# time - man(7) - [phpMan] [_time_(7)](https://www.chedong.com/phpMan.php/man/time/7/markdown) Miscellaneous Information Manual [_time_(7)](https://www.chedong.com/phpMan.php/man/time/7/markdown) ## NAME time - overview of time and timers ## 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 di‐ vided 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 be‐ half of the process (e.g., executing system calls). The [**time**(1)](https://www.chedong.com/phpMan.php/man/time/1/markdown) command can be used to de‐ termine the amount of CPU time consumed during the execution of a program. A program can de‐ termine the amount of CPU time it has consumed using [**times**(2)](https://www.chedong.com/phpMan.php/man/times/2/markdown), [**getrusage**(2)](https://www.chedong.com/phpMan.php/man/getrusage/2/markdown), or [**clock**(3)](https://www.chedong.com/phpMan.php/man/clock/3/markdown). ### 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)](https://www.chedong.com/phpMan.php/man/rtc/4/markdown) and [**hwclock**(8)](https://www.chedong.com/phpMan.php/man/hwclock/8/markdown). ### The software clock, HZ, and jiffies The accuracy of various system calls that set timeouts, (e.g., [**select**(2)](https://www.chedong.com/phpMan.php/man/select/2/markdown), [**sigtimedwait**(2)](https://www.chedong.com/phpMan.php/man/sigtimedwait/2/markdown)) and measure CPU time (e.g., [**getrusage**(2)](https://www.chedong.com/phpMan.php/man/getrusage/2/markdown)) 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 de‐ termined 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 Linux 2.4.x, HZ was 100, giving a jiffy value of 0.01 seconds; starting with Linux 2.6.0, HZ was raised to 1000, giving a jiffy of 0.001 seconds. Since Linux 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 Linux 2.6.20, a further frequency is available: 300, a number that di‐ vides evenly for the common video frame rates (PAL, 25 Hz; NTSC, 30 Hz). The [**times**(2)](https://www.chedong.com/phpMan.php/man/times/2/markdown) system call is a special case. It reports times with a granularity defined by the kernel constant _USER_HZ_. User-space applications can determine the value of this con‐ stant using _sysconf(_SC_CLK_TCK)_. ### System and process clocks; time namespaces The kernel supports a range of clocks that measure various kinds of elapsed and virtual (i.e., consumed CPU) time. These clocks are described in [**clock_gettime**(2)](https://www.chedong.com/phpMan.php/man/clockgettime/2/markdown). A few of the clocks are settable using [**clock_settime**(2)](https://www.chedong.com/phpMan.php/man/clocksettime/2/markdown). The values of certain clocks are virtualized by time namespaces; see [**time_namespaces**(7)](https://www.chedong.com/phpMan.php/man/timenamespaces/7/markdown). ### High-resolution timers Before Linux 2.6.21, the accuracy of timer and sleep system calls (see below) was also lim‐ ited 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 sys‐ tem calls is no longer constrained by the jiffy, but instead can be as accurate as the hard‐ ware 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)](https://www.chedong.com/phpMan.php/man/clockgetres/2/markdown) or looking at the "resolution" entries in _/proc/timer_list_. HRTs are not supported on all hardware architectures. (Support is provided 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_ via the [**clock_gettime**(2)](https://www.chedong.com/phpMan.php/man/clockgettime/2/markdown) **CLOCK_REALTIME **clock, which returns time (in seconds and nanoseconds) that have elapsed since the Epoch; [**time**(2)](https://www.chedong.com/phpMan.php/man/time/2/markdown) provides similar information, but only with accuracy to the nearest second. The system time can be changed using [**clock_settime**(2)](https://www.chedong.com/phpMan.php/man/clocksettime/2/markdown). ### 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 [**tm**(3type)](https://www.chedong.com/phpMan.php/man/tm/3type/markdown), 