2.9BSD/usr/include/sys/callout.h

/*
 * The callout structure is for
 * a routine arranging
 * to be called by the clock interrupt
 * (clock.c) with a specified argument,
 * in a specified amount of time.
 * Used, for example, to time tab
 * delays on typewriters.
 */

struct	callout
{
	short	c_time;			/* incremental time */
	caddr_t	c_arg;			/* argument to routine */
	short	(*c_func)();		/* routine */
};

#ifdef	KERNEL
struct	callout	callout[];
struct	callout	*callNCALL;
#endif