Using errno in C - Mini Project: Build & Apply

Using errno in C for Error Handling

📖 Scenario: Imagine you are writing a small C program that tries to open a file. Sometimes the file might not exist or you might not have permission to open it. The system sets a special variable called errno to tell you what went wrong.

🎯 Goal: You will write a program that tries to open a file named data.txt. If it fails, you will check the errno variable and print a friendly error message explaining why it failed.

📋 What You'll Learn

Include the errno.h and stdio.h headers

Create a FILE * variable called file and try to open data.txt in read mode

Create an int variable called errnum to store the value of errno

Use perror to print the error message if opening the file fails

Print a success message if the file opens correctly

💡 Why This Matters

🌍 Real World

In real programs, checking <code>errno</code> helps you understand why system calls or library functions fail, so you can handle errors gracefully.

💼 Career

Knowing how to use <code>errno</code> is important for system programming, debugging, and writing reliable C programs that interact with files and the operating system.

1

Include headers and declare file variable

Write the lines to include the stdio.h and errno.h headers. Then declare a variable called file of type FILE *.

C

# Include the standard input/output and errno headers
// Declare a FILE pointer called file

Need a hint?

Use #include <stdio.h> and #include <errno.h>. Declare FILE *file;.

2

Open the file and declare errno variable

Write the line to open the file data.txt in read mode using fopen and assign it to file. Then declare an int variable called errnum and set it to errno.

C

#include 
#include 

FILE *file;

// Open data.txt in read mode and assign to file
// Declare int errnum and assign errno

Need a hint?

Use file = fopen("data.txt", "r") and int errnum = errno;.

3

Check if file opened and print error with perror

Write an if statement to check if file is NULL. Inside the if, use perror with the message "Error opening file". Otherwise, print "File opened successfully.".

C

#include 
#include 

FILE *file;

file = fopen("data.txt", "r");
int errnum = errno;

// Check if file is NULL
// Use perror("Error opening file") if NULL
// Else print success message

Need a hint?

Use if (file == NULL) and inside it perror("Error opening file"). Use else to print success.

4

Print the errno value and close the file if opened

After the if-else, print the value of errnum using printf with the message "errno value: %d\n". If the file is not NULL, close it using fclose(file).

C

#include 
#include 

FILE *file;

file = fopen("data.txt", "r");
int errnum = errno;

if (file == NULL) {
    perror("Error opening file");
} else {
    printf("File opened successfully.\n");
}

// Print errno value
// Close file if not NULL

Need a hint?

Use printf("errno value: %d\n", errnum); and close the file with fclose(file); if it is not NULL.