~ chicken-core (master) /manual/Module (scheme time)

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 3
 4== Module (scheme time)
 5
 6<procedure>(current-second)</procedure>
 7
 8Returns an inexact number representing the current time on the International
 9Atomic Time (TAI) scale. The value 0.0 represents midnight on January 1, 1970
10TAI (equivalent to 8.000082 seconds before midnight Universal Time) and the
11value 1.0 represents one TAI second later. Neither high accuracy nor high
12precision are required; in particular, returning Coordinated Universal Time
13plus a suitable constant might be the best an implementation can do.
14
15<procedure>(current-jiffy)</procedure>
16
17Returns the number of jiffies as an exact integer that have elapsed since an
18arbitrary, implementation-defined epoch. A jiffy is an implementation-defined
19fraction of a second which is defined by the return value of the 
20jiffies-per-second procedure. The starting epoch is guaranteed to be constant
21during a run of the program, but may vary between runs.
22
23Rationale: Jiffies are allowed to be implementation-dependent so that
24current-jiffy can execute with minimum overhead. It should be very likely
25that a compactly represented integer will suffice as the returned
26value.  Any particular jiffy size will be inappropriate for some
27implementations: a microsecond is too long for a very fast machine,
28while a much smaller unit would force many implementations to
29return integers which have to be allocated for most calls, rendering
30current-jiffy less useful for accurate timing measurements.
31
32<procedure>(jiffies-per-second)</procedure>
33
34Returns an exact integer representing the number of jiffies per SI second. This
35value is an implementation-specified constant.
36
37 (define (time-length)
38   (let ((list (make-list 100000))
39         (start (current-jiffy)))
40     (length list)
41     (/ (- (current-jiffy) start)
42        (jiffies-per-second))))
43
44
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