PTHREAD_ONCE(3)            POSIX Programmer's Manual           PTHREAD_ONCE(3)



PPRROOLLOOGG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.


NNAAMMEE
       pthread_once -- dynamic package initialization

SSYYNNOOPPSSIISS
       #include <pthread.h>

       int pthread_once(pthread_once_t *_o_n_c_e___c_o_n_t_r_o_l,
           void (*_i_n_i_t___r_o_u_t_i_n_e)(void));
       pthread_once_t _o_n_c_e___c_o_n_t_r_o_l = PTHREAD_ONCE_INIT;

DDEESSCCRRIIPPTTIIOONN
       The first call to _p_t_h_r_e_a_d___o_n_c_e() by any thread in a process, with a
       given _o_n_c_e___c_o_n_t_r_o_l, shall call the _i_n_i_t___r_o_u_t_i_n_e with no arguments.
       Subsequent calls of _p_t_h_r_e_a_d___o_n_c_e() with the same _o_n_c_e___c_o_n_t_r_o_l shall not
       call the _i_n_i_t___r_o_u_t_i_n_e.  On return from _p_t_h_r_e_a_d___o_n_c_e(), _i_n_i_t___r_o_u_t_i_n_e
       shall have completed. The _o_n_c_e___c_o_n_t_r_o_l parameter shall determine
       whether the associated initialization routine has been called.

       The _p_t_h_r_e_a_d___o_n_c_e() function is not a cancellation point. However, if
       _i_n_i_t___r_o_u_t_i_n_e is a cancellation point and is canceled, the effect on
       _o_n_c_e___c_o_n_t_r_o_l shall be as if _p_t_h_r_e_a_d___o_n_c_e() was never called.

       The constant PTHREAD_ONCE_INIT is defined in the _<_p_t_h_r_e_a_d_._h_> header.

       The behavior of _p_t_h_r_e_a_d___o_n_c_e() is undefined if _o_n_c_e___c_o_n_t_r_o_l has
       automatic storage duration or is not initialized by PTHREAD_ONCE_INIT.

RREETTUURRNN VVAALLUUEE
       Upon successful completion, _p_t_h_r_e_a_d___o_n_c_e() shall return zero;
       otherwise, an error number shall be returned to indicate the error.

EERRRROORRSS
       The _p_t_h_r_e_a_d___o_n_c_e() function shall not return an error code of [[EEIINNTTRR]].

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       None.

RRAATTIIOONNAALLEE
       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:

           ssttaattiicc iinntt rraannddoomm__iiss__iinniittiiaalliizzeedd == 00;;
           eexxtteerrnn iinntt iinniittiiaalliizzee__rraannddoomm(());;

           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 _p_t_h_r_e_a_d___o_n_c_e() 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:

           ##iinncclluuddee <<pptthhrreeaadd..hh>>
           ssttaattiicc pptthhrreeaadd__oonnccee__tt rraannddoomm__iiss__iinniittiiaalliizzeedd == PPTTHHRREEAADD__OONNCCEE__IINNIITT;;
           eexxtteerrnn iinntt iinniittiiaalliizzee__rraannddoomm(());;

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

       Note that a pptthhrreeaadd__oonnccee__tt cannot be an array because some compilers do
       not accept the construct &&<<aarrrraayy__nnaammee>>.

       If an implementation detects that the value specified by the
       _o_n_c_e___c_o_n_t_r_o_l argument to _p_t_h_r_e_a_d___o_n_c_e() does not refer to a
       pptthhrreeaadd__oonnccee__tt object initialized by PTHREAD_ONCE_INIT, it is
       recommended that the function should fail and report an [[EEIINNVVAALL]] error.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       The Base Definitions volume of POSIX.1-2008, <<pptthhrreeaadd..hh>>

CCOOPPYYRRIIGGHHTT
       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(3)
