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

RUN_ONCE, INIT_ONCE, FINI_ONCE -- run a function exactly once
#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));
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.
On failure, RUN_ONCE() returns what init_func() returned. Otherwise, it returns 0.
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. */ }
pthread_once(3), condvar(9), intro(9) NetBSD 9.0 March 19, 2019 NetBSD 9.0

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