Lecture 24 — Error Handling in C

The Problem ⚡

What happens when things go wrong?

Unlike Java or Python, C does NOT have try-catch blocks. If you divide by zero or open a missing file, your program just crashes. We must handle errors manually!

⚠️ The Crash Scenario

Which of these is the developer's responsibility in C?

Checking for Divide by Zero

Checking Array Bounds

Checking if File Exists

✓ ALL OF THEM! The C compiler blindly trusts you. It is entirely up to you to write if conditions to prevent mathematical impossibilities, memory violations, and missing files!

crash.cDangerous Code 💀

// 1. Divide by Zero (Arithmetic Exception)
int a = 10 / 0; // CRASH! 💥

// 2. Array Out of Bounds (Segmentation Fault)
int arr[5];
arr[1000] = 50; // CRASH! 💥

// 3. Null Pointer Dereference (Segmentation Fault)
FILE *ptr = fopen("missing.txt", "r");
fgetc(ptr); // CRASH! 💥

Logic Checks

Handling Logical & Memory Errors

To prevent math errors and array bounds violations, we use simple if conditions to sanitize inputs before using them.

logic_safe.cHover to explore ✶

// --- Safe Division ---
int num = 10, den = 0;

if (den == 0) {
    printf("Error: Division by zero is not allowed!\n");
} else {
    printf("Result: %d\n", num / den);
}

// --- Safe Array Access ---
int arr[5] = {1, 2, 3, 4, 5};
int index = 10;

if (index < 0 || index >= 5) {
    printf("Error: Array index out of bounds!\n");
} else {
    printf("Value: %d\n", arr[index]);
}

🛡️ Defensive Programming

Don't trust user input. If a user provides an index or a denominator, always check it against the limits of your data structure before executing the logic.

File System Checks

Checking for File Existence

Similarly, to prevent file-related crashes, we must ALWAYS check if the file pointer is NULL immediately after calling fopen().

file_safe.cHover to explore ✶

FILE *ptr = fopen("data.txt", "r");

if (ptr == NULL) {
    printf("Error: File does not exist!\n");
    return 1; // Stop the program safely
}

// Safe to read now!
char ch = fgetc(ptr);
fclose(ptr);

💾 The Golden Rule of Files:

Open → Check for NULL → Process → Close

Why does fopen fail?
1. File not found ("r" mode)
2. Permission Denied
3. Hard Disk is Full

Advanced C

Robust Error Tools (perror & exit)

Instead of a simple printf, professional C programs use standard library tools like perror() and exit() to handle errors gracefully and provide exact system reasons.

robust_code.cProfessional Way

#include <stdio.h>
#include <stdlib.h> // Needed for exit()
#include <errno.h>  // Error number tracking

void openSystemFile() {
    FILE *ptr = fopen("C:\\Windows\\secret.txt", "r");

    if (ptr == NULL) {
        perror("File Error");
        exit(EXIT_FAILURE);
    }
    fclose(ptr);
}

Terminal Output (No rights)

File Error: Permission Denied

What is errno?
When a system call fails, C sets a global integer variable called errno. perror() looks at this number and prints the human-readable string associated with it!

Knowledge Check ✍

Let's test what you've learned!

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