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.

       If the call to init__routine is terminated by a call to longjmp(),
       __longjmp(), or siglongjmp(), the behavior is undefined.

       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
       If init__routine recursively calls pthread__once() with the same
       once__control, the recursive call will not call the specified
       init__routine, and thus the specified init__routine will not complete,
       and thus the recursive call to pthread__once() will not return. Use of
       longjmp(), __longjmp(), or siglongjmp() within an init__routine to jump
       to a point outside of init__routine prevents init__routine from
       returning.

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 void initialize_random(void);

           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, 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() which is designed for exactly this purpose, as follows:


           #include <pthread.h>
           static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
           extern void initialize_random(void);

           int random_function()
           {
               (void) pthread_once(&random_is_initialized, initialize_random);
               ... /* Operations performed after initialization. */
           }

       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.1-2017, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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                  2017                      PTHREAD_ONCE(3)
