source: GPL/include/linux/timer.h@ 18

#ifndef _LINUX_TIMER_H
#define _LINUX_TIMER_H

/*
 * Old-style timers. Please don't use for any new code.
 *
 * Numbering of these timers should be consecutive to minimize
 * processing delays. [MJ]
 */

#define BLANK_TIMER     0       /* Console screen-saver */
#define BEEP_TIMER      1       /* Console beep */
#define RS_TIMER        2       /* RS-232 ports */
#define SWAP_TIMER      3       /* Background pageout */
#define BACKGR_TIMER    4       /* io_request background I/O */
#define HD_TIMER        5       /* Old IDE driver */
#define FLOPPY_TIMER    6       /* Floppy */
#define QIC02_TAPE_TIMER 7      /* QIC 02 tape */
#define MCD_TIMER       8       /* Mitsumi CDROM */
#define GSCD_TIMER      9       /* Goldstar CDROM */
#define COMTROL_TIMER   10      /* Comtrol serial */
#define DIGI_TIMER      11      /* Digi serial */
#define GDTH_TIMER      12      /* Ugh - gdth scsi driver */

#define COPRO_TIMER     31      /* 387 timeout for buggy hardware (boot only) */

struct timer_struct {
        unsigned long expires;
        void (*fn)(void);
};

extern unsigned long timer_active;
extern struct timer_struct timer_table[32];

/*
 * This is completely separate from the above, and is the
 * "new and improved" way of handling timers more dynamically.
 * Hopefully efficient and general enough for most things.
 *
 * The "hardcoded" timers above are still useful for well-
 * defined problems, but the timer-list is probably better
 * when you need multiple outstanding timers or similar.
 *
 * The "data" field is in case you want to use the same
 * timeout function for several timeouts. You can use this
 * to distinguish between the different invocations.
 */
struct timer_list {
        struct timer_list *next; /* MUST be first element */
        struct timer_list *prev;
        unsigned long expires;
        unsigned long data;
        void (*function)(unsigned long);
};

extern void add_timer(struct timer_list * timer);
extern int  del_timer(struct timer_list * timer);

/*
 * mod_timer is a more efficient way to update the expire field of an
 * active timer (if the timer is inactive it will be activated)
 * mod_timer(a,b) is equivalent to del_timer(a); a->expires = b; add_timer(a)
 */
void mod_timer(struct timer_list *timer, unsigned long expires);

extern void it_real_fn(unsigned long);

#define init_timer(timer) \
        (timer)->next = NULL; \
        (timer)->prev = NULL; 


#define timer_pending(timer)    (timer)->prev != NULL;


/*
 *      These inlines deal with timer wrapping correctly. You are 
 *      strongly encouraged to use them
 *      1. Because people otherwise forget
 *      2. Because if the timer wrap changes in future you wont have to
 *         alter your driver code.
 *
 * Do this with "<0" and ">=0" to only test the sign of the result. A
 * good compiler would generate better code (and a really good compiler
 * wouldn't care). Gcc is currently neither.
 */
#define time_after(a,b)         ((long)(b) - (long)(a) < 0)
#define time_before(a,b)        time_after(b,a)

#define time_after_eq(a,b)      ((long)(a) - (long)(b) >= 0)
#define time_before_eq(a,b)     time_after_eq(b,a)

#endif