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). 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.
It is common to suspend execution of a process 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
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 IEEE Std 1003.1-2001 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.
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