Segmentation fault in Data file handling in c++ [duplicate] - c++

Question

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Why does C++ reject appending a structure to a binary file if it contains a member of the string class?
(2 answers)
Closed 5 years ago.
I've written a small code in c++ to understand data file handling. The program consists of accepting entries, writing them to a .dat file and searching for a particular entry. So far, only the writing function is working, the reading and the searching function give a segmentation fault error. What's going wrong?
#include<iostream>
#include<string>
#include<fstream>
#include<stdlib.h>
using namespace std;
class data_base
{
string name;
long int no;
public:
void input()
{
cout<<"\nEnter name:";
cin>>name;
cout<<"Enter ph number:";
cin>>no;
}
void display()
{
cout<<name<<"\t"<<no;
}
string retname()
{
return name;
}
long int retno()
{
return no;
}
};
void display_all()
{
data_base d;
fstream in;
in.open("database.dat",ios::in|ios::binary);
if(!in.is_open())
cout<<"Error opening file";
else{
while(in.read((char *)&d,sizeof(d)))
{
d.display();
}
}
in.close();
}
void search_name()
{
data_base d;
fstream in;
string s;
in.open("database.dat",ios::in|ios::binary);
if(!in.is_open())
cout<<"Error opening file";
else{
cout<<"\nEnter name to be searched:";
cin>>s;
while (in.read((char *) &d,sizeof(d))) {
if(s==d.retname())
{
d.display();
}
}
}
in.close();
}
void search_no()
{
data_base d;
fstream in;
long int l;
in.open("database.dat",ios::in|ios::binary);
if(!in.is_open())
cout<<"Error opening file";
else{
cout<<"\nEnter number to be searched:";
cin>>l;
while (in.read((char *) &d,sizeof(d)))
{
if(l==d.retno())
{
d.display();
}
}
}
in.close();
}
int main()
{
int ch;
fstream file;
data_base d,e;
string s;
while(1)
{
cout<<"1.Add entry\n2.Search by name\n3.Search by
number\n4.Display all entries\5.Exit"<<endl;
cin>>ch;
switch (ch) {
case 1: d.input();
file.open("database.dat",ios::out|ios::app|ios::binary);
if(!file.is_open())
cout<<"Error opening file";
else
{
file.write((char *)&d,sizeof(d));
cout<<"Entry added!"<<endl;
}
file.close();
break;
case 2:search_name();
break;
case 3:search_no();
break;
case 4:display_all();
break;
case 5: exit(0);
}
}
}

Answer 1

class data_base
{
string name;
...
}
...
data_base d;
...
file.write((char *)&d,sizeof(d));
You are already doomed here. The std::string struct has a pointer inside. Reinterpreting this pointer as a char array gives you some sequence of chars. You save it to a file and then read it some time later.
At that moment your newly read data_base variable has exact binary representation as the one before. But now the internal pointer of std::string no longer points to a valid memory address, hence segfault.
Note that segfaulting is actually good. In some bad case the memory address would be valid but it would point to some completely unrelated (random) data. That would be a bug extremely hard to find.
You are doing this wrong from the begining. Don't cast to/from char* for serialization/deserialization of data. Use proper serialization techniques, e.g. convert the structure to xml, json, google protobuf or even come up with something custom. Just don't reinterpret raw memory. Even if there are no pointers inside your struct the binary representation is compiler, os and cpu dependent.