diff options
Diffstat (limited to 'lib/qtime.c')
| -rw-r--r-- | lib/qtime.c | 33 |
1 files changed, 27 insertions, 6 deletions
diff --git a/lib/qtime.c b/lib/qtime.c index 789ced1e..d1283198 100644 --- a/lib/qtime.c +++ b/lib/qtime.c @@ -72,15 +72,37 @@ * (But seems unlikely that such a system would not support CLOCK_MONOTONIC !) * * When clock_t is a 32-bit integer must be at least ready for wrap around. - * There are two cases: + * We take the clock_t signed values and widen to 64-bit signed, so we have + * the current sample (this) and the previous one (last), and two cases to + * consider: * - * * +ve wrap around. new < old value, and new >= 0 + * * +ve wrap around -- so value is 31-bit unsigned, and wraps from large + * +ve value to small +ve value. * - * step = (INT32_MAX - old + 1) + new + * step = this - last will be -ve * - * * -ve wrap around. new < old value, and new < 0 (and old > 0) + * 'last' will be some value ((INT32_MAX + 1) - x), and 'this' will be some + * (relatively) small value y. The step is x + y, we have: * - * step = (INT32_MAX - old + 1) - (INT32_MIN - new) + * step = y - ((INT32_MAX + 1) - x) = (x + y) - (INT32_MAX + 1) + * + * so we correct by adding (INT32_MAX + 1). + * + * * -ve wrap around -- so value is 32-bit signed, and wraps from a large + * +ve value to a very -ve value. + * + * step = this - last will be -ve + * + * 'last will' be some value (INT32_MAX + 1) - x, and 'this' will be some + * value (y - (INT32_MAX + 1)). The step is x + y, we have: + * + * step = (y - (INT32_MAX + 1)) - ((INT32_MAX + 1) - x) + * = (x + y) - 2 * (INT32_MAX + 1) + * + * so we correct by adding (INT32_MAX + 1). + * + * In both cases the wrap around gives an apparently -ve 'step', and that is + * corrected by adding (INT32_MAX + 1) until it goes +ve. * * In any event, a step > 24 hours is taken to means that something has gone * very, very badly wrong. @@ -340,4 +362,3 @@ qt_random(uint32_t seed) * or the last entry, and have done with it. */ - |