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Linux 2.6 - man page for pthread_once (linux section 3posix)

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

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. Subsequent 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 may fail if:

       EINVAL If either once_control or init_routine is invalid.

       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 initializa-
       tion 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.  Other-
       wise,  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 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.

       The use of pthread_once() 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:

	      #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>.

FUTURE DIRECTIONS
       None.

SEE ALSO
       The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				  PTHREAD_ONCE(P)


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