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

    See also flockfile, ftrylockfile, and funlockfile.