finds the resolution (precision) of the specified clock
is non-NULL, stores it in the struct timespec pointed to by
The resolution of clocks depends on the implementation and cannot be
configured by a particular process.
If the time value pointed to by the argument
is not a multiple of
then it is truncated to a multiple of
retrieve and set the time of the specified clock
structures, as specified in
argument is the identifier of the particular clock on which to act.
A clock may be system-wide and hence visible for all processes, or
per-process if it measures time only within a single process.
All implementations support the system-wide real-time clock,
which is identified by
Its time represents seconds and nanoseconds since the Epoch.
When its time is changed, timers for a relative interval are
unaffected, but timers for an absolute point in time are affected.
More clocks may be implemented.
The interpretation of the
corresponding time values and the effect on timers is unspecified.
Sufficiently recent versions of glibc and the Linux kernel
support the following clocks:
System-wide real-time clock.
Setting this clock requires appropriate privileges.
Clock that cannot be set and represents monotonic time since
some unspecified starting point.
CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
but provides access to a raw hardware-based time
that is not subject to NTP adjustments.
High-resolution per-process timer from the CPU.
Thread-specific CPU-time clock.
return 0 for success, or -1 for failure (in which case
is set appropriately).
points outside the accessible address space.
specified is not supported on this system.
does not have permission to set the clock indicated.
On POSIX systems on which these functions are available, the symbol
is defined in <unistd.h> to a value greater than 0.
Note for SMP systems
clocks are realized on many platforms using timers from the CPUs
(TSC on i386, AR.ITC on Itanium).
These registers may differ between CPUs and as a consequence
these clocks may return
if a process is migrated to another CPU.
If the CPUs in an SMP system have different clock sources then
there is no way to maintain a correlation between the timer registers since
each CPU will run at a slightly different frequency.
If that is the case then
to signify this condition.
The two clocks will then only be useful if it
can be ensured that a process stays on a certain CPU.
The processors in an SMP system do not start all at exactly the same
time and therefore the timer registers are typically running at an offset.
Some architectures include code that attempts to limit these offsets on bootup.
However, the code cannot guarantee to accurately tune the offsets.
Glibc contains no provisions to deal with these offsets (unlike the Linux
Typically these offsets are small and therefore the effects may be
negligible in most cases.
According to POSIX.1-2001, a process with "appropriate privileges" may set the
On Linux, these clocks are not settable
(i.e., no process has "appropriate privileges").