The flockfile function locks a stdio stream so that a thread can
    have exclusive use of that stream for multiple I/O operations.
    Use the flockfile function for a thread that wants to ensure
    that the output of several printf functions, for example, is not
    garbled by another thread also trying to use printf.

    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 ftrylockfile and funlockfile.