nanosleep - high resolution sleep
The nanosleep() function shall cause the current thread to be suspended from execution until either the time interval specified by the rqtp argument has elapsed or a signal is delivered to the calling thread, and its action is to invoke a signal-catching function or to terminate the process. The suspension time may be longer than requested because the argument value is rounded up to an integer multiple of the sleep resolution or because of the scheduling of other activity by the system. But, except for the case of being interrupted by a signal, the suspension time shall not be less than the time specified by rqtp, as measured by the system clock CLOCK_REALTIME.
The use of the nanosleep() function has no effect on the action or blockage of any signal.
If the nanosleep() function returns because the requested time has elapsed, its return value shall be zero.
If the nanosleep() function returns because it has been interrupted by a signal, it shall return a value of -1 and set errno to indicate the interruption. If the rmtp argument is non-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). The rqtp and rmtp arguments can point to the same object. If the rmtp argument is NULL, the remaining time is not returned.
If nanosleep() fails, it shall return a value of -1 and set errno to indicate the error.
The nanosleep() function shall fail if:
- The nanosleep() function was interrupted by a signal.
- The rqtp argument specified a nanosecond value less than zero or greater than or equal to 1000 million.
It is common to suspend execution of a thread for an interval in order to poll the status of a non-interrupting function. A large number of actual needs can be met with a simple extension to sleep() that provides finer resolution.
In the POSIX.1-1990 standard and SVR4, it is possible to implement such a routine, but the frequency of wakeup is limited by the resolution of the alarm() and sleep() functions. In 4.3 BSD, it is possible to write such a routine using no static storage and reserving no system facilities. Although it is possible to write a function with similar functionality to sleep() using the remainder of the timer_*() functions, such a function requires the use of signals and the reservation of some signal number. This volume of POSIX.1-2017 requires that nanosleep() be non-intrusive of the signals function.
The nanosleep() function shall return a value of 0 on success and -1 on failure or if interrupted. This latter case is different from sleep(). This was done because the remaining time is returned via an argument structure pointer, rmtp, instead of as the return value.
First released in Issue 5. Included for alignment with the POSIX Realtime Extension.
The nanosleep() function is marked as part of the Timers option.
The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Timers option.
IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/37 is applied, updating the SEE ALSO section to include the clock_nanosleep() function.
IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/63 is applied, correcting text in the RATIONALE section.
SD5-XBD-ERN-33 is applied.
The nanosleep() function is moved from the Timers option to the Base.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0231  is applied.
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