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76 lines
3.2 KiB
Markdown
76 lines
3.2 KiB
Markdown
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## Time in tor ##
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### What time is it? ###
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We have several notions of the current time in Tor.
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The *wallclock time* is available from time(NULL) with
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second-granularity and tor_gettimeofday() with microsecond
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granularity. It corresponds most closely to "the current time and date".
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The *monotonic time* is available with the set of monotime_\*
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functions declared in compat_time.h. Unlike the wallclock time, it
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can only move forward. It does not necessarily correspond to a real
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world time, and it is not portable between systems.
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The *coarse monotonic time* is available from the set of
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monotime_coarse_\* functions in compat_time.h. It is the same as
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monotime_\* on some platforms. On others, it gives a monotonic timer
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with less precision, but which it's more efficient to access.
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### Cached views of time. ###
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On some systems (like Linux), many time functions use a VDSO to avoid
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the overhead of a system call. But on other systems, gettimeofday()
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and time() can be costly enough that you wouldn't want to call them
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tens of thousands of times. To get a recent, but not especially
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accurate, view of the current time, see approx_time() and
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tor_gettimeofday_cached().
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### Parsing and encoding time values ###
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Tor has functions to parse and format time in these formats:
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* RFC1123 format. ("Fri, 29 Sep 2006 15:54:20 GMT"). For this,
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use format_rfc1123_time() and parse_rfc1123_time.
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* ISO8601 format. ("2006-10-29 10:57:20") For this, use
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format_local_iso_time and format_iso_time. We also support the
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variant format "2006-10-29T10:57:20" with format_iso_time_nospace, and
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"2006-10-29T10:57:20.123456" with format_iso_time_nospace_usec.
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* HTTP format collections (preferably "Mon, 25 Jul 2016 04:01:11
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GMT" or possibly "Wed Jun 30 21:49:08 1993" or even "25-Jul-16
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04:01:11 GMT"). For this, use parse_http_time. Don't generate anything
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but the first format.
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Some of these functions use struct tm. You can use the standard
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tor_localtime_r and tor_gmtime_r() to wrap these in a safe way. We
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also have a tor_timegm() function.
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### Scheduling events ###
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The main way to schedule a not-too-frequent periodic event with
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respect to the Tor mainloop is via the mechanism in periodic.c.
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There's a big table of periodic_events in main.c, each of which gets
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invoked on its own schedule. You should not expect more than about
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one second of accuracy with these timers.
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You can create an independent timer using libevent directly, or using
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the periodic_timer_new() function. But you should avoid doing this
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for per-connection or per-circuit timers: Libevent's internal timer
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implementation uses a min-heap, and those tend to start scaling poorly
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once you have a few thousand entries.
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If you need to create a large number of fine-grained timers for some
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purpose, you should consider the mechanism in src/common/timers.c,
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which is optimized for the case where you have a large number of
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timers with not-too-long duration, many of which will be deleted
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before they actually expire. These timers should be reasonably
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accurate within a handful of milliseconds -- possibly better on some
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platforms. (The timers.c module uses William Ahern's timeout.c
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implementation as its backend, which is based on a hierarchical timing
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wheel algorithm. It's cool stuff; check it out.)
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