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 particu‐
lar 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 speci‐
fied 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 unspeci‐
fied 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 adj‐
time(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 per‐
formed by adjtime(3).
CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
Identical to CLOCK_MONOTONIC, except it also includes any time that the sys‐
tem 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_MONO‐
TONIC_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 architec‐
tures 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 privi‐
leges").
SEE ALSO
date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3), clock_getcpu‐
clockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3), sysconf(3), time(7)
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)