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)](https://www.chedong.com/phpMan.php/man/ctime/3/markdown), **strf‐** [**time**(3)](https://www.chedong.com/phpMan.php/man/time/3/markdown), and [**strptime**(3)](https://www.chedong.com/phpMan.php/man/strptime/3/markdown). ### Sleeping and setting timers Various system calls and functions allow a program to sleep (suspend execution) for a speci‐ fied period of time; see [**nanosleep**(2)](https://www.chedong.com/phpMan.php/man/nanosleep/2/markdown), [**clock_nanosleep**(2)](https://www.chedong.com/phpMan.php/man/clocknanosleep/2/markdown), and [**sleep**(3)](https://www.chedong.com/phpMan.php/man/sleep/3/markdown). 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)](https://www.chedong.com/phpMan.php/man/alarm/2/markdown), [**getitimer**(2)](https://www.chedong.com/phpMan.php/man/getitimer/2/markdown), [**timerfd_create**(2)](https://www.chedong.com/phpMan.php/man/timerfdcreate/2/markdown), and [**timer_create**(2)](https://www.chedong.com/phpMan.php/man/timercreate/2/markdown). ### 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 kernel 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 de‐ tails, see the description of **PR_SET_TIMERSLACK **in [**prctl**(2)](https://www.chedong.com/phpMan.php/man/prctl/2/markdown). ## SEE ALSO [**date**(1)](https://www.chedong.com/phpMan.php/man/date/1/markdown), [**time**(1)](https://www.chedong.com/phpMan.php/man/time/1/markdown), [**timeout**(1)](https://www.chedong.com/phpMan.php/man/timeout/1/markdown), [**adjtimex**(2)](https://www.chedong.com/phpMan.php/man/adjtimex/2/markdown), [**alarm**(2)](https://www.chedong.com/phpMan.php/man/alarm/2/markdown), [**clock_gettime**(2)](https://www.chedong.com/phpMan.php/man/clockgettime/2/markdown), [**clock_nanosleep**(2)](https://www.chedong.com/phpMan.php/man/clocknanosleep/2/markdown), [**getitimer**(2)](https://www.chedong.com/phpMan.php/man/getitimer/2/markdown), [**getrlimit**(2)](https://www.chedong.com/phpMan.php/man/getrlimit/2/markdown), [**getrusage**(2)](https://www.chedong.com/phpMan.php/man/getrusage/2/markdown), [**gettimeofday**(2)](https://www.chedong.com/phpMan.php/man/gettimeofday/2/markdown), [**nanosleep**(2)](https://www.chedong.com/phpMan.php/man/nanosleep/2/markdown), [**stat**(2)](https://www.chedong.com/phpMan.php/man/stat/2/markdown), [**time**(2)](https://www.chedong.com/phpMan.php/man/time/2/markdown), [**timer_create**(2)](https://www.chedong.com/phpMan.php/man/timercreate/2/markdown), [**timerfd_create**(2)](https://www.chedong.com/phpMan.php/man/timerfdcreate/2/markdown), [**times**(2)](https://www.chedong.com/phpMan.php/man/times/2/markdown), [**utime**(2)](https://www.chedong.com/phpMan.php/man/utime/2/markdown), [**adjtime**(3)](https://www.chedong.com/phpMan.php/man/adjtime/3/markdown), [**clock**(3)](https://www.chedong.com/phpMan.php/man/clock/3/markdown), [**clock_getcpuclockid**(3)](https://www.chedong.com/phpMan.php/man/clockgetcpuclockid/3/markdown), [**ctime**(3)](https://www.chedong.com/phpMan.php/man/ctime/3/markdown), [**ntp_adjtime**(3)](https://www.chedong.com/phpMan.php/man/ntpadjtime/3/markdown), [**ntp_gettime**(3)](https://www.chedong.com/phpMan.php/man/ntpgettime/3/markdown), [**pthread_getcpuclockid**(3)](https://www.chedong.com/phpMan.php/man/pthreadgetcpuclockid/3/markdown), [**sleep**(3)](https://www.chedong.com/phpMan.php/man/sleep/3/markdown), [**strftime**(3)](https://www.chedong.com/phpMan.php/man/strftime/3/markdown), [**strptime**(3)](https://www.chedong.com/phpMan.php/man/strptime/3/markdown), [**timeradd**(3)](https://www.chedong.com/phpMan.php/man/timeradd/3/markdown), [**usleep**(3)](https://www.chedong.com/phpMan.php/man/usleep/3/markdown), [**rtc**(4)](https://www.chedong.com/phpMan.php/man/rtc/4/markdown), [**time_namespaces**(7)](https://www.chedong.com/phpMan.php/man/timenamespaces/7/markdown), [**hwclock**(8)](https://www.chedong.com/phpMan.php/man/hwclock/8/markdown) Linux man-pages 6.7 2023-10-31 [_time_(7)](https://www.chedong.com/phpMan.php/man/time/7/markdown)