lib/libevent/event.3

.\"	$OpenBSD: event.3,v 1.56 2022/03/31 17:27:17 naddy Exp $
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.\" Copyright (c) 2000 Artur Grabowski <art@openbsd.org>
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.Dd $Mdocdate: March 31 2022 $
.Dt EVENT_INIT 3
.Os
.Sh NAME
.Nm event_init ,
.Nm event_dispatch ,
.Nm event_set ,
.Nm event_add ,
.Nm event_del ,
.Nm event_pending ,
.Nm event_initialized ,
.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 event_once ,
.Nm event_loop ,
.Nm event_loopexit ,
.Nm event_loopbreak ,
.Nm event_asr_run ,
.Nm event_asr_abort ,
.Nm event_priority_init ,
.Nm event_priority_set ,
.Nm event_base_dispatch ,
.Nm event_base_loop ,
.Nm event_base_loopexit ,
.Nm event_base_loopbreak ,
.Nm event_base_set ,
.Nm event_base_once ,
.Nm event_base_free ,
.Nm bufferevent_base_set ,
.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_setwatermark ,
.Nm EVBUFFER_INPUT ,
.Nm EVBUFFER_OUTPUT
.Nd execute a function when a specific event occurs
.Sh SYNOPSIS
.In sys/time.h
.In event.h
.Ft "struct event_base *"
.Fn "event_init" "void"
.Ft int
.Fn "event_dispatch" "void"
.Ft void
.Fn "event_set" "struct event *ev" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg"
.Ft int
.Fn "event_add" "struct event *ev" "const struct timeval *tv"
.Ft int
.Fn "event_del" "struct event *ev"
.Ft int
.Fn "event_pending" "struct event *ev" "short event" "struct timeval *tv"
.Ft int
.Fn "event_initialized" "struct event *ev"
.Ft void
.Fn "evtimer_set" "struct event *ev" "void (*fn)(int, short, void *)" "void *arg"
.Ft void
.Fn "evtimer_add" "struct event *ev" "const struct timeval *tv"
.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" "const struct timeval *tv"
.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 int
.Fn "event_once" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg" "const struct timeval *tv"
.Ft int
.Fn "event_loop" "int flags"
.Ft int
.Fn "event_loopexit" "const struct timeval *tv"
.Ft int
.Fn "event_loopbreak" "void"
.Ft "struct event_asr *"
.Fn event_asr_run "struct asr_query *aq" "void (*fn)(struct asr_result *, void *)" "void *arg"
.Ft "void"
.Fn event_asr_abort "struct event_asr *eva"
.Ft int
.Fn "event_priority_init" "int npriorities"
.Ft int
.Fn "event_priority_set" "struct event *ev" "int priority"
.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" "const 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 *ev"
.Ft int
.Fn "event_base_once" "struct event_base *base" "int fd" "short event" "void (*fn)(int, short, void *)" "void *arg" "const struct timeval *tv"
.Ft void
.Fn "event_base_free" "struct event_base *base"
.Ft int
.Fn "bufferevent_base_set" "struct event_base *base" "struct bufferevent *bufev"
.Ft "struct bufferevent *"
.Fn "bufferevent_new" "int fd" "evbuffercb readcb" "evbuffercb writecb" "everrorcb errorcb" "void *cbarg"
.Ft void
.Fn "bufferevent_free" "struct bufferevent *bufev"
.Ft int
.Fn "bufferevent_write" "struct bufferevent *bufev" "const 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 void
.Fn "bufferevent_setwatermark" "struct bufferevent *bufev" "short events" "size_t lowmark" "size_t highmark"
.Ft "struct evbuffer *"
.Fn "EVBUFFER_INPUT" "struct bufferevent *bufev"
.Ft "struct evbuffer *"
.Fn "EVBUFFER_OUTPUT" "struct bufferevent *bufev"
.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
preceding 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
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
The
.Fn event_asr_run
function is used to schedule the asynchronous resolver query
.Ar aq
to run within a libevent event loop, and call the
.Ar fn
callback when the result is available.
The extra
.Ar 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
.Fn event_asr_abort
function.
See
.Xr asr_run 3
for details on constructing asynchronous resolver queries.
.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
The
.Nm event
API 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
The
.Nm event
API 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
.Fn bufferevent_enable
and
.Fn 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_setwatermark
function can set the low and high watermarks
for read and write events.
The
.Fa events
can be either
.Va EV_READ ,
.Va EV_WRITE
or both.
When used with
.Va EV_READ ,
a bufferevent does not invoke the user read callback
unless there is at least
.Fa lowmark
data in the buffer.
If the read buffer is beyond
.Fa highmark ,
the bufferevent stops reading from the file descriptor.
When used with
.Va EV_WRITE ,
the user write callback is invoked whenever the buffered data
falls below
.Fa lowmark .
.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,
.Fn bufferevent_base_set
must be called before
enabling the bufferevent for the first time.
.Pp
The
.Fn EVBUFFER_INPUT
and
.Fn EVBUFFER_OUTPUT
macros return a pointer to evbuffer
.Fa input
and
.Fa output
respectively for the specified bufferevent
.Fa bufev .
.Sh ADDITIONAL NOTES
It is possible to disable support for
.Va kqueue , poll
or
.Va select
by setting the environment variable
.Va EVENT_NOKQUEUE , 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 asr_run 3 ,
.Xr evbuffer_new 3 ,
.Xr timeout 9
.Sh HISTORY
The
.Nm event
API manpage is based on the
.Xr timeout 9
manpage by Artur Grabowski.
Support for real-time signals was added by Taral.
.Sh AUTHORS
The
.Nm event
library was written by
.An Niels Provos .