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.\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
.\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
.\" Copyright (c) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
.\" 2003-08-24 aeb, large parts rewritten
.\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
.\"
.TH clock_getres 2 (date) "Linux man-pages (unreleased)"
.SH NAME
clock_getres, clock_gettime, clock_settime \- clock and time functions
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc ),
since glibc 2.17
.PP
Before glibc 2.17,
Real-time library
.RI ( librt ", " \-lrt )
.SH SYNOPSIS
.nf
.B #include <time.h>
.PP
.BI "int clock_getres(clockid_t " clockid ", struct timespec *_Nullable " res );
.PP
.BI "int clock_gettime(clockid_t " clockid ", struct timespec *" tp );
.BI "int clock_settime(clockid_t " clockid ", const struct timespec *" tp );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR clock_getres (),
.BR clock_gettime (),
.BR clock_settime ():
.nf
    _POSIX_C_SOURCE >= 199309L
.fi
.SH DESCRIPTION
The function
.BR clock_getres ()
finds the resolution (precision) of the specified clock
.IR clockid ,
and, if
.I res
is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
.IR 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
.I tp
of
.BR clock_settime ()
is not a multiple of
.IR res ,
then it is truncated to a multiple of
.IR res .
.PP
The functions
.BR clock_gettime ()
and
.BR clock_settime ()
retrieve and set the time of the specified clock
.IR clockid .
.PP
The
.I res
and
.I tp
arguments are
.BR timespec (3)
structures.
.PP
The
.I clockid
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.
.PP
All implementations support the system-wide real-time clock,
which is identified by
.BR 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.
.PP
More clocks may be implemented.
The interpretation of the
corresponding time values and the effect on timers is unspecified.
.PP
Sufficiently recent versions of glibc and the Linux kernel
support the following clocks:
.TP
.B CLOCK_REALTIME
A settable 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
.BR adjtime (3)
and NTP.
.TP
.BR CLOCK_REALTIME_ALARM " (since Linux 3.0; Linux-specific)"
Like
.BR CLOCK_REALTIME ,
but not settable.
See
.BR timer_create (2)
for further details.
.TP
.BR CLOCK_REALTIME_COARSE " (since Linux 2.6.32; Linux-specific)"
.\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
A faster but less precise version of
.BR CLOCK_REALTIME .
This clock is not settable.
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
.BR vdso (7).
.TP
.BR CLOCK_TAI " (since Linux 3.10; Linux-specific)"
.\" commit 1ff3c9677bff7e468e0c487d0ffefe4e901d33f4
A nonsettable system-wide clock derived from wall-clock time
but ignoring leap seconds.
This clock does
not experience discontinuities and backwards jumps caused by NTP
inserting leap seconds as
.B CLOCK_REALTIME
does.
.IP
The acronym TAI refers to International Atomic Time.
.TP
.B CLOCK_MONOTONIC
A nonsettable system-wide clock that
represents monotonic time since\(emas described
by POSIX\(em"some unspecified point in the past".
On Linux, that point corresponds to the number of seconds that the system
has been running since it was booted.
.IP
The
.B CLOCK_MONOTONIC
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
.BR adjtime (3)
and NTP.
This clock does not count time that the system is suspended.
All
.B CLOCK_MONOTONIC
variants guarantee that the time returned by consecutive calls will not go
backwards, but successive calls may\(emdepending on the architecture\(emreturn
identical (not-increased) time values.
.TP
.BR CLOCK_MONOTONIC_COARSE " (since Linux 2.6.32; Linux-specific)"
.\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
A faster but less precise version of
.BR 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
.BR vdso (7).
.TP
.BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
.\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
Similar to
.BR CLOCK_MONOTONIC ,
but provides access to a raw hardware-based time
that is not subject to NTP adjustments or
the incremental adjustments performed by
.BR adjtime (3).
This clock does not count time that the system is suspended.
.TP
.BR CLOCK_BOOTTIME " (since Linux 2.6.39; Linux-specific)"
.\" commit 7fdd7f89006dd5a4c702fa0ce0c272345fa44ae0
.\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
A nonsettable system-wide clock that is identical to
.BR CLOCK_MONOTONIC ,
except that 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
.BR CLOCK_REALTIME ,
which may have discontinuities if the time is changed using
.BR settimeofday (2)
or similar.
.TP
.BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0; Linux-specific)"
Like
.BR CLOCK_BOOTTIME .
See
.BR timer_create (2)
for further details.
.TP
.BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
This is a clock that measures CPU time consumed by this process
(i.e., CPU time consumed by all threads in the process).
On Linux, this clock is not settable.
.TP
.BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
This is a clock that measures CPU time consumed by this thread.
On Linux, this clock is not settable.
.PP
Linux also implements dynamic clock instances as described below.
.SS Dynamic clocks
In addition to the hard-coded System-V style clock IDs described above,
Linux also supports
POSIX clock operations on certain character devices.
Such devices are
called "dynamic" clocks, and are supported since Linux 2.6.39.
.PP
Using the appropriate macros, open file
descriptors may be converted into clock IDs and passed to
.BR clock_gettime (),
.BR clock_settime (),
and
.BR clock_adjtime (2).
The following example shows how to convert a file descriptor into a
dynamic clock ID.
.PP
.in +4n
.EX
#define CLOCKFD 3
#define FD_TO_CLOCKID(fd)   ((\(ti(clockid_t) (fd) << 3) | CLOCKFD)
#define CLOCKID_TO_FD(clk)  ((unsigned int) \(ti((clk) >> 3))

