|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | AVAILABILITY | NOTES | BUGS | SEE ALSO | COLOPHON |
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. 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 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.
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.
These system calls first appeared in Linux 2.6.
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() │ │ │
└─────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008, SUSv2.
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).)
POSIX.1 specifies the following:
Setting the value of the CLOCK_REALTIME clock via
clock_settime() 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, independently of the new
or old value of the clock.
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).
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), vdso(7), hwclock(8)
This page is part of release 4.12 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/.
2016-12-12 CLOCK_GETRES(2)
Pages that refer to this page: clock_nanosleep(2), getrusage(2), gettimeofday(2), nanosleep(2), recvmmsg(2), syscalls(2), timer_create(2), timer_delete(2), timerfd_create(2), timer_getoverrun(2), times(2), aio_suspend(3), clock(3), clock_getcpuclockid(3), ftime(3), pmdaeventarray(3), pthread_getcpuclockid(3), pthread_tryjoin_np(3), sd_bus_message_get_monotonic_usec(3), sd_event_add_time(3), sd_event_now(3), sd_journal_get_cutoff_realtime_usec(3), sd_journal_get_fd(3), sd_journal_get_realtime_usec(3), sd_login_monitor_new(3), sem_wait(3), signal-safety(7), time(7)