PTHREAD_ONCE(3)            POSIX Programmer's Manual           PTHREAD_ONCE(3)



PROLOG
       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.


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 == 00;;
           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
       The Base Definitions volume of POSIX.1-2008, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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                  2013                      PTHREAD_ONCE(3)
