NetBSD/lib/libevent/event.3

.\"	$NetBSD: event.3,v 1.2 2003/06/13 04:11:31 itojun Exp $
.\"	$OpenBSD: event.3,v 1.4 2002/07/12 18:50:48 provos Exp $
.\"
.\" Copyright (c) 2000 Artur Grabowski <art@openbsd.org>
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.Dd August 8, 2000
.Dt EVENT 3
.Os
.Sh NAME
.Nm event_init ,
.Nm event_dispatch ,
.Nm event_loop ,
.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
.Nd execute a function when a specific event occurs
.Sh SYNOPSIS
.Fd #include <sys/time.h>
.Fd #include <event.h>
.Ft void
.Fn "event_init"
.Ft int
.Fn "event_dispatch"
.Ft int
.Fn "event_loop" "int flags"
.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" "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" "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 int
.Fa (*event_sigcb)(void) ;
.Ft int
.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
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
.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.
.Pp
It is the responsibility of the caller to provide these functions with
pre-allocated event structures.
.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 .
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.
.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 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
.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
.Va 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 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 0, and the event type will be
.Va EV_TIMEOUT .
.Pp
.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
It is possible to disable support for
.Va epoll , kqueue , poll
or
.Va select
by setting the environment variable
.Va EVENT_NOEPOLL , EVENT_NOKQUEUE , EVENT_NOPOLL
or
.Va EVENT_NOSELECT .
By setting the environment variable
.Va EVENT_SHOW_METHOD ,
.Nm libevent
displays the kernel notification method that it uses.
.Pp
.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 timeout 9 ,
.Xr select 2 ,
.Xr kqueue 2
.Sh HISTORY
The
.Nm event
API manpage is based on the
.Xr timeout 9
manpage by Artur Grabowski.
.Sh AUTHORS
The
.Nm event
library was written by Niels Provos.