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pthread_once(3c) [opensolaris man page]

pthread_once(3C)					   Standard C Library Functions 					  pthread_once(3C)

NAME
pthread_once - initialize dynamic package SYNOPSIS
cc -mt [ flag... ] file... -lpthread [ library... ] #include <pthread.h> pthread_once_t once_control = PTHREAD_ONCE_INIT; int pthread_once(pthread_once_t *once_control, void (*init_routine)(void)); DESCRIPTION
If any thread in a process with a once_control parameter makes a call to pthread_once(), the first call will summon the init_routine(), but subsequent calls will not. The once_control parameter determines whether the associated initialization routine has been called. The init_routine() is complete upon return of pthread_once(). pthread_once() is not a cancellation point; however, if the function init_routine() is a cancellation point and is canceled, the effect on once_control is the same as if pthread_once() had never been called. The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header. If once_control has automatic storage duration or is not initialized by PTHREAD_ONCE_INIT, the behavior of pthread_once() is undefined. RETURN VALUES
Upon successful completion, pthread_once() returns 0. Otherwise, an error number is returned to indicate the error. ERRORS
EINVAL once_control or init_routine is NULL. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ SEE ALSO
attributes(5), standards(5) NOTES
Solaris threads do not offer this functionality. SunOS 5.11 23 Mar 2005 pthread_once(3C)

Check Out this Related Man Page

PTHREAD_ONCE(3P)					     POSIX Programmer's Manual						  PTHREAD_ONCE(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond- ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. NAME
pthread_once -- dynamic package initialization SYNOPSIS
#include <pthread.h> int pthread_once(pthread_once_t *once_control, void (*init_routine)(void)); pthread_once_t once_control = PTHREAD_ONCE_INIT; DESCRIPTION
The first call to pthread_once() by any thread in a process, with a given once_control, shall call the init_routine with no arguments. Sub- sequent calls of pthread_once() with the same once_control shall not call the init_routine. On return from pthread_once(), init_routine shall have completed. The once_control parameter shall determine whether the associated initialization routine has been called. The pthread_once() function is not a cancellation point. However, if init_routine is a cancellation point and is canceled, the effect on once_control shall be as if pthread_once() was never called. The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header. The behavior of pthread_once() is undefined if once_control has automatic storage duration or is not initialized by PTHREAD_ONCE_INIT. RETURN VALUE
Upon successful completion, pthread_once() shall return zero; otherwise, an error number shall be returned to indicate the error. ERRORS
The pthread_once() function shall not return an error code of [EINTR]. The following sections are informative. EXAMPLES
None. APPLICATION USAGE
None. 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: static int random_is_initialized = 0; extern int initialize_random(); int random_function() { if (random_is_initialized == 0) { initialize_random(); random_is_initialized = 1; } ... /* Operations performed after initialization. */ } 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. For dynamic library initialization in a multi-threaded process, a simple initialization flag is not sufficient; the flag needs to be pro- tected 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 solu- tion to this problem. The use of pthread_once() not only supplies an implementation-guaranteed means of dynamic initialization, it provides an aid to the reli- able construction of multi-threaded and realtime systems. The preceding example then becomes: #include <pthread.h> static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT; extern int initialize_random(); int random_function() { (void) pthread_once(&random_is_initialized, initialize_random); ... /* Operations performed after initialization. */ } Note that a pthread_once_t cannot be an array because some compilers do not accept the construct &<array_name>. If an implementation detects that the value specified by the once_control argument to pthread_once() does not refer to a pthread_once_t object initialized by PTHREAD_ONCE_INIT, it is recommended that the function should fail and report an [EINVAL] error. FUTURE DIRECTIONS
None. SEE ALSO
The Base Definitions volume of POSIX.1-2008, <pthread.h> COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol- ogy -- 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 Stan- dard 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 . IEEE
/The Open Group 2013 PTHREAD_ONCE(3P)
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