Error handling in files in C - Mini Project: Build & Apply

Error handling in files

📖 Scenario: You are writing a simple C program that reads a text file and prints its content to the screen. Sometimes the file might not exist or cannot be opened. You need to handle these errors gracefully.

🎯 Goal: Build a C program that tries to open a file called data.txt, reads its content line by line, and prints each line. If the file cannot be opened, the program should print an error message.

📋 What You'll Learn

Create a FILE* pointer variable called file.

Use fopen to open data.txt in read mode.

Check if file is NULL to detect errors.

If file is NULL, print Error: Cannot open file. and exit.

If the file opens successfully, read lines using fgets and print them.

Close the file after reading.

💡 Why This Matters

🌍 Real World

Reading files safely is important in many programs that process data from files, such as configuration files, logs, or user input files.

💼 Career

Understanding file error handling is essential for software developers to write robust programs that do not crash or behave unexpectedly when files are missing or inaccessible.

1

Create a FILE* pointer called file and open data.txt in read mode

Write code to create a FILE* pointer named file and open the file "data.txt" in read mode using fopen.

C

# Create a FILE* pointer called file and open "data.txt" in read mode
// Your code here

Need a hint?

Use FILE *file = fopen("data.txt", "r"); to open the file.

2

Check if file is NULL and print an error message if it is

Add an if statement to check if file is NULL. If it is, print Error: Cannot open file. using printf and return 1 to exit the program.

C

#include 

int main() {
    FILE *file = fopen("data.txt", "r");
    // Check if file is NULL and print error
    // Your code here
    return 0;
}

Need a hint?

Use if (file == NULL) to check for errors and printf to show the message.

3

Read lines from the file using fgets and print them

Declare a character array line with size 100. Use a while loop with fgets(line, 100, file) to read each line from the file. Inside the loop, print each line using printf.

C

#include 

int main() {
    FILE *file = fopen("data.txt", "r");
    if (file == NULL) {
        printf("Error: Cannot open file.\n");
        return 1;
    }
    char line[100];
    // Read each line from file and print it
    // Your code here
    return 0;
}

Need a hint?

Use while (fgets(line, 100, file) != NULL) to read lines and printf("%s", line) to print.

4

Close the file and finish the program

After the while loop, close the file using fclose(file). Then return 0 to end the program.

C

#include 

int main() {
    FILE *file = fopen("data.txt", "r");
    if (file == NULL) {
        printf("Error: Cannot open file.\n");
        return 1;
    }
    char line[100];
    while (fgets(line, 100, file) != NULL) {
        printf("%s", line);
    }
    // Close the file
    // Your code here
    return 0;
}

Need a hint?

Use fclose(file); to close the file.