get_string
    type:       character string
    access:     write only
    mechanism:  by descriptor

    Buffer to receive the line read from SYS$INPUT. The string is
    returned by a call to STR$COPY_DX.

 prompt_string
    type:       character string
    access:     read only
    mechanism:  by descriptor

    Prompt message that is displayed on the controlling terminal. A
    valid prompt consists of text followed by a colon (:), a space,
    and no carriage-return/line-feed combination. The maximum size
    of the prompt message is 255 characters. If the controlling input
    device is not a terminal, this argument is ignored.

 output_length
    type:       word
    access:     write only
    mechanism:  by reference

    Word to receive the actual length of the GET-STRING line, not
    counting any padding in the case of a fixed string. If the input
    line was truncated, this length reflects the truncated string.

 user_argument
    type:       _UNSPECIFIED
    access:     read only
    mechanism:  by reference

    User-specified value that was passed to the routine that called
    this action routine.