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 .