event.3

MPI stands for the Message Passing Interface. Written by the MPI Forum (a large committee comprising

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.\" Copyright (c) 2000 Artur Grabowski <[e-mail]>
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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_loopexit ,
.Nm event_base_loop ,
.Nm event_base_loopexit ,
.Nm event_set ,
.Nm event_add ,
.Nm event_del ,
.Nm event_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 evbuffer_new ,
.Nm evbuffer_free ,
.Nm evbuffer_add ,
.Nm evbuffer_add_buffer ,
.Nm evbuffer_add_printf ,
.Nm evbuffer_drain ,
.Nm evbuffer_write ,
.Nm evbuffer_read ,
.Nm evbuffer_find ,
.Nm evbuffer_readline
.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"
.Ft int
.Fn "event_dispatch"
.Ft int
.Fn "event_loop" "int flags"
.Ft int
.Fn "event_loopexit" "struct timeval *tv"
.Ft int
.Fn "event_base_loop" "struct event_base *" "int flags"
.Ft int
.Fn "event_base_loopexit" "struct event_base *" "struct timeval *tv"
.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_once" "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 "struct evbuffer *"
.Fn "evbuffer_new" "void"
.Ft void
.Fn "evbuffer_free" "struct evbuffer *buf"
.Ft int
.Fn "evbuffer_add" "struct evbuffer *buf" "u_char *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" "char *fmt" "..."
.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" "u_char *data" "size_t size"
.Ft "char *"
.Fn "evbuffer_readline" "struct evbuffer *buf"
.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.
The
.Nm event_loopexit
function allows the loop to be terminated after some amount of time
has passed.
The parameter indicates the time after which the loop should terminate.
.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.
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 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 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
.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 -1, 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 , devpoll, poll
or
.Va select
by setting the environment variable
.Va EVENT_NOEPOLL , EVENT_NOKQUEUE , EVENT_NODEVPOLL, 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 EVENT PRIORITIES
By default
.Nm libevent
schedules all active events with the same priority.
However, sometime 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.
.Pp
.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 ,
and
.Fn event_base_loopexit .
.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.
.Pp
.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 buffer 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 argument is specified by the fourth parameter
.Fa "cbarg" .
.Pp
By default the buffered event is read enabled and will try to read
from the file descriptor.
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.
The
.Fn bufferevent_read
function is used to read data from the input buffer.
Both functions return the amount of data written or read.
.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.
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.
.Pp
.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.