Arguments

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.