[plt-scheme] Read and write ports from pre-opened file descriptors?

Simpler:

  (module fd mzscheme
    (require (lib "foreign.ss")) (unsafe!)
    (provide fd->input-port
             fd->output-port)
    
    (define (fd->input-port fd name)
      (scheme_make_fd_input_port fd name 0 0))

    (define (fd->output-port fd name)
      (scheme_make_fd_output_port fd name 0 0 0))

    (define scheme_make_fd_input_port
      (get-ffi-obj "scheme_make_fd_input_port" #f
                   (_fun _int _scheme _int _int -> _scheme)))

    (define scheme_make_fd_output_port
      (get-ffi-obj "scheme_make_fd_output_port" #f
                   (_fun _int _scheme _int _int _int -> _scheme))))


At Wed, 27 Feb 2008 14:19:36 -0500, Eli Barzilay wrote:
> On Feb 27, David Fayram wrote:
> > Hello, everyone. I'm relatively new to scheme and I've been trying
> > to writing a binding between Erlang and Scheme for a clustering
> > application I've built in Erlang. Part of this entails reading and
> > writing to file descriptors 3 and 4 in a binary protocol for
> > communication with Erlang via its Ports abstraction.
> > 
> > It seems like this is possible to do at the C level, but I'm not
> > sure how to do it with MzScheme's ffi library. It would be best if
> > there were no external binary objects. Could someone suggest a way
> > that this might be accomplished?
> 
> A rough start at doing this:
> 
>   (module fd-read mzscheme
>     (require (lib "foreign.ss")) (unsafe!)
>     (provide fd-read)
>     (define buf-len 512)
>     (define fd-read
>       (get-ffi-obj "read" #f
>         (_fun ;; the fd argument is the only actual input for this
>               [fd : _int]
>               ;; the `buf' input is automatically set to a newly
>               ;; created byte-string
>               [buf : _bytes = (make-bytes buf-len)]
>               ;; len is also automatic -- the size of the buffer
>               [len : _int = buf-len]
>               ;; we get back a value that we bind to `n'
>               -> [n : _int]
>               ;; but this is what we actually return
>               -> (if (= n buf-len) buf (subbytes buf 0 n))))))
> 
> I'm saying `rough' because there might be some off-by-1 errors with
> the limits, and you might want to avoid allocating a new byte-string
> on every call and/or avoid copying the actual result from the buffer.
> (But you'll need to implement some locking if you want to do it
> properly.)
> 
> I tried it with bash, and the only way I could get it to get input on
> 4 (from "x") and keep 0 as stdin is
> 
>   mzscheme 5<&0 0<x 4<&0- 0<&5
> 
> It seems to work with that.
> 
> (This obviously is not intended for Windows...)
> 
> -- 
>           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                   http://www.barzilay.org/                 Maze is Life!
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