[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
ports.
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,
-Ben
;; Typical use of this code:
(with-input-from-u8vector
(c-buffer->u8vector foreign-pointer-to-pointer 0 buffer-size )
(lambda () ...))
(u8vector->c-buffer
(with-output-to-u8vector
'(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;
eo-declare
)
(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
(c-lambda
(scheme-object (pointer char) offset-type size-type) void
#<<end-lambda
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;
}
end-lambda
))
(define %c-buffer->u8vector
(c-lambda
(const-char-buffer scheme-object offset-type size-type)
void
#<<end-lambda
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);
}
end-lambda
))
(define (c-buffer->u8vector buff offset size)
(let ((v (make-u8vector size)))
(%c-buffer->u8vector buff v offset size)
v))
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