Defensive programming practices - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of buffer overflow prevention code

What is the output of this C code that uses defensive programming to prevent buffer overflow?

C

#include <stdio.h>
#include <string.h>

int main() {
    char buffer[10];
    const char *input = "HelloWorld!";
    if (strlen(input) < sizeof(buffer)) {
        strcpy(buffer, input);
        printf("Buffer content: %s\n", buffer);
    } else {
        printf("Input too large for buffer\n");
    }
    return 0;
}

A Buffer content: HelloWorld!

B Buffer content: HelloWorl

C Input too large for buffer

D Segmentation fault

💡 Hint

Check the length of the input string compared to the buffer size.

The input string "HelloWorld!" has 11 characters plus the null terminator, which is too large for the 10-byte buffer. The code correctly detects this and prints the warning message.

🧠 Conceptual

intermediate

Purpose of defensive null pointer checks

Why is it important to check if a pointer is NULL before using it in C?

A To avoid dereferencing a null pointer which causes a crash

B To improve program speed by skipping pointer usage

C To automatically allocate memory for the pointer

D To allow the pointer to point to multiple variables

💡 Hint

Think about what happens if you try to use a pointer that points to nothing.

Dereferencing a null pointer causes a runtime crash (segmentation fault). Checking for NULL prevents this by ensuring the pointer is valid before use.

🔧 Debug

advanced

Identify the defensive programming mistake

What is the mistake in this code that tries to safely open a file?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r");
    if (fp == NULL) {
        printf("File not found\n");
    }
    fprintf(fp, "Hello\n");
    fclose(fp);
    return 0;
}

A The file is not closed after writing

B The file is opened in read mode but written to

C The file pointer is not checked for NULL before fclose

D The code writes to the file even if fopen failed

💡 Hint

Look at what happens after the if (fp == NULL) check.

The code prints an error if the file failed to open but continues to write to the file pointer anyway, which is invalid and causes undefined behavior.

📝 Syntax

advanced

Which option causes a compilation error?

Which of the following code snippets will cause a compilation error due to defensive programming misuse?

A if ptr != NULL { *ptr = 10; }

B if (ptr) *ptr = 10;

C if (ptr != NULL) { *ptr = 10; }

D if (ptr == NULL) { return; } *ptr = 10;

💡 Hint

Check the syntax of the if statement.

Option A is missing parentheses around the condition, which is required in C syntax for if statements.

🚀 Application

expert

Result of integer overflow check function

What is the output of this C program that uses defensive programming to check for integer overflow before addition?

C

#include <stdio.h>
#include <limits.h>

int safe_add(int a, int b, int *result) {
    if ((b > 0) && (a > INT_MAX - b)) {
        return 0; // overflow
    }
    if ((b < 0) && (a < INT_MIN - b)) {
        return 0; // underflow
    }
    *result = a + b;
    return 1; // success
}

int main() {
    int x = INT_MAX;
    int y = 1;
    int res;
    if (safe_add(x, y, &res)) {
        printf("Sum: %d\n", res);
    } else {
        printf("Overflow detected\n");
    }
    return 0;
}

A Sum: 2147483647

B Overflow detected

C Sum: -2147483648

D Undefined behavior

💡 Hint

Consider what happens when adding 1 to INT_MAX.

The function detects that adding 1 to INT_MAX would overflow and returns 0, so the program prints "Overflow detected".