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 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
       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>.

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 Technol-
       ogy -- 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(3P)