git.distorted.org.uk Git - u/mdw/putty/blob - timing.c

   1 /*
   2  * timing.c
   3  *
   4  * This module tracks any timers set up by schedule_timer(). It
   5  * keeps all the currently active timers in a list; it informs the
   6  * front end of when the next timer is due to go off if that
   7  * changes; and, very importantly, it tracks the context pointers
   8  * passed to schedule_timer(), so that if a context is freed all
   9  * the timers associated with it can be immediately annulled.
  10  *
  11  *
  12  * The problem is that computer clocks aren't perfectly accurate.
  13  * The GETTICKCOUNT function returns a 32bit number that normally
  14  * increases by about 1000 every second. On windows this uses the PC's
  15  * interrupt timer and so is only accurate to around 20ppm.  On unix it's
  16  * a value that's calculated from the current UTC time and so is in theory
  17  * accurate in the long term but may jitter and jump in the short term.
  18  *
  19  * What PuTTY needs from these timers is simply a way of delaying the
  20  * calling of a function for a little while, if it's occasionally called a
  21  * little early or late that's not a problem. So to protect against clock
  22  * jumps schedule_timer records the time that it was called in the timer
  23  * structure. With this information the run_timers function can see when
  24  * the current GETTICKCOUNT value is after the time the event should be
  25  * fired OR before the time it was set. In the latter case the clock must
  26  * have jumped, the former is (probably) just the normal passage of time.
  27  *
  28  */
  29
  30 #include <assert.h>
  31 #include <stdio.h>
  32
  33 #include "putty.h"
  34 #include "tree234.h"
  35
  36 struct timer {
  37     timer_fn_t fn;
  38     void *ctx;
  39     long now;
  40     long when_set;
  41 };
  42
  43 static tree234 *timers = NULL;
  44 static tree234 *timer_contexts = NULL;
  45 static long now = 0L;
  46
  47 static int compare_timers(void *av, void *bv)
  48 {
  49     struct timer *a = (struct timer *)av;
  50     struct timer *b = (struct timer *)bv;
  51     long at = a->now - now;
  52     long bt = b->now - now;
  53
  54     if (at < bt)
  55         return -1;
  56     else if (at > bt)
  57         return +1;
  58
  59     /*
  60      * Failing that, compare on the other two fields, just so that
  61      * we don't get unwanted equality.
  62      */
  63 #ifdef __LCC__
  64     /* lcc won't let us compare function pointers. Legal, but annoying. */
  65     {
  66         int c = memcmp(&a->fn, &b->fn, sizeof(a->fn));
  67         if (c < 0)
  68             return -1;
  69         else if (c > 0)
  70             return +1;
  71     }
  72 #else
  73     if (a->fn < b->fn)
  74         return -1;
  75     else if (a->fn > b->fn)
  76         return +1;
  77 #endif
  78
  79     if (a->ctx < b->ctx)
  80         return -1;
  81     else if (a->ctx > b->ctx)
  82         return +1;
  83
  84     /*
  85      * Failing _that_, the two entries genuinely are equal, and we
  86      * never have a need to store them separately in the tree.
  87      */
  88     return 0;
  89 }
  90
  91 static int compare_timer_contexts(void *av, void *bv)
  92 {
  93     char *a = (char *)av;
  94     char *b = (char *)bv;
  95     if (a < b)
  96         return -1;
  97     else if (a > b)
  98         return +1;
  99     return 0;
 100 }
 101
 102 static void init_timers(void)
 103 {
 104     if (!timers) {
 105         timers = newtree234(compare_timers);
 106         timer_contexts = newtree234(compare_timer_contexts);
 107         now = GETTICKCOUNT();
 108     }
 109 }
 110
 111 long schedule_timer(int ticks, timer_fn_t fn, void *ctx)
 112 {
 113     long when;
 114     struct timer *t, *first;
 115
 116     init_timers();
 117
 118     now = GETTICKCOUNT();
 119     when = ticks + now;
 120
 121     /*
 122      * Just in case our various defences against timing skew fail
 123      * us: if we try to schedule a timer that's already in the
 124      * past, we instead schedule it for the immediate future.
 125      */
 126     if (when - now <= 0)
 127         when = now + 1;
 128
 129     t = snew(struct timer);
 130     t->fn = fn;
 131     t->ctx = ctx;
 132     t->now = when;
 133     t->when_set = now;
 134
 135     if (t != add234(timers, t)) {
 136         sfree(t);                      /* identical timer already exists */
 137     } else {
 138         add234(timer_contexts, t->ctx);/* don't care if this fails */
 139     }
 140
 141     first = (struct timer *)index234(timers, 0);
 142     if (first == t) {
 143         /*
 144          * This timer is the very first on the list, so we must
 145          * notify the front end.
 146          */
 147         timer_change_notify(first->now);
 148     }
 149
 150     return when;
 151 }
 152
 153 /*
 154  * Call to run any timers whose time has reached the present.
 155  * Returns the time (in ticks) expected until the next timer after
 156  * that triggers.
 157  */
 158 int run_timers(long anow, long *next)
 159 {
 160     struct timer *first;
 161
 162     init_timers();
 163
 164     now = GETTICKCOUNT();
 165
 166     while (1) {
 167         first = (struct timer *)index234(timers, 0);
 168
 169         if (!first)
 170             return FALSE;              /* no timers remaining */
 171
 172         if (find234(timer_contexts, first->ctx, NULL) == NULL) {
 173             /*
 174              * This timer belongs to a context that has been
 175              * expired. Delete it without running.
 176              */
 177             delpos234(timers, 0);
 178             sfree(first);
 179         } else if (first->now - now <= 0 ||
 180                    now - (first->when_set - 10) < 0) {
 181             /*
 182              * This timer is active and has reached its running
 183              * time. Run it.
 184              */
 185             delpos234(timers, 0);
 186             first->fn(first->ctx, first->now);
 187             sfree(first);
 188         } else {
 189             /*
 190              * This is the first still-active timer that is in the
 191              * future. Return how long it has yet to go.
 192              */
 193             *next = first->now;
 194             return TRUE;
 195         }
 196     }
 197 }
 198
 199 /*
 200  * Call to expire all timers associated with a given context.
 201  */
 202 void expire_timer_context(void *ctx)
 203 {
 204     init_timers();
 205
 206     /*
 207      * We don't bother to check the return value; if the context
 208      * already wasn't in the tree (presumably because no timers
 209      * ever actually got scheduled for it) then that's fine and we
 210      * simply don't need to do anything.
 211      */
 212     del234(timer_contexts, ctx);
 213 }