This routine notifies the thread scheduler that the current
    thread is willing to release its processor to other threads of
    equivalent or greater scheduling precedence. (A thread generally
    will release its processor to a thread of a greater scheduling
    precedence without calling this routine.) If no other threads of
    equivalent or greater scheduling precedence are ready to execute,
    the thread continues.

    This routine can allow knowledge of the details of an application
    to be used to improve its performance. If a thread does not call
    pthread_yield_np(), other threads may be given the opportunity
    to run at arbitrary points (possibly even when the interrupted
    thread holds a required resource). By making strategic calls to
    pthread_yield_np(), other threads can be given the opportunity
    to run when the resources are free. This improves performance by
    reducing contention for the resource.

    As a general guideline, consider calling this routine after a
    thread has released a resource (such as a mutex) which is heavily
    contended for by other threads. This can be especially important
    either if the program is running on a uniprocessor machine, or
    if the thread acquires and releases the resource inside a tight
    loop.

    Use this routine carefully and sparingly, because misuse can
    cause unnecessary context switching which will increase overhead
    and actually degrade performance. For example, it is counter-
    productive for a thread to yield while it holds a resource that
    the threads to which it is yielding will need. Likewise, it is
    pointless to yield unless there is likely to be another thread
    that is ready to run.

                                   NOTE

       pthread_yield_np() is equivalent to sched_yield(). Use
       sched_yield() since it is part of the standard portable
       POSIX Threads Library.