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.

       If the call to _i_n_i_t___r_o_u_t_i_n_e is terminated by a call to _l_o_n_g_j_m_p(),
       ___l_o_n_g_j_m_p(), or _s_i_g_l_o_n_g_j_m_p(), the behavior is undefined.

       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
       If _i_n_i_t___r_o_u_t_i_n_e recursively calls _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, the recursive call will not call the specified
       _i_n_i_t___r_o_u_t_i_n_e, and thus the specified _i_n_i_t___r_o_u_t_i_n_e will not complete,
       and thus the recursive call to _p_t_h_r_e_a_d___o_n_c_e() will not return. Use of
       _l_o_n_g_j_m_p(), ___l_o_n_g_j_m_p(), or _s_i_g_l_o_n_g_j_m_p() within an _i_n_i_t___r_o_u_t_i_n_e to jump
       to a point outside of _i_n_i_t___r_o_u_t_i_n_e prevents _i_n_i_t___r_o_u_t_i_n_e from
       returning.

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:


           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
       _p_t_h_r_e_a_d___o_n_c_e() 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
       _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.1-2017, <<pptthhrreeaadd..hh>>

CCOOPPYYRRIIGGHHTT
       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 .



IEEE/The Open Group                  2017                      PTHREAD_ONCE(3)
