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A soft-port is a port based on a vector of procedures capable of accepting or delivering characters. It allows emulation of I/O ports.

— Scheme Procedure: make-soft-port pv modes
— C Function: scm_make_soft_port (pv, modes)

Return a port capable of receiving or delivering characters as specified by the modes string (see open-file). pv must be a vector of length 5 or 6. Its components are as follows:

  1. procedure accepting one character for output
  2. procedure accepting a string for output
  3. thunk for flushing output
  4. thunk for getting one character
  5. thunk for closing port (not by garbage collection)
  6. (if present and not #f) thunk for computing the number of characters that can be read from the port without blocking.

For an output-only port only elements 0, 1, 2, and 4 need be procedures. For an input-only port only elements 3 and 4 need be procedures. Thunks 2 and 4 can instead be #f if there is no useful operation for them to perform.

If thunk 3 returns #f or an eof-object (see eof-object?) it indicates that the port has reached end-of-file. For example:

          (define stdout (current-output-port))
          (define p (make-soft-port
                      (lambda (c) (write c stdout))
                      (lambda (s) (display s stdout))
                      (lambda () (display "." stdout))
                      (lambda () (char-upcase (read-char)))
                      (lambda () (display "@" stdout)))
          (write p p) ⇒ #<input-output: soft 8081e20>