CLOCK_GETRES(2) Linux Programmer's Manual CLOCK_GETRES(2)
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 pro-
cesses, 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 nanosec-
onds 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 correspond-
ing 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 time-
stamps.
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 adjust-
ments or the incremental adjustments performed by adj-
time(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 applica-
tions to get a suspend-aware monotonic clock without hav-
ing to deal with the complications of CLOCK_REALTIME,
which may have discontinuities if the time is changed
using settimeofday(2).
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 appropri-
ately).
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 <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
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 reg-
isters since each CPU will run at a slightly different fre-
quency. 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 run-
ning 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 Ker-
nel). 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 privi-
leges" 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").
SEE ALSO
date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3),
clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpu-
clockid(3), sysconf(3), time(7)
COLOPHON
This page is part of release 3.53 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/.
2013-02-25 CLOCK_GETRES(2)
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