Reading and Writing Binary File in C/C++

Writing a Binary File

To write a binary file in C/C++ use fwrite()/write() method. It is used to write a given number of bytes on the given stream, starting at the position of the "put" pointer. The file is extended if the put pointer is currently at the end of the file. If this pointer points into the middle of the file, characters in the file are overwritten with the new data.

If any error has occurred during writing in the file, the stream is placed in an error state.

Syntax of write() method

Following is the basic syntax of writing a binary file:

ostream& write(const char*, int);

Reading a Binary File

To read a binary file in C/C++ use fread()/read() method. It extracts a given number of bytes from the given stream and place them into the memory, pointed to the first parameter. If any error is occurred during reading in the file, the stream is placed in an error state, all future read operation will be failed then.

gcount() can be used to count the number of characters has already read. Then clear() can be used to reset the stream to a usable state.

Syntax of read() method

Following is the syntax to read the binary file:

ifstream& write(const char*, int);

Algorithm

Following is the list of steps to read and write binary file in C/C++:

• Begin
• Create a structure Student to declare variables.
• Open a binary file for writing.
• Check if any error occurs in file opening.
• Initialize the variables with data.
• If the file opens successfully, write the binary data using the write method.
• Close the file after writing.
• Open the binary file for reading.
• Check if any error occurs in file opening.
• If the file opens successfully, read the binary data using the read method.
• Close the file after reading.
• Check if any error occurs.
• Print the data.
• End

Code for Reading and Writing Binary File

In this example, we perform file handling functions like fread()/read() and write ()/write() to read and write binary files in C/C++:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

// Define the structure to store student details
struct Student {
    int roll_no;
    char name[50];
};

int main() {
   // Open the binary file for writing
   FILE *wf = fopen("student.dat", "wb");
   
   // Check if file open successfully
   if (!wf) { 
       printf("Cannot open file!\n");
       return 1;
   }

   // Initialize student data
   struct Student wstu[3];
   wstu[0].roll_no = 1;
   strcpy(wstu[0].name, "Ram");
   wstu[1].roll_no = 2;
   strcpy(wstu[1].name, "Shyam");
   wstu[2].roll_no = 3;
   strcpy(wstu[2].name, "Madhu");

   // Write student data to the file
   for (int i = 0; i < 3; i++)
       fwrite(&wstu[i], sizeof(struct Student), 1, wf);

   // Close the file after writing
   fclose(wf);

   // Open the binary file for reading
   wf = fopen("student.dat", "rb");
   
   // Check if file open sucessfully
   if (!wf) { 
       printf("Cannot open file!\n");
       return 1;
   }

   // Read student data from the file
   struct Student rstu[3];
   for (int i = 0; i < 3; i++)
       fread(&rstu[i], sizeof(struct Student), 1, wf);

   // Close the file after reading
   fclose(wf);

   // Display student details
   printf("Students Details:\n");
   for (int i = 0; i < 3; i++) {
       printf("Roll No: %d\n", rstu[i].roll_no);
       printf("Name: %s\n", rstu[i].name);
       printf("\n");
   }

   return 0;
}

Students Details:
Roll No: 1
Name: Ram

Roll No: 2
Name: Shyam

Roll No: 3
Name: Madhu

#include <iostream>
#include <fstream>
using namespace std;

// Define the structure to store student details
struct Student {
  int roll_no;
  string name;
};

int main() {
  // Open a binary file for writing
  ofstream wf("student.dat", ios::out | ios::binary);

  // Check if file open successfully
  if (!wf) {
    cout << "Cannot open file!" << endl;
    return 1;
  }

  // Initialize student data
  Student wstu[3];
  wstu[0].roll_no = 1;
  wstu[0].name = "Ram";
  wstu[1].roll_no = 2;
  wstu[1].name = "Shyam";
  wstu[2].roll_no = 3;
  wstu[2].name = "Madhu";

  // Write student data to the file
  for (int i = 0; i < 3; i++)
    wf.write((char*)&wstu[i], sizeof(Student));

  // Close the file after writing
  wf.close();

  // Check if writing was successful
  if (!wf.good()) {
    cout << "Error occurred at writing time!" << endl;
    return 1;
  }

  // Open the binary file for reading
  ifstream rf("student.dat", ios::in | ios::binary);

  // Check if file open successfully
  if (!rf) {
    cout << "Cannot open file!" << endl;
    return 1;
  }
  
  // Read student data from the file
  Student rstu[3];
  for (int i = 0; i < 3; i++)
    rf.read((char*)&rstu[i], sizeof(Student));

  // Close the file after reading
  rf.close();

  // Check if reading was successful
  if (!rf.good()) {
    cout << "Error occurred at reading time!" << endl;
    return 1;
  }
  
  // Display student details
  cout << "Students Details:" << endl;
  for (int i = 0; i < 3; i++) {
    cout << "Roll No: " << rstu[i].roll_no << endl;
    cout << "Name: " << rstu[i].name << endl;
    cout << endl;
  }
  
  return 0;
}

Students Details:
Roll No: 1
Name: Ram

Roll No: 2
Name: Shyam

Roll No: 3
Name: Madhu