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 descrip-
       tion 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 com-
       ponents.   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  various system calls that set timeouts, (e.g., select(2), sigtimedwait(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 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 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 configurable 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 modern hardware).  You can deter-
       mine 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, 1970-01-01 00:00:00 +0000 (UTC).

       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.  Functions for converting between bro-
       ken-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 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2010-02-25                                                    TIME(7)