mirror of https://github.com/mkerrisk/man-pages
359 lines
11 KiB
Groff
359 lines
11 KiB
Groff
.\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
|
|
.\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.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.\" manual page may be incorrect or out-of-date.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 <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 2019-03-06 "" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
clock_getres, clock_gettime, clock_settime \- clock and time functions
|
|
.SH SYNOPSIS
|
|
.B #include <time.h>
|
|
.PP
|
|
.BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res );
|
|
.PP
|
|
.BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp );
|
|
.PP
|
|
.BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp );
|
|
.PP
|
|
Link with \fI\-lrt\fP (only for glibc versions before 2.17).
|
|
.PP
|
|
.in -4n
|
|
Feature Test Macro Requirements for glibc (see
|
|
.BR feature_test_macros (7)):
|
|
.in
|
|
.PP
|
|
.ad l
|
|
.BR clock_getres (),
|
|
.BR clock_gettime (),
|
|
.BR clock_settime ():
|
|
.RS
|
|
_POSIX_C_SOURCE\ >=\ 199309L
|
|
.RE
|
|
.ad b
|
|
.SH DESCRIPTION
|
|
The function
|
|
.BR clock_getres ()
|
|
finds the resolution (precision) of the specified clock
|
|
.IR clk_id ,
|
|
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 clk_id .
|
|
.PP
|
|
The
|
|
.I res
|
|
and
|
|
.I tp
|
|
arguments are
|
|
.I timespec
|
|
structures, as specified in
|
|
.IR <time.h> :
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
struct timespec {
|
|
time_t tv_sec; /* seconds */
|
|
long tv_nsec; /* nanoseconds */
|
|
};
|
|
.EE
|
|
.in
|
|
.PP
|
|
The
|
|
.I 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.
|
|
.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
|
|
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_COARSE " (since Linux 2.6.32; Linux-specific)"
|
|
.\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
|
|
A faster but less precise version of
|
|
.BR 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
|
|
.BR vdso (7).
|
|
.TP
|
|
.B CLOCK_MONOTONIC
|
|
Clock that cannot be set and 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.
|
|
.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
|
|
Identical to
|
|
.BR 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
|
|
.BR CLOCK_REALTIME ,
|
|
which may have discontinuities if the time is changed using
|
|
.BR settimeofday (2)
|
|
or similar.
|
|
.TP
|
|
.BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
|
|
Per-process CPU-time clock
|
|
(measures CPU time consumed by all threads in the process).
|
|
.TP
|
|
.BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
|
|
Thread-specific CPU-time clock.
|
|
.SH RETURN VALUE
|
|
.BR clock_gettime (),
|
|
.BR clock_settime (),
|
|
and
|
|
.BR clock_getres ()
|
|
return 0 for success, or \-1 for failure (in which case
|
|
.I errno
|
|
is set appropriately).
|
|
.SH ERRORS
|
|
.TP
|
|
.B EFAULT
|
|
.I tp
|
|
points outside the accessible address space.
|
|
.TP
|
|
.B EINVAL
|
|
The
|
|
.I clk_id
|
|
specified is not supported on this system.
|
|
.\" 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
|
|
.BR EINVAL " (since Linux 4.3)"
|
|
.\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
|
|
A call to
|
|
.BR clock_settime ()
|
|
with a
|
|
.I clk_id
|
|
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 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).
|
|
.TS
|
|
allbox;
|
|
lbw32 lb lb
|
|
l l l.
|
|
Interface Attribute Value
|
|
T{
|
|
.BR clock_getres (),
|
|
.BR clock_gettime (),
|
|
.BR clock_settime ()
|
|
T} Thread safety MT-Safe
|
|
.TE
|
|
.sp 1
|
|
.SH CONFORMING TO
|
|
POSIX.1-2001, POSIX.1-2008, SUSv2.
|
|
.SH AVAILABILITY
|
|
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
|
|
.\"
|
|
.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 BUGS
|
|
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
|
|
.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 time (7),
|
|
.BR vdso (7),
|
|
.BR hwclock (8)
|