\" $NetBSD: call_once.3,v 1.2 2019/04/27 10:57:11 wiz Exp $
\"
\" Copyright (c) 2016 The NetBSD Foundation, Inc.
\" All rights reserved.
\"
\" This code is derived from software contributed to The NetBSD Foundation
\" by Kamil Rytarowski.
\"
\" Redistribution and use in source and binary forms, with or without
\" modification, are permitted provided that the following conditions
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\" 1. Redistributions of source code must retain the above copyright
\" notice, this list of conditions and the following disclaimer.
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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
[email protected]
