clock_settime


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.

   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 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
       affected.

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

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

       CLOCK_REALTIME
              High-resolution per-process timer from the CPU.

       CLOCK_THREAD_CPUTIME_ID
              Thread-specific CPU-time clock.

RETURN VALUE
       clock_gettime(), clock_settime() and clock_getres() return 0  for  suc-
       cess, 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 indi-
              cated.

CONFORMING TO
       SUSv2, POSIX.1-2001.

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
   Note for SMP systems
       The CLOCK_PROCESS_CPUTIME_ID  and  CLOCK_THREAD_CPUTIME_ID  clocks  are
       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
       therefore the effects may be negligible in most cases.

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.,


                                  2010-02-03                   CLOCK_GETRES(2)
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