C语言strlen,strcpy,strcmp,strcat,strstr字符串操作函数实现

int my_strlen(const char* p) {
	int size = 0;
	//找到末尾停止
	while (*p != '\0') {
		p++;
		size++;
	}
	return size;
}

char* my_strcpy(char* dest, const char* src) {

	//判断传入是否为空
	if (src == NULL || dest == NULL) {
		return dest;
	}
	char* pdest = dest;
	//不为空打印
	while (*src != '\0') {
		*pdest = *src;
		pdest++;
		src++;
	}
	//末尾加'\0'
	*pdest = '\0';
	return dest;

}

int my_strcmp(const char* str1, const char* str2) {
	//一直往后查找直到其中一个结尾或者他们二者不相等为止
	while (*str1 != '\0' && *str2 != '\0' && (*str1 == *str2)) {
		str1++;
		str2++;
	}
	return *str1 - *str2;
}

char* my_strcat(char* dest, const char* src) {

	char* pdest = dest;
	//pdset先走到末尾
	while (*pdest != '\0') {
		pdest++;
	}
	//链接
	while (*src != '\0') {
		*pdest = *src;
		pdest++;
		src++;
	}
	//加结尾标志'\0'
	*pdest = '\0';
	return dest;
}

char* my_strstr(const char* dest, const char* src) {

	//断言非空
	assert(dest && src);

	while (*dest != '\0') {
		//记录dest src初始位置
		char* pdest = dest;
		char* psrc = src;
		//若相等则继续比较
		while (*pdest == *psrc || *psrc == '\0') {
			//如果psrc=='\0' 则说明找到子串
			if (*psrc == '\0') {
				return dest;
			}
			//psrc 未到 \0 则继续比较是否相等
			pdest++;
			psrc++;
		}
		dest++;
	}
	return NULL;
}