1PTHREAD_ONCE(3P) POSIX Programmer's Manual PTHREAD_ONCE(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 pthread_once — dynamic package initialization
13
15 #include <pthread.h>
16 int pthread_once(pthread_once_t *once_control,
18 void (*init_routine)(void));
19 pthread_once_t once_control = PTHREAD_ONCE_INIT;
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22 The first call to pthread_once() by any thread in a process, with a
23 given once_control, shall call the init_routine with no arguments. Sub‐
24 sequent calls of pthread_once() with the same once_control shall not
25 call the init_routine. On return from pthread_once(), init_routine
26 shall have completed. The once_control parameter shall determine
27 whether the associated initialization routine has been called.
28 The pthread_once() function is not a cancellation point. However, if
30 init_routine is a cancellation point and is canceled, the effect on
31 once_control shall be as if pthread_once() was never called.
32 If the call to init_routine is terminated by a call to longjmp(),
34 _longjmp(), or siglongjmp(), the behavior is undefined.
35 The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header.
37 The behavior of pthread_once() is undefined if once_control has auto‐
39 matic storage duration or is not initialized by PTHREAD_ONCE_INIT.
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42 Upon successful completion, pthread_once() shall return zero; other‐
43 wise, an error number shall be returned to indicate the error.
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46 The pthread_once() function shall not return an error code of [EINTR].
47 The following sections are informative.
49
51 None.
52
54 If init_routine recursively calls pthread_once() with the same
55 once_control, the recursive call will not call the specified init_rou‐
56 tine, and thus the specified init_routine will not complete, and thus
57 the recursive call to pthread_once() will not return. Use of longjmp(),
58 _longjmp(), or siglongjmp() within an init_routine to jump to a point
59 outside of init_routine prevents init_routine from returning.
60
62 Some C libraries are designed for dynamic initialization. That is, the
63 global initialization for the library is performed when the first pro‐
64 cedure in the library is called. In a single-threaded program, this is
65 normally implemented using a static variable whose value is checked on
66 entry to a routine, as follows:
67 static int random_is_initialized = 0;
70 extern void initialize_random(void);
71 int random_function()
73 {
74 if (random_is_initialized == 0) {
75 initialize_random();
76 random_is_initialized = 1;
77 }
78 ... /* Operations performed after initialization. */
79 }
80 To keep the same structure in a multi-threaded program, a new primitive
82 is needed. Otherwise, library initialization has to be accomplished by
83 an explicit call to a library-exported initialization function prior to
84 any use of the library.
85 For dynamic library initialization in a multi-threaded process, if an
87 initialization flag is used the flag needs to be protected against mod‐
88 ification by multiple threads simultaneously calling into the library.
89 This can be done by using a mutex (initialized by assigning
90 PTHREAD_MUTEX_INITIALIZER). However, the better solution is to use
91 pthread_once() which is designed for exactly this purpose, as follows:
92 #include <pthread.h>
95 static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
96 extern void initialize_random(void);
97 int random_function()
99 {
100 (void) pthread_once(&random_is_initialized, initialize_random);
101 ... /* Operations performed after initialization. */
102 }
103 If an implementation detects that the value specified by the once_con‐
105 trol argument to pthread_once() does not refer to a pthread_once_t
106 object initialized by PTHREAD_ONCE_INIT, it is recommended that the
107 function should fail and report an [EINVAL] error.
108
110 None.
111
113 The Base Definitions volume of POSIX.1‐2017, <pthread.h>
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116 Portions of this text are reprinted and reproduced in electronic form
117 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
118 table Operating System Interface (POSIX), The Open Group Base Specifi‐
119 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
120 Electrical and Electronics Engineers, Inc and The Open Group. In the
121 event of any discrepancy between this version and the original IEEE and
122 The Open Group Standard, the original IEEE and The Open Group Standard
123 is the referee document. The original Standard can be obtained online
124 at http://www.opengroup.org/unix/online.html .
125 Any typographical or formatting errors that appear in this page are
127 most likely to have been introduced during the conversion of the source
128 files to man page format. To report such errors, see https://www.ker‐
129 nel.org/doc/man-pages/reporting_bugs.html .
130IEEE/The Open Group 2017 PTHREAD_ONCE(3P)