GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
GNU/Linux |
CentOS 5.1 |
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time(7) |
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time − overview of time
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 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.
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.
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).
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), ctime(3), strftime(3), strptime(3), usleep(3), rtc(4), hwclock(8).
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time(7) | |