C语言模拟实现字符串操作

/*
模拟实现strlen的三种方式
*/
int myStrlen(const char *str)
{//方式1，计数器方式
	int count = 0;
	while (*str != '\0')
	{
		count++;
		str++;
	}
	return count;
}

int myStrlen2(const char *str)
{//方式2：不创建临时变量
	if (*str == '\0')
	{
		return 0;
	}
	else
	{
		return 1 + myStrlen2(str+1);
	}
}

int myStrlen3(const char *str)
{//方式3：指针-指针方式
	char *p = str;
	while (*p != '\0')
	{
		p++;
	}
	return p - str;
}

//模拟实现strcpy
char *my_strcpy(char *dest, const char *src)
{
	char *ret = dest;
	assert(*dest);
	assert(src);
	while (*dest++ = *src++)//注意这里不能是(*dest)++ = (*src)++，要分清这两者的区别
	{
		;
	}
	return ret;
}

//模拟实现strcat
char *my_strcat(char *dest, const char *src)
{
	char *ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	while (*dest)
	{
		dest++;
	}
	while ((*dest++ = *src++))
	{
		;
	}
	return ret;
	
}

//模拟实现strstr
char *my_strstr(const char*str1, const char *str2)
{
	assert(str1);
	assert(str2);
	char *cp = (char *)str1;
	char *substr = (char *)str2;
	char *s1 = NULL;
	if (*str2 == '\0')
	{
		return NULL;
	}
	while (*cp)
	{
		s1 = cp;
		substr = str2;
		while (*s1&&*substr && (*s1 == *substr))
		{
			s1++;
			substr++;
		}
		if (*substr == '\0')
		{
			return cp;
		}
		cp++;
	}
}

//模拟实现strcmp
int my_strcmp(const char *src, const char *dst)
{//将src中的字符串拷贝进dst
	int ret = 0;
	while (!(ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
	{
		++src;
		++dst;
	}
	if (ret < 0)
	{
		ret = -1;
	}
	else if (ret>0)
	{
		ret = 1;
	}
	return ret;
}

//模拟实现memcpy
void *my_memcpy(void *dst, const void *src, int count)
{//注意：以memcpy开头的函数操作的基本单位都是字节
	void *ret = dst;
	while (count--)
	{
		*(char *)dst = *(char *)src;
		dst = (char *)dst + 1;
		src = (char *)src + 1;
	}
	return ret;
}

//模拟实现memmove
void *my_memmove(void *dst,const void *src,int 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;
}