[gambit-list] FFI with (char *) buffer

Ben Weaver weaver at coptix.com
Sat Feb 24 16:06:44 EST 2007

Hello Everyone,

I'm creating an FFI that requires writing to and reading from a C
(char *) buffer.  The buffer may contain arbitrary binary data (not
necessarily a character string).  I wrote a few procedures (see below)
to copy the data to and from a u8vector so I can operate on it using

I'm hoping for comments or suggestions from you all about the code.
Is there a cleaner/better/faster way to do this?  Ideally there would
be some way to open a byte-port directly to the buffer so it wouldn't
have to be copied.  Is this possible?

Thanks in advance,


;; Typical use of this code:

 (c-buffer->u8vector foreign-pointer-to-pointer 0 buffer-size )
 (lambda () ...))

  '(char-encoding: ASCII)
  (lambda () ...))
 foreign-pointer-to-buffer 0 buffer-size)

;; The code

(declare (fixnum) (not safe))

(c-declare #<<eo-declare

typedef const char * const_char_ptr;


(c-define-type const-char-buffer "const_char_ptr")
(c-define-type size-type unsigned-int32)
; Using -D_FILE_OFFSET_BITS=64 makes off_t 8 bytes instead of 4
(c-define-type offset-type "off_t" "S64_TO_SCMOBJ" "SCMOBJ_TO_S64" #f)

(define u8vector->c-buffer
      (scheme-object (pointer char) offset-type size-type) void

    off_t i;
    ___U8 c;
    for (i = 0; i < ___arg4; i++) {
           ___SCMOBJ_to_U8(___U8VECTORREF(___arg1, ___FIX(i)), &c, i);
	      ___CAST(char *,___arg2)[i + ___arg3] = c;

(define %c-buffer->u8vector
      (const-char-buffer scheme-object offset-type size-type)

    off_t i;
    ___SCMOBJ c;
    ___U8* buff = ___CAST(___U8*, ___arg1);
    for (i = 0; i < ___arg4; i++) {
           ___U8_to_SCMOBJ(buff[i + ___arg3], &c, i);
           ___U8VECTORSET(___arg2, ___FIX(i), c);

(define (c-buffer->u8vector buff offset size)
  (let ((v (make-u8vector size)))
    (%c-buffer->u8vector buff v offset size)

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