clock_getres, clock_gettime, clock_settime — clock and time functions
#include <time.h>
int
clock_getres( |
clockid_t clockid, |
struct timespec *res) ; |
int
clock_gettime( |
clockid_t clockid, |
struct timespec *tp) ; |
int
clock_settime( |
clockid_t clockid, |
const struct timespec *tp) ; |
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The function clock_getres
()
finds the resolution (precision) of the specified clock
clockid
, 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 clockid
.
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 clockid
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
A settable 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_ALARM
(since Linux 3.0;
Linux-specific)Like CLOCK_REALTIME
,
but not settable. See timer_create(2) for
further details.
CLOCK_REALTIME_COARSE
(since Linux
2.6.32; Linux-specific)A faster but less precise version of CLOCK_REALTIME
. This clock is not
settable. 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_TAI
(since Linux 3.10;
Linux-specific)A nonsettable system-wide clock derived from
wall-clock time but ignoring leap seconds. This clock
does not experience discontinuities and backwards jumps
caused by NTP inserting leap seconds as CLOCK_REALTIME
does.
The acronym TAI refers to International Atomic Time.
CLOCK_MONOTONIC
A nonsettable system-wide clock that 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. All CLOCK_MONOTONIC
variants guarantee
that the time returned by consecutive calls will not go
backwards, but successive calls may—depending on
the architecture—return identical (not-increased)
time values.
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)A nonsettable system-wide clock that is identical to
CLOCK_MONOTONIC
, except
that 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_BOOTTIME_ALARM
(since Linux 3.0;
Linux-specific)Like CLOCK_BOOTTIME
.
See timer_create(2) for
further details.
CLOCK_PROCESS_CPUTIME_ID
(since Linux
2.6.12)This is a clock that measures CPU time consumed by this process (i.e., CPU time consumed by all threads in the process). On Linux, this clock is not settable.
CLOCK_THREAD_CPUTIME_ID
(since Linux
2.6.12)This is a clock that measures CPU time consumed by this thread. On Linux, this clock is not settable.
Linux also implements dynamic clock instances as described below.
In addition to the hard-coded System-V style clock IDs described above, Linux also supports POSIX clock operations on certain character devices. Such devices are called "dynamic" clocks, and are supported since Linux 2.6.39.
Using the appropriate macros, open file descriptors may
be converted into clock IDs and passed to clock_gettime
(), clock_settime
(), and clock_adjtime(2). The
following example shows how to convert a file descriptor
into a dynamic clock ID.
#define CLOCKFD 3 #define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD) #define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3)) struct timespec ts; clockid_t clkid; int fd; fd = open("/dev/ptp0", O_RDWR); clkid = FD_TO_CLOCKID(fd); clock_gettime(clkid, &ts);
clock_gettime
(),
clock_settime
(), and
clock_getres
() return 0 for
success. On error, −1 is returned and errno
is set to indicate the error.
clock_settime
() does
not have write permission for the dynamic POSIX clock
device indicated.
tp
points
outside the accessible address space.
The clockid
specified is invalid for one of two reasons. Either the
System-V style hard coded positive value is out of
range, or the dynamic clock ID does not refer to a
valid instance of a clock object.
(clock_settime
()):
tp.tv_sec
is
negative or tp.tv_nsec
is outside
the range [0..999,999,999].
The clockid
specified in a call to clock_settime
() is not a settable
clock.
A call to clock_settime
() with a clockid
of CLOCK_REALTIME
attempted to set the
time to a value less than the current value of the
CLOCK_MONOTONIC
clock.
The hot-pluggable device (like USB for example)
represented by a dynamic clk_id
has disappeared
after its character device was opened.
The operation is not supported by the dynamic POSIX clock device specified.
clock_settime
() does
not have permission to set the clock indicated.
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 |
POSIX.1-2001, POSIX.1-2008, SUSv2.
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).)
POSIX.1 specifies the following:
Setting the value of the
CLOCK_REALTIME
clock viaclock_settime
() shall have no effect on threads that are blocked waiting for a relative time service based upon this clock, including thenanosleep
() 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.
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").
On some architectures, an implementation of clock_gettime
() is provided in the
vdso(7).
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).
The program below demonstrates the use of clock_gettime
() and clock_getres
() with various clocks. This is
an example of what we might see when running the program:
$ ./clock_times x CLOCK_REALTIME : 1585985459.446 (18356 days + 7h 30m 59s) resolution: 0.000000001 CLOCK_TAI : 1585985496.447 (18356 days + 7h 31m 36s) resolution: 0.000000001 CLOCK_MONOTONIC: 52395.722 (14h 33m 15s) resolution: 0.000000001 CLOCK_BOOTTIME : 72691.019 (20h 11m 31s) resolution: 0.000000001
/* clock_times.c Licensed under GNU General Public License v2 or later. */ #define _XOPEN_SOURCE 600 #include <time.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <stdbool.h> #include <unistd.h> #define SECS_IN_DAY (24 * 60 * 60) static void displayClock(clockid_t clock, const char *name, bool showRes) { struct timespec ts; if (clock_gettime(clock, &ts) == −1) { perror("clock_gettime"); exit(EXIT_FAILURE); } printf("%−15s: %10jd.%03ld (", name, (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000); long days = ts.tv_sec / SECS_IN_DAY; if (days > 0) printf("%ld days + ", days); printf("%2dh %2dm %2ds", (int) (ts.tv_sec % SECS_IN_DAY) / 3600, (int) (ts.tv_sec % 3600) / 60, (int) ts.tv_sec % 60); printf(")\n"); if (clock_getres(clock, &ts) == −1) { perror("clock_getres"); exit(EXIT_FAILURE); } if (showRes) printf(" resolution: %10jd.%09ld\n", (intmax_t) ts.tv_sec, ts.tv_nsec); } int main(int argc, char *argv[]) { bool showRes = argc > 1; displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes); #ifdef CLOCK_TAI displayClock(CLOCK_TAI, "CLOCK_TAI", showRes); #endif displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes); #ifdef CLOCK_BOOTTIME displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes); #endif exit(EXIT_SUCCESS); }
date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3), clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3), sysconf(3), time(7), time_namespaces(7), vdso(7), hwclock(8)
This page is part of release 5.11 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 Copyright (c) 2020 Michael Kerrisk <mtk.manpagesgmail.com> %%%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 |