The funlockfile function unlocks a stdio stream, causing the
    thread that had been holding the lock to relinquish exclusive use
    of the stream.

    File pointers passed are assumed to be valid; flockfile will
    perform locking even on invalid file pointers. Also, the
    funlockfile function will not fail if the calling thread does
    not own a lock on the file pointer passed.

    Matching flockfile and funlockfile calls can be nested. If the
    stream has been locked recursively, it will remain locked until
    the last matching funlockfile is called.

    All C RTL file-pointer I/O functions lock their file pointers as
    if calling flockfile and funlockfile.

    See also flockfile and ftrylockfile.