学步园

代码一：

void *memcpy( void *dest, const void *src, size_t count )
{
	ASSERT((dest != NULL)&&(src != NULL));
	char *temp_dest = (char *)dest;
	char *temp_src = (char *)src;
	while(count--)	                                  // 不对是否存在重叠区域进行判断
	{
		*temp_dest++ = *temp_src++;
	}
	return dest;
}

代码二：

void* memcpy(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--;l
     }
     return ret;
   }

代码三：

void memcpy(void* dest, const void* src, size_t n)
{
    assert((((uintptr_t) dest | (uintptr_t) src | n) & 0x03) == 0);
    uint32_t* d = (uint32_t*) dest;
    const uint32_t* s = (uint32_t*) src;

    n /= sizeof(uint32_t);

   if (d < s) {
       /* copy forward */
        while (n--) {
            *d++ = *s++;
        }
    } else {
        /* copy backward */
        d += n;
        s += n;
        while (n--) {
            *--d = *--s;
        }
    }
}

ref：http://snake-hand.iteye.com/blog/1522972