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October 1994

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1994.10.13 18:44 "Kudos + possible bug in 3.3 PackBits", by Mike Stanfill

Hello all, 

	First off, thanks to the many people who gave me advice on 
	compiling libtiff 3.3 under gcc.  You were right -- the 
	problem was my 2.2 version of gcc.  The 2.5.8 version works
	wonderfully, though I would not recommend 2.6.0, at least 
	for ISC (I got assembler errors when compiling with it).

	Incase nobody has mentioned this yet, there may be a bug in 
	libtiff/tif_packbits.c.  It looks like line 226 (cc -= n;) 
	should not be there.  Here's the $Header for the version I 
	have:

/* $Header: /usr/people/sam/tiff/libtiff/RCS/tif_packbits.c,v 1.39
1994/07/27 01:59:44 sam Exp $ */

	I compared this file to an old 3.0 version and found that 
	line 226 and the extra cc-- on line 213 were the only 
	changes that might have caused the errors I was getting 
	(PackBitsDecode: Not enough data for scanline n).

	When I commented out line 226, everything suddenly started 
	working again.  I tend to think line 213 is a bug fix for 
	3.0, since it makes sense that you want to decrement the 
	input byte count on every byte read from the buffer.  Such
	a bug in 3.0 would not have affected anything so long as 
	the scanlines actually encoded the right amount of data.
	This would explain why my 3.0 libtiff works just as well 
	as my (modified) 3.3.

	The following is my modified copy of PackBitsDecode():

	( Sam:  Sorry, I would have given a diff, but I understand 
		you've already moved on to a new beta version. )

					Regards,
					-Mike Stanfill

-----valuable-coupon-----clip'n'save-----valuable-coupon-----clip'n'save-----

static int
PackBitsDecode(TIFF* tif, tidata_t op, tsize_t occ, tsample_t s)
{
	char *bp;
	tsize_t cc;

	bp = (char*) tif->tif_rawcp;
	cc = tif->tif_rawcc;
	while (cc > 0 && (long)occ > 0) {
		long n;
		int b;
		
		n = (long) *bp++, cc--;
		/*
		 * Watch out for compilers that
		 * don't sign extend chars...
		 */
		if (n >= 128)
			n -= 256;
		if (n < 0) {		/* replicate next byte -n+1 times */
			cc--;
			if (n == -128)	/* nop */
				continue;
			n = -n + 1;
			occ -= n;
/*  bug? ----->		cc -= n; 	*/
			for (b = *bp++; n-- > 0;)
				*op++ = b;
		} else {		/* copy next n+1 bytes literally */
			memcpy(op, bp, ++n);
			op += n; occ -= n;
			bp += n; cc -= n;
		}
	}
	tif->tif_rawcp = (tidata_t) bp;
	tif->tif_rawcc = cc;
	if (occ > 0) {
		TIFFError(tif->tif_name,
		    "PackBitsDecode: Not enough data for scanline %ld",
		    (long) tif->tif_row);
		return (0);
	}
	/* check for buffer overruns? */
	return (1);
}