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6.14.10.8 Binary Input

R6RS binary input ports can be created with the procedures described below.

Scheme Procedure: open-bytevector-input-port bv [transcoder]
C Function: scm_open_bytevector_input_port (bv, transcoder)

Return an input port whose contents are drawn from bytevector bv (see section Bytevectors).

The transcoder argument is currently not supported.

Scheme Procedure: make-custom-binary-input-port id read! get-position set-position! close
C Function: scm_make_custom_binary_input_port (id, read!, get-position, set-position!, close)

Return a new custom binary input port(13) named id (a string) whose input is drained by invoking read! and passing it a bytevector, an index where bytes should be written, and the number of bytes to read. The read! procedure must return an integer indicating the number of bytes read, or 0 to indicate the end-of-file.

Optionally, if get-position is not #f, it must be a thunk that will be called when port-position is invoked on the custom binary port and should return an integer indicating the position within the underlying data stream; if get-position was not supplied, the returned port does not support port-position.

Likewise, if set-position! is not #f, it should be a one-argument procedure. When set-port-position! is invoked on the custom binary input port, set-position! is passed an integer indicating the position of the next byte is to read.

Finally, if close is not #f, it must be a thunk. It is invoked when the custom binary input port is closed.

Using a custom binary input port, the open-bytevector-input-port procedure could be implemented as follows:

(define (open-bytevector-input-port source)
  (define position 0)
  (define length (bytevector-length source))

  (define (read! bv start count)
    (let ((count (min count (- length position))))
      (bytevector-copy! source position
                        bv start count)
      (set! position (+ position count))
      count))

  (define (get-position) position)

  (define (set-position! new-position)
    (set! position new-position))

  (make-custom-binary-input-port "the port" read!
                                  get-position
                                  set-position!))

(read (open-bytevector-input-port (string->utf8 "hello")))
⇒ hello

Binary input is achieved using the procedures below:

Scheme Procedure: get-u8 port
C Function: scm_get_u8 (port)

Return an octet read from port, a binary input port, blocking as necessary, or the end-of-file object.

Scheme Procedure: lookahead-u8 port
C Function: scm_lookahead_u8 (port)

Like get-u8 but does not update port’s position to point past the octet.

Scheme Procedure: get-bytevector-n port count
C Function: scm_get_bytevector_n (port, count)

Read count octets from port, blocking as necessary and return a bytevector containing the octets read. If fewer bytes are available, a bytevector smaller than count is returned.

Scheme Procedure: get-bytevector-n! port bv start count
C Function: scm_get_bytevector_n_x (port, bv, start, count)

Read count bytes from port and store them in bv starting at index start. Return either the number of bytes actually read or the end-of-file object.

Scheme Procedure: get-bytevector-some port
C Function: scm_get_bytevector_some (port)

Read from port, blocking as necessary, until data are available or and end-of-file is reached. Return either a new bytevector containing the data read or the end-of-file object.

Scheme Procedure: get-bytevector-all port
C Function: scm_get_bytevector_all (port)

Read from port, blocking as necessary, until the end-of-file is reached. Return either a new bytevector containing the data read or the end-of-file object (if no data were available).


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