Question

I have this following program which gets the number of files from user, then the filename and reads the content of the file in to a priority queue. When I execute the program, after I enter the first filename it gives segmentation fault.

#include <cstdlib>
#include <ctime>
#include <functional>
#include <iostream>
#include <queue>
#include <fstream>
using namespace std;

int main() {
    char *filename;
    int fnum;

    cout<<"Number of files:"<<endl;
    cin>>fnum;

    int i;
    priority_queue<int, vector<int>, greater<int> > pqi;
    for(i = 0; i<fnum;i++){
        cout <<"Enter Filename:"<<endl;
        cin>>filename;
        ifstream inFile(filename);
        long n;
        while(!inFile.eof()){
            inFile >> n;
            pqi.push(n);
        }
        inFile.close();
        inFile.clear();
    }
    while(!pqi.empty()) {
        cout << pqi.top() << ' ';
        pqi.pop();
    }
}

Not able to figure out why.

Answer 1

The problem is with your char* definition. You just define a pointer and don’t allocate any memory to it. You have to allocate memory to it using the new keyword:

char *filename = new char[256];
//... rest of your code ...
//When you no longer need filename (usually at the end of the code)
//you have to free the memory used by it manually:
delete[] filename;

In this simple case you can also use a static array:

char filename[256];
//No need to delete[] anything in this way.

Both of the above ways allocate a fixed amount of memory to filename, which means if user enters a file name longer than 256 bytes in the above examples we encounter a buffer overflow. You can use string type which automatically does the memory management for you and is easy to use:

#include <string>
string filename;
cin >> filename;