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

       time - overview of time and timers

   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  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, respec‐
       tively,	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  com‐
       mon 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.  User-space	appli‐
       cations	  can	determine   the	  value	  of   this   constant	 using

   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 determine  whether  high-resolution  timers  are	 supported  by
       checking	 the resolution returned by a call to clock_getres(2) or look‐
       ing at the "resolution" entries in /proc/timer_list.

       HRTs are not supported on all hardware architectures.  (Support is pro‐
       vided 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 settimeof‐

   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),
       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).

   Timer slack
       Since Linux 2.6.28, it is possible to control the "timer	 slack"	 value
       for  a thread.  The timer slack is the length of time by which the ker‐
       nel may delay the wake-up of certain system calls  that	block  with  a
       timeout.	  Permitting  this delay allows the kernel to coalesce wake-up
       events, thus possibly reducing the number of system wake-ups and saving
       power.	For  more details, see the description of PR_SET_TIMERSLACK in

       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)

       This page is part of release 3.54 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				  2012-10-28			       TIME(7)

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