114 lines
2.9 KiB
Groff
114 lines
2.9 KiB
Groff
.\" $NetBSD: call_once.3,v 1.2 2019/04/27 10:57:11 wiz Exp $
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.\"
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.\" Copyright (c) 2016 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Kamil Rytarowski.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd October 16, 2016
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.Dt CALL_ONCE 3
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.Os
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.Sh NAME
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.Nm call_once
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.Nd calls function exactly once
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.Sh LIBRARY
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.Lb libpthread
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.Sh SYNOPSIS
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.In threads.h
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.Ft void
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.Fn call_once "once_flag *flag" "void (*func)(void)"
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.Vt #define ONCE_FLAG_INIT /* implementation specified */
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.Sh DESCRIPTION
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The
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.Nm
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function uses the
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.Fa flag
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parameter to ensure that
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.Fa func
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is called exactly once,
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even if called from several threads.
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.Pp
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The
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.Dv ONCE_FLAG_INIT
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definition expands to a value that can be used to initialize an object of type
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.Dv once_flag .
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.Pp
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This portable interface is implemented on top of the
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.Xr pthread_once 3
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functionality.
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.Sh RETURN VALUES
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None.
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.Sh EXAMPLES
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The following calls
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.Nm
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from two threads using the portable
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.Xr thrd 3
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interface.
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.Bd -literal -offset indent
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#include <stdio.h>
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#include <threads.h>
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static once_flag oflag = ONCE_FLAG_INIT;
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void
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called_once(void)
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{
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printf("called once\n");
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}
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int
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tfun(void *ptr)
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{
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call_once(&oflag, called_once);
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}
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int
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main(int argc, char **argv)
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{
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thrd_t th1, th2;
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thrd_create(&th1, tfun, NULL);
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thrd_create(&th2, tfun, NULL);
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thrd_join(th1, NULL);
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thrd_join(th2, NULL);
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return 0;
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}
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.Ed
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.Sh SEE ALSO
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.Xr pthread_once 3 ,
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.Xr threads 3
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.Sh STANDARDS
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The
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.Nm
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function conforms to
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.St -isoC-2011 .
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.Sh HISTORY
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This interface first appeared in
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.Nx 9 .
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.Sh AUTHORS
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.An Kamil Rytarowski Aq Mt kamil@NetBSD.org
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