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CLOCK_GETRES(2) Linux Programmer's Manual CLOCK_GETRES(2)
clock_getres, clock_gettime, clock_settime - clock and time functions
#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
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
High-resolution per-process timer from the CPU.
CLOCK_THREAD_CPUTIME_ID
Thread-specific CPU-time clock.
clock_gettime(), clock_settime() and clock_getres() return 0
for success, or -1 for failure (in which case errno is set
appropriately).
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.
SUSv2, POSIX.1-2001.
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).)
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 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.
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").
date(1), gettimeofday(2), settimeofday(2), time(2),
adjtime(3), clock_getcpuclockid(3), ctime(3), ftime(3),
pthread_getcpuclockid(3), sysconf(3), time(7)
This page is part of release 3.51 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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