memcpy的实现（转载）

/* $NetBSD: bcopy.c,v 1.3 2007/06/04 18:19:27 christos Exp $ */ /*- * Copyright (c) 1990, 1993 * The Regents of the University of California. All rights reserved. * * This code is derived from software contributed to Berkeley by * Chris Torek. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. Neither the name of the University nor the names of its contributors * may be used to endorse or promote products derived from this software * without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ #include <sys/cdefs.h> #if defined(LIBC_SCCS) && !defined(lint) #if 0 static char sccsid[] = "@(#)bcopy.c 8.1 (Berkeley) 6/4/93"; #else __RCSID("$NetBSD: bcopy.c,v 1.3 2007/06/04 18:19:27 christos Exp ___FCKpd___1quot;); #endif #endif /* LIBC_SCCS and not lint */ #if !defined(_KERNEL) && !defined(_STANDALONE) #include <assert.h> #include <string.h> #else #include <lib/libkern/libkern.h> #endif #ifdef _FORTIFY_SOURCE #undef bcopy #undef memcpy #undef memmove #endif /* * 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; unsigned long u; #if !defined(_KERNEL) _DIAGASSERT(dst0 != 0); _DIAGASSERT(src0 != 0); #endif 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. */ u = (unsigned long)src; /* only need low bits */ if ((u | (unsigned long)dst) & wmask) { /* * Try to align operands. This cannot be done * unless the low bits match. */ if ((u ^ (unsigned long)dst) & wmask || length < wsize) t = length; else t = wsize - (size_t)(u & wmask); length -= t; TLOOP1(*dst++ = *src++); } /* * Copy whole words, then mop up any trailing bytes. */ t = length / wsize; TLOOP(*(word *)(void *)dst = *(const word *)(const void *)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; _DIAGASSERT((unsigned long)dst >= (unsigned long)dst0); _DIAGASSERT((unsigned long)src >= (unsigned long)src0); u = (unsigned long)src; if ((u | (unsigned long)dst) & wmask) { if ((u ^ (unsigned long)dst) & wmask || length <= wsize) t = length; else t = (size_t)(u & wmask); length -= t; TLOOP1(*--dst = *--src); } t = length / wsize; TLOOP(src -= wsize; dst -= wsize; *(word *)(void *)dst = *(const word *)(const void *)src); t = length & wmask; TLOOP(*--dst = *--src); } done: #if defined(MEMCOPY) || defined(MEMMOVE) return (dst0); #else return; #endif }