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clock_getres(2) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|ATTRIBUTES \|VERSIONS \|STANDARDS \|HISTORY \|EXAMPLES \|SEE ALSO \|COLOPHON

clock_getres(2)            System Calls Manualclock_getres(2)

NAME top

       clock_getres, clock_gettime, clock_settime - clock and time       functions

LIBRARY top

       Standard C library (libc,-lc), since glibc 2.17       Before glibc 2.17, Real-time library (librt,-lrt)

SYNOPSIS top

#include <time.h>int clock_getres(clockid_tclockid, struct timespec *_Nullableres);int clock_gettime(clockid_tclockid, struct timespec *tp);int clock_settime(clockid_tclockid, const struct timespec *tp);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):clock_getres(),clock_gettime(),clock_settime():           _POSIX_C_SOURCE >= 199309L

DESCRIPTION top

       The functionclock_getres() finds the resolution (precision) of       the specified clockclockid, and, ifres is non-NULL, stores it in       thestruct timespec pointed to byres.  The resolution of clocks       depends on the implementation and cannot be configured by a       particular process.  If the time value pointed to by the argumenttp ofclock_settime() is not a multiple ofres, then it is       truncated to a multiple ofres.       The functionsclock_gettime() andclock_settime() retrieve and set       the time of the specified clockclockid.       Theres andtp arguments aretimespec(3) structures.       Theclockid 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 byCLOCK_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 viaadjtime(3),adjtimex(2),clock_adjtime(2), andntp_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)              LikeCLOCK_REALTIME, but not settable.  Seetimer_create(2)              for further details.CLOCK_REALTIME_COARSE(since Linux 2.6.32; Linux-specific)              A faster but less precise version ofCLOCK_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 thevdso(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 asCLOCK_REALTIMEdoes.              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.              TheCLOCK_MONOTONICclock 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.  AllCLOCK_MONOTONICvariants              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 ofCLOCK_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 thevdso(7).CLOCK_MONOTONIC_RAW(since Linux 2.6.28; Linux-specific)              Similar toCLOCK_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 toCLOCK_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 ofCLOCK_REALTIME, which may have              discontinuities if the time is changed usingsettimeofday(2) or similar.CLOCK_BOOTTIME_ALARM(since Linux 3.0; Linux-specific)              LikeCLOCK_BOOTTIME.  Seetimer_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 toclock_gettime(),clock_settime(), andclock_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 top

clock_gettime(),clock_settime(), andclock_getres() return 0 for       success.  On error, -1 is returned anderrno is set to indicate       the error.

ERRORS top

EACCES clock_settime() does not have write permission for the              dynamic POSIX clock device indicated.EFAULTtp points outside the accessible address space.EINVALTheclockid 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 ortp.tv_nsec is              outside the range [0, 999,999,999].EINVALTheclockid specified in a call toclock_settime() is not a              settable clock.EINVAL(since Linux 4.3)              A call toclock_settime() with aclockid ofCLOCK_REALTIME              attempted to set the time to a value less than the current              value of theCLOCK_MONOTONICclock.ENODEVThe hot-pluggable device (like USB for example) represented              by a dynamicclk_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 intime_t range.  This can              happen if an executable with 32-bittime_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 oftime_t              range in other situations, the behavior is undefined.EPERM  clock_settime() does not have permission to set the clock              indicated.

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │clock_getres(),clock_gettime(),     │ Thread safety │ MT-Safe │       │clock_settime()                      │               │         │       └──────────────────────────────────────┴───────────────┴─────────┘

VERSIONS top

       POSIX.1 specifies the following:              Setting the value of theCLOCK_REALTIMEclock 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 theCLOCK_PROCESS_CPUTIME_IDandCLOCK_THREAD_CPUTIME_ID       clocks usingclock_settime().  On Linux, these clocks are not       settable (i.e., no process has "appropriate privileges").C library/kernel differences       On some architectures, an implementation ofclock_gettime() is       provided in thevdso(7).

STANDARDS top

       POSIX.1-2008.

HISTORY top

POSIX.1-2001, SUSv2. Linux 2.6. On POSIX systems on which these functions are available, the symbol_POSIX_TIMERSis 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_CPUTIMEindicate thatCLOCK_MONOTONIC,CLOCK_PROCESS_CPUTIME_ID,CLOCK_THREAD_CPUTIME_IDare available. (See alsosysconf(3).)Historical note for SMP systems Before Linux added kernel support forCLOCK_PROCESS_CPUTIME_IDandCLOCK_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 returnbogus resultsif 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, thenclock_getcpuclockid(0) will returnENOENTto 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 ofCLOCK_PROCESS_CPUTIME_ID andCLOCK_THREAD_CPUTIME_ID, on systems that provide such an implementation (i.e., Linux 2.6.12 and later).

EXAMPLES top

       The program below demonstrates the use ofclock_gettime() andclock_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:          0.000000001Program 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 <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 *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);       }

COLOPHON top

       This page is part of theman-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-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  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 isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17clock_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), timespec_get(3), systemd.service(5), systemd.timer(5), signal-safety(7), time(7), time_namespaces(7)