Ubuntu Manpage: clock_getres, clock_gettime, clock_settime

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)
Per-process CPU-time clock (measures CPU time consumed by all threads in the
process).

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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌─────────────────────────────────┬───────────────┬─────────┐
       │Interface                        │ Attribute     │ Value   │
       ├─────────────────────────────────┼───────────────┼─────────┤
       │clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
       │clock_settime()                  │               │         │
       └─────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SUSv2.

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 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

                                            2015-12-28                            CLOCK_GETRES(2)