CLOCK_GETRES(2)                                              Linux Programmer's Manual                                             CLOCK_GETRES(2)

NAME

       clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS

       #include <time.h>

       int clock_getres(clockid_t clk_id, struct timespec *res);

       int clock_gettime(clockid_t clk_id, struct timespec *tp);

       int clock_settime(clockid_t clk_id, 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

DESCRIPTION

       The  function  clock_getres()  finds  the  resolution  (precision) of the specified clock clk_id, 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 clk_id.

       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 clk_id argument is the identifier of the particular clock on which to act.  A clock may be system-wide and hence visible for  all  pro-
       cesses, 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
              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 incre-
              mental adjustments performed by adjtime(3) and NTP.

       CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
              A  faster  but less precise version of 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_MONOTONIC
              Clock that cannot be set and represents monotonic time since some unspecified starting point.  This clock is not affected by discon-
              tinuous  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.

       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 incremen-
              tal adjustments performed by adjtime(3).

       CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
              Identical to CLOCK_MONOTONIC, except it also includes any time that the system is suspended.  This allows applications to get a sus-
              pend-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_PROCESS_CPUTIME_ID (since Linux 2.6.12)
              Per-process CPU-time clock (measures CPU time consumed by all threads in the process).

       CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
              Thread-specific CPU-time clock.

RETURN VALUE

       clock_gettime(), clock_settime(), and clock_getres() return 0 for success, or -1 for failure (in which case errno is set appropriately).

ERRORS

       EFAULT tp points outside the accessible address space.

       EINVAL The clk_id specified is not supported on this system.

       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(), | Thread safety | MT-Safe |
       |clock_settime()                  |               |         |
       +---------------------------------+---------------+---------+

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SUSv2.

AVAILABILITY

       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.

   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  plat-
       forms  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 ker-
       nel 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).

BUGS

       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").

SEE ALSO

       date(1), gettimeofday(2), settimeofday(2),  time(2),  adjtime(3),  clock_getcpuclockid(3),  ctime(3),  ftime(3),  pthread_getcpuclockid(3),
       sysconf(3), time(7), vdso(7), hwclock(8)

COLOPHON

       This  page is part of release 4.15 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/.

                                                                    2017-09-15                                                     CLOCK_GETRES(2)