c++ - 当遇到很​​长的循环时，得到 Segmentation Fault :11 or memory allocation error

Question

代码如下，代码中比较麻烦的部分是getTop()函数:calculate() 函数中有一个 while 循环，输入文件非常大，当我尝试使用 2000 行输入文件的小版本来处理时，程序从未出错。但是当我尝试一个有 130,000 行输入文件的大版本时，我基本上得到了两种错误。

#include "UserUserSim.h"
UserUserSim::UserUserSim(string &query_url):
    _query_url(query_url)
{

}

void UserUserSim::calculate(ostream &out){
    ifstream infile(_query_url.c_str());
    string line;
    int movie_id;
    int user_id;
    MovieList* ml;
    while(infile>>line){
        if (line[line.length()-1]==':'){
            movie_id=atoi(line.c_str());
            ml=MovieDictionary::getInstance().getMovie(movie_id);
            ml->sortList();
            out<<line<<endl;
        }
        else{
            user_id=atoi(line.c_str());
            if (_map.find(user_id)==_map.end())
                getTop(user_id);
            float score=getScore(user_id,ml);
            out<<score<<endl;
        }
    }
}
float UserUserSim::getScore(int &user_id, MovieList* ml){
    vector<USim>* p=_map[user_id];
    vector<UserScore>::iterator it=ml->begin();
    vector<USim>::iterator sim_it=p->begin();
    float score=0;
    float score2=0;
    int total_num=0;
    float total_weight=0;
    int it_user_id;
    int sim_it_user_id;
    while( it != ml->end() && sim_it != p->end()){
        it_user_id=(*it).user_id;
        sim_it_user_id=(*sim_it).user_id;
        //cout<<it_user_id<<"  sdfsd "<<sim_it_user_id<<endl;
        if (it_user_id>sim_it_user_id)
            sim_it++;
        else if (it_user_id<sim_it_user_id)
            it++;
        else{ // when the user id matches
            score+=(*it).rating*(*sim_it).score;
            score2+=(*it).rating;
            total_num++;
            total_weight+=(*sim_it).score;
            sim_it++;
            it++;
        }
    }
    if (total_weight!=0)
        score=score/total_weight;
    else
        score=3.37827;//score2/total_num;
    return score;
}
typedef pair<int, float> mapPair;
bool compareSim(USim p1, USim p2){
    return p1.score>p2.score;
}
bool compareID(USim p1, USim p2){
    return p1.user_id<p2.user_id;
}
void UserUserSim::getTop(int user_id){
    vector<USim>* p=new vector<USim>;
    _map.insert(pair<int,vector<USim>*>(user_id,p));

    UserList* ul=UserDictionary::getInstance().getUser(user_id);
    vector<MovieScore>::iterator it;
    vector<UserScore>::iterator it_movie; // the iterator for the movielist
    vector<USim> score_list;
    vector<USim>::iterator it_s1;
    vector<USim>::iterator it_s2;
    for (it=ul->begin();it!=ul->end();++it){
        vector<USim> new_score_list;
        int movie_id=(*it).movie_id;
        it_s1=score_list.begin();
        MovieList* ml=MovieDictionary::getInstance().getMovie(movie_id);
        for(it_movie=ml->begin();it_movie!=ml->end();it_movie++){
            int user_id=(*it_movie).user_id;
            int rating=(*it_movie).rating;
            while( it_s1!=score_list.end() && user_id>(*it_s1).user_id){
                new_score_list.push_back((*it_s1));
                it_s1++;
            }
            USim us;
            us.user_id=user_id;     
            if (it_s1!=score_list.end() && user_id==(*it_s1).user_id){
                us.score=(*it_s1).score+rating*(*it).rating;// old allocated score+ new score
                it_s1++;
            }else{
                us.score=rating*(*it).rating;
            }
            new_score_list.push_back(us);// the vector's user rating x the rating of the movie,user
        }
        //copy the new_score_list into the score_list:
        score_list.clear();
        for (it_s2=new_score_list.begin();it_s2!=new_score_list.end();++it_s2)
            score_list.push_back(*it_s2);
    }
    int k=10;
    partial_sort(score_list.begin(),score_list.begin()+k,score_list.end(),compareSim);
    vector<USim>::iterator v_it;
    //store these users into the user_sim vector
    for (v_it=score_list.begin();v_it<score_list.begin()+k;++v_it){
        USim us=(*v_it);
        p->push_back(us);
    }
    sort(p->begin(),p->end(),compareID);
}

这是错误 I:

recommend(71304) malloc: *** error for object 0x7f96d3ec8618: incorrect checksum for freed object - object was probably modified after being freed.
    *** set a breakpoint in malloc_error_break to debug
    Abort trap: 6

或错误二:

Segmentation Fault:11

我检查了每次报错时的断点，都是不一样的，在第50000行到第100000行左右。所以我猜这是一个内存问题。而且我在运行代码之前也输入了ulimit，错误仍然发生。真的希望有人告诉我错误在哪里，或者至少教我如何调试内存问题。

Answer 1

在方法 UserUserSim::calculate 中你有

MovieList* ml;

并且您从未对其进行初始化，也许当您调用 UserUserSim::getScore 方法时，您正在传递一个可能导致段错误的空指针。