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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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clock_getres(2) System Calls Manual clock_getres(2)
clock_getres, clock_gettime, clock_settime - clock and time
functions
Standard C library (libc, -lc), since glibc 2.17
Before glibc 2.17, Real-time library (librt, -lrt)
#include <time.h>
int clock_getres(clockid_t clockid, struct timespec *_Nullable res);
int clock_gettime(clockid_t clockid, struct timespec *tp);
int clock_settime(clockid_t clockid, const struct timespec *tp);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
clock_getres(), clock_gettime(), clock_settime():
_POSIX_C_SOURCE >= 199309L
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(3) structures.
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 frequency adjustments
performed by NTP and similar applications via adjtime(3),
adjtimex(2), clock_adjtime(2), and ntp_adjtime(3). This
clock normally counts the number of seconds since
1970-01-01 00:00:00 Coordinated Universal Time (UTC)
except that it ignores leap seconds; near a leap second it
is typically adjusted by NTP to stay roughly in sync with
UTC.
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 counting leap seconds. This clock does not
experience discontinuities or frequency adjustments caused
by 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 frequency adjustments. 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 frequency
adjustments. 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.
Dynamic clocks
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.
EACCES clock_settime() does not have write permission for the
dynamic POSIX clock device indicated.
EFAULT tp points outside the accessible address space.
EINVAL 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.
EINVAL (clock_settime()): tp.tv_sec is negative or tp.tv_nsec is
outside the range [0, 999,999,999].
EINVAL The clockid specified in a call to clock_settime() is not
a settable clock.
EINVAL (since Linux 4.3)
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.
ENODEV The hot-pluggable device (like USB for example)
represented by a dynamic clk_id has disappeared after its
character device was opened.
ENOTSUP
The operation is not supported by the dynamic POSIX clock
device specified.
EOVERFLOW
The timestamp would not fit in time_t range. This can
happen if an executable with 32-bit time_t is run on a
64-bit kernel when the time is 2038-01-19 03:14:08 UTC or
later. However, when the system time is out of time_t
range in other situations, the behavior is undefined.
EPERM 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(), │ Thread safety │ MT-Safe │
│ clock_settime() │ │ │
└─────────────────────────────────────┴───────────────┴─────────┘
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.
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").
C library/kernel differences
On some architectures, an implementation of clock_gettime() is
provided in the vdso(7).
POSIX.1-2008.
POSIX.1-2001, SUSv2. Linux 2.6.
On POSIX systems on which these functions are available, the
symbol _POSIX_TIMERS is defined in <unistd.h> to a value greater
than 0. POSIX.1-2008 makes these functions mandatory.
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).)
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).
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
Program source
/* clock_times.c
Licensed under GNU General Public License v2 or later.
*/
#define _XOPEN_SOURCE 600
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define SECS_IN_DAY (24 * 60 * 60)
static void
displayClock(clockid_t clock, const char *name, bool showRes)
{
long days;
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);
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), timespec(3), time(7),
time_namespaces(7), vdso(7), hwclock(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 clock_getres(2)
Pages that refer to this page: lsclocks(1), strace(1), adjtimex(2), clock_nanosleep(2), getrusage(2), gettimeofday(2), nanosleep(2), recvmmsg(2), seccomp(2), stime(2), syscalls(2), timer_create(2), timer_delete(2), timerfd_create(2), timer_getoverrun(2), times(2), aio_suspend(3), clock(3), clock_getcpuclockid(3), clockid_t(3type), ftime(3), pmdaeventarray(3), pthread_getcpuclockid(3), pthread_tryjoin_np(3), sd_bus_message_get_monotonic_usec(3), sd_event_add_time(3), sd_event_now(3), sd_journal_get_cutoff_realtime_usec(3), sd_journal_get_fd(3), sd_journal_get_realtime_usec(3), sd_login_monitor_new(3), sem_wait(3), timespec(3type), systemd.service(5), systemd.timer(5), signal-safety(7), time(7), time_namespaces(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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