mirror of https://github.com/mkerrisk/man-pages
220 lines
7.0 KiB
Groff
220 lines
7.0 KiB
Groff
.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" 2008-06-24, mtk: added some details about where jiffies come into
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.\" play; added section on high-resolution timers.
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.\"
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.TH TIME 7 2012-10-28 "Linux" "Linux Programmer's Manual"
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.SH NAME
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time \- overview of time and timers
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.SH DESCRIPTION
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.SS Real time and process time
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.I "Real time"
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is defined as time measured from some fixed point,
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either from a standard point in the past
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(see the description of the Epoch and calendar time below),
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or from some point (e.g., the start) in the life of a process
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.RI ( "elapsed time" ).
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.I "Process time"
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is defined as the amount of CPU time used by a process.
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This is sometimes divided into
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.I user
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and
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.I system
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components.
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User CPU time is the time spent executing code in user mode.
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System CPU time is the time spent by the kernel executing
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in system mode on behalf of the process (e.g., executing system calls).
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The
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.BR time (1)
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command can be used to determine the amount of CPU time consumed
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during the execution of a program.
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A program can determine the amount of CPU time it has consumed using
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.BR times (2),
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.BR getrusage (2),
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or
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.BR clock (3).
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.SS The hardware clock
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Most computers have a (battery-powered) hardware clock which the kernel
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reads at boot time in order to initialize the software clock.
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For further details, see
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.BR rtc (4)
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and
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.BR hwclock (8).
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.SS The software clock, HZ, and jiffies
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The accuracy of various system calls that set timeouts,
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(e.g.,
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.BR select (2),
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.BR sigtimedwait (2))
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.\" semtimedop(), mq_timedwait(), io_getevents(), poll() are the same
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.\" futexes and thus sem_timedwait() seem to use high-res timers.
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and measure CPU time (e.g.,
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.BR getrusage (2))
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is limited by the resolution of the
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.IR "software clock" ,
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a clock maintained by the kernel which measures time in
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.IR jiffies .
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The size of a jiffy is determined by the value of the kernel constant
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.IR HZ .
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The value of
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.I HZ
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varies across kernel versions and hardware platforms.
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On i386 the situation is as follows:
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on kernels up to and including 2.4.x, HZ was 100,
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giving a jiffy value of 0.01 seconds;
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starting with 2.6.0, HZ was raised to 1000, giving a jiffy of
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0.001 seconds.
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Since kernel 2.6.13, the HZ value is a kernel
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configuration parameter and can be 100, 250 (the default) or 1000,
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yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds.
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Since kernel 2.6.20, a further frequency is available:
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300, a number that divides evenly for the common video
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frame rates (PAL, 25 HZ; NTSC, 30 HZ).
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The
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.BR times (2)
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system call is a special case.
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It reports times with a granularity defined by the kernel constant
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.IR USER_HZ .
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User-space applications can determine the value of this constant using
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.IR sysconf(_SC_CLK_TCK) .
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.\" glibc gets this info with a little help from the ELF loader;
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.\" see glibc elf/dl-support.c and kernel fs/binfmt_elf.c.
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.\"
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.SS High-resolution timers
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Before Linux 2.6.21, the accuracy of timer and sleep system calls
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(see below) was also limited by the size of the jiffy.
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Since Linux 2.6.21, Linux supports high-resolution timers (HRTs),
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optionally configurable via
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.BR CONFIG_HIGH_RES_TIMERS .
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On a system that supports HRTs, the accuracy of sleep and timer
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system calls is no longer constrained by the jiffy,
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but instead can be as accurate as the hardware allows
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(microsecond accuracy is typical of modern hardware).
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You can determine whether high-resolution timers are supported by
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checking the resolution returned by a call to
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.BR clock_getres (2)
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or looking at the "resolution" entries in
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.IR /proc/timer_list .
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HRTs are not supported on all hardware architectures.
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(Support is provided on x86, arm, and powerpc, among others.)
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.SS The Epoch
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UNIX systems represent time in seconds since the
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.IR Epoch ,
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1970-01-01 00:00:00 +0000 (UTC).
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A program can determine the
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.I "calendar time"
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using
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.BR gettimeofday (2),
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which returns time (in seconds and microseconds) that have
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elapsed since the Epoch;
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.BR time (2)
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provides similar information, but only with accuracy to the
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nearest second.
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The system time can be changed using
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.BR settimeofday (2).
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.SS Broken-down time
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Certain library functions use a structure of
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type
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.I tm
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to represent
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.IR "broken-down time" ,
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which stores time value separated out into distinct components
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(year, month, day, hour, minute, second, etc.).
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This structure is described in
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.BR ctime (3),
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which also describes functions that convert between calendar time and
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broken-down time.
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Functions for converting between broken-down time and printable
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string representations of the time are described in
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.BR ctime (3),
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.BR strftime (3),
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and
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.BR strptime (3).
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.SS Sleeping and setting timers
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Various system calls and functions allow a program to sleep
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(suspend execution) for a specified period of time; see
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.BR nanosleep (2),
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.BR clock_nanosleep (2),
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and
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.BR sleep (3).
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Various system calls allow a process to set a timer that expires
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at some point in the future, and optionally at repeated intervals;
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see
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.BR alarm (2),
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.BR getitimer (2),
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.BR timerfd_create (2),
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and
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.BR timer_create (2).
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.SS Timer slack
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Since Linux 2.6.28, it is possible to control the "timer slack"
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value for a thread.
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The timer slack is the length of time by
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which the kernel may delay the wake-up of certain
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system calls that block with a timeout.
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Permitting this delay allows the kernel to coalesce wake-up events,
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thus possibly reducing the number of system wake-ups and saving power.
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For more details, see the description of
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.B PR_SET_TIMERSLACK
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in
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.BR prctl (2).
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.SH SEE ALSO
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.ad l
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.nh
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.BR date (1),
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.BR time (1),
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.BR adjtimex (2),
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.BR alarm (2),
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.BR clock_gettime (2),
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.BR clock_nanosleep (2),
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.BR getitimer (2),
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.BR getrlimit (2),
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.BR getrusage (2),
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.BR gettimeofday (2),
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.BR nanosleep (2),
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.BR stat (2),
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.BR time (2),
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.BR timer_create (2),
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.BR timerfd_create (2),
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.BR times (2),
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.BR utime (2),
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.BR adjtime (3),
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.BR clock (3),
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.BR clock_getcpuclockid (3),
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.BR ctime (3),
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.BR pthread_getcpuclockid (3),
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.BR sleep (3),
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.BR strftime (3),
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.BR strptime (3),
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.BR timeradd (3),
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.BR usleep (3),
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.BR rtc (4),
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.BR hwclock (8)
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