CLOCK_NANOSLEEP(3P) POSIX Programmer's Manual CLOCK_NANOSLEEP(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
clock_nanosleep - high resolution sleep with specifiable clock (ADVANCED REALTIME)
int clock_nanosleep(clockid_t clock_id, int flags,
const struct timespec *rqtp, struct timespec *rmtp);
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 by clock_id.
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 sig-
nal, 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 con-
tain 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 value.
The clock_nanosleep() function shall fail if:
EINTR The clock_nanosleep() function was interrupted by a signal.
EINVAL 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 following sections are informative.
Calling clock_nanosleep() with the value TIMER_ABSTIME not set in the flags argument and with a clock_id of CLOCK_REALTIME is equivalent to
calling nanosleep() with the same rqtp and rmtp arguments.
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 required.
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 relative nanosleep().
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 perform-
ing 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() func-
tion, 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
clock_getres(), nanosleep(), pthread_cond_timedwait(), sleep(), the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
ogy -- 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 .
/The Open Group 2003 CLOCK_NANOSLEEP(3P)