ClockPeriod(), ClockPeriod_r()

Get or set a clock period


#include <sys/neutrino.h>

int ClockPeriod( clockid_t id,
                 const struct _clockperiod * new,
                 struct _clockperiod * old,
                 int reserved );

int ClockPeriod_r( clockid_t id,
                   const struct _clockperiod * new,
                   struct _clockperiod * old,
                   int reserved );


The clock ID of the clock; one of the following:
  • CLOCK_REALTIME — the clock that maintains the system time.
  • CLOCK_HARMONIC — the clock that maintains the harmonic timer boundary. Timers whose remaining time before expiry is greater than this boundary are considered for timer harmonization; for more information, see "Clocks, timers, and power management" in the Tick, Tock: Understanding the Neutrino Microkernel's Concept of Time chapter of the QNX Neutrino Programmer's Guide.
NULL, or a pointer to a _clockperiod structure that contains the period to set the clock to. This structure contains at least the following members:
  • unsigned long nsec—the period of the clock, in nanoseconds.
    Note: For CLOCK_HARMONIC, this period is in seconds, not nanoseconds.
  • long fract—reserved for future fractional nanoseconds. Set this member to zero.
NULL, or a pointer to a _clockperiod structure where the function can store the current period (before being changed by a non-NULL new).
Set this argument to 0.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


You can use the ClockPeriod() and ClockPeriod_r() kernel calls to get or set the clock period of the clock. These functions are identical except in the way they indicate errors. See the Returns section for details.

  • If you want to get the clock period, consider calling clock_getres() instead of using these kernel calls directly.
  • In order to set the clock period, your process must have the PROCMGR_AID_CLOCKPERIOD ability enabled. For more information, see procmgr_ability().
  • If you're using adaptive partitioning, and you change the tick size of the system at runtime, do so before defining the adaptive partitioning scheduler's window size. That's because Neutrino converts the window size from milliseconds to clock ticks for internal use. For more information, see the Adaptive Partitioning User's Guide.

All the timer_*() calls operate with an accuracy no better than the clock period. Every moment within the Neutrino microkernel is referred to as a tick. A tick's initial length is determined by the clock rate of your processor:

CPU clock speed: Default value:
≥ 40MHz 1 millisecond
< 40MHz 10 milliseconds

Since a very small tick size imposes an interrupt load on the system, and can consume all available processor cycles, the kernel call limits how small a period can be specified. The lowest clock period that can currently be set on any machine is 10 microseconds.

If an attempt is made to set a value that the kernel believes to be unsafe, the call fails with an EINVAL. The timeslice rate (for "round-robin" and "other" scheduling policies) is always four times the clock period (this isn't changeable).

Blocking states

These calls don't block.


The only difference between these functions is the way they indicate errors:

If an error occurs, this function returns -1 is and sets errno. Any other value returned indicates success.
EOK is returned on success. This function does NOT set errno. If an error occurs, the function can return any value in the Errors section.


A fault occurred when the kernel tried to access the buffers provided.
Invalid clock ID. A period was set which wasn't in a range considered safe.
The process tried to change the period of the clock without having the required permission; see procmgr_ability().


QNX Neutrino

Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes