File Operations in C

In the previous lesson, you learned how to open and close files.

Now we will learn how to actually work with file data — writing data, reading data, and appending data.

These operations are the foundation of file-based programs.

Types of File Operations

There are three basic file operations:

• Writing data to a file
• Reading data from a file
• Appending data to a file

Writing Data to a File

Writing means storing data into a file. If the file already exists, its content will be erased.

We use "w" mode for writing.

#include <stdio.h>

int main() {
    FILE *fp;
    fp = fopen("info.txt", "w");

    fprintf(fp, "C Programming File Operations");

    fclose(fp);
    return 0;
}

This program creates info.txt and writes text into it.

Reading Data from a File

Reading means fetching data from an existing file.

We use "r" mode for reading.

#include <stdio.h>

int main() {
    FILE *fp;
    char ch;

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

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

    fclose(fp);
    return 0;
}

Each character is read one by one until the end of the file.

Appending Data to a File

Appending means adding data at the end of an existing file.

We use "a" mode for appending.

#include <stdio.h>

int main() {
    FILE *fp;
    fp = fopen("info.txt", "a");

    fprintf(fp, "\nLearning at Dataplexa");

    fclose(fp);
    return 0;
}

The new text is added without deleting existing content.

Difference Between File Modes

• w → Write (overwrite existing content)
• r → Read (file must exist)
• a → Append (add data at end)

Real-World Example

Think of a diary:

• Write → start a new diary
• Read → read old pages
• Append → add today's entry

Mini Practice

• Create a file named log.txt
• Write a message into it
• Append another message
• Read and display full content

Quick Quiz

Q1. Which mode is used to write data?

w

Q2. Which function reads characters from a file?

fgetc()

Q3. Which mode keeps old content and adds new data?

a

Q4. What happens if you open a file in "w" mode?

Existing content is erased.

Q5. Is file reading possible if the file does not exist?

No.