内存拷贝函数memcpy函数深入剖析

memcpy（拷贝内存内容）  

定义函数：void * memcpy( void * dest, const void *src, size_t n );

memcpy()用来拷贝src所指的内存内容前n个字节到dest所指的内存地址上。与strcpy()不同的是，memcpy()会完整的复制n个字节，不会因为遇到字符串结束'/0'而结束。memcpy()函数可以拷贝任意类型的数据。memcpy()函数返回指向dest的指针。指针src和dest所指的内存区域不可重叠。在拷贝字符串时，通常都使用strcpy()函数；在拷贝其它数据(例如结构)时，通常都使用memcpy()函数。

void* mymemcpy(void* dest, void* source, size_t count){
    void* ret = dest;
    if (dest <= source || dest >= (source + count)){
         while (count--)  *dest++ = *source++;
     }else{
        //Overlapping Buffers
       //copy from higher addresses to lower addresses
        dest += count - 1;
        source += count - 1;
        while (count--) *dest-- = *source--;
     }
    return ret;
}

另一种诡异解法：

void* mymemcpy( void* dest, const void* src, size_t count ) { 
    char* d = (char*)dest; 
    const char* s = (const char*)src; 
    //int n = (count + 7) / 8; // count > 0 assumed 
    int n = count >> 3; 
    switch( count & 7 ){ 
        do {   
			*d++ = *s++; 
			case 7:         *d++ = *s++; 
			case 6:         *d++ = *s++; 
			case 5:         *d++ = *s++; 
			case 4:         *d++ = *s++; 
			case 3:         *d++ = *s++; 
			case 2:         *d++ = *s++; 
			case 1:         *d++ = *s++; 
			case 0           } //while (--n > 0); 
        }while (n-- > 0) 
     } 
    return dest; 
}