add and Fn 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 Fa ev structure has been added to libevent using Fn event_add the structure must persist until the event occurs (assuming Fa EV_PERSIST is not set) or is removed using Fn event_del . You may not reuse the same Fa ev structure for multiple monitored descriptors; each descriptor needs its own Fa ev .

The function Fn event_add schedules the execution of the Fa ev event when the event specified in Fn event_set occurs or in at least the time specified in the Fa tv . If Fa 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 Fa ev argument must be already initialized by Fn event_set and may not be used in calls to Fn event_set until it has timed out or been removed with Fn event_del . If the event in the Fa ev argument already has a scheduled timeout, the old timeout will be replaced by the new one.

The function Fn event_del will cancel the event in the argument Fa ev . If the event has already executed or has never been added the call will have no effect.

The functions Fn evtimer_set , Fn evtimer_add , Fn evtimer_del , Fn evtimer_initialized , and Fn 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 Fn signal_set , Fn signal_add , Fn signal_del , Fn signal_initialized , and Fn signal_pending are abbreviations. The event type will be a persistent EV_SIGNAL That means Fn 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 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 Fn event_dispatch to terminate with errno set to Er EINTR .

The function Fn event_once is similar to Fn event_set . However, it schedules a callback to be called exactly once and does not require the caller to prepare an Fa event structure. This function supports Fa EV_TIMEOUT , Fa EV_READ , and Fa EV_WRITE .

The Fn event_pending function can be used to check if the event specified by Fa event is pending to run. If EV_TIMEOUT was specified and Fa tv is not NULL the expiration time of the event will be returned in Fa tv .

The Fn 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 Fn 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 Fn event_loop will proceed normally. The event_loopbreak function exits from the event loop immediately. Fn event_loop will abort after the next event is completed; Fn event_loopbreak is typically invoked from this event's callback. This behavior is analogous to the "break;" statement. Subsequent invocations of Fn event_loop will proceed normally.

It is the responsibility of the caller to provide these functions with pre-allocated event structures.

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 Fn event_priority_init function. This function should be called before the first call to Fn event_dispatch . The Fn 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 Fn event_init , an event base is returned. This event base can be used in conjunction with calls to Fn event_base_set , Fn event_base_dispatch , Fn event_base_loop , Fn event_base_loopexit , Fn bufferevent_base_set and Fn event_base_free . Fn event_base_set should be called after preparing an event with Fn event_set , as Fn event_set assigns the provided event to the most recently created event base. Fn bufferevent_base_set should be called after preparing a bufferevent with Fn bufferevent_new . Fn 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 instead is reading from input and writing to output buffers.

A new bufferevent is created by Fn bufferevent_new . The parameter Fa 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: Ft void Fn (*cb) struct bufferevent *bufev void *arg . The error callback has the following form: Ft void Fn (*cb) struct bufferevent *bufev short what void *arg . The argument is specified by the fourth parameter Fa cbarg . A Fa 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 Fn 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. Fn bufferevent_write returns 0 on success or -1 on failure. The Fn 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 Fn evhttp_new . It can be bound to any port and address with the Fn evhttp_bind_socket function. When the HTTP server is no longer used, it can be freed via Fn evhttp_free .

To be notified of HTTP requests, a user needs to register callbacks with the HTTP server. This can be done by calling Fn 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.

ADDITIONAL NOTES

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.

RETURN VALUES

Upon successful completion Fn event_add and Fn event_del return 0. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

HISTORY

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

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.