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. 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 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>.

       If an implementation detects that the value specified by the once_control 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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2013                                  PTHREAD_ONCE(3POSIX)