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plurals

PO
primarykey
Id
11681069
data
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0
AnswerCount
0
ClosedDate
CommentCount
6
CommunityOwnedDate
CreationDate
2012-07-27T03:31:29.960
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0
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2014-11-25T16:39:19.927
LastEditDate
2014-11-25T16:39:19.927
LastEditorUserId
22079
OwnerUserId
251153
ParentId
11680461
PostTypeId
2
Score
22
ViewCount
0
LastEditorDisplayName
text
Body
The Mach kernel provides access to system clocks, out of which at least one (<code>SYSTEM_CLOCK</code>) is <a href="https://opensource.apple.com/source/xnu/xnu-2422.1.72/osfmk/mach/clock_types.h">advertised by the documentation</a> as being monotonically incrementing. <pre><code>#include <mach/clock.h> #include <mach/mach.h> clock_serv_t cclock; mach_timespec_t mts; host_get_clock_service(mach_host_self(), SYSTEM_CLOCK, &cclock); clock_get_time(cclock, &mts); mach_port_deallocate(mach_task_self(), cclock); </code></pre> <code>mach_timespec_t</code> has nanosecond precision. I'm not sure about the accuracy, though. Mac OS X supports three clocks: <ul> <li><code>SYSTEM_CLOCK</code> returns the time since boot time;</li> <li><code>CALENDAR_CLOCK</code> returns the UTC time since 1970-01-01;</li> <li><code>REALTIME_CLOCK</code> is deprecated and is the same as <code>SYSTEM_CLOCK</code> in its current implementation.</li> </ul> The <a href="https://opensource.apple.com/source/xnu/xnu-2422.1.72/osfmk/man/clock_get_time.html">documentation for <code>clock_get_time</code></a> says the clocks are monotonically incrementing unless someone calls <code>clock_set_time</code>. Calls to <code>clock_set_time</code> are <a href="https://opensource.apple.com/source/xnu/xnu-2422.1.72/osfmk/man/clock_set_time.html">discouraged</a> as it could break the monotonic property of the clocks, and in fact, <a href="https://opensource.apple.com/source/xnu/xnu-2422.1.72/osfmk/kern/clock_oldops.c">the current implementation returns <code>KERN_FAILURE</code></a> without doing anything.
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1. POMonotonic clock on OSX
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1. PTAnswer
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1. USJeff Allen
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1. USzneak
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1. POMonotonic clock on OSX
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