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PTHREAD_ONCE(3) PTHREAD_ONCE(3) 名前 pthread_once - 1 回きりの初期化 書式 #include pthread_once_t once_control = PTHREAD_ONCE_INIT; int pthread_once(pthread_once_t *once_control, void (*init_routine) (void)); 説明 pthread_once の目的は、初期化コードが高々 1 回しか実行されないように す る ことである。引数 once_control は静的変数または外部変数を指し示し、静 的に PTHREAD_ONCE_INIT という値に初期化されていなければならない。 最初に引数 once_control とともに pthread_once が呼び出されたときには 、 init_routine を引数なしで呼び出し、変数 once_control の値を変更して初期 化が行なわれたことを記録する。その後同じ引数 once_control と と も に pthread_once が呼び出されたときは何もしない。 返り値 pthread_once は常に 0 を返す。 エラー なし。 著者 Xavier Leroy [訳注] glibc-linuxthreads の最新のドキュメントは Texinfo 形式で提供され ている。以下は glibc-linuxthreads-2.3.1 の Texinfo ファイルからの引用で 、「説明」への追記にあたるものである。 ス レッドが init_routine を実行中に取り消しされた場合、 once_control は リセットされ、将来 pthread_once が呼び出されたとき初期化ルーチンは再 び 実行される。 1 つまたは複数のスレッドが pthread_once の初期化ルーチンを実行中にプロ セスが fork したときには、子プロセスではそれぞれの once_control 変数 は リ セットされたように見え、子プロセスで pthread_once が呼び出されたとき 初期化ルーチンは実行される。 LinuxThreads PTHREAD_ONCE(3)

PTHREAD_ONCE(3P) POSIX Programmer’s Manual PTHREAD_ONCE(3P) 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 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. Sub- sequent 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 header. The behavior of pthread_once() is undefined if once_control has auto- matic storage duration or is not initialized by PTHREAD_ONCE_INIT. RETURN VALUE Upon successful completion, pthread_once() shall return zero; other- wise, an error number shall be returned to indicate the error. ERRORS The pthread_once() function may fail if: EINVAL If either once_control or init_routine is invalid. 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 pro- cedure 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 sim- ple initialization flag is not sufficient; the flag needs to be pro- tected against modification by multiple threads simultaneously calling into the library. Protecting the flag requires the use of a mutex; how- ever, 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-guaran- teed means of dynamic initialization, it provides an aid to the reli- able construction of multi-threaded and realtime systems. The preced- ing example then becomes: #include 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 &. FUTURE DIRECTIONS None. SEE ALSO The Base Definitions volume of IEEE Std 1003.1-2001, COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 . IEEE/The Open Group 2003 PTHREAD_ONCE(3P)