CLOCK_GETRES
Section: Linux Programmer's Manual (2)
Updated: 2010-02-03
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.
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 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 real-time clock.
Setting this clock requires appropriate privileges.
- CLOCK_MONOTONIC
-
Clock that cannot be set and represents monotonic time since
some unspecified starting point.
- 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.
- CLOCK_PROCESS_CPUTIME_ID
-
High-resolution per-process timer from the CPU.
- CLOCK_THREAD_CPUTIME_ID
-
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
-
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.
CONFORMING TO
SUSv2, POSIX.1-2001.
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
Note for SMP systems
The
CLOCK_PROCESS_CPUTIME_ID
and
CLOCK_THREAD_CPUTIME_ID
clocks are realized on many platforms using timers 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 only be useful 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.
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),
adjtimex(2),
gettimeofday(2),
settimeofday(2),
time(2),
clock_getcpuclockid(3),
ctime(3),
ftime(3),
pthread_getcpuclockid(3),
sysconf(3),
time(7)
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- AVAILABILITY
-
- NOTES
-
- Note for SMP systems
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:34:56 GMT, March 26, 2013