Linux man pages

Section (2) clock_getres

Linux manual pages	Section 2

Name

clock_getres, clock_gettime, clock_settime — clock and time functions

Synopsis

#include <time.h>

`int clock_getres(`	clockid_t `clk_id`,
	struct timespec *`res)`;

`int clock_gettime(`	clockid_t `clk_id`,
	struct timespec *`tp)`;

`int clock_settime(`	clockid_t `clk_id`,
	const struct timespec *`tp)`;

Note

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
`clock_getres`(), `clock_gettime`(), `clock_settime`():
`_POSIX_C_SOURCE` >= 199309L

	Note
	Link with `−lrt` (only for glibc versions before 2.17).

DESCRIPTION

The function clock_getres() finds the resolution (precision) of the specified clock clk_id, and, if res is non-NULL, stores it in the struct timespec pointed to by res. The resolution of clocks depends on the implementation and cannot be configured by a particular process. If the time value pointed to by the argument tp of clock_settime() is not a multiple of res, then it is truncated to a multiple of res.

The functions clock_gettime() and clock_settime() retrieve and set the time of the specified clock clk_id.

The res and tp arguments are timespec structures, as specified in <time.h>

struct timespec {

time_t tv_sec;
/* seconds */

long tv_nsec;
/* nanoseconds */

};

The clk_id argument is the identifier of the particular clock on which to act. A clock may be system-wide and hence visible for all processes, or per-process if it measures time only within a single process.

All implementations support the system-wide real-time clock, which is identified by CLOCK_REALTIME. Its time represents seconds and nanoseconds since the Epoch. When its time is changed, timers for a relative interval are unaffected, but timers for an absolute point in time are affected.

More clocks may be implemented. The interpretation of the corresponding time values and the effect on timers is unspecified.

Sufficiently recent versions of glibc and the Linux kernel support the following clocks:

CLOCK_REALTIME

System-wide clock that measures real (i.e., wall-clock) time. Setting this clock requires appropriate privileges. This clock is affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), and by the incremental adjustments performed by adjtime(3) and NTP.

CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)

A faster but less precise version of CLOCK_REALTIME. Use when you need very fast, but not fine-grained timestamps. Requires per-architecture support, and probably also architecture support for this flag in the vdso(7).

CLOCK_MONOTONIC

Clock that cannot be set and represents monotonic time since—as described by POSIX—some unspecified point in the past. On Linux, that point corresponds to the number of seconds that the system has been running since it was booted.

The CLOCK_MONOTONIC clock is not affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), but is affected by the incremental adjustments performed by adjtime(3) and NTP. This clock does not count time that the system is suspended.

CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)

A faster but less precise version of CLOCK_MONOTONIC. Use when you need very fast, but not fine-grained timestamps. Requires per-architecture support, and probably also architecture support for this flag in the vdso(7).

CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)

Similar to CLOCK_MONOTONIC, but provides access to a raw hardware-based time that is not subject to NTP adjustments or the incremental adjustments performed by adjtime(3). This clock does not count time that the system is suspended.

CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)

Identical to CLOCK_MONOTONIC, except it also includes any time that the system is suspended. This allows applications to get a suspend-aware monotonic clock without having to deal with the complications of CLOCK_REALTIME, which may have discontinuities if the time is changed using settimeofday(2) or similar.

CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)

Per-process CPU-time clock (measures CPU time consumed by all threads in the process).

CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)

Thread-specific CPU-time clock.

RETURN VALUE

clock_gettime(), clock_settime(), and clock_getres() return 0 for success, or −1 for failure (in which case errno is set appropriately).

ERRORS

EFAULT: tp points outside the accessible address space.
EINVAL: The clk_id specified is not supported on this system.
EINVAL: (clock_settime()): tp.tv_sec is negative or tp.tv_nsec is outside the range [0..999,999,999].
EINVAL (since Linux 4.3): A call to clock_settime() with a clk_id of CLOCK_REALTIME attempted to set the time to a value less than the current value of the CLOCK_MONOTONIC clock.
EPERM: clock_settime() does not have permission to set the clock indicated.

VERSIONS

These system calls first appeared in Linux 2.6.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
`clock_getres`(), `clock_gettime`(), `clock_settime`()	Thread safety	MT-Safe

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SUSv2.

AVAILABILITY

On POSIX systems on which these functions are available, the symbol _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID, CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)

NOTES

POSIX.1 specifies the following:

Setting the value of the CLOCK_REALTIME clock via clock_settime() shall have no effect on threads that are blocked waiting for a relative time service based upon this clock, including the nanosleep() function; nor on the expiration of relative timers based upon this clock. Consequently, these time services shall expire when the requested relative interval elapses, independently of the new or old value of the clock.

C library/kernel differences

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

Historical note for SMP systems

Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many platforms using timer registers from the CPUs (TSC on i386, AR.ITC on Itanium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to another CPU.

If the CPUs in an SMP system have different clock sources, then there is no way to maintain a correlation between the timer registers since each CPU will run at a slightly different frequency. If that is the case, then clock_getcpuclockid(0) will return ENOENT to signify this condition. The two clocks will then be useful only if it can be ensured that a process stays on a certain CPU.

The processors in an SMP system do not start all at exactly the same time and therefore the timer registers are typically running at an offset. Some architectures include code that attempts to limit these offsets on bootup. However, the code cannot guarantee to accurately tune the offsets. Glibc contains no provisions to deal with these offsets (unlike the Linux Kernel). Typically these offsets are small and therefore the effects may be negligible in most cases.

Since glibc 2.4, the wrapper functions for the system calls described in this page avoid the abovementioned problems by employing the kernel implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID, on systems that provide such an implementation (i.e., Linux 2.6.12 and later).

BUGS

According to POSIX.1-2001, a process with appropriate privileges may set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks using clock_settime(). On Linux, these clocks are not settable (i.e., no process has appropriate privileges).

COLOPHON

This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

  Copyright (c) 2003 Nick Clifford (zafnrc.co.nz), Jan 25, 2003
Copyright (c) 2003 Andries Brouwer (aebcwi.nl), Aug 24, 2003

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

2003-08-23 Martin Schulze <joeyinfodrom.org> improvements
2003-08-24 aeb, large parts rewritten
2004-08-06 Christoph Lameter <clametersgi.com>, SMP note

struct	timespec {
	time_t	`tv_sec`;	/* seconds */
	long	`tv_nsec`;	/* nanoseconds */
};