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 may fail if:

       EINVAL If either once_control or init_routine is invalid.

       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>.

FUTURE DIRECTIONS

       None.

SEE ALSO

       The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .