event_base_loop.3

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lib/libevent/event_base_loop.3
.\" $OpenBSD: event_base_loop.3,v 1.6 2025/06/07 20:50:40 schwarze Exp $
.\" Copyright (c) 2023 Ted Bullock <tbullock@comlore.com>
.\"
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.Dd $Mdocdate: June 7 2025 $
.Dt EVENT_BASE_LOOP 3
.Os
.Sh NAME
.Nm event_base_loop ,
.Nm event_loop ,
.Nm event_base_dispatch ,
.Nm event_dispatch
.Nd run an event loop
.Sh SYNOPSIS
.Lb libevent
.In event.h
.Ft int
.Fn event_base_loop "struct event_base *base" "int flags"
.Ft int
.Fn event_loop "int flags"
.Ft int
.Fn event_base_dispatch "struct event_base *base"
.Ft int
.Fn event_dispatch void
.Sh DESCRIPTION
An event loop waits for events
and invokes an associated callback function whenever one occurs.
This enables asynchronous programming, allowing a program to perform other
tasks while waiting for an event to occur.
Asynchronous programming is a useful idiom for handling multiple I/O
operations, such as network connections, user interfaces, or disk operations
without blocking the main loop of a program.
.Pp
The
.Fn event_base_loop
family of functions run an event loop.
By default, they return when there are no more scheduled events.
.Pp
There are three types of events these functions monitor:
signal, kernel, and timer events.
Events involving POSIX signals are configured with
.Xr signal_set 3 .
Kernel events such as network activity and changes to file descriptors are
configured with
.Xr event_set 3 .
Timer events are configured with
.Xr evtimer_set 3 .
.Pp
The event library categorizes event states as:
.Bl -tag -width initialized
.It Em initialized
These have been configured with
.Xr event_set 3
and not yet registered with an event loop.
.It Em scheduled
These are registered to a loop using
.Xr event_add 3
and are idle, waiting to trigger and be processed by the loop.
They can be queried with
.Fn event_pending 3 .
.It Em activated
These are triggered events.
The loop processes events until all activated events are resolved.
Some of these, such as signals, may persist and become scheduled again.
.El
.Pp
The arguments are as follows:
.Bl -tag -width flags
.It Fa base
A pointer to an
.Vt event_base
structure returned from
.Xr event_base_new 3
or
.Xr event_init 3 .
.It Fa flags
Zero or more of the following flags OR'ed together:
.Bl -tag -width EVLOOP_NONBLOCK
.It Dv EVLOOP_ONCE
Run the event loop for one iteration and then return.
This is useful for a main program that needs to perform actions outside
the event loop.
.It Dv EVLOOP_NONBLOCK
Run the event loop in non-blocking mode.
The loop returns after processing activated signal and kernel events and does
not wait for timer events to expire.
.El
.El
.Pp
The function
.Fn event_loop
is a version of
.Fn event_base_loop
that requires the library to be initialized by
.Xr event_init 3 .
.Pp
.Fn event_base_dispatch
is equivalent to calling
.Fn event_base_loop
with
.Fa flags
set to zero.
Likewise,
.Fn event_dispatch
is the same as invoking
.Fn event_loop
with
.Fa flags
set to zero.
.Sh RETURN VALUES
The event loop functions return:
.Pp
.Bl -tag -compact -offset 3n -width 3n
.It \-1
on error,
.Va errno
is set.
.It 0
on success, asked to terminate loop.
.It 1
on success, no scheduled events remain.
.El
.Sh ERRORS
These functions have complex interactions with
.Va errno .
Error conditions that set
.Va errno
change corresponding to the kernel notification method the
.Fa base
structure is using.
These values directly correspond to
.Xr kevent 2 ,
.Xr poll 2
or
.Xr select 2
system calls and default to
.Xr kevent 2
on
.Ox .
Refer to
.Xr event_base_new 3
for details on kernel notification methods.
.Bl -tag -width Er
.It Bq Er EINTR
Although the kernel notification methods can set
.Va errno
to
.Er EINTR ,
.Fn event_base_loop
and related functions never fail with
.Va errno
set to
.Er EINTR .
.El
.Sh SEE ALSO
.Xr kevent 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr event_base_new 3 ,
.Xr event_set 3
.Sh HISTORY
.Fn event_dispatch
first appeared in libevent-0.1 and has been available since
.Ox 3.2 .
.Pp
.Fn event_loop
first appeared in libevent-0.4 and has been available since
.Ox 3.2 .
.Pp
.Fn event_base_dispatch
and
.Fn event_base_loop
first appeared in libevent-1.0 and have been available since
.Ox 3.8 .
.Sh AUTHORS
The event library and these functions were written by
.An -nosplit
.An Niels Provos .
.Pp
This manual page was written by
.An Ted Bullock Aq Mt tbullock@comlore.com .
IABSD.fr/src/lib/libevent/event_base_loop.3

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