Callouts allow drivers to asynchronously execute a function after a specified amount of time (or at regular intervals). These functions are known as callout functions.

The FreeBSD kernel provides the following seven functions for working with callouts:

#include <sys/types.h>
#include <sys/systm.h>

typedef void timeout_t (void *);

void
callout_init(struct callout *c, int mpsafe);

void
callout_init_mtx(struct callout *c, struct mtx *mtx, int flags);

void
callout_init_rw(struct callout *c, struct rwlock *rw, int flags);

int
callout_stop(struct callout *c);

int
callout_drain(struct callout *c);

int
callout_reset(struct callout *c, int ticks, timeout_t *func,
    void *arg);

int
callout_schedule(struct callout *c, int ticks);