Defensive programming practices - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the pointer is NULL before using it.

C

if ([1] != NULL) {
    printf("Pointer is valid.\n");
}

Drag options to blanks, or click blank then click option'

Aptr

BNULL

C0

Dptr == NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing NULL to NULL instead of the pointer variable.

Using 'ptr == NULL' inside the if condition which checks for non-null.

We check if the pointer ptr is not NULL before using it to avoid errors.

2fill in blank

medium

Complete the code to safely open a file and check if it opened successfully.

C

FILE *file = fopen("data.txt", "r");
if ([1] == NULL) {
    printf("Failed to open file.\n");
}

Drag options to blanks, or click blank then click option'

ANULL

Bfopen

Cfile

Ddata.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Checking fopen directly instead of the file pointer variable.

Comparing the filename string to NULL.

We check if file is NULL to know if fopen failed.

3fill in blank

hard

Fix the error in the code to prevent buffer overflow by limiting input size.

C

char buffer[10];
printf("Enter text: ");
scanf("%[1]s", buffer);

Drag options to blanks, or click blank then click option'

A10

Bs

C99

D9

Attempts:

3 left

💡 Hint

Common Mistakes

Using %s without size limit causes buffer overflow.

Using %10s reads 10 characters but buffer can only hold 10 including null terminator.

Using %9s limits input to 9 characters plus the null terminator, preventing overflow.

4fill in blank

hard

Fill both blanks to safely allocate memory and check if allocation succeeded.

C

int *arr = (int *)malloc([1] * sizeof(int));
if (arr [2] NULL) {
    printf("Memory allocation failed.\n");
}

Drag options to blanks, or click blank then click option'

A10

B==

C!=

Dsizeof

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking if malloc returned NULL.

Using '!=' instead of '==' in the if condition.

Allocate memory for 10 integers and check if arr is NULL to detect failure.

5fill in blank

hard

Fill all three blanks to safely copy a string with length check.

C

char src[] = "Hello";
char dest[6];
if (strlen(src) [1] sizeof(dest) - 1) {
    strncpy(dest, src, [2]);
    dest[[3]] = '\0';
} else {
    printf("Source string too long.\n");
}

Drag options to blanks, or click blank then click option'

A<=

Bsizeof(dest) - 1

Cstrlen(src)

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Not adding null terminator after strncpy.

Using wrong comparison operator causing buffer overflow.

Check if source length is less or equal to destination size minus 1, copy that many characters, and add null terminator at last index.

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