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   pthread_once.3p    ( 3 )

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Пролог (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.

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Имя (Name)

pthread_once — dynamic package initialization

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Синопсис (Synopsis)

#include <pthread.h>

       int pthread_once(pthread_once_t *once_control,
           void (*init_routine)(void));
       pthread_once_t once_control = PTHREAD_ONCE_INIT;

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Описание (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.

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Возвращаемое значение (Return value)

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

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Ошибки (Error)

The pthread_once() function shall not return an error code of
       [EINTR].

       The following sections are informative.

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Примеры (Examples)

None.

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Использование в приложениях (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.

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Обоснование (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.

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Будущие направления (Future directions)

None.

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Смотри также (See also)

The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)

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