Re: [Chicken-users] Reading binary files

[Felix Winkelmann]

Magnus Therning wrote:
> My first post :-)

Very good! I hope it isn't the last. ;-)

> I need to read a file that has the following basic layout:
>
>  table-size - 32 bits
>  [ frequency - 32 bits
>    byte - 8 bits]+
>  message-size - 32 bits
>  message ...
>
> Now, the problem I have is that I have never used scheme for programming
> on this low level before -- I have never had to read a specific number
> of bytes from a file and convert those bytes to an integer. I can't find
> any information on how to do it, I guess SRFI-4 is related, but from the
> webpage that the chicken manual refers to[1] I can't figure it out. I
> also can't find any kind of read-byte, read-32bits, or similar function
> in the manual.

You could simply use `read-char' and convert the character into a byte
with `char->integer'. If you are using Windows, you should also open
the file with `#:binary'

(open-input-file filename #:binary)

to avoid CRLF translation of line terminators.

`read-string' might also be helpful. So for example:

(declare (uses extras lolevel srfi-4)); for `read-string', 
`string->byte-vector' and
  `byte-vector->u8vector'
(define i (open-input-file "myfile" #:binary))
(define s (read-string 32 i)); read 32 bytes from file
(define v (string->byte-vector s)); gives you a byte-vector
(define u (byte-vector->u8vector v)); if you want a SRFI-4 bytevector instead


cheers,
felix