C ++指针和变量分配

Question

So, I am facing a very very unusual problem with my program. Here is my program

    void insertItem (Song *&, Song *&, char ar [], int);
    void printList (Song *);
    void deleteList (Song *&, Song *&);
    void processList (Song *&);

    int main (int argc, char * argv[]) {    
    char * filename = argv[1]; char line [31];


int m, n, a;
------------


    /*
        Head and tail pointer to maintain a list of songs
    */
    Song *head;
    Song *tail;

    if (filename == NULL)
        exit(1);    

    fstream fio;
    fio.open (filename, ios::in);



**fio >> m; fio >> n;**
-----------------------


    // Reading the file
    while (!fio.eof()){


**fio >> a;**
-------------

        fio >> line;


**insertItem (head, tail, line, a);**
-------------------------------------

    }
    fio.close();

    printList (head);



**for (int b = 0; b < n; ++b) processList (head);**
---------------------------------------------------


    cout << "\nDeleting List";
    deleteList (head, tail);
    return 0;
    }

I declare 3 static variables (m,n, and a) and I use all three of them. The problem I am facing is that as soon as try to initialize a variable b in the for loop, I get a segmentation fault. I debugged it using gdb and the value of the head pointer changes when I try to use the variable b in the for loop. On the contrary, if I use a previously used variable like m or a instead of making a new variable for the for loop, it runs absolutely fine. The address of the head pointer doesn't change. Any clue on what might be happening here?

Answer 1

There are at least two errors in your code: you never initialize head and tail (so anything you do with them is undefined behavior), and you don't check that your input has succeeded before using it. (Your first loop should probably be while ( fio >> line ) . But you don't want to use m or n either, unless you've checked that the input succeeded.)