C++查找子串

void sub_string(char str[],char sub[])
{

 int num=0,j=0;
 int len = strlen(str);
 int sublen =strlen(sub);


 if (sublen>len)
 {//子串比总串长
  cout<<"sublen more than len ,error";
  exit(1);
 }


 for(int i = 0;i<=len-sublen+1;i++)
 {
  if (str[i] == sub[0])
  {
   for (j=0;j<sublen&&(i+j)<len&&(str[i+j]==sub[j]);j++)  ;
   if(j==sublen)  {num = i;j=0;}
  }
  
 }
 if(num!=0)
     cout<<num;
 else
  cout<<"not found";
}