编译结果总是蛋的情况，求分析。

//在字符串str中找字符串substr出现的次数
#include<iostream>
#include<cstring>
using namespace std;
void  SubStrNum(char *str,char *substr);
int main()
{
    char str1[1000];
    char substr[] = {'s','u','b','s','t','r','\0'};
    cin>>str1;
    SubStrNum(str1,substr);
    return 0;
}
void  SubStrNum(char *str,char *substr)
{
    int m = 0;
    char *sp = str;
    while(*sp != '\0')
    {
        char *rp = substr;
        int k = 0;
        while(*sp == *rp && k<6)
        {
            sp++;
            rp++;
            k++;
        }
        if(k == 5)
        m++;
    }
    cout<<m<<endl;
}