Originální popis anglicky:
clock_getres, clock_gettime, clock_settime - clock and timer functions (
REALTIME)
Návod, kniha: POSIX Programmer's Manual
#include <time.h>
int clock_getres(clockid_t
clock_id, struct timespec
* res);
int clock_gettime(clockid_t
clock_id, struct
timespec *tp);
int clock_settime(clockid_t
clock_id, const struct
timespec *tp);
The
clock_getres() function shall return the resolution of any clock.
Clock resolutions are implementation-defined and cannot be set by a process.
If the argument
res is not NULL, the resolution of the specified clock
shall be stored in the location pointed to by
res. If
res is
NULL, the clock resolution is not returned. If the
time argument of
clock_settime() is not a multiple of
res, then the value is
truncated to a multiple of
res.
The
clock_gettime() function shall return the current value
tp for
the specified clock,
clock_id.
The
clock_settime() function shall set the specified clock,
clock_id, to the value specified by
tp. Time values that are
between two consecutive non-negative integer multiples of the resolution of
the specified clock shall be truncated down to the smaller multiple of the
resolution.
A clock may be system-wide (that is, visible to all processes) or per-process
(measuring time that is meaningful only within a process). All implementations
shall support a
clock_id of CLOCK_REALTIME as defined in
<time.h>. This clock represents the realtime clock for the
system. For this clock, the values returned by
clock_gettime() and
specified by
clock_settime() represent the amount of time (in seconds
and nanoseconds) since the Epoch. An implementation may also support
additional clocks. The interpretation of time values for these clocks is
unspecified.
If the value of the CLOCK_REALTIME clock is set via
clock_settime(), the
new value of the clock shall be used to determine the time of expiration for
absolute time services based upon the CLOCK_REALTIME clock. This applies to
the time at which armed absolute timers expire. If the absolute time requested
at the invocation of such a time service is before the new value of the clock,
the time service shall expire immediately as if the clock had reached the
requested time normally.
Setting the value of the CLOCK_REALTIME clock via
clock_settime() shall
have no effect on threads that are blocked waiting for a relative time service
based upon this clock, including the
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.
If the Monotonic Clock option is supported, all implementations shall support a
clock_id of CLOCK_MONOTONIC defined in
<time.h>. This
clock represents the monotonic clock for the system. For this clock, the value
returned by
clock_gettime() represents the amount of time (in seconds
and nanoseconds) since an unspecified point in the past (for example, system
start-up time, or the Epoch). This point does not change after system start-up
time. The value of the CLOCK_MONOTONIC clock cannot be set via
clock_settime(). This function shall fail if it is invoked with a
clock_id argument of CLOCK_MONOTONIC.
The effect of setting a clock via
clock_settime() on armed per-process
timers associated with a clock other than CLOCK_REALTIME is
implementation-defined.
If the value of the CLOCK_REALTIME clock is set via
clock_settime(), the
new value of the clock shall be used to determine the time at which the system
shall awaken a thread blocked on an absolute
clock_nanosleep() call
based upon the CLOCK_REALTIME clock. If the absolute time requested at the
invocation of such a time service is before the new value of the clock, the
call shall return immediately as if the clock had reached the requested time
normally.
Setting the value of the CLOCK_REALTIME clock via
clock_settime() shall
have no effect on any thread that is blocked on a relative
clock_nanosleep() call. Consequently, the call shall return when the
requested relative interval elapses, independently of the new or old value of
the clock.
The appropriate privilege to set a particular clock is implementation-defined.
If _POSIX_CPUTIME is defined, implementations shall support clock ID values
obtained by invoking
clock_getcpuclockid(), which represent the
CPU-time clock of a given process. Implementations shall also support the
special
clockid_t value CLOCK_PROCESS_CPUTIME_ID, which represents the
CPU-time clock of the calling process when invoking one of the
clock_*() or
timer_*() functions. For these clock IDs, the
values returned by
clock_gettime() and specified by
clock_settime() represent the amount of execution time of the process
associated with the clock. Changing the value of a CPU-time clock via
clock_settime() shall have no effect on the behavior of the sporadic
server scheduling policy (see
Scheduling Policies ).
If _POSIX_THREAD_CPUTIME is defined, implementations shall support clock ID
values obtained by invoking
pthread_getcpuclockid(), which represent
the CPU-time clock of a given thread. Implementations shall also support the
special
clockid_t value CLOCK_THREAD_CPUTIME_ID, which represents the
CPU-time clock of the calling thread when invoking one of the
clock_*()
or
timer_*() functions. For these clock IDs, the values returned by
clock_gettime() and specified by
clock_settime() shall represent
the amount of execution time of the thread associated with the clock. Changing
the value of a CPU-time clock via
clock_settime() shall have no effect
on the behavior of the sporadic server scheduling policy (see
Scheduling
Policies ).
A return value of 0 shall indicate that the call succeeded. A return value of -1
shall indicate that an error occurred, and
errno shall be set to
indicate the error.
The
clock_getres(),
clock_gettime(), and
clock_settime()
functions shall fail if:
- EINVAL
- The clock_id argument does not specify a known
clock.
The
clock_settime() function shall fail if:
- EINVAL
- The tp argument to clock_settime() is outside
the range for the given clock ID.
- EINVAL
- The tp argument specified a nanosecond value less
than zero or greater than or equal to 1000 million.
- EINVAL
- The value of the clock_id argument is
CLOCK_MONOTONIC.
The
clock_settime() function may fail if:
- EPERM
- The requesting process does not have the appropriate
privilege to set the specified clock.
The following sections are informative.
None.
These functions are part of the Timers option and need not be available on all
implementations.
Note that the absolute value of the monotonic clock is meaningless (because its
origin is arbitrary), and thus there is no need to set it. Furthermore,
realtime applications can rely on the fact that the value of this clock is
never set and, therefore, that time intervals measured with this clock will
not be affected by calls to
clock_settime().
None.
None.
clock_getcpuclockid() ,
clock_nanosleep() ,
ctime() ,
mq_timedreceive() ,
mq_timedsend() ,
nanosleep() ,
pthread_mutex_timedlock() ,
sem_timedwait() ,
time() ,
timer_create() ,
timer_getoverrun() , the Base Definitions
volume of IEEE Std 1003.1-2001,
<time.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
