bionic (3) pthread_once.3posix.gz

PROLOG

       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

       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

       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 .