If the flag TIMER_ABSTIME is not set in the flags argument, the
clock_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 the process
is terminated. The clock used to measure the time shall be the clock specified
If the flag TIMER_ABSTIME is set in the flags argument, the
clock_nanosleep() function shall cause the current thread to be
suspended from execution until either the time value of the clock specified by
clock_id reaches the absolute time specified by the rqtp
argument, or a signal is delivered to the calling thread and its action is to
invoke a signal-catching function, or the process is terminated. 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()
shall return immediately and the calling process shall not be suspended.
The suspension time caused by this function 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 for
the relative clock_nanosleep() function (that is, with the
TIMER_ABSTIME flag not set) shall not be less than the time interval specified
by rqtp, as measured by the corresponding clock. The suspension for the
absolute clock_nanosleep() function (that is, with the TIMER_ABSTIME
flag set) shall be in effect at least until the value of the corresponding
clock reaches the absolute time specified by rqtp, except for the case
of being interrupted by a signal.
The use of the clock_nanosleep() function shall have no effect on the
action or blockage of any signal.
The clock_nanosleep() function shall fail if the clock_id argument
refers to the CPU-time clock of the calling thread. It is unspecified whether
clock_id values of other CPU-time clocks are allowed.
If the clock_nanosleep() function returns because the requested time has
elapsed, its return value shall be zero.
If the clock_nanosleep() function returns because it has been interrupted
by a signal, it shall return the corresponding error value. For the relative
clock_nanosleep() function, if the rmtp argument is non-NULL,
the timespec structure referenced by it shall be 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. The absolute clock_nanosleep() function has no effect on
the structure referenced by rmtp.
If clock_nanosleep() fails, it shall return the corresponding error
The clock_nanosleep() function was interrupted by a
The rqtp argument specified a nanosecond value less
than zero or greater than or equal to 1000 million; or the TIMER_ABSTIME
flag was specified in flags and the rqtp argument is outside the
range for the clock specified by clock_id; or the clock_id
argument does not 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() is not supported, such as a CPU-time clock.
The nanosleep() function specifies that the system-wide clock
CLOCK_REALTIME is used to measure the elapsed time for this time service.
However, with the introduction of the monotonic clock CLOCK_MONOTONIC a new
relative sleep function is needed to allow an application to take advantage of
the special characteristics of this clock.
There are many applications in which a process needs to be suspended and then
activated multiple times in a periodic way; for example, to poll the status of
a non-interrupting device or to refresh a display device. For these cases, it
is known that precise periodic activation cannot be achieved with a relative
sleep() or nanosleep() function call. Suppose, for example, a
periodic process that is activated at time T0, executes for a while,
and then wants to suspend itself until time T0+ T, the period
being T. If this process wants to use the nanosleep() function,
it must first call clock_gettime() to get the current time, then
calculate the difference between the current time and T0+ T and,
finally, call nanosleep() using the computed interval. However, the
process could be preempted by a different process between the two function
calls, and in this case the interval computed would be wrong; the process
would wake up later than desired. This problem would not occur with the
absolute clock_nanosleep() function, since only one function call would
be necessary to suspend the process until the desired time. In other cases,
however, a relative sleep is needed, and that is why both functionalities are
Although it is possible to implement periodic processes using the timers
interface, this implementation would require the use of signals, and the
reservation of some signal numbers. In this regard, the reasons for including
an absolute version of the clock_nanosleep() function in
IEEE Std 1003.1-2001 are the same as for the inclusion of the
It is also possible to implement precise periodic processes using
pthread_cond_timedwait(), in which an absolute timeout is specified
that takes effect if the condition variable involved is never signaled.
However, the use of this interface is unnatural, and involves performing other
operations on mutexes and condition variables that imply an unnecessary
overhead. Furthermore, pthread_cond_timedwait() is not available in
implementations that do not support threads.
Although the interface of the relative and absolute versions of the new high
resolution sleep service is the same clock_nanosleep() function, the
rmtp argument is only used in the relative sleep. This argument is
needed in the relative clock_nanosleep() function to reissue the
function call if it is interrupted by a signal, but it is not needed in the
absolute clock_nanosleep() function call; if the call is interrupted by
a signal, the absolute clock_nanosleep() function can be invoked again
with the same rqtp argument used in the interrupted call.
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