The reentrant version of the clearerr function is locked against
    multiple threads calling it simultaneously. This incurs overhead
    to ensure integrity of the stream. The unlocked version of this
    call, clearerr_unlocked can be used to avoid the overhead. The
    clearerr_unlocked macro is functionally identical to the clearerr
    macro, except that it is not required to be implemented in a
    thread-safe manner. The clearerr_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 clearerr_unlocked is used.

    See also flockfile, ftrylockfile, and funlockfile.