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C File Handling
In this tutorial, you will learn about file handling in C. You will learn to handle standard I/O in C using fprintf(), fscanf(), fread(), fwrite(), fseek() etc. with the help of examples.


A file is a container in computer storage devices used for storing data.

Why files are needed?
When a program is terminated, the entire data is lost. Storing in a file will preserve your data even if the program terminates.
If you have to enter a large number of data, it will take a lot of time to enter them all.
However, if you have a file containing all the data, you can easily access the contents of the file using a few commands in C.
You can easily move your data from one computer to another without any changes.
Types of Files
When dealing with files, there are two types of files you should know about:

Text files
Binary files
1. Text files
Text files are the normal .txt files. You can easily create text files using any simple text editors such as Notepad.

When you open those files, you'll see all the contents within the file as plain text. You can easily edit or delete the contents.

They take minimum effort to maintain, are easily readable, and provide the least security and takes bigger storage space.

2. Binary files
Binary files are mostly the .bin files in your computer.

Instead of storing data in plain text, they store it in the binary form (0's and 1's).

They can hold a higher amount of data, are not readable easily, and provides better security than text files.

File Operations
In C, you can perform four major operations on files, either text or binary:

Creating a new file
Opening an existing file
Closing a file
Reading from and writing information to a file
Working with files
When working with files, you need to declare a pointer of type file. This declaration is needed for communication between the file and the program.

FILE *fptr;
Opening a file - for creation and edit
Opening a file is performed using the fopen() function defined in the stdio.h header file.

The syntax for opening a file in standard I/O is:

ptr = fopen("fileopen","mode");
For example,

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");
Let's suppose the file newprogram.txt doesn't exist in the location E:\cprogram. The first function creates a new file named newprogram.txt and opens it for writing as per the mode 'w'.
The writing mode allows you to create and edit (overwrite) the contents of the file.
Now let's suppose the second binary file oldprogram.bin exists in the location E:\cprogram. The second function opens the existing file for reading in binary mode 'rb'.
The reading mode only allows you to read the file, you cannot write into the file.
Opening Modes in Standard I/O
Mode	Meaning of Mode	During Inexistence of file
r	Open for reading.	If the file does not exist, fopen() returns NULL.
rb	Open for reading in binary mode.	If the file does not exist, fopen() returns NULL.
w	Open for writing.	If the file exists, its contents are overwritten.

				If the file does not exist, it will be created.
wb	Open for writing in binary mode.	If the file exists, its contents are overwritten.

				If the file does not exist, it will be created.
a	Open for append.

				Data is added to the end of the file.	If the file does not exist, it will be created.
ab	Open for append in binary mode.

				Data is added to the end of the file.	If the file does not exist, it will be created.
r+	Open for both reading and writing.	If the file does not exist, fopen() returns NULL.
rb+	Open for both reading and writing in binary mode.	If the file does not exist, fopen() returns NULL.
w+	Open for both reading and writing.	If the file exists, its contents are overwritten.

				If the file does not exist, it will be created.
wb+	Open for both reading and writing in binary mode.	If the file exists, its contents are overwritten.

				If the file does not exist, it will be created.
a+	Open for both reading and appending.	If the file does not exist, it will be created.
ab+	Open for both reading and appending in binary mode.	If the file does not exist, it will be created.
Closing a File
The file (both text and binary) should be closed after reading/writing.

Closing a file is performed using the fclose() function.

fclose(fptr);
Here, fptr is a file pointer associated with the file to be closed.

Reading and writing to a text file
For reading and writing to a text file, we use the functions fprintf() and fscanf().


They are just the file versions of printf() and scanf(). The only difference is that fprint() and fscanf() expects a pointer to the structure FILE.

Example 1: Write to a text file
#include <stdio.h>
#include <stdlib.h>

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}
This program takes a number from the user and stores in the file program.txt.

After you compile and run this program, you can see a text file program.txt created in C drive of your computer. When you open the file, you can see the integer you entered.

Example 2: Read from a text file
#include <stdio.h>
#include <stdlib.h>

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}
This program reads the integer present in the program.txt file and prints it onto the screen.

If you successfully created the file from Example 1, running this program will get you the integer you entered.

Other functions like fgetchar(), fputc() etc. can be used in a similar way.

Reading and writing to a binary file
Functions fread() and fwrite() are used for reading from and writing to a file on the disk respectively in case of binary files.

Writing to a binary file
To write into a binary file, you need to use the fwrite() function. The functions take four arguments:

address of data to be written in the disk
size of data to be written in the disk
number of such type of data
pointer to the file where you want to write.
fwrite(addressData, sizeData, numbersData, pointerToFile);
Example 3: Write to a binary file using fwrite()
#include <stdio.h>
#include <stdlib.h>

struct threeNum
{
   int n1, n2, n3;
};

int main()
{
   int n;
   struct threeNum num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.bin","wb")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   for(n = 1; n < 5; ++n)
   {
      num.n1 = n;
      num.n2 = 5*n;
      num.n3 = 5*n + 1;
      fwrite(&num, sizeof(struct threeNum), 1, fptr); 
   }
   fclose(fptr); 
  
   return 0;
}