#include <time.h> int clock_nanosleep(clockid_t clock_id, int flags, const struct timespec *rqtp, struct timespec *rmtp);
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.
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.
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 POSIX.1-2008 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 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.
The Base Definitions volume of POSIX.1-2008, <time.h>
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