RUN_ONCE(9) - NetBSD Manual Pages

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RUN_ONCE(9)            NetBSD Kernel Developer's Manual            RUN_ONCE(9)


NAME

     RUN_ONCE, INIT_ONCE, FINI_ONCE -- run a function exactly once


SYNOPSIS

     #include <sys/once.h>

     ONCE_DECL(control);

     int
     RUN_ONCE(once_t *control, int (*init_func)(void));

     int
     INIT_ONCE(once_t *control, int (*init_func)(void));

     void
     FINI_ONCE(once_t *control, void (*fini_func)(void));


DESCRIPTION

     RUN_ONCE() provides a functionality similar to pthread_once(3).  It
     ensures that, for a given control, init_func() is executed (successfully)
     exactly once.  It is considered as a successful execution if and only if
     init_func() returned 0.  As long as there was no successful execution,
     RUN_ONCE() will try again each time it is called.

     RUN_ONCE() can sleep if it's called concurrently.

     INIT_ONCE() is used in pair with FINI_ONCE().  INIT_ONCE() will only be
     run once similar to RUN_ONCE().  FINI_ONCE() will only be run at last
     time if it is called as many times as calling INIT_ONCE().  When
     FINI_ONCE() is executed, the next call to INIT_ONCE() will be executed
     again.  That is, INIT_ONCE() and FINI_ONCE() can be nested.


RETURN VALUES

     On failure, RUN_ONCE() returns what init_func() returned.  Otherwise, it
     returns 0.


EXAMPLES

     The following example shows how RUN_ONCE() is used.  Regardless of how
     many times some_func() is executed, init_func() will be executed exactly
     once.

     static int
     init_func(void)
     {

             /*
              * do some initialization.
              */

             return 0; /* success */
     }

     int
     some_func(void)
     {
             static ONCE_DECL(control);

             RUN_ONCE(&control, init_func);

             /*
              * we are sure that init_func has already been completed here.
              */
     }


SEE ALSO

     pthread_once(3), condvar(9), intro(9)

NetBSD 9.4                      March 19, 2019                      NetBSD 9.4