Prompt String
     QP_stream:      char *prompt

A prompt string is a character string that will be written out when a tty stream (a stream with QP_DELIM_TTY format) reads input at the beginning of a line. The prompt string is stored in this field. This field is only used for an input tty stream. The prompt string will not be written out if there are no output tty streams connected to the same tty as the input tty stream (see fli-ios-tty). A prompt string of a stream may be changed after the stream is created through prompt/[2,3] or by assigning a new character string to this field directly.