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



NAME
       time - overview of time and timers

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 some-
       times 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 exe-
       cuting 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 con-
       sumed 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 various system calls that set timeouts, (e.g., select(2), sigtimed-
       wait(2)) and measure CPU time (e.g., getrusage(2)) 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  configura-
       tion  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 times(2) system call is a special case.  It reports times  with  a  granularity
       defined  by  the kernel constant USER_HZ.  Userspace applications can determine the
       value of this constant using sysconf(_SC_CLK_TCK).

   High-Resolution Timers
       Before Linux 2.6.21, the accuracy of timer and sleep system calls (see  below)  was
       also limited by the size of the jiffy.

       Since  Linux  2.6.21, Linux supports high-resolution timers (HRTs), optionally con-
       figurable via CONFIG_HIGH_RES_TIMERS.  On a system that supports HRTs, the accuracy
       of  sleep and timer system calls is no longer constrained by the jiffy, but instead
       can be as accurate as the hardware allows (microsecond accuracy is typical of  mod-
       ern  hardware).   You can determine whether high-resolution timers are supported by
       checking the resolution returned by a call to clock_getres(2)  or  looking  at  the
       "resolution" entries in /proc/timer_list.

       HRTs are not supported on all hardware architectures.  (Support is provided on x86,
       arm, and powerpc, among others.)

   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 settimeofday(2).

   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.  Func-
       tions 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), clock_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),
       timerfd_create(2), and timer_create(2).

SEE ALSO
       date(1),  time(1),  adjtimex(2),  alarm(2),  clock_gettime(2),  clock_nanosleep(2),
       getitimer(2),  getrlimit(2),  getrusage(2), gettimeofday(2), nanosleep(2), stat(2),
       time(2),  timer_create(2),  timerfd_create(2),  times(2),   utime(2),   adjtime(3),
       clock(3),  clock_getcpuclockid(3),  ctime(3),  pthread_getcpuclockid(3),  sleep(3),
       strftime(3), strptime(3), timeradd(3), usleep(3), rtc(4), hwclock(8)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-02-10                           TIME(7)

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