time man page on CentOS

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TIME(7)			   Linux Programmer's Manual		       TIME(7)

NAME
       time - overview of time

DESCRIPTION
   Real time and process time
       Real  time  is  defined	as time measured from some fixed point, either
       from a standard point in the past (see the description of the Epoch and
       calendar	 time below), or from some point (e.g., the start) in the life
       of a process (elapsed time).

       Process time is defined as the amount of CPU time used  by  a  process.
       This  is	 sometimes  divided into user and system components.  User CPU
       time is the time spent executing code in user mode.  System CPU time is
       the  time spent by the kernel executing in system mode on behalf of the
       process (e.g., executing system calls).	The  time(1)  command  can  be
       used  to determine the amount of CPU time consumed during the execution
       of a program.  A program can determine the amount of CPU	 time  it  has
       consumed using times(2), getrusage(2), or clock(3).

   The Hardware Clock
       Most computers have a (battery-powered) hardware clock which the kernel
       reads at boot time in order to initialize the software clock.  For fur‐
       ther details, see rtc(4) and hwclock(8).

   The Software Clock, HZ, and Jiffies
       The accuracy of many system calls and timestamps is limited by the res‐
       olution of the software clock, a clock maintained by the	 kernel	 which
       measures	 time  in  jiffies.   The size of a jiffy is determined by the
       value of the kernel constant HZ.	 The value of HZ varies across	kernel
       versions	 and  hardware platforms.  On x86 the situation is as follows:
       on kernels up to and including 2.4.x, HZ was 100, giving a jiffy	 value
       of  0.01	 seconds; starting with 2.6.0, HZ was raised to 1000, giving a
       jiffy of 0.001 seconds; since kernel 2.6.13, the HZ value is  a	kernel
       configuration  parameter	 and  can  be  100, 250 (the default) or 1000,
       yielding a jiffies value of, respectively, 0.01, 0.004, or  0.001  sec‐
       onds.

   The Epoch
       Unix  systems  represent	 time  in  seconds  since  the Epoch, which is
       defined as 0:00:00 UTC on the morning of 1 January 1970.

       A program can determine the calendar time using gettimeofday(2),	 which
       returns	time (in seconds and microseconds) that have elapsed since the
       Epoch; time(2) provides similar information, but only with accuracy  to
       the  nearest  second.   The system time can be changed using settimeof‐
       day(2).

   Broken-down time
       Certain library functions use a structure of type tm to represent  bro‐
       ken-down time, which stores time value separated out into distinct com‐
       ponents (year, month, day, hour, minute, second, etc.).	This structure
       is  described  in ctime(3), which also describes functions that convert
       between calendar time and broken-down time.  Functions  for  converting
       between	broken-down  time  and printable string representations of the
       time are described in ctime(3), strftime(3), and strptime(3).

   Sleeping and Setting Timers
       Various system calls and functions allow a program  to  sleep  (suspend
       execution)  for	a  specified  period  of  time;	 see  nanosleep(2) and
       sleep(3).

       Various system calls allow a process to set a  timer  that  expires  at
       some  point  in	the  future, and optionally at repeated intervals; see
       alarm(2), getitimer(2), and timer_create(3).

SEE ALSO
       date(1), time(1), adjtimex(2),  alarm(2),  getitimer(2),	 getrlimit(2),
       getrusage(2),	gettimeofday(2),   nanosleep(2),   stat(2),   time(2),
       times(2), utime(2), adjtime(3),	clock(3),  sleep(3),  ctime(3),	 strf‐
       time(3), strptime(3), usleep(3), rtc(4), hwclock(8).

Linux 2.6.16			  2006-04-28			       TIME(7)
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