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;
20
23 The first call to pthread_once() by any thread in a process, with a
24 given once_control, shall call the init_routine with no arguments. Sub‐
25 sequent calls of pthread_once() with the same once_control shall not
26 call the init_routine. On return from pthread_once(), init_routine
27 shall have completed. The once_control parameter shall determine
28 whether the associated initialization routine has been called.
29 The pthread_once() function is not a cancellation point. However, if
31 init_routine is a cancellation point and is canceled, the effect on
32 once_control shall be as if pthread_once() was never called.
33 The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header.
35 The behavior of pthread_once() is undefined if once_control has auto‐
37 matic storage duration or is not initialized by PTHREAD_ONCE_INIT.
38
40 Upon successful completion, pthread_once() shall return zero; other‐
41 wise, an error number shall be returned to indicate the error.
42
44 The pthread_once() function may fail if:
45 EINVAL If either once_control or init_routine is invalid.
47 The pthread_once() function shall not return an error code of [EINTR].
50 The following sections are informative.
52
54 None.
55
57 None.
58
60 Some C libraries are designed for dynamic initialization. That is, the
61 global initialization for the library is performed when the first pro‐
62 cedure in the library is called. In a single-threaded program, this is
63 normally implemented using a static variable whose value is checked on
64 entry to a routine, as follows:
65 static int random_is_initialized = 0;
68 extern int initialize_random();
69 int random_function()
72 {
73 if (random_is_initialized == 0) {
74 initialize_random();
75 random_is_initialized = 1;
76 }
77 ... /* Operations performed after initialization. */
78 }
79 To keep the same structure in a multi-threaded program, a new primitive
81 is needed. Otherwise, library initialization has to be accomplished by
82 an explicit call to a library-exported initialization function prior to
83 any use of the library.
84 For dynamic library initialization in a multi-threaded process, a sim‐
86 ple initialization flag is not sufficient; the flag needs to be pro‐
87 tected against modification by multiple threads simultaneously calling
88 into the library. Protecting the flag requires the use of a mutex; how‐
89 ever, mutexes have to be initialized before they are used. Ensuring
90 that the mutex is only initialized once requires a recursive solution
91 to this problem.
92 The use of pthread_once() not only supplies an implementation-guaran‐
94 teed means of dynamic initialization, it provides an aid to the reli‐
95 able construction of multi-threaded and realtime systems. The preced‐
96 ing example then becomes:
97 #include <pthread.h>
100 static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
101 extern int initialize_random();
102 int random_function()
105 {
106 (void) pthread_once(&random_is_initialized, initialize_random);
107 ... /* Operations performed after initialization. */
108 }
109 Note that a pthread_once_t cannot be an array because some compilers do
111 not accept the construct &<array_name>.
112
114 None.
115
117 The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>
118
120 Portions of this text are reprinted and reproduced in electronic form
121 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
122 -- Portable Operating System Interface (POSIX), The Open Group Base
123 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
124 Electrical and Electronics Engineers, Inc and The Open Group. In the
125 event of any discrepancy between this version and the original IEEE and
126 The Open Group Standard, the original IEEE and The Open Group Standard
127 is the referee document. The original Standard can be obtained online
128 at http://www.opengroup.org/unix/online.html .
129IEEE/The Open Group 2003 PTHREAD_ONCE(3P)