The reentrant version of the getchar function is locked against
    multiple threads calling it simultaneously. This incurs overhead
    to ensure integrity of the input stream. The unlocked version of
    this call, getchar_unlocked can be used to avoid the overhead.
    The getchar_unlocked function is functionally identical to
    the getchar function, except that it is not required to be
    implemented in a thread-safe manner. The getchar_unlocked
    function can be safely used only within a scope that is protected
    by the flockfile and funlockfile functions used as a pair. The
    caller must ensure that the stream is locked before getchar_
    unlocked is used.

    See also flockfile, ftrylockfile, and funlockfile.