Segmentation Fault while appending and reading a binary file

Question

I'm making a login system which will have saved usernames and passwords in a binary file so if any username or password is matched, it should show "matched!" it works fine with txt file but giving segmentation faults in binary mode

I tried to put f.read() outside the loop and read it manually. i also tried to terminate loop when !f.eof() becomes true but in all cases the same problem happens

here is the code

#include<iostream>
#include<fstream>

using namespace std;
class Login {
 public:
 string username;
 int pass;
 Login() {
 }
 Login(string u,int p) {
  username = u;
  pass =p;
 }
};

int main() {
 fstream f("sarah.bin",ios::in|ios::app|ios::binary);
 if(!f) cout<<"error"<<endl;
 //adding data,take data from user
 Login l("ahmed",123),k;
 f.write((char*)&l,sizeof(l));
 bool logged = false;
 string s = "ahmed";
 int p = 123;
 f.seekg(0,ios::beg);
 //verification
 while(!f.eof() && f.read((char*)&k,sizeof(l))) {
    if(k.pass==p && k.username == s) {
        cout<<"logged";
        logged = true;
        break;
     }
 }
 if(!logged)
 cout<<"not matched";
 f.close();
}

It is not giving expected result but giving segmentation fault and sometimes no output at all.

Writing/reading non-POD types with `std::fstream::write`/`std::fstream::read` can cause UB. — Algirdas Preidžius, Apr 30 '19 at 16:38
@MuhammadAhmed You don't have a POD type, there's `std::string` involved. — πάντα ῥεῖ, Apr 30 '19 at 16:42
***how can i overcome the problem ?*** Use proper serialization possibly getting rid of the binary file and using text instead. — drescherjm, Apr 30 '19 at 16:43
@MuhammadAhmed You can look into [serialization](https://stackoverflow.com/questions/234724/is-it-possible-to-serialize-and-deserialize-a-class-in-c). — Algirdas Preidžius, Apr 30 '19 at 16:44
Why are passwords `int`egers? And why are you seemingly storing credentials in the plain, rather than hashing them? — Jesper Juhl, Apr 30 '19 at 16:50
And why the `using namespace std;` and all the C-style casts and magic numbers all over? This code needs a clean-up pass or two. — Jesper Juhl, Apr 30 '19 at 16:55
the problem is f.write((char*)&l,sizeof(l)); .. std::string eventually will hold only a pointer to the data (implementation dependent of multiple factors, like string length etc.) .. for this kind of problem just use text serialization with some encrypt/decrypt algorithm, like blowfish for example — StPiere, Apr 30 '19 at 16:59

score 0 · Accepted Answer · answered Apr 30 '19 at 17:25

0

Actually the problem was: I was using non-POD data type .i.e string which was causing the error so the solution I did was I replaced string with character array. another solution is to write it in txt file.

answered Apr 30 '19 at 17:25

Graphics Engineer

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Segmentation Fault while appending and reading a binary file

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