memcpy实现

#include <string.h>

/*
 * sizeof(word) MUST BE A POWER OF TWO
 * SO THAT wmask BELOW IS ALL ONES
 */
typedef	long word;		/* "word" used for optimal copy speed */

#define	wsize	sizeof(word)
#define	wmask	(wsize - 1)

/*
 * Copy a block of memory, handling overlap.
 * This is the routine that actually implements
 * (the portable versions of) bcopy, memcpy, and memmove.
 */
#ifdef MEMCOPY
void *
memcpy(void *dst0, const void *src0, size_t length)
#else
#ifdef MEMMOVE
void *
memmove(void *dst0, const void *src0, size_t length)
#else
void
bcopy(const void *src0, void *dst0, size_t length)
#endif
#endif
{
	char *dst = dst0;
	const char *src = src0;
	size_t t;

	if (length == 0 || dst == src)		/* nothing to do */
		goto done;

	/*
	 * Macros: loop-t-times; and loop-t-times, t>0
	 */
#define	TLOOP(s) if (t) TLOOP1(s)
#define	TLOOP1(s) do { s; } while (--t)

	if ((unsigned long)dst < (unsigned long)src) {
		/*
		 * Copy forward.
		 */
		t = (long)src;	/* only need low bits */
		if ((t | (long)dst) & wmask) {
			/*
			 * Try to align operands.  This cannot be done
			 * unless the low bits match.
			 */
			if ((t ^ (long)dst) & wmask || length < wsize)
				t = length;
			else
				t = wsize - (t & wmask);
			length -= t;
			TLOOP1(*dst++ = *src++);
		}
		/*
		 * Copy whole words, then mop up any trailing bytes.
		 */
		t = length / wsize;
		TLOOP(*(word *)dst = *(word *)src; src += wsize; dst += wsize);
		t = length & wmask;
		TLOOP(*dst++ = *src++);
	} else {
		/*
		 * Copy backwards.  Otherwise essentially the same.
		 * Alignment works as before, except that it takes
		 * (t&wmask) bytes to align, not wsize-(t&wmask).
		 */
		src += length;
		dst += length;
		t = (long)src;
		if ((t | (long)dst) & wmask) {
			if ((t ^ (long)dst) & wmask || length <= wsize)
				t = length;
			else
				t &= wmask;
			length -= t;
			TLOOP1(*--dst = *--src);
		}
		t = length / wsize;
		TLOOP(src -= wsize; dst -= wsize; *(word *)dst = *(word *)src);
		t = length & wmask;
		TLOOP(*--dst = *--src);
	}
done:
#if defined(MEMCOPY) || defined(MEMMOVE)
	return (dst0);
#else
	return;
#endif
}