PROLOG<\/a> This manual page is part of the POSIX Programmer\u2019s 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.<\/p>\n pthread_once \u2014 dynamic package initialization<\/p>\n #include int pthread_once(pthread_once_t *once_control<\/i>, The first call to pthread_once<\/i>() by any thread in a process, with a given once_control<\/i>, shall call the init_routine<\/i> with no arguments. Subsequent calls of pthread_once<\/i>() with the same once_control<\/i> shall not call the init_routine<\/i>. On return from pthread_once<\/i>(), init_routine<\/i> shall have completed. The once_control<\/i> parameter shall determine whether the associated initialization routine has been called.<\/p>\n The pthread_once<\/i>() function is not a cancellation point. However, if init_routine<\/i> is a cancellation point and is canceled, the effect on once_control<\/i> shall be as if pthread_once<\/i>() was never called.<\/p>\n If the call to init_routine<\/i> is terminated by a call to longjmp<\/i>(), _longjmp<\/i>(), or siglongjmp<\/i>(), the behavior is undefined.<\/p>\n The constant PTHREAD_ONCE_INIT is defined in the The behavior of pthread_once<\/i>() is undefined if once_control<\/i> has automatic storage duration or is not initialized by PTHREAD_ONCE_INIT.<\/p>\n Upon successful completion, pthread_once<\/i>() shall return zero; otherwise, an error number shall be returned to indicate the error.<\/p>\n The pthread_once<\/i>() function shall not return an error code of [EINTR]<\/b>.<\/p>\n The following sections are informative.<\/i><\/p>\n None.<\/p>\n If init_routine<\/i> recursively calls pthread_once<\/i>() with the same once_control<\/i>, the recursive call will not call the specified init_routine<\/i>, and thus the specified init_routine<\/i> will not complete, and thus the recursive call to pthread_once<\/i>() will not return. Use of longjmp<\/i>(), _longjmp<\/i>(), or siglongjmp<\/i>() within an init_routine<\/i> to jump to a point outside of init_routine<\/i> prevents init_routine<\/i> from returning.<\/p>\n 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:<\/p>\n static int random_is_initialized = 0; int random_function() 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.<\/p>\n 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<\/i>() which is designed for exactly this purpose, as follows:<\/p>\n #include int random_function() If an implementation detects that the value specified by the once_control<\/i> argument to pthread_once<\/i>() does not refer to a pthread_once_t<\/b> object initialized by PTHREAD_ONCE_INIT, it is recommended that the function should fail and report an [EINVAL]<\/b> error.<\/p>\n None.<\/p>\n
NAME<\/a>
SYNOPSIS<\/a>
DESCRIPTION<\/a>
RETURN VALUE<\/a>
ERRORS<\/a>
EXAMPLES<\/a>
APPLICATION USAGE<\/a>
RATIONALE<\/a>
FUTURE DIRECTIONS<\/a>
SEE ALSO<\/a>
COPYRIGHT<\/a> <\/p>\n
\nPROLOG <\/a> <\/h2>\n
NAME <\/a> <\/h2>\n
SYNOPSIS <\/a> <\/h2>\n
void (*init_routine<\/i>)(void));
pthread_once_t once_control<\/i> = PTHREAD_ONCE_INIT;<\/p>\nDESCRIPTION <\/a> <\/h2>\n
RETURN VALUE <\/a> <\/h2>\n
ERRORS <\/a> <\/h2>\n
EXAMPLES <\/a> <\/h2>\n
APPLICATION USAGE <\/a> <\/h2>\n
RATIONALE <\/a> <\/h2>\n
extern void initialize_random(void);<\/p>\n
{
if (random_is_initialized == 0) {
initialize_random();
random_is_initialized = 1;
}
… \/bin \/boot \/dead.letter \/dev \/etc \/home \/initrd \/lib \/lib64 \/lost+found \/media \/mnt \/opt \/proc \/release-notes.html \/release-notes.txt \/root \/run \/sbin \/srv \/sys \/tmp \/usr \/var Operations performed after initialization. bodies\/ usr\/
}<\/p>\n
extern void initialize_random(void);<\/p>\n
{
(void) pthread_once(&random_is_initialized, initialize_random);
… \/bin \/boot \/dead.letter \/dev \/etc \/home \/initrd \/lib \/lib64 \/lost+found \/media \/mnt \/opt \/proc \/release-notes.html \/release-notes.txt \/root \/run \/sbin \/srv \/sys \/tmp \/usr \/var Operations performed after initialization. bodies\/ usr\/
}<\/p>\nFUTURE DIRECTIONS <\/a> <\/h2>\n
SEE ALSO <\/a> <\/h2>\n