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. Requires per-architecture support, and probably
also architecture support for this flag in the vdso(7).

CLOCK_MONOTONIC
Clock that cannot be set and represents monotonic time since—as described by
POSIX—"some unspecified point in the past". On Linux, that point corresponds to
the number of seconds that the system has been running since it was booted.

The CLOCK_MONOTONIC 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. This clock does not
count time that the system is suspended.

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. Requires per-architecture support, and probably
also architecture support for this flag in the vdso(7).

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). This clock does not count time that the system is suspended.

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) or similar.

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.

       EINVAL (clock_settime()):  tp.tv_sec  is  negative  or  tp.tv_nsec  is  outside  the range
              [0..999,999,999].

       EINVAL (since Linux 4.3)
              A call to clock_settime() with a clk_id of CLOCK_REALTIME attempted to set the time
              to a value less than the current value of the CLOCK_MONOTONIC clock.

       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

POSIX.1 specifies the following:

Setting the value of the CLOCK_REALTIME clock via clock_settime() shall have no
effect on threads that are blocked waiting for a relative time service based upon
this clock, including the nanosleep() function; nor on the expiration of relative
timers based upon this clock. Consequently, these time services shall expire when
the requested relative interval elapses, independently of the new or old value of
the clock.

C library/kernel differences
On some architectures, an implementation of clock_gettime() is provided in the vdso(7).

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 5.05 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 https://www.kernel.org/doc/man-pages/.

                                            2019-03-06                            CLOCK_GETRES(2)