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#### 21.1.1.2 Input, Output, and Bidirectional Streams

A stream, whether a character stream or a binary stream, can be an input stream (source of data), an output stream (sink for data), both, or (e.g., when “`:direction :probe`” is given to open) neither.

Figure 21–2 shows operators relating to input streams.

      clear-input  read-byte            read-from-string
      listen       read-char            read-line
      peek-char    read-char-no-hang    read-preserving-whitespace
      read         read-delimited-list  unread-char

            Figure 21–2: Operators relating to Input Streams.

Figure 21–3 shows operators relating to output streams.

      clear-output   prin1            write
      finish-output  prin1-to-string  write-byte
      force-output   princ            write-char
      format         princ-to-string  write-line
      fresh-line     print            write-string
      pprint         terpri           write-to-string

      Figure 21–3: Operators relating to Output Streams.

A stream that is both an input stream and an output stream is called a bidirectional stream . See the functions input-stream-p and output-stream-p.

Any of the operators listed in Figure~21–2 or Figure~21–3 can be used with bidirectional streams. In addition, Figure 21–4 shows a list of operators that relate specificaly to bidirectional streams.

      y-or-n-p  yes-or-no-p

      Figure 21–4: Operators relating to Bidirectional Streams.