Re: reading/writing binary data

Roland Orre (orre@nada.kth.se)
Sat, 13 Dec 1997 07:05:03 +0100 (MET)

Maciej Stachowiak <mstachow@mit.edu> wrote:
> I'd like to write a guile script to communicate with a program that
> reads and writes data in binary format - 32 bit numeric values are
> written as four bytes in host byte order. I can get Guile to read this
> in as a string of four chars, but is there a reasonably portable way
> to convert that into a Scheme number (and vice versa for communicating
> the other way)? I can think of the simple-minded approach of using
> char->integer on each byte, bitshifting and adding, but I'd rather not
> put knowledge of the host's endianness into the script, and I'm hoping
> there's some primitives that can help do it more efficiently, although
> I don't know of any.

;The easiest for you is to use the uniform vector type. A very useful
;data type available in scm/guile. In guile there are also structs which
;allows for more complicated binary patterns.

;A very simple test program that writes and read two 32 bit numbers
;Make a uniform vector of type and size you want.
;The -1 is a prototype which means signed integer, 1 is unsigned

(define twonumber (make-uniform-vector 2 -1))

;Fill them with some numbers
(array-set! twonumber 255 0)
(array-set! twonumber 65280 1)

;Write the numbers binary to file
(define out (open-output-file "twonumber"))
(uniform-vector-write twonumber out)
(close-output-port out)

;Read them back again in another vector
(define numbers (make-uniform-vector 2 -1))

(define in (open-input-file "twonumber"))
(close-input-port out)

(uniform-vector-read! numbers in)

; Try
(apropos "uniform-vector")

; and check docs about uniform vectors. If you are communicating data
; between different endian machines the cleanest way is to use an
; intermediate file for the conversion.

Best reagards
Roland
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