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diff --git a/man3p/pthread_once.3p b/man3p/pthread_once.3p new file mode 100644 index 000000000..4220b3167 --- /dev/null +++ b/man3p/pthread_once.3p @@ -0,0 +1,144 @@ +.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved +.TH "PTHREAD_ONCE" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual" +.\" pthread_once +.SH NAME +pthread_once \- dynamic package initialization +.SH SYNOPSIS +.LP +\fB#include <pthread.h> +.br +.sp +int pthread_once(pthread_once_t *\fP\fIonce_control\fP\fB, +.br +\ \ \ \ \ \ void (*\fP\fIinit_routine\fP\fB)(void)); +.br +pthread_once_t\fP \fIonce_control\fP \fB= PTHREAD_ONCE_INIT; \fP +\fB +.br +\fP +.SH DESCRIPTION +.LP +The first call to \fIpthread_once\fP() by any thread in a process, +with a given \fIonce_control\fP, shall call the +\fIinit_routine\fP with no arguments. Subsequent calls of \fIpthread_once\fP() +with the same \fIonce_control\fP shall not call +the \fIinit_routine\fP. On return from \fIpthread_once\fP(), \fIinit_routine\fP +shall have completed. The \fIonce_control\fP +parameter shall determine whether the associated initialization routine +has been called. +.LP +The \fIpthread_once\fP() function is not a cancellation point. However, +if \fIinit_routine\fP is a cancellation point and is +canceled, the effect on \fIonce_control\fP shall be as if \fIpthread_once\fP() +was never called. +.LP +The constant PTHREAD_ONCE_INIT is defined in the \fI<pthread.h>\fP +header. +.LP +The behavior of \fIpthread_once\fP() is undefined if \fIonce_control\fP +has automatic storage duration or is not initialized +by PTHREAD_ONCE_INIT. +.SH RETURN VALUE +.LP +Upon successful completion, \fIpthread_once\fP() shall return zero; +otherwise, an error number shall be returned to indicate +the error. +.SH ERRORS +.LP +The \fIpthread_once\fP() function may fail if: +.TP 7 +.B EINVAL +If either \fIonce_control\fP or \fIinit_routine\fP is invalid. +.sp +.LP +The \fIpthread_once\fP() function shall not return an error code of +[EINTR]. +.LP +\fIThe following sections are informative.\fP +.SH EXAMPLES +.LP +None. +.SH APPLICATION USAGE +.LP +None. +.SH RATIONALE +.LP +Some C libraries are designed for dynamic initialization. That is, +the global initialization for the library is performed when +the first procedure in the library is called. In a single-threaded +program, this is normally implemented using a static variable +whose value is checked on entry to a routine, as follows: +.sp +.RS +.nf + +\fBstatic int random_is_initialized = 0; +extern int initialize_random(); +.sp + +int random_function() +{ + if (random_is_initialized == 0) { + initialize_random(); + random_is_initialized = 1; + } + ... /* Operations performed after initialization. */ +} +\fP +.fi +.RE +.LP +To keep the same structure in a multi-threaded program, a new primitive +is needed. Otherwise, library initialization has to be +accomplished by an explicit call to a library-exported initialization +function prior to any use of the library. +.LP +For dynamic library initialization in a multi-threaded process, a +simple initialization flag is not sufficient; the flag needs +to be protected against modification by multiple threads simultaneously +calling into the library. Protecting the flag requires the +use of a mutex; however, mutexes have to be initialized before they +are used. Ensuring that the mutex is only initialized once +requires a recursive solution to this problem. +.LP +The use of \fIpthread_once\fP() not only supplies an implementation-guaranteed +means of dynamic initialization, it provides an +aid to the reliable construction of multi-threaded and realtime systems. +The preceding example then becomes: +.sp +.RS +.nf + +\fB#include <pthread.h> +static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT; +extern int initialize_random(); +.sp + +int random_function() +{ + (void) pthread_once(&random_is_initialized, initialize_random); + ... /* Operations performed after initialization. */ +} +\fP +.fi +.RE +.LP +Note that a \fBpthread_once_t\fP cannot be an array because some compilers +do not accept the construct +\fB&<array_name>\fP. +.SH FUTURE DIRECTIONS +.LP +None. +.SH SEE ALSO +.LP +The Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<pthread.h>\fP +.SH COPYRIGHT +Portions of this text are reprinted and reproduced in electronic form +from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology +-- Portable Operating System Interface (POSIX), The Open Group Base +Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of +Electrical and Electronics Engineers, Inc and The Open Group. In the +event of any discrepancy between this version and the original IEEE and +The Open Group Standard, the original IEEE and The Open Group Standard +is the referee document. The original Standard can be obtained online at +http://www.opengroup.org/unix/online.html . |