struct timespec ts;
clockid_t clkid;
int fd;

fd = open("/dev/ptp0", O_RDWR);
clkid = FD_TO_CLOCKID(fd);
clock_gettime(clkid, &ts);
.EE
.in
.SH RETURN VALUE
.BR clock_gettime (),
.BR clock_settime (),
and
.BR clock_getres ()
return 0 for success.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
.BR clock_settime ()
does not have write permission for the dynamic POSIX
clock device indicated.
.TP
.B EFAULT
.I tp
points outside the accessible address space.
.TP
.B EINVAL
The
.I clockid
specified is invalid for one of two reasons.
Either the System-V style
hard coded positive value is out of range, or the dynamic clock ID
does not refer to a valid instance of a clock object.
.\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
.\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
.\" EPERM.
.TP
.B EINVAL
.RB ( clock_settime ()):
.I tp.tv_sec
is negative or
.I tp.tv_nsec
is outside the range [0..999,999,999].
.TP
.B EINVAL
The
.I clockid
specified in a call to
.BR clock_settime ()
is not a settable clock.
.TP
.BR EINVAL " (since Linux 4.3)"
.\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
A call to
.BR clock_settime ()
with a
.I clockid
of
.B CLOCK_REALTIME
attempted to set the time to a value less than
the current value of the
.B CLOCK_MONOTONIC
clock.
.TP
.B ENODEV
The hot-pluggable device (like USB for example) represented by a
dynamic
.I clk_id
has disappeared after its character device was opened.
.TP
.B ENOTSUP
The operation is not supported by the dynamic POSIX clock device
specified.
.TP
.B EPERM
.BR clock_settime ()
does not have permission to set the clock indicated.
.SH VERSIONS
These system calls first appeared in Linux 2.6.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR clock_getres (),
.BR clock_gettime (),
.BR clock_settime ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008, SUSv2.
.PP
On POSIX systems on which these functions are available, the symbol
.B _POSIX_TIMERS
is defined in \fI<unistd.h>\fP to a value greater than 0.
The symbols
.BR _POSIX_MONOTONIC_CLOCK ,
.BR _POSIX_CPUTIME ,
.B _POSIX_THREAD_CPUTIME
indicate that
.BR CLOCK_MONOTONIC ,
.BR CLOCK_PROCESS_CPUTIME_ID ,
.B CLOCK_THREAD_CPUTIME_ID
are available.
(See also
.BR sysconf (3).)
.SH NOTES
POSIX.1 specifies the following:
.RS
.PP
Setting the value of the
.B CLOCK_REALTIME
clock via
.BR clock_settime ()
shall have no effect on threads that are blocked waiting for a relative time
service based upon this clock, including the
.BR 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.
.RE
.PP
According to POSIX.1-2001, a process with "appropriate privileges" may set the
.B CLOCK_PROCESS_CPUTIME_ID
and
.B CLOCK_THREAD_CPUTIME_ID
clocks using
.BR clock_settime ().
On Linux, these clocks are not settable
(i.e., no process has "appropriate privileges").
.\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
.\"
.SS C library/kernel differences
On some architectures, an implementation of
.BR clock_gettime ()
is provided in the
.BR vdso (7).
.\"
.SS Historical note for SMP systems
Before Linux added kernel support for
.B CLOCK_PROCESS_CPUTIME_ID
and
.BR 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
.B bogus results
if a process is migrated to another CPU.
.PP
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
.I clock_getcpuclockid(0)
will return
.B 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.
.PP
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.
.PP
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
.B CLOCK_PROCESS_CPUTIME_ID
and
.BR CLOCK_THREAD_CPUTIME_ID ,
on systems that provide such an implementation
(i.e., Linux 2.6.12 and later).
.SH EXAMPLES
The program below demonstrates the use of
.BR clock_gettime ()
and
.BR clock_getres ()
with various clocks.
This is an example of what we might see when running the program:
.PP
.in +4n
.EX
$ \fB./clock_times x\fP
CLOCK_REALTIME : 1585985459.446 (18356 days +  7h 30m 59s)
     resolution:          0.000000001
CLOCK_TAI      : 1585985496.447 (18356 days +  7h 31m 36s)
     resolution:          0.000000001
CLOCK_MONOTONIC:      52395.722 (14h 33m 15s)
     resolution:          0.000000001
CLOCK_BOOTTIME :      72691.019 (20h 11m 31s)
     resolution:          0.000000001
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (clock_getres.c)
.EX
/* clock_times.c

