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); |
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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;long tv_nsec;};
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 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 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 ofCLOCK_REALTIME, which may have discontinuities if the time is changed using settimeofday(2). 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
-
tppoints outside the accessible address space. - EINVAL
-
The
clk_idspecified is not supported on this system. - EPERM
-
clock_settime() does not have permission to set the clock indicated.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
clock_getres(), clock_gettime(), clock_settime() |
Thread safety | MT-Safe |
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
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").
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)
COLOPHON
This page is part of release 4.07 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/.
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Copyright (c) 2003 Nick Clifford (zafnrc.co.nz), Jan 25, 2003 Copyright (c) 2003 Andries Brouwer (aebcwi.nl), Aug 24, 2003 %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END 2003-08-23 Martin Schulze <joeyinfodrom.org> improvements 2003-08-24 aeb, large parts rewritten 2004-08-06 Christoph Lameter <clametersgi.com>, SMP note |