Ubuntu Manpage: clock_getres, clock_gettime, clock_settime

Provided by: manpages-dev_3.54-1ubuntu1_all

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.

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.

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).

CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
High-resolution per-process timer from the CPU.

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.

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

   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").

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

                                                   2013-09-04                                    CLOCK_GETRES(2)