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NAME
clock_getres, clock_gettime, clock_settime - clock and timer functions (REALTIME)
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 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 ).
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_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.
EXAMPLES
None.
APPLICATION USAGE
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().
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
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>
COPYRIGHT
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 .
IEEE/The Open Group 2003 CLOCK_GETRES(P)