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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CLOCK_GETRES(3P) POSIX Programmer's Manual CLOCK_GETRES(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
clock_getres, clock_gettime, clock_settime — clock and timer
functions
#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
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).
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_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.
None.
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.
Scheduling Policies, clock_getcpuclockid(3p), clock_nanosleep(3p),
ctime(3p), mq_receive(3p), mq_send(3p), nanosleep(3p),
pthread_mutex_timedlock(3p), sem_timedwait(3p), time(3p),
timer_create(3p), timer_getoverrun(3p)
The Base Definitions volume of POSIX.1‐2017, time.h(0p)
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 .
IEEE/The Open Group 2017 CLOCK_GETRES(3P)
Pages that refer to this page: time.h(0p), clock(3p), clock_getcpuclockid(3p), clock_nanosleep(3p), clock_settime(3p), gettimeofday(3p), posix_trace_create(3p), pthread_getcpuclockid(3p), time(3p), timer_create(3p), timer_getoverrun(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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