This routine obtains and returns the value of the "concurrency
    level" global setting for the calling thread's process. Because
    the Threads Library automatically manages the concurrency of all
    threads in a multithreaded process, it ignores this concurrency
    level value.

    The concurrency level value has no effect on the behavior of a
    multithreaded program that uses the Threads Library. This routine
    is provided for Single UNIX Specification, Version 2, source code
    compatibility and has no other effect when called.

    The initial concurrency level is zero (0), indicating that the
    Threads Library controls the concurrency level.

    The concurrency level can be set using the pthread_
    setconcurrency() routine.