C library/kernel differences

On some architectures, an implementation of time() is provided in the vdso(7).

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 various system calls that set timeouts, (e.g., select(2), sigtimedwait(2)) and measure CPU time (e.g., getrusage(2)) 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 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).

The times(2) 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 constant using sysconf(_SC_CLK_TCK).

High-resolution timers

Before Linux 2.6.21, the accuracy of timer and sleep system calls (see below) was also limited 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 system calls is no longer constrained by the jiffy, but instead can be as accurate as the hardware 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) 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) CLOCK_REALTIME clock, which returns time (in seconds and nanoseconds) 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 clock_settime(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), clock_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), timerfd_create(2), and timer_create(2).

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 details, see the description of PR_SET_TIMERSLACK in prctl(2).