//功能:在s1中如果不能找到s2,则返回"Substring is not found.\n",反之.
#include <stdio.h>

int find_substr(char *s1, char *s2);//函数声明

void main()
{
if(find_substr("C is fun","un") != -1)
{
printf("Substring is found.\n");
}
else
{
printf("Substring is not found.\n");
}
}

int find_substr(char *s1, char *s2)
{
int t;
char *p, *p2;

for(t = 0; s1[t]; t++)//字符串常量是存在数组中的s1[0]就是字符串第一个字符
{
p = &s1[t];
p2 = s2;

while(*p2 && (*p2 == *p))//先比较s1,s2的值是不是相等的,不相等就跳过
//这里的与运算是测试P2是不是到最后字符了就是'\0'
{
p++;
p2++;
}

if(*p2 == '\0')
return t;
}

return -1;
}