   Licensed under GNU General Public License v2 or later.
*/
#define _XOPEN_SOURCE 600
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define SECS_IN_DAY (24 * 60 * 60)

static void
displayClock(clockid_t clock, const char *name, bool showRes)
{
    long             days;
    struct timespec  ts;

    if (clock_gettime(clock, &ts) == \-1) {
        perror("clock_gettime");
        exit(EXIT_FAILURE);
    }

    printf("%\-15s: %10jd.%03ld (", name,
           (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);

    days = ts.tv_sec / SECS_IN_DAY;
    if (days > 0)
        printf("%ld days + ", days);

    printf("%2dh %2dm %2ds",
           (int) (ts.tv_sec % SECS_IN_DAY) / 3600,
           (int) (ts.tv_sec % 3600) / 60,
           (int) ts.tv_sec % 60);
    printf(")\en");

    if (clock_getres(clock, &ts) == \-1) {
        perror("clock_getres");
        exit(EXIT_FAILURE);
    }

    if (showRes)
        printf("     resolution: %10jd.%09ld\en",
               (intmax_t) ts.tv_sec, ts.tv_nsec);
}

int
main(int argc, char *argv[])
{
    bool showRes = argc > 1;

    displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
#ifdef CLOCK_TAI
    displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
#endif
    displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
#ifdef CLOCK_BOOTTIME
    displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);
#endif
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR date (1),
.BR gettimeofday (2),
.BR settimeofday (2),
.BR time (2),
.BR adjtime (3),
.BR clock_getcpuclockid (3),
.BR ctime (3),
.BR ftime (3),
.BR pthread_getcpuclockid (3),
.BR sysconf (3),
.BR timespec (3),
.BR time (7),
.BR time_namespaces (7),
.BR vdso (7),
.BR hwclock (8)