The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group

#### NAME

> pthread_once - dynamic package initialization

#### SYNOPSIS

> ``#include <pthread.h>
> 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. 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.

> 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.

#### RETURN VALUE

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

#### ERRORS

> 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 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 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 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 pthread_once() 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:

>     #include <pthread.h>
>     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 &<array_name>.

> 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.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> XBD <pthread.h>

#### CHANGE HISTORY

> First released in Issue 5. Included for alignment with the POSIX Threads Extension.

#### Issue 6

> The pthread_once() function is marked as part of the Threads option.

> The [EINVAL] error is added as a ``may fail'' case for if either argument is invalid.

#### Issue 7

> The pthread_once() function is moved from the Threads option to the Base.

> The [EINVAL] error for an uninitialized pthread_once_t object is removed; this condition results in undefined behavior.

End of informative text.

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