struct timespec的使用

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



NAME
       clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS
       #include

       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 :

           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 real-time clock.  Setting this clock requires appropriate privileges.

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

       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.

       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 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.

CONFORMING TO
       SUSv2, POSIX.1-2001.

AVAILABILITY
       On  POSIX  systems  on  which  these  functions  are  available,  the  symbol  _POSIX_TIMERS  is defined in 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 condition.  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 offset.   Some  archi‐
       tectures  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.

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_set‐
       time().  On Linux, these clocks are not settable (i.e., no process has "appropriate privileges").

SEE ALSO
       date(1), adjtimex(2), gettimeofday(2), settimeofday(2), time(2), clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3), sysconf(3), time(7)

COLOPHON
       This  page  is  part  of  release  3.35  of  the Linux man-pages project.  A description of the project, and information about reporting bugs, can be found at
      



                                                                              2010-02-03                                                              CLOCK_GETRES(2)