On Tue, 8 Aug 2006 09:37:09 -0400, Kuba Ober kuba@mareimbrium.org wrote:
Fix rather unusual bug in LZ77 decompressor. We cannot use memcpy with overlapped areas because of unpredictable result. We must copy byte-by-byte.
Why don't you use memmove instead? The man page for memcpy says: Use memmove(3) if the memory areas do overlap.
We cannot use memmove. It provides different functionality. It moves blocks, but LZ77 decompress algorithm copies bytes.
Let's suppose: 012 XYZ
When we say 'memmove(1, 0, 2)' we will get: 012 XXY
But we (LZ77 algorithm) expect: 012 XXX (three 'X').
memcpy is rather clever (it optmizes process by coping INT instead of BYTES). This optimization is fatal for LZ77.
Why?
Er, if the memcpy worked (or almost worked) and failed due to overlap problems, then memmove will do the trick. Did it ever work at all before? Either this code never worked, or you're either seeing a problem that doesn't exist/misunderstanding things.
Cheers, Kuba
memmove provides OTHER functionality - it GUARANTEES, that if blocks overlaps, then destination area will the same as source area before. LZ77 decompress algorithms needs copiing byte-by-byte - of course, if areas overlap, then dest area WILL NOT match the src area before. It IS correct for LZ77!
The simpliest version of memcpy routine, like this:
void my_memcpy(char *dst, char *src, int len) { while (len-->0) *dst++=*src++; )
WILL work correctly for LZ77.
memcpy provided by gcc is optimized - it NOT copies byte-by-byte from beginning till end.
If you mediate over lz77 decompress algo (dlls/kernel/lzexpand.c) the things will be clean.
-
HolyLich