File Handling in C | Basics of File Handling in C | File Management in C

File handling in C refers to the task of storing data in the form of input or output produced by running C programs in data files for future reference and analysis.

When a collection of related data is stored on any secondary storage medium, it is called data file. Data files are permanent in nature and they allow large quantities of data to be stored on-line and used in pieces as needed.

Once we compile and run the program, the output is obtained, but this output is not stored in the form of information anywhere in the system. If we want to store the output produced for future references, we have to implement file handling.

A file is nothing but a source of storing information permanently in the form of a sequence of bytes on a disk where a group of related data is stored. File is created for permanent storage of data.

Need for File Handling in C:

There is a time when the output generated by compiling and running the program does not serve the purpose. If we want to check the output of the program several times, it becomes a tedious task to compile and run the same program multiple times. This is where file handling comes into play. Here are some of the following reasons behind the popularity of file handling:

·    Reusability: It helps in preserving the data or information generated after running the program.

·    Large storage capacity: Using files, you need not worry about the problem of storing data in bulk.

·    Saves time: There are certain programs that require a lot of input from the user. You can easily access any part of the code with the help of certain commands.

·    Portability: You can easily transfer the contents of a file from one computer system to another without having to worry about the loss of data.

 

Types of Files in C:

In C, data files are of two types: Text Files and Binary Files

Text Files:

A text file contains textual information. These are the simplest files a user can create when dealing with file handling in C.  A carriage return and a line-feed character tab is the data delimiter (marks end of input string). Such files are referred to as sequential files. E.g. program source code, a file created through word processor.

It is created with a .txt extension using any simple text editor. Generally, we use notepads to create text files. Text file stores information in the form of ASCII characters internally, but when we open the text file, we would find the content of the text readable to humans.

Hence, it is safe to say that text files are simple to use and access. But, along with advantages comes disadvantages as well. Since it is easily readable, it does not provide any security of information. Moreover, it consumes large storage space. Hence, there is a different type of file available called binary files which helps us solve this problem.

Binary Files:

A binary file stores information in the form of the binary number system (0’s and 1’s) and hence occupies less storage space. In simple words, it stores information in the same way as the information is held in computer memory. Therefore, it proves to be much easier to access.

In a binary file, the data is not delimited. The entire file is one piece of data. Hence, it is the programmer’s responsibility to dissect the bytes and impart logic to them. Since each byte is data, the file pointer position can be maintained easily. This allows the programmer to retrieve data from and write data to anywhere in the file. It is created with a .bin extension. It overcomes the drawback offered by text files. Since it is not readable to humans, the information is more secure. Hence, it is safe to say that binary files prove to be the best way to store information in a data file.

File Handling Operations:

There are various operations associated with file handling in C. They are:

• Creation of  a new file
• Opening an existing file
• Reading  data from a file
• Writing data to a file
• Moving to a specific location
• Closing a file

 

File Declaration:

Each file used in a program must have a pointer of FILE Type associated with it.

Syntax:  FILE *file_ptr;

Example: FILE *fp;       // variable fp is pointer to type FILE

There are a number of functions that helps to perform basic file operations. Following are the functions:

Function	Description
fopen()	create a new file or open an existing file
fclose()	closes an opened file
getc()	reads a character from a file
putc()	writes a character to a file
fscanf()	reads a set of data from a file
fprintf()	writes a set of data to a file
getw()	reads a integer from a file
putw()	writes a integer to a file
fseek()	Move the file position to a desired point
ftell()	gives current position in the file
rewind()	set the position to the starting point
fread()	Read from a file
fwrite()	Write to a file

Opening a File or Creating a File:

The first step in using a file is to open it. A file can be used in different ways, for reading from or writing to, or perhaps both. Before performing any type of operation, a file must be opened. We open the file by using the function fopen(). This has two parameters, the name of the file to be opened and the mode to use. fopen() returns a file pointer on success or NULL on failure.

Syntax:   

file-pointer=fopen(“FILE NAME ”, “Mode”);

Example:  FILE *fp=fopen( “tstdata.txt”, “r”);  

Here, *fp is the FILE pointer, which will hold the reference to the opened file.

If the files do not exist, or there is a problem opening them, fopen() returns a special value called NULL. When you open a file for reading, you are assuming that the file exists. If it does not, fopen() will give you an error. When you open a file for writing the file may or may not exist, and fopen() will create one for you if it needs to

The modes and their implications are given below:

Mode	Description
r	open existing file for reading
w	open or create a file for writing. If the file specified exists, it is overwritten. If the file does not exist it is created
a	open a text file for appending(adding) data
r+	open file for reading and writing(updating)
w+	open a text file in both reading and writing mode
a+	open a text file in both reading and writing mode
rb	open a binary file in reading mode
wb	open or create a binary file in writing mode
ab	open a binary file in append mode
rb+	open a binary file in both reading and writing mode
wb+	open a binary file in both reading and writing mode
ab+	open a binary file in both reading and writing mode

Closing a File:

Every file must be closed after all I/O operations related to that file are completed. This is to ensure flushing the buffer of that file. Flushing a buffer is essential as it prevents loss of data when writing to disk. When a file is closed, various system resources are freed. The fclose() function is used to close an opened file. It has one parameter, the file pointer whose file needs to be closed.

fclose() returns zero on success and EOF on failure. This would be done using a statement like: 

fclose(file_ptr);

Example:

FILE *p1, *p2;

p1 = fopen(“INPUT.txt”, “r”);

p2 =fopen(“OUTPUT.txt”, “w”);

……..

……..

fclose(p1);

fclose(p2);

Example: Program to open and close a file

#include<stdio.h>

#include<conio.h>

int main()

{

FILE *fp;

fp=fopen(“tstdata.txt”, “w”);

if(fp==0)

printf(“Error in file opening”);

else

printf(“File tstdata.txt opened successfully);

fclose(fp);

return 0;

} 

Reading and Writing a Text File in C:

The input/output operations in a file help us read and write in a file. The simplest functions used while performing operations on reading and writing characters in a file are getc() and putc() respectively.

In order to read and write a set of data in a file, we use the fscanf() and fprintf() functions.

Text File: Reading

Reading a file means copying the contents of the file from secondary storage. The file must be opened in the “r” mode. The function getc(), fgetc(), or fscanf() can be used to achieve this.

getc() and fgetc() functions : Both these functions are to get a character from the stream.

Syntax: getc(stream);

Syntax: fgetc(stream);

Text File: Writing

Write a file means putting information into a file. The stream must be opened in the “w” or the “a” mode. The function putc(), fputc() or fprintf() are available in stdio.h to achieve this.

putc() and fputc() function(): The putc() and fputc() function output a character to a stream. They are similar to the putchar() function, only the putchar() function outputs the character to stdout, while putc() and fputc() output the character to a stream.

Random Access Files:

In ‘C’ language, the data stored in the file can be accessed in two ways:

• Sequential access
• Random access

If we want to read, access record in the middle of the file and if the file size is too large sequential access is not preferable. In this case, random access to file can be used, which allows to access any record directly at any position in the file. We can randomly position file pointer at any position in the data file. Random access means you can move to any part of a file and read or write data from it without having to read through the entire file.

In general, with small files, we access the files sequentially. In sequential access, we access the file record by record or character by character. This approach is appropriate if the file size is small, but what if the file has tens of thousands of records? In random file access, if a file has 10,000 records and if we want to go to the 9,999th record, we can directly point the file pointer to this record using the random file access technique. Random accessing of files in C language can be done with the help of the following functions:

• fseek()
• ftell()
• rewind()

fseek() function: fseek() function is used to take file pointer to a particular position of the data file.

Syntax: 

fseek(Fpointer, Position, Initial);

Fpointer is name of file pointer variable.

Position number of characters to be jumped

Initial specifies the position from where file pointer should be jumped. It can three values :

 

0: From the beginning of file.
1: Current position. 

2: End of file 

 

ftell() function: This function returns the current position of file pointer.

Syntax: ftell(Fpointer)

Fpointer is the name of file pointer variable.

 

rewind(): This function is used to move the file pointer to the beginning of the file. This function is useful when we open file for update.

Syntax: rewind(file pointer);