长度不受限制的字符串函数：strcpy,strcat,strcmp
长度受限制的字符串函数：strncpy,strncat,strncmp

strcpy函数模拟实现

#include<stdio.h>
#include<string.h>
#include<assert.h>
char* my_strcpy(char* dest,const char* src)
{ 
	assert(dest != NULL);
	assert(src != NULL);
	int ret = dest;
	while (*dest++ = *src++)
	{
		;
	}
	return ret;
}
int main()
{
	char arr1[] = "abcdefghi";
	char arr2[] = "bit";
	my_strcpy(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

strcat函数模拟实现

#include<stdio.h>
#include<assert.h>
void my_strcat(char* dest, const char* src)
{
	assert(dest && src);
	char* len = strlen(dest)+ dest;
	while (*len++ = *src++)
	{
		;
	}

}
int main()
{
	char arr1[] = "hello";
	char arr2[] = "world";
	my_strcat(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

#include<stdio.h>
#include<assert.h>
char* my_strcat(char* dest, const char* src)
{
	char* ret = dest;
	assert(dest && src);
	//1.找到目的字符串'\0'
	while (*dest != '\0')
	{
		dest++;
	}
	while (*dest++ = *src++)
	{
		;
	}
	return ret;

}
int main()
{
	char arr1[] = "hello";
	char arr2[] = "world";
	char* i = my_strcat(arr1, arr2);
	printf("%s\n",i);
	return 0;
}

strcmp
//strcmp函数的模拟实现
//string 1>string 2 return >0
//string 1<string 2 return <0
//string 1=string 2 return 0
 

#include<stdio.h>
#include<assert.h>
int my_strcmp(const char* str1,const char* str2)
{
	assert(str1 && str2);
	//比较
	while (*str1!='\0'&&*str2!='\0')
	{
		if (*str1 == *str2)
		{
			str1++;
			str2++;
		}
		else if (*str1 > *str2)
		{
			return 1;
		}
		else
		{
			return -1;
		}
	}
	return 0;
}
int main()
{
	char* p1 = "abcdef";
	char* p2 = "abcasd";
	int ret = my_strcmp(p1, p2);
	printf("%d", ret);
	return 0;
}

#include<stdio.h>
#include<assert.h>
int my_strcmp(const char* str1,const char* str2)
{
	assert(str1 && str2);
	//比较
	while (*str1==*str2)
	{
		if (*str1 == '\0')
		{
			return 0;
		}
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;
	else 
		return -1;
    //return (*str1 - *str2);
}
int main()
{
	char* p1 = "abcdef";
	char* p2 = "abcdef";
	int ret = my_strcmp(p1, p2);
	printf("%d", ret);
	return 0;
}

strstr函数的模拟实现

#include<stdio.h>
#include<assert.h>
char* my_strstr(const char* p1, const char* p2)
{
	assert(p1 != NULL);
	assert(p2 != NULL);
	if (*p2 == '\0')
	{
		return p1;
	}
	char* p3 = p2;
	int count = 0;//记录连续相等字符的长度
	while (*p1)
	{
		if (*p1 == *p2)//遇到字符相等时，两个指针同时进一位
		{
			p1++;
			p2++;
			count++;
		}
		else//字符串不相等
		{
			p1++;//p1继续向前
			p2 = p3;//p2还原到开始位置
			count = 0;//此时两个字符不相等而不连续，所以归零
		}
		if (*p2 == '\0')
			return p1-count;
	}
}
int main()
{
	char* p1 = "abdcdefghi";
	char* p2 = "def";
	char*ret = my_strstr(p1, p2);
	if (ret == NULL)
	{
		printf("子串不存在\n");
	}
	else
	{
		printf("子串存在:%s\n", ret);
	}
	return 0;
}

此法当 p1为"abdcddefghi" 也就时存在两个连续的d 时行不通

库函数

#include<stdio.h>
#include<assert.h>
char* my_strstr(const char* p1, const char* p2)
{
	assert(p1 != NULL);
	assert(p2 != NULL);
	if (*p2 == '\0')
	{
		return (char*)p1;
	}
	char* s1 = p1;
	char* s2 = p2;
	char* cur = p1;
	while (*cur)
	{
		s1 = cur;
		s2 = p2;
		while ((*s2!='\0') && (*s1 != '\0') && (*s1 == *s2))//while(*s1 && *s2 && !(s1-*s2))
		{
			s1++;
			s2++;
		}
		if (!*s2)//if(*s2=='\0')
		{
			return cur; //找到子串		
		}
		cur++;
	}
	return NULL;//找不到空指针
}
int main()
{
	char* p1 = "abcddefghi";
	char* p2 = "def";
	char*ret = my_strstr(p1, p2);
	if (ret == NULL)
	{
		printf("子串不存在\n");
	}
	else
	{
		printf("子串存在:%s\n", ret);
	}
	return 0;
}