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NAME
clock_getres, clock_gettime, clock_settime - clock and time functions
LIBRARY
Standard C library (libc, -lc), since glibc 2.17
Before glibc 2.17, Real-time library (librt, -lrt)
SYNOPSIS
#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
DESCRIPTION
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 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.
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);
RETURN VALUE
clock_gettime(), clock_settime(), and clock_getres() return 0 for success. On error, -1
is returned and errno is set to indicate the error.
ERRORS
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.
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 │
└───────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
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).)
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.
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).
EXAMPLES
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);
}
SEE ALSO
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)