Description

   This routine changes both the current scheduling policy and
   associated scheduling parameters of the thread specified by
   thread to the policy and associated parameters provided in policy
   and param, respectively.

   All currently implemented scheduling policies have one scheduling
   parameter called sched_priority. For the policy you choose, you
   must specify an appropriate value in the sched_priority field of
   the sched_param structure.

   Changing the scheduling policy or priority, or both, of a thread
   can cause it either to start executing or to be preempted by
   another thread. A thread changes its own scheduling policy and
   priority by using the handle returned by the pthread_self()
   routine.

   This routine differs from pthread_attr_setschedpolicy() and
   pthread_attr_setschedparam(), in that those routines set the
   scheduling policy and parameter attributes that are used to
   establish the scheduling priority and scheduling policy of a
   new thread when it is created. However, this routine changes the
   scheduling policy and parameters of an existing thread.