Defensive programming practices

Introduction

Defensive programming helps your code avoid mistakes and crashes by checking for problems early.

When reading input from users or files that might be wrong.

When working with pointers that could be NULL.

When dividing numbers to avoid dividing by zero.

When handling memory allocation to check if it worked.

When writing functions that others will use to prevent misuse.

Syntax

C

if (condition) {
    // handle error or unexpected case
}

Use if statements to check for bad inputs or errors.

Always check pointers before using them to avoid crashes.

Examples

This checks if a pointer is NULL before using it.

C

if (ptr == NULL) {
    printf("Pointer is NULL, cannot proceed.\n");
    return;
}

This prevents division by zero errors.

C

if (denominator == 0) {
    printf("Cannot divide by zero!\n");
    return;
}

This checks if memory allocation worked before using the pointer.

C

int *p = malloc(sizeof(int));
if (p == NULL) {
    printf("Memory allocation failed.\n");
    exit(1);
}

Sample Program

This program shows defensive checks for division by zero and memory allocation failure.

C

#include <stdio.h>
#include <stdlib.h>

int divide(int a, int b) {
    if (b == 0) {
        printf("Error: Cannot divide by zero.\n");
        return 0; // Return safe value
    }
    return a / b;
}

int main() {
    int x = 10;
    int y = 0;
    int result = divide(x, y);
    printf("Result: %d\n", result);
    
    int *ptr = malloc(sizeof(int));
    if (ptr == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }
    *ptr = 5;
    printf("Value pointed by ptr: %d\n", *ptr);
    free(ptr);

    return 0;
}

Important Notes

Always check inputs and pointers before using them.

Return safe values or stop the program if something goes wrong.

Defensive programming makes your code stronger and easier to fix.

Summary

Defensive programming means checking for errors before they cause problems.

Use if statements to catch bad inputs or NULL pointers.

This helps your program avoid crashes and unexpected behavior.

Practice

1. What is the main goal of defensive programming in C?

easy

A. To check for errors before they cause problems

B. To write code faster without testing

C. To ignore invalid inputs

D. To make the program run without any comments

Solution

1. Step 1: Understand the purpose of defensive programming

   Defensive programming is about preventing errors by checking inputs and conditions early.

2. Step 2: Identify the correct goal from options

   To check for errors before they cause problems clearly states checking for errors before problems happen, which matches defensive programming.

3. Final Answer:

   To check for errors before they cause problems -> Option A

4. Quick Check:

   Defensive programming = check errors early [OK]

Hint: Defensive programming means catching errors early [OK]

Common Mistakes:

• Thinking it means ignoring errors
• Confusing it with faster coding
• Believing it removes need for comments

2. Which of the following is the correct way to check for a NULL pointer before using it in C?

easy

A. if (ptr = NULL) { /* use ptr */ }

B. if (ptr != NULL) { /* use ptr */ }

C. if (ptr == 0) { /* use ptr */ }

D. if (ptr = 0) { /* use ptr */ }

Solution

1. Step 1: Identify correct NULL pointer check syntax

   In C, to check if a pointer is not NULL, use 'ptr != NULL'. Assignment '=' is wrong here.

2. Step 2: Analyze each option

   'if (ptr != NULL) { /* use ptr */ }' is correct because it checks that ptr is not NULL before use. B and D use '=' for assignment, setting ptr to NULL/0 and making the condition false. 'if (ptr == 0) { /* use ptr */ }' checks if ptr IS null (equivalent to NULL) and then uses it, causing a null dereference crash.

3. Final Answer:

   if (ptr != NULL) { /* use ptr */ } -> Option B

4. Quick Check:

   Use '!=' to compare pointer with NULL [OK]

Hint: Use '!=' to compare pointer with NULL, not '=' [OK]

Common Mistakes:

• Using '=' instead of '==' or '!=' in conditions
• Confusing NULL with zero without clarity
• Not checking pointer before use

3. What will be the output of this C code snippet?

int *ptr = NULL;
if (ptr != NULL) {
    printf("Pointer is valid\n");
} else {
    printf("Pointer is NULL\n");
}

medium

A. Pointer is NULL

B. Pointer is valid

C. Compilation error

D. No output

Solution

1. Step 1: Analyze pointer initialization

   The pointer 'ptr' is set to NULL, meaning it points to nothing.

2. Step 2: Check the if condition

   The condition 'ptr != NULL' is false because ptr is NULL, so the else block runs.

3. Final Answer:

   Pointer is NULL -> Option A

4. Quick Check:

   NULL pointer triggers else block [OK]

Hint: NULL pointer means else block runs [OK]

Common Mistakes:

• Assuming NULL pointer is valid
• Expecting compilation error
• Thinking no output occurs

4. Identify the error in this defensive programming snippet:

void print_length(char *str) {
    if (str = NULL) {
        printf("String is NULL\n");
        return;
    }
    printf("Length: %d\n", strlen(str));
}

medium

A. strlen() cannot be used on strings

B. Missing semicolon after printf

C. Using assignment '=' instead of comparison '==' in if condition

D. Function return type should be int

Solution

1. Step 1: Check the if condition syntax

   The condition uses 'str = NULL' which assigns NULL to str instead of comparing it.

2. Step 2: Understand impact of assignment in condition

   Assignment returns NULL (false), so the if block never runs as intended. It should be 'str == NULL'.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition -> Option C

4. Quick Check:

   Use '==' to compare pointers, not '=' [OK]

Hint: Use '==' to compare, '=' is assignment [OK]

Common Mistakes:

• Confusing '=' and '==' in conditions
• Thinking strlen() is invalid here
• Ignoring function return type

5. You want to safely read an integer from user input in C. Which defensive programming practice should you apply?

hard

A. Use scanf without checking its return value

B. Use gets() to read input without validation

C. Assume input is always correct and proceed

D. Check scanf's return value and validate the input range

Solution

1. Step 1: Understand safe input reading

   Using scanf requires checking its return value to confirm input was read correctly.

2. Step 2: Validate input range

   After reading, check if the integer is within expected limits to avoid errors later.

3. Final Answer:

   Check scanf's return value and validate the input range -> Option D

4. Quick Check:

   Check input success and validity [OK]

Hint: Always check scanf return and input validity [OK]

Common Mistakes:

• Ignoring scanf return value
• Using unsafe input functions like gets()
• Assuming user input is always valid