CLOCK_GETRES(3)            Linux Programmer's Manual           CLOCK_GETRES(3)

       clock_getres, clock_gettime, clock_settime - clock and time functions

       #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);

       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  structs,  as specified  in

       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  pro-

       All  implementations  support  the system-wide realtime clock, which is
       identified by CLOCK_REALTIME.  Its time represents seconds and nanosec-
       onds  since the Epoch.  When its time is changed, timers for a relative
       interval are unaffected, but timers for an absolute point in  time  are

       More clocks may be implemented. The interpretation of the corresponding
       time values and the effect on timers is unspecified.

       Sufficiently recent versions of GNU libc and the Linux  kernel  support
       the following clocks:

              System-wide  realtime clock.  Setting this clock requires appro-
              priate privileges.

              Clock that cannot be set and  represents  monotonic  time  since
              some unspecified starting point.

              High-resolution per-process timer from the CPU.

              Thread-specific CPU-time clock.

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

       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  indi-

       Most  systems  require  the program be linked with the librt library to
       use these functions.

NOTE for SMP systems
       realized  on  many  platforms  using timers 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 condi-
       tion. The two clocks will then only be useful 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 off-
       set.  Some architectures include code that attempts to limit these off-
       sets 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 there-
       fore the effects may be negligible in most cases.

       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
       indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
       CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)

       SUSv2, POSIX.1-2001.

       date(1),   adjtimex(2),   gettimeofday(2),   settimeofday(2),  time(2),
       ctime(3), ftime(3), sysconf(3)

                                  2003-08-24                   CLOCK_GETRES(3)

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