Originální popis anglicky:
clock_getres, clock_gettime, clock_settime - clock and time functions
Návod, kniha: Linux Programmer's Manual
#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);
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 GNU libc 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).
- 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.
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.
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).)
SUSv2, POSIX 1003.1-2001.
date(1),
adjtimex(2),
gettimeofday(2),
settimeofday(2),
time(2),
ctime(3),
ftime(3),
sysconf(3)
