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 .