perror and strerror functions - Mini Project: Build & Apply

Using perror and strerror Functions in C

📖 Scenario: Imagine you are writing a small program that tries to open a file. If the file does not exist or cannot be opened, you want to show a clear error message to the user.

🎯 Goal: You will create a program that attempts to open a file and uses perror and strerror functions to display error messages when the file cannot be opened.

📋 What You'll Learn

Create a variable to hold the filename

Attempt to open the file using fopen

Use perror to print an error message if opening fails

Use strerror with errno to get the error message string

Print the error message string using printf

💡 Why This Matters

🌍 Real World

When programs try to open files or perform system calls, errors can happen. Showing clear error messages helps users understand what went wrong.

💼 Career

Understanding how to handle and display system errors is important for debugging and writing reliable C programs in many software development jobs.

1

Set up the filename variable

Create a char pointer variable called filename and set it to the string "nonexistent.txt".

C

# Your code here

Need a hint?

Use char *filename = "nonexistent.txt"; to store the file name.

2

Try to open the file

Add a FILE pointer variable called file and use fopen(filename, "r") to try opening the file for reading.

C

char *filename = "nonexistent.txt";
# Your code here

Need a hint?

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

3

Use perror to print error if file open fails

Add an if statement to check if file is NULL. Inside it, call perror("Error opening file") to print the error message.

C

char *filename = "nonexistent.txt";
FILE *file = fopen(filename, "r");
# Your code here

Need a hint?

Use if (file == NULL) and inside it perror("Error opening file").

4

Use strerror and printf to show error message

Inside the if block, include #include <errno.h> at the top (if not already). Then declare a char pointer err_msg and set it to strerror(errno). Finally, print the error message with printf("strerror message: %s\n", err_msg);.

C

#include 
#include 
#include 

char *filename = "nonexistent.txt";
FILE *file = fopen(filename, "r");
if (file == NULL) {
    perror("Error opening file");
    # Your code here
}

Need a hint?

Use char *err_msg = strerror(errno); and then printf("strerror message: %s\n", err_msg); inside the if block.