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	 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 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 void 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 preceding
       example then becomes:

	   #include <pthread.h>
	   static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
	   extern void 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_con-
       trol  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 Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal  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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group		     2013		      PTHREAD_ONCE(3P)