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.