从图中可以看出，src的内存区域和dest的内存区域相对位置和重叠关系有四种情况，memcpy没有考虑重叠的情况，而memmove考虑到了全部情况，因此memcpy函数的时候可能出现意向不到的结果。
这两个函数的实现：
***********下面两个是错误的实现**************
void* memcpy(void* dest, void* source, size_t count)
{
void* ret = dest;
//copy from lower address to higher address
while (count--)
*dest++ = *source++; //不知道两个指针的类型，不可以这样自加。
return ret;
}
void* memmove(void* dest, void* source, size_t count)
{
void* ret = dest;
if (dest <= source || dest >= (source + count))
{
//Non-Overlapping Buffers
//copy from lower addresses to higher addresses
while (count --)
*dest++ = *source++;
} else{
//Overlapping Buffers
//copy from higher addresses to lower addresses
dest += count - 1;
source += count - 1;
while (count--)
*dest-- = *source--; // 情况同上
}
return ret;
}
***********************正确的如下**************************
void* mymemcpy(void* dest, void* source, size_t count)
{
char *ret = (char *)dest;
char *dest_t = ret;
char *source_t = (char *)source;
while (count--){
*dest_t++ = *source_t++;
}
return ret;
}
void *my_memmove(void *dst,const void *src,int count)
{
char *ret;
char *dst_t;
char *src_t;
ret = (char *)dst;
if ((unsigned char*)dst <= (unsigned char*)src
|| (unsigned char *)dst >= ((unsigned char *)src + count))
{
dst_t = (char *)dst;
src_t = (char *)src;
while (count--)
{
*dst_t++ = *src_t++;
}
}else{
dst_t = (char *)dst + count - 1;
src_t = (char *)src + count - 1;
while (count--)
{
*dst_t-- = *src_t--;
}
}
return(ret);
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