Description

   This routine sets the scheduling parameters associated with
   the scheduling policy attribute of the thread attributes object
   specified by the attr argument.
   Scheduling Priority
   Use the sched_priority field of a sched_param structure to set
   a thread's execution priority. The effect of the scheduling
   priority you assign depends on the scheduling policy specified
   for the attributes object specified by the attr argument.

   By default, a created thread inherits the priority of the thread
   calling pthread_create(). To specify a priority using this
   routine, scheduling inheritance must be disabled at the time the
   thread is created. Before calling pthread_create(), call pthread_
   attr_setinheritsched() and specify the value PTHREAD_EXPLICIT_
   SCHED for the inherit argument.

   An application specifies priority only to express the urgency
   of executing the thread relative to other threads. Do not use
   priority to control mutual exclusion when you are accessing
   shared data. With a sufficient number of processors present, all
   ready threads, regardless of priority, execute simultaneously.
   Even on a uniprocessor, a lower priority thread could either
   execute before or be interleaved with a higher priority thread,
   for example due to page fault behavior. See <REFERENCE>(intro_
   threads_chap) and <REFERENCE>(threads_concepts_chap) for more
   information.

   Valid values of the sched_priority scheduling parameter depend on
   the chosen scheduling policy. Use the POSIX routines sched_get_
   priority_min() or sched_get_priority_max()  to determine the low
   and high limits of each policy.

   Additionally, the Threads Library provides nonportable priority
   range constants, as follows:

   Policy          Low            High

   SCHED_FIFO      PRI_FIFO_MIN   PRI_FIFO_MAX
   SCHED_RR        PRI_RR_MIN     PRI_RR_MAX
   SCHED_OTHER     PRI_OTHER_MIN  PRI_OTHER_MAX
   SCHED_FG_NP     PRI_FG_MIN_NP  PRI_FG_MAX_NP
   SCHED_BG_NP     PRI_BG_MIN_NP  PRI_BG_MAX_NP

   The default priority varies by platform. On Tru64 UNIX, the
   default is 19 (that is, the POSIX priority of a normal timeshare
   process). On other platforms, the default priority is the
   midpoint between PRI_FG_MIN_NP and PRI_FG_MAX_NP.