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



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.

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 auto-
       matic storage duration or is not initialized by PTHREAD_ONCE_INIT.

RETURN VALUE
       Upon successful completion, pthread_once() shall	 return	 zero;	other-
       wise, 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  initialization for the library is performed when the first pro-
       cedure 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 sim-
       ple 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; how-
       ever,  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-guaran-
       teed means of dynamic initialization, it provides an aid to  the	 reli-
       able  construction of multi-threaded and realtime systems.  The preced-
       ing 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 original 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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