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. 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
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. 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 hard-
ware-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) 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 suc-
cess, 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 indi-
cated.
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_set-
time() 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, indepen-
dently 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 plat-
forms using timer registers from the CPUs (TSC on i386, AR.ITC on Ita-
nium). 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 off-
set. Some architectures include code that attempts to limit these off-
sets 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").
SEE ALSO
date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3),
clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
sysconf(3), time(7), vdso(7), hwclock(8)
COLOPHON
This page is part of release 4.15 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/.
2017-09-15 CLOCK_GETRES(2)
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