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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.
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 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.
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.
SUSv2, POSIX.1-2001.
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).)
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.
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), adjtimex(2), gettimeofday(2), settimeofday(2), time(2),
clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
sysconf(3), time(7)
This page is part of release 3.32 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/.
2010-02-03 CLOCK_GETRES(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface