.\" $OpenBSD: pthread_once.3,v 1.13 2013/06/05 03:44:50 tedu Exp $
.\"
.\" Copyright (c) 1996 John Birrell <jb@cimlogic.com.au>.
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.\"	This product includes software developed by John Birrell.
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.\" $FreeBSD: pthread_once.3,v 1.5 1999/08/28 00:03:09 peter Exp $
.\"
.Dd $Mdocdate: June 5 2013 $
.Dt PTHREAD_ONCE 3
.Os
.Sh NAME
.Nm pthread_once
.Nd dynamic package initialization
.Sh SYNOPSIS
.In pthread.h
.Pp
.Ft pthread_once_t
.Fa once_control
\&=
.Dv PTHREAD_ONCE_INIT ;
.Pp
.Ft int
.Fn pthread_once "pthread_once_t *once_control" "void (*init_routine)(void)"
.Sh DESCRIPTION
The first call to
.Fn pthread_once
by any thread in a process, with a given
.Fa once_control ,
will call the
.Fn init_routine
with no arguments.
Subsequent calls to
.Fn pthread_once
with the same
.Fa once_control
will not call the
.Fn init_routine .
On return from
.Fn pthread_once ,
it is guaranteed that
.Fn init_routine
has completed.
The
.Fa once_control
parameter is used to determine whether the associated initialization
routine has been called.
.Pp
The function
.Fn pthread_once
is not a cancellation point.
However, if
.Fn init_routine
is a cancellation point and is cancelled, the effect on
.Fa once_control
is as if
.Fn pthread_once
was never called.
.Pp
The constant
.Dv PTHREAD_ONCE_INIT
is defined in the header file
.Aq Pa pthread.h .
.Pp
The behavior of
.Fn pthread_once
is undefined if
.Fa once_control
has automatic storage duration or is not initialized by
.Dv PTHREAD_ONCE_INIT .
.Sh RETURN VALUES
If successful, the
.Fn pthread_once
function will return zero.
Otherwise an error number will be returned to indicate the error.
.Sh ERRORS
None.
.Sh STANDARDS
.Fn pthread_once
conforms to
.St -p1003.1-96 .