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 (con-
       sult 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. 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  can-
       celed, 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  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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                              PTHREAD_ONCE(3P)