man3p/pthread_once.3p

   1 .\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
   2 .TH "PTHREAD_ONCE" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
   3 .\" pthread_once
   4 .SH NAME
   5 pthread_once \- dynamic package initialization
   6 .SH SYNOPSIS
   7 .LP
   8 \fB#include <pthread.h>
   9 .br
  10 .sp
  11 int pthread_once(pthread_once_t *\fP\fIonce_control\fP\fB,
  12 .br
  13 \ \ \ \ \ \  void (*\fP\fIinit_routine\fP\fB)(void));
  14 .br
  15 pthread_once_t\fP \fIonce_control\fP \fB= PTHREAD_ONCE_INIT; \fP
  16 \fB
  17 .br
  18 \fP
  19 .SH DESCRIPTION
  20 .LP
  21 The first call to \fIpthread_once\fP() by any thread in a process,
  22 with a given \fIonce_control\fP, shall call the
  23 \fIinit_routine\fP with no arguments. Subsequent calls of \fIpthread_once\fP()
  24 with the same \fIonce_control\fP shall not call
  25 the \fIinit_routine\fP. On return from \fIpthread_once\fP(), \fIinit_routine\fP
  26 shall have completed. The \fIonce_control\fP
  27 parameter shall determine whether the associated initialization routine
  28 has been called.
  29 .LP
  30 The \fIpthread_once\fP() function is not a cancellation point. However,
  31 if \fIinit_routine\fP is a cancellation point and is
  32 canceled, the effect on \fIonce_control\fP shall be as if \fIpthread_once\fP()
  33 was never called.
  34 .LP
  35 The constant PTHREAD_ONCE_INIT is defined in the \fI<pthread.h>\fP
  36 header.
  37 .LP
  38 The behavior of \fIpthread_once\fP() is undefined if \fIonce_control\fP
  39 has automatic storage duration or is not initialized
  40 by PTHREAD_ONCE_INIT.
  41 .SH RETURN VALUE
  42 .LP
  43 Upon successful completion, \fIpthread_once\fP() shall return zero;
  44 otherwise, an error number shall be returned to indicate
  45 the error.
  46 .SH ERRORS
  47 .LP
  48 The \fIpthread_once\fP() function may fail if:
  49 .TP 7
  50 .B EINVAL
  51 If either \fIonce_control\fP or \fIinit_routine\fP is invalid.
  52 .sp
  53 .LP
  54 The \fIpthread_once\fP() function shall not return an error code of
  55 [EINTR].
  56 .LP
  57 \fIThe following sections are informative.\fP
  58 .SH EXAMPLES
  59 .LP
  60 None.
  61 .SH APPLICATION USAGE
  62 .LP
  63 None.
  64 .SH RATIONALE
  65 .LP
  66 Some C libraries are designed for dynamic initialization. That is,
  67 the global initialization for the library is performed when
  68 the first procedure in the library is called. In a single-threaded
  69 program, this is normally implemented using a static variable
  70 whose value is checked on entry to a routine, as follows:
  71 .sp
  72 .RS
  73 .nf
  74
  75 \fBstatic int random_is_initialized = 0;
  76 extern int initialize_random();
  77 .sp
  78
  79 int random_function()
  80 {
  81     if (random_is_initialized == 0) {
  82         initialize_random();
  83         random_is_initialized = 1;
  84     }
  85     ... /* Operations performed after initialization. */
  86 }
  87 \fP
  88 .fi
  89 .RE
  90 .LP
  91 To keep the same structure in a multi-threaded program, a new primitive
  92 is needed. Otherwise, library initialization has to be
  93 accomplished by an explicit call to a library-exported initialization
  94 function prior to any use of the library.
  95 .LP
  96 For dynamic library initialization in a multi-threaded process, a
  97 simple initialization flag is not sufficient; the flag needs
  98 to be protected against modification by multiple threads simultaneously
  99 calling into the library. Protecting the flag requires the
 100 use of a mutex; however, mutexes have to be initialized before they
 101 are used. Ensuring that the mutex is only initialized once
 102 requires a recursive solution to this problem.
 103 .LP
 104 The use of \fIpthread_once\fP() not only supplies an implementation-guaranteed
 105 means of dynamic initialization, it provides an
 106 aid to the reliable construction of multi-threaded and realtime systems.
 107 The preceding example then becomes:
 108 .sp
 109 .RS
 110 .nf
 111
 112 \fB#include <pthread.h>
 113 static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
 114 extern int initialize_random();
 115 .sp
 116
 117 int random_function()
 118 {
 119     (void) pthread_once(&random_is_initialized, initialize_random);
 120     ... /* Operations performed after initialization. */
 121 }
 122 \fP
 123 .fi
 124 .RE
 125 .LP
 126 Note that a \fBpthread_once_t\fP cannot be an array because some compilers
 127 do not accept the construct
 128 \fB&<array_name>\fP.
 129 .SH FUTURE DIRECTIONS
 130 .LP
 131 None.
 132 .SH SEE ALSO
 133 .LP
 134 The Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<pthread.h>\fP
 135 .SH COPYRIGHT
 136 Portions of this text are reprinted and reproduced in electronic form
 137 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
 138 -- Portable Operating System Interface (POSIX), The Open Group Base
 139 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
 140 Electrical and Electronics Engineers, Inc and The Open Group. In the
 141 event of any discrepancy between this version and the original IEEE and
 142 The Open Group Standard, the original IEEE and The Open Group Standard
 143 is the referee document. The original Standard can be obtained online at
 144 http://www.opengroup.org/unix/online.html .