memmove与memcpy的区别在于memmove可是实现数据重叠的拷贝，而memcpy只能拷贝不重叠的数据。

memmove的实现

void* memmove(void* dst, const void* src, size_t count)
{
    void* ret = dst;
    if (dst <= src || (char*)dst >= ((char*)src + count)) 
    {
        while (count--) 
        {
            *(char*)dst = *(char*)src;
            dst = (char*)dst + 1;
            src = (char*)src + 1;
        }
    }
    else
    {
        dst = (char*)dst + count - 1;
        src = (char*)src + count - 1;
        while (count--) 
        {
             *(char*)dst = *(char*)src;
             dst = (char*)dst - 1;
             src = (char*)src - 1;
        }
    }
    return ret;
}

memcpy的实现

void* memcpy(void* dst, const void* src, size_t count)
{
    void* ret = dst;
    assert(dst);
    assert(src);
    while (count--) {
        *(char*)dst = *(char*)src;
        dst = (char*)dst + 1;
        src = (char*)src + 1;
    }
    return ret;
}

strcmp的实现

int strcmp(const char* str1, const char* str2) 
{
	assert(str1 != NULL && str2 != NULL);
	while (*str1 && *str1 == *str2)
	{
		str1++;
		str2++;
	}
	if (*(unsigned char*)str1 < *(unsigned char*)str2) 
		return -1;
	else if (*(unsigned char*)str1 > *(unsigned char*)str2) 
		return 1;
	else 
		return 0;
}

strcat的实现

char* my_strcat(char* arr1, const char* arr2)
{
	assert(arr1 != NULL && arr2 != NULL);
	char* address = arr1;
	while (*arr1 != '\0') 
		arr1++;
	while (*arr2 != '\0')
		*arr1++ = *arr2++;
	*arr1 = '\0';
	return address;
}

strlen的实现

int my_strlen(char* arr)
{
	int count = 0;
	while (*arr != '\0')
	{
		arr++;
		count++;
	}
	return count;
}

strcpy的实现

strcpy与memcpy的区别在于memcpy是拷贝内存空间，而strcpy是字符串的拷贝

char* my_strcpy(char* arr1, const char* arr2)
{
	assert(arr1 != NULL && arr2 != NULL);
	char* tmp = arr1;
	while (*arr1++ = *arr2++)
	{
		;
	}
		return tmp;
}