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1.\" $OpenBSD: pthread_once.3,v 1.16 2025/06/07 00:16:52 schwarze Exp $
2.\"
3.\" Copyright (c) 1996 John Birrell <jb@cimlogic.com.au>.
4.\" All rights reserved.
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16.\" This product includes software developed by John Birrell.
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33.\" $FreeBSD: pthread_once.3,v 1.5 1999/08/28 00:03:09 peter Exp $
34.\"
35.Dd $Mdocdate: June 7 2025 $
36.Dt PTHREAD_ONCE 3
37.Os
38.Sh NAME
39.Nm pthread_once
40.Nd dynamic package initialization
41.Sh SYNOPSIS
42.Lb libpthread
43.In pthread.h
44.Vt pthread_once_t once_control No = Dv PTHREAD_ONCE_INIT ;
45.Ft int
46.Fn pthread_once "pthread_once_t *once_control" "void (*init_routine)(void)"
47.Sh DESCRIPTION
48The first call to
49.Fn pthread_once
50by any thread in a process, with a given
51.Fa once_control ,
52will call the
53.Fn init_routine
54with no arguments.
55Subsequent calls to
56.Fn pthread_once
57with the same
58.Fa once_control
59will not call the
60.Fn init_routine .
61On return from
62.Fn pthread_once ,
63it is guaranteed that
64.Fn init_routine
65has completed.
66The
67.Fa once_control
68parameter is used to determine whether the associated initialization
69routine has been called.
70.Pp
71The function
72.Fn pthread_once
73is not a cancellation point.
74However, if
75.Fn init_routine
76is a cancellation point and is cancelled, the effect on
77.Va once_control
78is as if
79.Fn pthread_once
80was never called.
81.Pp
82The constant
83.Dv PTHREAD_ONCE_INIT
84is defined in the header file
85.In pthread.h .
86.Pp
87The behavior of
88.Fn pthread_once
89is undefined if
90.Fa once_control
91has automatic storage duration or is not initialized by
92.Dv PTHREAD_ONCE_INIT .
93.Sh RETURN VALUES
94If successful, the
95.Fn pthread_once
96function will return zero.
97Otherwise an error number will be returned to indicate the error.
98.Sh ERRORS
99None.
100.Sh STANDARDS
101.Fn pthread_once
102conforms to
103.St -p1003.1-96 .