PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       pthread_once — dynamic package initialization

SYNOPSIS         top

       #include <pthread.h>

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

DESCRIPTION         top

       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         top

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

ERRORS         top

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

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         top

       None.

RATIONALE         top

       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         top

       None.

SEE ALSO         top

       The Base Definitions volume of POSIX.1‐2008, pthread.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: pthread.h(0p)