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.

       If  the  call  to  init_routine  is  terminated  by  a  call  to longjmp(), _longjmp(), or
       siglongjmp(), the behavior is undefined.

       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

       If init_routine recursively calls pthread_once() with the same once_control, the recursive
       call  will  not  call the specified init_routine, and thus the specified init_routine will
       not complete, and thus the recursive call  to  pthread_once()  will  not  return.  Use  of
       longjmp(),  _longjmp(),  or siglongjmp() within an init_routine to jump to a point outside
       of init_routine prevents init_routine from returning.

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 void initialize_random(void);

           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, if an initialization flag
       is used  the  flag  needs  to  be  protected  against  modification  by  multiple  threads
       simultaneously calling into the library. This can be done by using a mutex (initialized by
       assigning  PTHREAD_MUTEX_INITIALIZER).  However,   the   better   solution   is   to   use
       pthread_once() which is designed for exactly this purpose, as follows:

           #include <pthread.h>
           static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
           extern void initialize_random(void);

           int random_function()
           {
               (void) pthread_once(&random_is_initialized, initialize_random);
               ... /* Operations performed after initialization. */
           }

       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.1‐2017, <pthread.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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 .