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+.\"	$OpenBSD: event.3,v 1.4 2002/07/12 18:50:48 provos Exp $
+.\"
+.\" Copyright (c) 2000 Artur Grabowski <art@openbsd.org>
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\"
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. The name of the author may not be used to endorse or promote products
+.\"    derived from this software without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
+.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+.\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+.\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+.\" EXEMPLARY, OR CONSEQUENTIAL  DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+.\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+.\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+.\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+.\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd August 8, 2000
+.Dt EVENT 3
+.Os
+.Sh NAME
+.Nm event_init ,
+.Nm event_dispatch ,
+.Nm event_loop ,
+.Nm event_loopexit ,
+.Nm event_loopbreak ,
+.Nm event_set ,
+.Nm event_base_dispatch ,
+.Nm event_base_loop ,
+.Nm event_base_loopexit ,
+.Nm event_base_loopbreak ,
+.Nm event_base_set ,
+.Nm event_base_free ,
+.Nm event_add ,
+.Nm event_del ,
+.Nm event_once ,
+.Nm event_base_once ,
+.Nm event_pending ,
+.Nm event_initialized ,
+.Nm event_priority_init ,
+.Nm event_priority_set ,
+.Nm evtimer_set ,
+.Nm evtimer_add ,
+.Nm evtimer_del ,
+.Nm evtimer_pending ,
+.Nm evtimer_initialized ,
+.Nm signal_set ,
+.Nm signal_add ,
+.Nm signal_del ,
+.Nm signal_pending ,
+.Nm signal_initialized ,
+.Nm bufferevent_new ,
+.Nm bufferevent_free ,
+.Nm bufferevent_write ,
+.Nm bufferevent_write_buffer ,
+.Nm bufferevent_read ,
+.Nm bufferevent_enable ,
+.Nm bufferevent_disable ,
+.Nm bufferevent_settimeout ,
+.Nm bufferevent_base_set ,
+.Nm evbuffer_new ,
+.Nm evbuffer_free ,
+.Nm evbuffer_add ,
+.Nm evbuffer_add_buffer ,
+.Nm evbuffer_add_printf ,
+.Nm evbuffer_add_vprintf ,
+.Nm evbuffer_drain ,
+.Nm evbuffer_write ,
+.Nm evbuffer_read ,
+.Nm evbuffer_find ,
+.Nm evbuffer_readline ,
+.Nm evhttp_new ,
+.Nm evhttp_bind_socket ,
+.Nm evhttp_free
+.Nd execute a function when a specific event occurs
+.Sh 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 "unsigned char *"
+.Fn "evbuffer_find" "struct evbuffer *buf" "const unsigned 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" "unsigned short port"
+.Ft "void"
+.Fn "evhttp_free" "struct evhttp *http"
+.Ft int
+.Fa (*event_sigcb)(void) ;
+.Ft volatile sig_atomic_t
+.Fa event_gotsig ;
+.Sh DESCRIPTION
+The
+.Nm 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.
+.Pp
+The
+.Nm event
+API needs to be initialized with
+.Fn event_init
+before it can be used.
+.Pp
+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.
+.Pp
+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
+.Va EV_READ ,
+.Va EV_WRITE ,
+or both,
+indicating that an application can read or write from the file descriptor
+respectively without blocking.
+.Pp
+The function
+.Fa fn
+will be called with the file descriptor that triggered the event and
+the type of event which will be either
+.Va EV_TIMEOUT ,
+.Va EV_SIGNAL ,
+.Va EV_READ ,
+or
+.Va 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
+.Va EV_PERSIST
+makes an
+.Fn event_add
+persistent until
+.Fn event_del
+has been called.
+.Pp
+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 .
+.Pp
+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
+.Dv 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.
+.Pp
+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.
+.Pp
+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
+.Va EV_TIMEOUT .
+.Pp
+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
+.Va EV_SIGNAL .
+That means
+.Fn signal_set
+adds
+.Va EV_PERSIST .
+.Pp
+In order to avoid races in signal handlers, the
+.Nm event
+API provides two variables:
+.Va event_sigcb
+and
+.Va event_gotsig .
+A signal handler
+sets
+.Va event_gotsig
+to indicate that a signal has been received.
+The application sets
+.Va event_sigcb
+to a callback function.
+After the signal handler sets
+.Va event_gotsig ,
+.Nm 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
+.Nm event
+library, causing
+.Fn event_dispatch
+to terminate with
+.Va errno
+set to
+.Er EINTR .
+.Pp
+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 .
+.Pp
+The
+.Fn event_pending
+function can be used to check if the event specified by
+.Fa event
+is pending to run.
+If
+.Va EV_TIMEOUT
+was specified and
+.Fa tv
+is not
+.Dv NULL ,
+the expiration time of the event will be returned in
+.Fa tv .
+.Pp
+The
+.Fn event_initialized
+macro can be used to check if an event has been initialized.
+.Pp
+The
+.Nm event_loop
+function provides an interface for single pass execution of pending
+events.
+The flags
+.Va EVLOOP_ONCE
+and
+.Va EVLOOP_NONBLOCK
+are recognized.
+The
+.Nm 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
+.Nm 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.
+.Pp
+It is the responsibility of the caller to provide these functions with
+pre-allocated event structures.
+.Pp
+.Sh EVENT PRIORITIES
+By default
+.Nm 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,
+.Nm libevent
+supports strict priority queues.
+Active events with a lower priority are always processed before events
+with a higher priority.
+.Pp
+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,
+.Nm libevent
+assigns the middle priority to all events unless their priority
+is explicitly set.
+.Sh THREAD SAFE EVENTS
+.Nm 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.
+.Sh BUFFERED EVENTS
+.Nm libevent
+provides an abstraction on top of the regular event callbacks.
+This abstraction is called a
+.Va "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.
+.Pp
+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
+.Xr 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.
+.Pp
+Once initialized, the bufferevent structure can be used repeatedly with
+bufferevent_enable() and bufferevent_disable().
+The flags parameter can be a combination of
+.Va EV_READ
+and
+.Va 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
+.Va 0
+by default.
+.Pp
+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.
+.Pp
+If multiple bases are in use, bufferevent_base_set() must be called before
+enabling the bufferevent for the first time.
+.Sh NON-BLOCKING HTTP SUPPORT
+.Nm 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 .
+.Pp
+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
+.Va struct evhttp_request
+object that contains all information about the request.
+.Pp
+This section does not document all the possible function calls; please
+check
+.Va event.h
+for the public interfaces.
+.Sh ADDITIONAL NOTES
+It is possible to disable support for
+.Va epoll , kqueue , devpoll , poll
+or
+.Va select
+by setting the environment variable
+.Va EVENT_NOEPOLL , EVENT_NOKQUEUE , EVENT_NODEVPOLL , EVENT_NOPOLL
+or
+.Va EVENT_NOSELECT ,
+respectively.
+By setting the environment variable
+.Va EVENT_SHOW_METHOD ,
+.Nm libevent
+displays the kernel notification method that it uses.
+.Sh 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.
+.Sh SEE ALSO
+.Xr kqueue 2 ,
+.Xr poll 2 ,
+.Xr select 2 ,
+.Xr evdns 3 ,
+.Xr timeout 9
+.Sh HISTORY
+The
+.Nm event
+API manpage is based on the
+.Xr timeout 9
+manpage by Artur Grabowski.
+The port of
+.Nm libevent
+to Windows is due to Michael A. Davis.
+Support for real-time signals is due to Taral.
+.Sh AUTHORS
+The
+.Nm event
+library was written by Niels Provos.
+.Sh 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.