Embedded C Interview Guide

Embedded C is a powerful language for developing embedded systems, known for its efficiency and portability. This guide provides common interview questions and answers to help you excel in your Embedded C interview.

Interview Questions for Freshers

1. What is Embedded C Programming, and How Does It Differ from C?

Answer: Embedded C is an extension of C for embedded systems, using the same syntax. Differences:

• Standard C: Hardware-independent, used for desktop applications.
• Embedded C: Hardware-dependent, optimized for limited resources.

2. What is Startup Code?

Answer: Startup code runs before main, initializing the stack and interrupt vectors, typically in assembly.

3. What is an ISR?

Answer: An Interrupt Service Routine (ISR) handles interrupts and must be short and fast.

4. What is a Void Pointer, and Why is It Used?

Answer: A void pointer can point to any data type for flexibility:

int num = 20;
void *ptr = #
printf("%d", *(int*)ptr);

5. Why Use the volatile Keyword?

Answer: volatile prevents compiler optimization for variables that may change unexpectedly:

volatile int sensor_value;

6. Differences Between const and volatile

Answer:

• const: Makes a variable read-only.
• volatile: Prevents compiler optimization.

7. What is the Concatenation Operator?

Answer: The ## operator concatenates macro arguments:

#define CONCAT(x, y) x##y
int main() {
    int xValue = 20;
    printf("%d", CONCAT(x, Value));
}

8. What is Interrupt Latency?

Answer: Time taken for an ISR to respond to an interrupt.

9. Using a Variable from One Source File in Another

Answer: Use extern:

// file1.c
int global_variable = 10;

// file2.c
extern int global_variable;
printf("%d", global_variable);

10. What is a Segmentation Fault?

Answer: Occurs when accessing prohibited memory, like NULL pointers or out-of-bounds arrays.

11. Inline vs. Macro Functions

Answer: Macros are preprocessor-expanded, lack type checking; inline functions are compiler-expanded, safer.

12. Can a Variable Be volatile and const?

Answer: Yes, const prevents program modification, while volatile allows external changes.

13. Declaring static in a Header File

Answer: Possible but not recommended, as each source file gets a separate copy.

14. Pre-Decrement vs. Post-Decrement

Answer:

int x = 100, y;
y = --x; // y = 99, x = 99
y = x--; // y = 100, x = 99

15. What is a Reentrant Function?

Answer: A function that can be interrupted and re-entered safely.

Interview Questions for Experienced Candidates

16. Counting Up vs. Down Loop

Answer: Counting down to zero is faster due to optimized zero comparison.

17. What is a Null Pointer?

Answer: Points to no valid memory:

int *ptr = NULL;

18. Meaning of These Declarations

Answer:

• const int x;: Read-only integer.
• const int *x;: Pointer to read-only integer.
• int *const x;: Constant pointer to an integer.
• int const *x const;: Constant pointer to a read-only integer.

19. Why is ++i Faster Than i+1?

Answer: ++i uses a single machine instruction, while i+1 requires loading and addition.

20. What Causes Segmentation Faults, and How to Avoid Them?

Answer: Causes: Dereferencing NULL/freed pointers, accessing out-of-bounds arrays. Prevention: Initialize pointers, check array bounds.

21. Is printf Recommended in an ISR?

Answer: No, printf is non-reentrant and causes delays.

22. Can You Pass Parameters to or Return Values from an ISR?

Answer: No, ISRs cannot accept parameters or return values.

23. What is Virtual Memory in Embedded C?

Answer: Virtual memory uses paging to reduce memory usage.

24. What’s Wrong with This Code?

int square(volatile int *p) {
    return (*p) * (*p);
}

Answer: *p may change between dereferences. Corrected:

long square(volatile int *p) {
    int x = *p;
    return (long)x * x;
}

25. Is This ISR Code Correct?

__interrupt double calculate_circle_area(double radius) {
    double circle_area = 3.14 * radius * radius;
    printf("Area = %f", circle_area);
    return circle_area;
}

Answer: No, ISRs cannot return values, accept parameters, or use printf.

26. What is the Output of This Code?

void demo(void) {
    unsigned int x = 10;
    int y = -40;
    if (x + y > 10) {
        printf("Greater than 10");
    } else {
        printf("Less than or equals 10");
    }
}

Answer: Output: Greater than 10, as y is converted to unsigned.

27. Causes of Interrupt Latency and Reduction

Answer: Causes: Hardware synchronization, CPU pipeline, long ISRs. Reduction: Short ISRs, better scheduling.

28. Protecting a Character Pointer

Answer: Use char *const.

29. Wild vs. Dangling Pointers

Answer:

• Wild: Uninitialized pointer.
• Dangling: Pointer to freed memory.

30. Difference Between #include "..." and #include <...>

Answer: "..." searches the current directory first, <...> searches standard directories.

31. What is a Memory Leak, and How to Avoid It?

Answer: Memory leaks occur when allocated memory isn’t freed. Avoid with free and tools like Valgrind.

Embedded C Programming

32. Multiply a Number by 9 Efficiently

#include <stdio.h>
int main() {
    int num;
    printf("Enter number: ");
    scanf("%d", &num);
    printf("%d", (num << 3) + num);
    return 0;
}

33. Swap Two Variables

int a = 20, b = 30, temp;
temp = a;
a = b;
b = temp;

34. Check If a Number is a Power of 2

#include <stdio.h>
int main() {
    int num;
    printf("Enter number: ");
    scanf("%d", &num);
    if (num && !(num & (num - 1)))
        printf("Number is a power of 2");
    else
        printf("Number is not a power of 2");
    return 0;
}

35. Print Numbers 1 to 100 Without Conditionals

#include <stdio.h>
int main() {
    int i = 0;
    while (100 - i++)
        printf("%d ", i);
    return 0;
}

36. MIN Macro

#define MIN(a, b) ((a) <= (b) ? (a) : (b))

Conclusion

Preparing for an Embedded C interview requires understanding C concepts and optimization for embedded systems. Practice these questions to excel in your interview.