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clock_getres(3) - clock_getres, clock_gettime, clock_settime, clock_getres, clock_gettime, clock_settime - clock and time functions - man 3 clock_getres

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



NAME
       clock_getres, clock_gettime, clock_settime - clock(3,n) and time(1,2,n) 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);

DESCRIPTION
       The  function  clock_getres()  finds  the resolution (precision) of the
       specified clock(3,n) clk_id, and, if(3,n) res  is  non-NULL,  stores  it  in(1,8)  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(1,2,n) 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(7,n,1 builtins)  the
       time(1,2,n) of the specified clock(3,n) clk_id.

       The  res  and  tp  arguments  are  timespec  structs,  as specified  in(1,8)
       <time.h>:

       struct timespec {
               time_t   tv_sec;        /* seconds */
               long     tv_nsec;       /* nanoseconds */
       };

       The clk_id argument is the identifier of the particular clock(3,n) on  which
       to  act.   A  clock(3,n)  may  be system-wide and hence visible for all pro-
       cesses, or per-process  if(3,n)  it  measures  time(1,2,n)  only  within  a  single
       process.

       All  implementations  support  the system-wide realtime clock(3,n), which is
       identified by CLOCK_REALTIME.  Its time(1,2,n) represents seconds and nanosec-
       onds  since the Epoch.  When its time(1,2,n) is changed, timers for a relative
       interval are unaffected, but timers for an absolute point in(1,8)  time(1,2,n)  are
       affected.

       More clocks may be implemented. The interpretation of the corresponding
       time(1,2,n) values and the effect on timers is unspecified.

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

       CLOCK_REALTIME
              System-wide  realtime clock.  Setting this clock(3,n) requires appro-
              priate privileges.

       CLOCK_MONOTONIC
              Clock that cannot be set(7,n,1 builtins) and  represents  monotonic  time(1,2,n)  since
              some unspecified starting point.

       CLOCK_PROCESS_CPUTIME_ID
              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(1,8) which case errno is set(7,n,1 builtins) 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(7,n,1 builtins) the clock(3,n)  indi-
              cated.

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

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(3,n) a process is
       migrated to another CPU.

       If the CPUs in(1,8) an SMP system have different clock(3,n) 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(3,n) it can be ensured that
       a process stays on a certain CPU.

       The  processors  in(1,8)  an SMP system do not start all at exactly the same
       time(1,2,n) 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(1,8) most cases.

AVAILABILITY
       On  POSIX  systems  on  which these functions are available, the symbol
       _POSIX_TIMERS is defined in(1,8) <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).)

CONFORMING TO
       SUSv2, POSIX 1003.1-2001.

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



                                  2003-08-24                   CLOCK_GETRES(3)

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