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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 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
              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_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 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.

       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).

       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 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_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).

       ┌─────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├─────────────────────────────────┼───────────────┼─────────┤
       │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 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).

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 4.16 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)