.\"	$NetBSD: RUN_ONCE.9,v 1.7 2008/10/03 16:19:46 pooka Exp $
.\"
.\" Copyright (c)2005 YAMAMOTO Takashi,
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    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
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.\" ------------------------------------------------------------
.Dd October 3, 2008
.Dt RUN_ONCE 9
.Os
.\" ------------------------------------------------------------
.Sh NAME
.Nm RUN_ONCE
.Nd Run a function exactly once
.\" ------------------------------------------------------------
.Sh SYNOPSIS
.In sys/once.h
.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.Vt ONCE_DECL(control);
.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.Ft int
.Fn RUN_ONCE \
"once_t *control" "int (*init_func)(void)"
.\" ------------------------------------------------------------
.Sh DESCRIPTION
.Fn RUN_ONCE
provides a functionality similar to
.Fn pthread_once 3 .
It ensures that, for a given
.Fa control ,
.Fn init_func
is executed (successfully) exactly once.
It is considered as a successful execution if and only if
.Fn init_func
returned 0.
As long as there was no successful execution,
.Fn RUN_ONCE
will try again each time it is called.
.Pp
.Fn RUN_ONCE
can sleep if it's called concurrently.
.\" ------------------------------------------------------------
.Sh RETURN VALUES
On failure,
.Fn RUN_ONCE
returns what
.Fn init_func
returned.
Otherwise, it returns 0.
.\" ------------------------------------------------------------
.Sh EXAMPLES
The following example shows how
.Fn RUN_ONCE
is used.
Regardless of how many times
.Fn some_func
is executed,
.Fn init_func
will be executed exactly once.
.Bd -literal
static int
init_func(void)
{

	/*
	 * do some initialization.
	 */

	return 0; /* success */
}

int
some_func(void)
{
	static ONCE_DECL(control);

	RUN_ONCE(\*[Am]control, init_func);

	/*
	 * we are sure that init_func has already been completed here.
	 */
}
.Ed
.\" ------------------------------------------------------------
.Sh SEE ALSO
.Xr pthread_once 3 ,
.Xr condvar 9 ,
.Xr intro 9