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TIME(7) Linux Programmer's Manual TIME(7)
NAME
time - overview of time
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 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 execu-
tion 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 x86 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 settimeof-
day(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 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) 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), and timer_create(3).
SEE ALSO
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)
COLOPHON
This page is part of release 2.69 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 2006-04-28 TIME(7) |
07/12/2007, 20h27
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