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       time - overview of time


   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
       further 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
       resolution 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 i386 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 seconds.  Since kernel 2.6.20, a further frequency is
       available: 300, a number that divides evenly for the common video frame
       rates (PAL, 25 HZ; NTSC, 30 HZ).

   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

   Broken-down time
       Certain library functions use a  structure  of  type  tm  to  represent
       broken-down  time,  which stores time value separated out into distinct
       components (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

   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

       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).


       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),  timeradd(3),
       ctime(3), strftime(3), strptime(3), usleep(3), rtc(4), hwclock(8)


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