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) ; |
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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 structs, 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 realtime
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 realtime clock. Setting this clock requires appropriate privileges.
CLOCK_MONOTONIC
Clock that cannot be set and represents monotonic time since some unspecified starting point.
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).
tp
points
outside the accessible address space.
The clk_id
specified is not supported on this system.
clock_settime
() does
not have permission to set the clock indicated.
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).)
Most systems require the program be linked with the librt library to use these functions.
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.
date(1), adjtimex(2), gettimeofday(2), settimeofday(2), time(2), ctime(3), ftime(3), sysconf(3)