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Top Forums UNIX for Dummies Questions & Answers Timekeeping in Linux question ... Post 302513658 by Corona688 on Wednesday 13th of April 2011 07:42:11 PM
Old 04-13-2011
Quote:
Originally Posted by newlinuxuser1
So I'm summing up:
1. When machine boots, the hardware clock (otherwise accessible via BIOS) sets the system time with "date" command (from somewhere in rc files I guess).
I don't see anything in my boot scripts that uses hwclock to read the clock, so I think the Linux kernel actually consults the hardware clock itself on boot. But you've got the right idea: Read the hardware clock once, then only use the system clock from then on out.
Quote:
3. When NTP adjustments are received, and/or when "shutdown" command is issues, the updated time value is used for adjusting hardware clock.
Mostly nothing touches the hardware clock. It's likely to be much lower resolution than the system clock, after all. (resolution and accuracy being not quite the same thing. the hardware clock might lose less seconds per month than the system one but can't give you numbers down to microseconds.) Your system may set it on power-off.

And most of this is configurable, so it might not always be the same. I had to edit a config file to tell my system to set the hardware clock on power-off. This is just the big picture.
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clock(3C)																 clock(3C)

NAME
clock() - report CPU time used SYNOPSIS
DESCRIPTION
returns the amount of CPU time (in microseconds) used since the first call to The time reported is the sum of the user and system times of the calling process and its terminated child processes for which it has executed , or (see wait(2) , system(3S), and popen(3S)). To deter- mine the time in seconds, the value returned by should be divided by the value of the macro The resolution of the clock varies, depending on the hardware and on software configuration. If the processor time used is not available or its value cannot be represented, the function returns the value WARNINGS
The value returned by is defined in microseconds for compatibility with systems that have CPU clocks with much higher resolution. Because of this, the value returned wraps around after accumulating only 4295 seconds of CPU time (about 72 minutes). DEPENDENCIES
The default clock resolution is 10 milliseconds. SEE ALSO
times(2), wait(2), system(3S), thread_safety(5). STANDARDS CONFORMANCE
clock(3C)
All times are GMT -4. The time now is 03:35 AM.
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