.\"	$NetBSD: call_once.3,v 1.2 2019/04/27 10:57:11 wiz Exp $
.\"
.\" Copyright (c) 2016 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Kamil Rytarowski.
.\"
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.Dd October 16, 2016
.Dt CALL_ONCE 3
.Os
.Sh NAME
.Nm call_once
.Nd calls function exactly once
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In threads.h
.Ft void
.Fn call_once "once_flag *flag" "void (*func)(void)"
.Vt #define ONCE_FLAG_INIT /* implementation specified */
.Sh DESCRIPTION
The
.Nm
function uses the
.Fa flag
parameter to ensure that
.Fa func
is called exactly once,
even if called from several threads.
.Pp
The
.Dv ONCE_FLAG_INIT
definition expands to a value that can be used to initialize an object of type
.Dv once_flag .
.Pp
This portable interface is implemented on top of the
.Xr pthread_once 3
functionality.
.Sh RETURN VALUES
None.
.Sh EXAMPLES
The following calls
.Nm
from two threads using the portable
.Xr thrd 3
interface.
.Bd -literal -offset indent
#include <stdio.h>
#include <threads.h>

static once_flag oflag = ONCE_FLAG_INIT;

void
called_once(void)
{
	printf("called once\n");
}

int
tfun(void *ptr)
{
	call_once(&oflag, called_once);
}

int
main(int argc, char **argv)
{
	thrd_t th1, th2;

	thrd_create(&th1, tfun, NULL);
	thrd_create(&th2, tfun, NULL);

	thrd_join(th1, NULL);
	thrd_join(th2, NULL);

	return 0;
}
.Ed
.Sh SEE ALSO
.Xr pthread_once 3 ,
.Xr threads 3
.Sh STANDARDS
The
.Nm
function conforms to
.St -isoC-2011 .
.Sh HISTORY
This interface first appeared in
.Nx 9 .
.Sh AUTHORS
.An Kamil Rytarowski Aq Mt kamil@NetBSD.org