1、memcpy()

一般的函数实现

void* my_memcpy(void *dest, void *src, unsigned long count)
{
	char *temp = (char *)dest;
	const char* s = (const char*)src;
	while (count--)
	{
		*temp++ = *s++;
	}
	return dest;
}

上面的实现存在缺陷

内存重叠会出现拷贝异常，举例：

int a[10] = {0,1,2,3,4,5,6,7,8,9,10};

//高地址向地址拷贝,拷贝正常
memcpy(a, a+3, sizeof(int) * 4);

//低地址向高地址拷贝,拷贝不正常
memcpy(a+3, a, sizeof(int) * 4);

预计结果：0,1,2,0,1,2,3,7,8,9,10
实际结果: 0,1,2,0,1,2,0,7,8,9,10

解决方法

• 直接使用memmove()函数

• 使用memcpy前做些判断

void* memcpy_s(void* dest, const void* src, unsigned int count)
{
	assert((dest!=NULL) && (src != NULL));
	if (dest >= src)
	{
		return memcpy(dest, src, count);
	}
	else
	{
		//优先拷贝重叠部分内容
		char* temp;
		const char* s;
		temp = (char*)dest;
		temp += count;

		s = (const char*)src;
		s += count;

		while(count--)
		{
			*--temp = *--s;
		}
	}
	return dest;
}

2、strcpy()

一般的函数实现

char* strcpy(char* dest, const char*src)
{
	char* temp = dest;
	const char* s = src;
	while((*temp++ = *s++) != '\0');
	return dest;
}