reading bytes using fstream c++

Question

I created a read / write of bytes into a binary file using fstream object

#include <iostream>
#include <fstream>

using namespace std;

#define COL_WIDTH 20

int writeBinaryFile(char *desPath);
int readBinaryFile(char *desPath);

int age;
char name[COL_WIDTH];
int recsize = sizeof(int)+sizeof(name);
int n;

int main(){

    char desPath[MAX_PATH+1];
    char input[2];

    while(true){
        cout  << "Main Menu:\n1 Write Binary File\n2 Read Binary File\n3 EXIT" << endl;
        cin.getline(input, 2);          


        if(atoi(input)== 1){
            cout << "Enter destination path:" << endl;
            cin.getline(desPath, MAX_PATH+1);
            for(;;){
                int output = writeBinaryFile(desPath);
                if(output == 1) break;
                else if(output == 2) { *input = '4'; break;}
            }
        }
        else if(atoi(input) == 2){
            cout << "Enter destination path:" << endl;
            cin.getline(desPath, MAX_PATH+1);
            for(;;){
                int output = readBinaryFile(desPath);
                if(output == 1) break;
                else if(output == 2){ *input = '4'; break;}
            }
        }

        else if(atoi(input) == 3)
            break;
        else cout << "You entered wrong input, enter only 1 or 2." << endl;

        if(atoi(input) == 4) continue;
        else break;

    }

    system("pause");
    return 0;   
}

int writeBinaryFile(char *desPath){
    char option[2];
    fstream wBin(desPath, ios::binary | ios::out);
    if(!wBin){
        cout << desPath << " is not good path."<< endl;
        return 1;
    }
    cout << "Enter name: " << endl;
    cin.getline(name,COL_WIDTH);
    cout << "Enter age: " << endl;
    cin >> age;

    cout << "Enter record number: " << endl;
    cin >> n;

    wBin.seekp(n*recsize); 
    wBin.write(name, sizeof(name));
    wBin.write((char*)&age, sizeof(age));
    wBin.close();

    cout << "Enter record again? Press:" << endl
         << "Enter to continue" << endl
         << "Q to quit" << endl
         << "M to go back to main menu" << endl;

    cin.ignore(256,'\n');
    cin.getline(option,2);

    if(option[0] == 'q' ||option[0] == 'Q') return 1;
    else if(option[0] == 'm' ||option[0] == 'M') return 2;

    return 0;
}


int readBinaryFile(char *desPath){       
    char option[2];
    fstream rBin(desPath, ios:: binary | ios:: in);
    if(! rBin){
        cout << "File not found!" << endl;
        return 1;
    }
    cout << "What record number?" <<endl;
    cin >> n;
    rBin.seekp((n*recsize));
    rBin.read(name,sizeof(name));
    rBin.read((char*)&age, sizeof(int));


    cout << name <<endl;
    cout << age << endl;
    rBin.close();
    cout << "Print more? Press enter. Press Q to QUIT or M to go back." << endl;
    cin.clear();
    cin.sync();
    cin.getline(option, 2);

    if(option[0] == 'q'|| option[0] == 'Q'){rBin.close(); return 1;}
    else if(option[0] == 'm' || option[0] == 'M'){rBin.close(); return 2;}

    return 0;
}

i created a binary file first with name, age, recordnumber ( position of bytes which is 0) then on second loop i input name, age, recordnumber 2. When i tried to read the first pair(char*, int) of bytes pos(0) but it returns wrong result but when i tried to read the last pair in pos(1) it returns correct result.

I also tried making 3 times the input, but only the last char* and int is correct.

Why is it getting wrong results in first bytes(name(20bytes),age(4bytes)) but getting correct it last bytes?

What is your question? What are your wrong and right results? — Stealth Rabbi
– Stealth Rabbi, Commented Nov 3, 2011 at 20:17
I don't immediately see anything wrong with it. I'd have to run it to find the problem. Have you stepped through it as it ran? — Mooing Duck
– Mooing Duck, Commented Nov 3, 2011 at 20:21
when i created the bin file i input ff: name:name1, age:1, recordnum:0 then on second loop name2,2,1 then on third loop name3,3,2...i expect to get when i enter 0 for recordnum ..name1, 1 but i just got blank,0...same with recordnum 1..but with recordnum 2, i did get correct which is name3, 3... — jko
– jko, Commented Nov 3, 2011 at 20:28
In each call to writeBinaryFile the output is reopened, destroying the previous contents of the file. — Robᵩ
– Robᵩ, Commented Nov 3, 2011 at 20:29
Yes but as @Rob said, since you're opening it with only ios::out, the previous file contents will be destroyed. You'll want to use ios::binary | ios::ate | ios::out | ios::in — JoeFish
– JoeFish, Commented Nov 3, 2011 at 20:37

2 revs · Accepted Answer · 2011-11-03 21:04:50Z

2

In each call to writeBinaryFile the output is reopened, destroying the previous contents of the file.

You'll want to use ios::binary | ios::ate | ios::out | ios::in.

But, this bitmask will not create the file if it does not exist. You can work around that problem with this code sequence:

int writeBinaryFile(char *desPath) {
  ...
  // Create the file only if it doesn't exist
  fstream(desPath, ios::binary|ios::out|ios::app);

  // Now it exists -- open it
  fstream wBin(desPath, ios::binary|ios::out|ios::in);
  ...
}

Credit @JoeFish for the useful part of the answer.

edited Nov 3, 2011 at 21:04

community wiki

2 revs
Robᵩ

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