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NAME
clock_getres, clock_gettime, clock_settime — clock and timer functions
SYNOPSIS
#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);
DESCRIPTION
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 clock
measuring real time 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.
Appropriate privileges to set a particular clock are 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).
RETURN VALUE
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.
ERRORS
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_gettime() function shall fail if:
EOVERFLOW
The number of seconds will not fit in an object of type time_t.
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 appropriate privileges to set the specified
clock.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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().
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
Scheduling Policies, clock_getcpuclockid(), clock_nanosleep(), ctime(), mq_receive(),
mq_send(), nanosleep(), pthread_mutex_timedlock(), sem_timedwait(), time(),
timer_create(), timer_getoverrun()
The Base Definitions volume of POSIX.1‐2017, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .