File Handling in C

Until now, all the data you've worked with in C programs existed only while the program was running. Once the program terminated, all variables stored in memory were lost.

Imagine you're developing KauraX, where you need to store student details, quiz scores, or course progress. If everything disappears after closing the program, the application becomes useless.

To solve this problem, C provides File Handling, allowing programs to store data permanently on disk.

Files make it possible to save information, retrieve it later, update records, and share data between different programs.

🎯 Learning Objectives

After completing this lesson, you will be able to:

Understand files and file handling.
Open and close files.
Read from files.
Write to files.
Append data to existing files.
Understand different file modes.
Handle file errors.
Work with text and binary files.
Build simple file-based applications.

📖 What is a File?

A file is a collection of data stored permanently on a storage device such as a hard disk or SSD.

Unlike variables stored in RAM, file data remains available even after the program ends.

Examples:

Student records
Employee database
Marksheets
Configuration files
Log files

Why Use Files?

Without files:

text
Run Program

↓

Enter Data

↓

Close Program

↓

Data Lost

With files:

text
Run Program

↓

Enter Data

↓

Save to File

↓

Close Program

↓

Data Still Exists

File Pointer

In C, every file is handled using a file pointer.

Syntax

c
FILE *fp;

FILE is a structure defined in:

c
#include <stdio.h>

Opening a File

The fopen() function opens a file.

Syntax

c
FILE *fp;

fp = fopen("filename.txt", "mode");

Example

c
FILE *fp;

fp = fopen("students.txt", "w");

If the file cannot be opened,

c
fp == NULL

Closing a File

Always close files after use.

Syntax

c
fclose(fp);

Example

c
fclose(fp);

File Modes

Mode	Description
`"r"`	Read existing file
`"w"`	Write (creates new file or overwrites existing file)
`"a"`	Append data
`"r+"`	Read and Write
`"w+"`	Read, Write, Create
`"a+"`	Read and Append

Writing to a File

Use fprintf().

Example

c
#include <stdio.h>

int main()
{
    FILE *fp;

    fp = fopen("student.txt","w");

    if(fp == NULL)
    {
        printf("Unable to open file");
        return 1;
    }

    fprintf(fp,"Welcome to KauraX\n");
    fprintf(fp,"C Programming Course\n");

    fclose(fp);

    printf("Data Written Successfully");

    return 0;
}

Reading from a File

Use fscanf().

Example

c
#include <stdio.h>

int main()
{
    FILE *fp;

    char name[50];

    fp = fopen("student.txt","r");

    if(fp == NULL)
    {
        printf("File Not Found");
        return 1;
    }

    fscanf(fp,"%s",name);

    printf("%s",name);

    fclose(fp);

    return 0;
}

Reading an Entire Line

Use fgets().

Example

c
char line[100];

fgets(line,sizeof(line),fp);

printf("%s",line);

Writing a Character

Use fputc().

Example

c
FILE *fp;

fp = fopen("letters.txt","w");

fputc('A',fp);

fclose(fp);

Reading a Character

Use fgetc().

Example

c
char ch;

ch = fgetc(fp);

Writing a String

Use fputs().

Example

c
fputs("KauraX Learning Platform",fp);

Reading Until End of File

c
char ch;

while((ch=fgetc(fp))!=EOF)
{
    printf("%c",ch);
}

EOF means End Of File.

Appending Data

Append mode keeps existing data and adds new content.

Example

c
FILE *fp;

fp = fopen("student.txt","a");

fprintf(fp,"New Student Added\n");

fclose(fp);

Error Handling

Always check whether a file was opened successfully.

Example

c
FILE *fp;

fp = fopen("data.txt","r");

if(fp==NULL)
{
    printf("Unable to Open File");
    return 1;
}

Text Files vs Binary Files

Text File

Stores readable text.

Example

text
Pritee
95
Bangalore

Functions:

fprintf()
fscanf()
fgets()
fputs()

Binary File

Stores raw bytes.

Functions:

c
fwrite()

fread()

Binary files are:

Faster
Smaller
Not human-readable

Writing Binary Data

Example

c
#include <stdio.h>

struct Student
{
    char name[30];
    int marks;
};

int main()
{
    FILE *fp;

    struct Student s =
    {
        "Pritee",
        95
    };

    fp = fopen("student.dat","wb");

    fwrite(&s,sizeof(s),1,fp);

    fclose(fp);

    return 0;
}

Reading Binary Data

Example

c
#include <stdio.h>

struct Student
{
    char name[30];
    int marks;
};

int main()
{
    FILE *fp;

    struct Student s;

    fp = fopen("student.dat","rb");

    fread(&s,sizeof(s),1,fp);

    printf("%s\n",s.name);

    printf("%d",s.marks);

    fclose(fp);

    return 0;
}

Real-World Example

Suppose KauraX needs to save student progress.

Instead of storing data in variables,

text
Student Name

↓

Marks

↓

Course Progress

the program writes these values into a file so they remain available even after restarting the application.

Common Beginner Mistakes

Forgetting to Close Files

Wrong

c
fopen(...);

Always use

c
fclose(fp);

Not Checking NULL

Wrong

c
fp = fopen(...);

fprintf(fp,...);

Always verify

c
if(fp==NULL)

Opening Wrong Mode

Example

c
"w"

overwrites existing data.

If you want to preserve old data, use

c
"a"

Reading Beyond EOF

Always stop when

c
EOF

is reached.

⚠️Warning

Opening a file in "w" mode deletes all existing contents before writing new data. Use "a" if you want to preserve existing information.

Best Practices

Always check if fopen() succeeds.
Close every file using fclose().
Use append mode when adding records.
Validate user input before writing to files.
Prefer binary files for large structured data.

🧠 Interview Questions

What is file handling?
What is a file pointer?
Explain fopen().
Explain different file modes.
Difference between "w" and "a" mode.
Explain fprintf() and fscanf().
Explain fread() and fwrite().
What is EOF?
Difference between text and binary files?
Why should files always be closed?

✍ Practice Questions

Create a file and write your name.
Read data from a text file.
Copy contents from one file to another.
Count characters in a file.
Count words in a file.
Store student details in a file.
Read student records from a file.
Append new records to an existing file.
Write a structure into a binary file.
Read a structure from a binary file.

📚 Lesson Summary

In this lesson, you learned:

What files are.
File pointers.
Opening and closing files.
Reading and writing text files.
Reading and writing binary files.
File modes.
EOF.
Error handling.
Best practices.

File handling enables programs to store and retrieve data permanently, making applications practical and useful beyond a single execution.

🚀 What's Next?

In the next lesson, Preprocessor Directives & Macros, you'll learn:

#include
#define
Macros
Header Files
Conditional Compilation (#ifdef, #ifndef, #if, #endif)
Include Guards
Built-in Macros
Practical Examples

The C preprocessor prepares your source code before compilation, making programs more modular, reusable, and configurable.

Happy Coding with KauraX 💙

File Handling

File Handling in C

🎯 Learning Objectives

📖 What is a File?

Why Use Files?

File Pointer

Opening a File

Closing a File

File Modes

Writing to a File

Reading from a File

Reading an Entire Line

Writing a Character

Reading a Character

Writing a String

Reading Until End of File

Appending Data

Error Handling

Text Files vs Binary Files

Text File

Binary File

Writing Binary Data

Reading Binary Data

Real-World Example

Common Beginner Mistakes

Forgetting to Close Files

Not Checking NULL

Opening Wrong Mode

Reading Beyond EOF

Best Practices

🧠 Interview Questions

✍ Practice Questions

📚 Lesson Summary

🚀 What's Next?

Practice in KauraC

Dynamic Memory Allocation

Preprocessor Directives