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本帖最后由 JohnBull 于 2011-05-11 17:39 编辑
#include <sys/cdefs.h>;
#include <string.h>;
/*
* sizeof(word) MUST BE A POWER OF TWO
* SO THAT wmask BELOW IS ALL ONES
*/
typedef int 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.
* length 以Byte作为度量单位
*/
#ifdef MEMCOPY
void *
memcpy(dst0, src0, length)
#else
#ifdef MEMMOVE
void *
memmove(dst0, src0, length)
#else
void
bcopy(src0, dst0, length)
#endif
#endif
void *dst0;
const void *src0;
register size_t length;
{
register char *dst = dst0;
register const char *src = src0;
register 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 = (int)src; /* only need low bits */
/*这样做的目的是由于下面的TLOOP(*(word *)dst = *(word *)src; src += wsize; dst += wsize)拷贝用的是32位为一个单位
*所以 这是的指针所指向的地址是最后两位是00(这个想法我还没验证)所以在匹配之前一定要将DES 和 SRC指针对齐到最后两位为00才行
*/
if ((t | (int)dst) & wmask) { // 如果源和目标地址的最后两位不是00的话
/* 对齐操作数???
* Try to align operands. This cannot be done
* unless the low bits match.
*/
if ((t ^ (int)dst) & wmask || length < wsize)
t = length; //如果目标和源的地址最后两位参差不齐如 11 10 只能以Byte为单位拷贝
else
t = wsize - (t & wmask); //如果目标和源的地址最后两位如 10 10相等的话 把他们同时移到00来做
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 = (int)src;
if ((t | (int)dst) & wmask) {
if ((t ^ (int)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
} 其中中文注释是我自己加的,不知道对不对
不过我对函数中先将DES和SRC指针对齐(align) 就是我加注释的那部分的意义还不是很明白,我觉得直接进行字赋值操作 最后把结尾的部分补齐就行了
呵呵,不明白为什么作者要加这一段 大家讨论讨论 |
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