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High resolution sleep with specifiable clock


#include <time.h>

int clock_nanosleep( clockid_t clock_id,
                     int flags,
                     const struct timespec * rqtp,
                     struct timespec * rmtp );


The ID of the clock to use to measure the time. The possible clock types are:
  • CLOCK_REALTIME — the standard POSIX-defined clock. Timers based on this clock wake up the processor if it's in a power-saving mode.
  • CLOCK_SOFTTIME — this clock is active only when the processor isn't in a power-saving mode. For example, an application using a CLOCK_SOFTTIME timer to sleep wouldn't wake up the processor when the application was due to wake up. This will allow the processor to enter a power-saving mode.

    While the processor isn't in a power-saving mode, CLOCK_SOFTTIME behaves the same as CLOCK_REALTIME.

  • CLOCK_MONOTONIC — this clock always increases at a constant rate and can't be adjusted.

The clock_nanosleep() function fails if the clock_id argument refers to the CPU-time clock of the calling thread.

Flags that specify when the current thread is to be suspended from execution:
  • when the time interval specified by the rqtp argument has elapsed (TIMER_ABSTIME is not set).
  • when the time value of the clock specified by clock_id reaches the absolute time specified by the rqtp argument (TIMER_ABSTIME is set).

    If, at the time of the call, the time value specified by rqtp is less than or equal to the time value of the specified clock, then clock_nanosleep() returns immediately, and the calling process isn't suspended.

  • when a signal is delivered to the calling thread, and the signal's action is to invoke a signal-catching function or terminate the process.

Calling clock_nanosleep() with TIMER_ABSTIME not set, and clock_id set to CLOCK_REALTIME is the equivalent to calling nanosleep() with the same rqtp and rmtp arguments.

A pointer to a timespec structure that specifies the time interval between the requested time and the time actually slept.
NULL, or a pointer to a timespec in which the function can store the amount of time remaining in an interval.

For the relative clock_nanosleep() function, if rmtp isn't NULL, the timespec structure referenced by it is updated to contain the amount of time remaining in the interval (the requested time minus the time actually slept). If it's NULL, the remaining time isn't returned.

The absolute clock_nanosleep() function has no effect on the structure referenced by rmtp.



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


The clock_nanosleep() function suspends the current thread from execution until:

The nanosleep() function always uses CLOCK_REALTIME.

The suspension time may be longer than requested because the argument value is rounded up to an integer multiple of the sleep resolution (see the Tick, Tock: Understanding the Neutrino Microkernel's Concept of Time chapter of the QNX Neutrino Programmer's Guide) or because of scheduling and other system activity. Except for the case of being interrupted by a signal, the suspension time for:

Using the clock_nanosleep() function has no effect on the action or blockage of any signal.


Zero if the requested time has elapsed, or a corresponding error value if clock_nanosleep() has been interrupted by a signal, or fails.


The call was interrupted by a signal.
The rqtp argument specified a nanosecond value less than zero or greater than or equal to 1000 million; or TIMER_ABSTIME is specified in flags and the rqtp argument is outside the range for the clock specified by clock_id; or the clock_id argument doesn't specify a known clock, or specifies the CPU-time clock of the calling thread.
The clock_id argument specifies a clock for which clock_nanosleep() isn't supported, such as a CPU-time clock.


POSIX 1003.1 CS

Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes

See also:

clock_settime(), nanosleep(), sleep(), timespec

Clocks, Timers, and Getting a Kick Every So Often chapter of Getting Started with QNX Neutrino

Tick, Tock: Understanding the Neutrino Microkernel's Concept of Time chapter of the QNX Neutrino Programmer's Guide