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数据拷贝的方法解析

memcpy 与 memmove

区别：两个函数都是进行n字节内存内容的拷贝，入口参数和返回参数也都一样，可是这两个函数在内部实现上是有一定区别的，这主要是因为dest内存区域和src内存区域可能有一下四种不同的情况，

注意count的影响：

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); }