memory_cpy
// 版本 1
void *memcpy(void *dest, const void *src, size_t count)
{
char *tmp = dest;
const char *s = src;
while (count--)
*tmp++ = *s++ ; // 先取出s的内容,后s+1
return dest;
}
//版本 2 考虑重叠
void *memcpy(void *dest, const void *src, size_t count)
{
char *d;
const char *s;
if (dest > (src+size)) || (dest < src))
{
d = dest;
s = src;
while (count--)
*d++ = *s++;
}
else /* overlap 重叠情况从高位到低位复制*/
{
d = (char *)(dest + count - 1); /* offset of pointer is from 0 */
s = (char *)(src + count -1);
while (count --)
*d-- = *s--;
}
return dest;
}