clock_getres2

Usage

#include <time.h>

    _POSIX_C_SOURCE >= 199309L

Library

Standard C library (libc, -lc), since glibc 2.17

Before glibc 2.17, Real-time library (librt, -lrt)

Overview

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 settable only indirectly by setting CLOCK_REALTIME, and behaves differently if the system is suspended. See timer_create(2).

CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)

A faster but less precise version of CLOCK_REALTIME. It is settable only indirectly by setting 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_TAI (since Linux 3.10; Linux-specific)

A system-wide clock derived from wall-clock time but counting leap seconds. This clock does not experience clock adjustments caused by inserting leap seconds as CLOCK_REALTIME does. It counts seconds since 1970-01-01 00:00 TAI (1969-12-31 23:59:50 UTC), so its epoch is ten seconds before CLOCK_REALTIME's. However, its implementation requires help from NTP, and if that help is not available it behaves like CLOCK_REALTIME instead.

Its relationship to CLOCK_REALTIME for timestamps before 1972 is only approximate, as the leap second regime was introduced on 1972-01-01. Because leap seconds are planned to be discontinued, its value from now on will likely remain exactly 37 seconds greater than CLOCK_REALTIME's for many years, after which its offset from CLOCK_REALTIME's will likely be adjusted by some other mechanism as yet unspecified. It is settable only indirectly by setting CLOCK_REALTIME.

The acronym TAI stands for the French "temps atomique international", or 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, but behaves differently if the system is suspended. See timer_create(2).

CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)

This clock 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 clock measures CPU time consumed by this thread. On Linux, this clock is not settable.

Linux also implements dynamic clock instances as described below.

#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.

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.

Attributes

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

Versions

POSIX.1 specifies the following:

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").

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

/* 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 <sys/types.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 *[])
{
    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);
}