Difference between revisions of "Interview Preparation Articles"

Revision as of 23:01, 7 December 2016

Interview preparation requires proficient knowledge of C/C++. This article has just been recently written (July 2013) and will be elaborated soon; the articles in RED are waiting to be written. The hope is that this article will be "one-stop-shop" for most common C/C++ interview questions.

Frequently Asked Topics

• What is a volatile variable?
• What is a static keyword in C/C++?
• Linked List
• Linked List

C++ Object Oriented Topics

• What is "virtual", "polymorphism" and "abstract class"?

Operating Systems Topics

FreeRTOS Tutorial

FreeRTOS is a real-time OS that has many ports for various different controllers. This is a great system to learn about because it gives you the fundamental knowledge of an operating system while making it incredibly easy to learn the material. Here's a must-read tutorial :

• FreeRTOS Tutorial

Other OS Topics

• Synchronization Primitives - Mutexes, Spinlocks, Queues etc.
• Common Problems in Multitasking OS

Miscellaneous Topics

Bit Fiddling

• Bit Fiddling
• Bitmasking Tutorial
• Bit Structures and Unions

BSS and Data Segments

What are text,.bss and data sections in an Embedded C program?

Where do your program recide?Obviously memory,but how is it organized?
In embedded systems ,which are RAM-memory constrained ,memory map is divided into segments
called text,data and bss.
	
Text segment:Contains code and constants of the program.Text section is allocated on flash.
	Eg1: a.out :executable code 
	Eg2:const uint8_t i=8 :contant
	Eg3:
	
	#include <stdio.h>
	const int global_var=20;
	int data_variable = 500;
	static int static_var=10;
	void foo(){
	const int local_constant=100;	
	int local;
	local = 3;
	int un_initialized;
	int another_initialized=0;
	static int local_static=9;
	local_static++;
	printf("local varaible is %d\n", local);
	}
	
In the above example, executable code of the program is stored in flash and const variable global_var=20 and local_constant=100 are stored in flash.
	
Data segment:Initialized variables are stored in this section.
Initially values are stored in flash and copied into RAM during execution
through copy down process.
	
Eg: uint8_t j=10; initialized variable
	
In the code given above int data_variable = 500,static int static_var=10,local = 3, and local_static=9 are stored in data segment of flash.
	
.bss segment:Uninitialized variables are stored in bss section.
	
In embedded software, the bss segment is mapped into memory that is initialized to zero by the C run-time system before main() is entered.
(wiki :https://en.wikipedia.org/wiki/.bss#BSS_in_C)
	
	Eg:int newvar;
	
In the above code un_initialized(after initializing it to zero) and another_initialized are stored in bss section ;
	
	Simple example:

	#include <stdio.h>
	int main(void)
	{
		return 0;
	}
	Memory occupied by the above program:
	
	$ gcc -o just_main just_main.c
	$ size just_main.exe
	text    data     bss     dec     hex filename
	3153    1976     448    5577    15c9 just_main.exe
	After modifying the above program with one global variable:
	
	#include <stdio.h>
	int global_var;
	int main(void)
	{
		return 0;
	}
	
	$ gcc -o just_main just_main.c
	$ size just_main.exe
	text    data     bss     dec     hex filename
	3153    1976     464    5593    15d9 just_main.exe
	bss section is increased by 2 bytes;
	
	Modify the above program with global variable initialized to zero;
	
	#include <stdio.h>
	int global_var=0;
	int main(void)
	{
		return 0;
	}
	
	$ gcc -o just_main just_main.c
	$ size just_main.exe
	text    data     bss     dec     hex filename
	3153    1976     480    5609    15e9 just_main.exe
	bss segment is increased by 32 bits(4 bytes)
	
	Modify the above program with global variable initialized to non-zero;
	
	#include <stdio.h>
	int global_var=8;
	int main(void)
	{
		return 0;
	}
	$ gcc -o just_main just_main
	$ size just_main.exe
	text    data     bss     dec     hex filename
	3153    1976     448    5577    15c9 just_main.exe
    	bss section is not modified.
	
	
FAQ:
What is the size of bss segment?
bss does not take any space in the object file and stores the count of variables that can be given initial values.Hence occupy 4 or 8 bytes
depending on the implementation.
	
What is the amount of RAM being used?
Memory occupied by bss and data segments added together.
	

Others

• String Manipulation
• All about pointers

Sorting Topics

Sorting means arrangement of elements in a defined manner(Ascending/Descending order). This is widely used in the complex algorithms to decrease the time complexity. There are six types of sorting algorithms namely, Bubble Sort, Insertion Sort, Merge Sort, Quick Sort, Heap Sort and Selection Sort.

• Bubble Sort
• Insertion Sort
• