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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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CLOCK_GETRES(2) Linux Programmer's Manual CLOCK_GETRES(2)
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);
Link with -lrt (only for glibc versions before 2.17).
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 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 corre‐
sponding 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 privi‐
leges. 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 per‐
formed 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 sec‐
onds 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 incremen‐
tal 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 architec‐
ture 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 hard‐
ware-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 discontinu‐
ities 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 charac‐
ter devices. Such devices are called "dynamic" clocks, and are sup‐
ported 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 timeval tv;
clockid_t clkid;
int fd;
fd = open("/dev/ptp0", O_RDWR);
clkid = FD_TO_CLOCKID(fd);
clock_gettime(clkid, &tv);
clock_gettime(), clock_settime(), and clock_getres() return 0 for
success, or -1 for failure (in which case errno is set
appropriately).
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.
ENOTSUP
The operation is not supported by the dynamic POSIX clock
device specified.
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.
EPERM clock_settime() does not have permission to set the clock
indicated.
EACCES clock_settime() does not have write permission for the dynamic
POSIX clock device indicated.
These system calls first appeared in Linux 2.6.
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.1-2001, 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 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).
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 <time.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, char *name, bool showRes)
{
struct timespec ts;
if (clock_gettime(clock, &ts) == -1) {
perror("clock_gettime");
exit(EXIT_FAILURE);
}
printf("%-15s: %10ld.%03ld (", name,
(long) ts.tv_sec, ts.tv_nsec / 1000000);
long days = ts.tv_sec / SECS_IN_DAY;
if (days > 0)
printf("%ld days + ", days);
printf("%2ldh %2ldm %2lds", (ts.tv_sec % SECS_IN_DAY) / 3600,
(ts.tv_sec % 3600) / 60, ts.tv_sec % 60);
printf(")\n");
if (clock_getres(clock, &ts) == -1) {
perror("clock_getres");
exit(EXIT_FAILURE);
}
if (showRes)
printf(" resolution: %10ld.%09ld\n",
(long) 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 5.07 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/.
2020-04-11 CLOCK_GETRES(2)
Pages that refer to this page: aio_suspend(3), ETF(8), tc-etf(8)
Copyright and license for this manual page
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