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memcpy 與 memmove

區別：兩個函數都是進行n字節內存內容的拷貝，入口參數和返回參數也都一樣，可是這兩個函數在內部實現上是有一定區別的，這主要是因為dest內存區域和src內存區域可能有一下四種不同的情況，

注意count的影響：

從圖中可以看出，src的內存區域和dest的內存區域相對位置和重疊關系有四種情況，memcpy沒有考慮重疊的情況，而memmove考慮到了全部情況，因此memcpy函數的時候可能出現意向不到的結果。

這兩個函數的實現：

***********下面兩個是錯誤的實現**************

void* memcpy(void* dest, void* source, size_t count) {

void* ret = dest;

//copy from lower address to higher address

while (count--)

*dest++ = *source++; //不知道兩個指針的類型，不可以這樣自加。

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--; // 情況同上

}

return ret;

}

***********************正確的如下**************************

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); }