Introduction to Programming in C

📘 Error Handling and errno in C

Error handling in C is essential to ensure a program doesn't crash or behave unexpectedly. C provides basic error reporting through return values and advanced error information using the errno global variable.

🔹 Why Handle Errors?

• To detect and respond to failures (e.g., file not found, memory not allocated).
• To prevent undefined behavior or program crashes.
• To make debugging easier and provide user-friendly messages.

🔹 Common Error Sources

• File operations (e.g., fopen() failure)
• Memory allocation issues (e.g., malloc() returns NULL)
• Division by zero (logic error)
• Array index out-of-bound (undefined behavior)

🔹 Using Return Values for Error Detection

FILE *fp = fopen("example.txt", "r");
if (fp == NULL) {
    printf("Error opening file.\n");
}

🔹 Introducing errno

errno is a global variable defined in <errno.h> that stores the last error code when a library function fails.

#include <stdio.h>
#include <errno.h>

int main() {
    FILE *fp = fopen("missing.txt", "r");
    if (fp == NULL) {
        printf("Error code: %d\n", errno);
        perror("File opening failed");
    }
    return 0;
}

📚 UdaanPath Example: Read Scores from File

FILE *scoreFile = fopen("scores.txt", "r");
if (scoreFile == NULL) {
    perror("UdaanPath Error");
    printf("Use 'Contact Us' if the file is missing.\n");
    return 1;
}

🔹 perror() and strerror()

• perror("msg"): Prints system error with your message
• strerror(errno): Returns a string describing the error

🔁 Example with strerror()

#include <string.h>
#include <errno.h>

int main() {
    FILE *f = fopen("abc.txt", "r");
    if (!f) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

📌 List of Common errno Values

📝 Practice Questions

1. Write a program to open a non-existent file and print errno.
2. Use malloc() to allocate a huge array and check for NULL.
3. Display a custom error message using strerror(errno).
4. Write a function that checks if a file can be opened for reading.

✅ Summary

• Always check return values of I/O and memory functions.
• errno helps identify specific issues after failure.
• perror() and strerror() provide detailed error messages.

🚀 Coming Next

Now that you've completed the fundamentals, it's time to level up! In the next section, we'll begin with advanced concepts starting with Bitwise Operators and Bit Manipulation — powerful tools for performance-oriented C programming, system-level logic, and embedded development.