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diff --git a/man-pages-posix-2013/man3p/pthread_once.3p b/man-pages-posix-2013/man3p/pthread_once.3p new file mode 100644 index 0000000..316b1e9 --- /dev/null +++ b/man-pages-posix-2013/man3p/pthread_once.3p @@ -0,0 +1,175 @@ +'\" et +.TH PTHREAD_ONCE "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual" +.SH PROLOG +This manual page is part of the POSIX Programmer's Manual. +The Linux implementation of this interface may differ (consult +the corresponding Linux manual page for details of Linux behavior), +or the interface may not be implemented on Linux. + +.SH NAME +pthread_once +\(em dynamic package initialization +.SH SYNOPSIS +.LP +.nf +#include <pthread.h> +.P +int pthread_once(pthread_once_t *\fIonce_control\fP, + void (*\fIinit_routine\fP)(void)); +pthread_once_t \fIonce_control\fP = PTHREAD_ONCE_INIT; +.fi +.SH DESCRIPTION +The first call to +\fIpthread_once\fR() +by any thread in a process, with a given +.IR once_control , +shall call the +.IR init_routine +with no arguments. Subsequent calls of +\fIpthread_once\fR() +with the same +.IR once_control +shall not call the +.IR init_routine . +On return from +\fIpthread_once\fR(), +.IR init_routine +shall have completed. The +.IR once_control +parameter shall determine whether the associated initialization +routine has been called. +.P +The +\fIpthread_once\fR() +function is not a cancellation point. However, if +.IR init_routine +is a cancellation point and is canceled, the effect on +.IR once_control +shall be as if +\fIpthread_once\fR() +was never called. +.P +The constant PTHREAD_ONCE_INIT is defined in the +.IR <pthread.h> +header. +.P +The behavior of +\fIpthread_once\fR() +is undefined if +.IR once_control +has automatic storage duration or is not initialized by +PTHREAD_ONCE_INIT. +.SH "RETURN VALUE" +Upon successful completion, +\fIpthread_once\fR() +shall return zero; otherwise, an error number shall be returned to +indicate the error. +.SH ERRORS +The +\fIpthread_once\fR() +function shall not return an error code of +.BR [EINTR] . +.LP +.IR "The following sections are informative." +.SH EXAMPLES +None. +.SH "APPLICATION USAGE" +None. +.SH RATIONALE +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 4 +.nf +\fB +static int random_is_initialized = 0; +extern int initialize_random(); +.P +int random_function() +{ + if (random_is_initialized == 0) { + initialize_random(); + random_is_initialized = 1; + } + ... /* Operations performed after initialization. */ +} +.fi \fR +.P +.RE +.P +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. +.P +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. +.P +The use of +\fIpthread_once\fR() +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 4 +.nf +\fB +#include <pthread.h> +static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT; +extern int initialize_random(); +.P +int random_function() +{ + (void) pthread_once(&random_is_initialized, initialize_random); + ... /* Operations performed after initialization. */ +} +.fi \fR +.P +.RE +.P +Note that a +.BR pthread_once_t +cannot be an array because some compilers do not accept the construct +\fB&<array_name>\fP. +.P +If an implementation detects that the value specified by the +.IR once_control +argument to +\fIpthread_once\fR() +does not refer to a +.BR pthread_once_t +object initialized by PTHREAD_ONCE_INIT, it is recommended that the +function should fail and report an +.BR [EINVAL] +error. +.SH "FUTURE DIRECTIONS" +None. +.SH "SEE ALSO" +The Base Definitions volume of POSIX.1\(hy2008, +.IR "\fB<pthread.h>\fP" +.SH COPYRIGHT +Portions of this text are reprinted and reproduced in electronic form +from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology +-- Portable Operating System Interface (POSIX), The Open Group Base +Specifications Issue 7, Copyright (C) 2013 by the Institute of +Electrical and Electronics Engineers, Inc and The Open Group. +(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html . + +Any typographical or formatting errors that appear +in this page are most likely +to have been introduced during the conversion of the source files to +man page format. To report such errors, see +https://www.kernel.org/doc/man-pages/reporting_bugs.html . |