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.TH TIME 7 2006-04-28 "Linux" "Linux Programmer's Manual"
.SH NAME
time \- overview of time
.SH DESCRIPTION
.SS "Real time and process time"
.I "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
.RI ( "elapsed time" ).

.I "Process time"
is defined as the amount of CPU time used by a process.
This is sometimes divided into
.I user
and
.I 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
.BR 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
.BR times (2),
.BR getrusage (2),
or
.BR clock (3).
.SS "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
.BR rtc (4)
and
.BR hwclock (8).
.SS "The Software Clock, HZ, and Jiffies"
The accuracy of many system calls and timestamps is limited by
the resolution of the
.IR "software clock" ,
a clock maintained by the kernel which measures time in
.IR jiffies .
The size of a jiffy is determined by the value of the kernel constant
.IR HZ .
The value of
.I 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).
.SS "The Epoch"
Unix systems represent time in seconds since the
.IR Epoch ,
which is defined as 0:00:00 UTC on the morning of 1 January 1970.

A program can determine the
.I "calendar time"
using
.BR gettimeofday (2),
which returns time (in seconds and microseconds) that have
elapsed since the Epoch;
.BR time (2)
provides similar information, but only with accuracy to the
nearest second.
The system time can be changed using
.BR settimeofday (2).
.SS "Broken-down time"
Certain library functions use a structure of
type
.I tm
to represent
.IR "broken-down time" ,
which stores time value separated out into distinct components
(year, month, day, hour, minute, second, etc.).
This structure is described in
.BR 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
.BR ctime (3),
.BR strftime (3),
and
.BR strptime (3).
.SS "Sleeping and Setting Timers"
Various system calls and functions allow a program to sleep
(suspend execution) for a specified period of time; see
.BR nanosleep (2)
and
.BR 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
.BR alarm (2),
.BR getitimer (2),
and
.BR timer_create (3).
.\" FIXME . timer_create() and friends are not yet in man-pages
.SH "SEE ALSO"
.BR date (1),
.BR time (1),
.BR adjtimex (2),
.BR alarm (2),
.BR getitimer (2),
.BR getrlimit (2),
.BR getrusage (2),
.BR gettimeofday (2),
.BR nanosleep (2),
.BR stat (2),
.BR time (2),
.\" FIXME . timer_create (2),
.BR timerfd_create (2),
.BR times (2),
.BR utime (2),
.BR adjtime (3),
.BR clock (3),
.BR sleep (3),
.BR timeradd (3),
.BR ctime (3),
.BR strftime (3),
.BR strptime (3),
.BR usleep (3),
.BR rtc (4),
.BR hwclock (8)