[Larceny-users] Passing bytevectors to C via the FFI

[Ray Racine]

I just finished typing up the code below, and it works, but as I looked
at it I'm worried it only works sometimes.

The sig of the C function gethostname is

 int gethostname(char *name, size_t len);

It populates name with up to len chars of the hostname.  Below I use a
boxed arg for name and pass a scheme heap allocated bytevector.

I'm wondering if this is safe.  i,e, A GC won't move the bytevector or
some other problem.  Or is a locally allocated heap object effectively
pinned for the duration of the call?

Q. When passing a char * buffer to a C function via the FFI is using a
scheme allocated bytevector safe or should one be doing some kind of
malloc call to allocate memory outside the auspices of the GC?

Sorry if this is kind of obvious, my Scheme/C foo is not strong.

Thanks,

Ray


(define c-gethostname (foreign-procedure "gethostname" '(boxed int)
'unsigned))

(define get-hostname
  (lambda ()
    (define MAX-HOST 256)

    (define null-offset
      (lambda (bytes)
        (let loop ((idx 0))
          (if (fx=? (bytevector-u8-ref bytes idx) 0)
              idx
              (loop (fx+ idx 1))))))
    
    (let ((buff (make-bytevector MAX-HOST 0)))
      (let ((result (c-gethostname buff MAX-HOST)))
        ;; man page says truncation is unspecified         
        (bytevector-u8-set! buff (fx- MAX-HOST 1) 0) 
        (substring (utf8->string buff) 0 (null-offset buff))))))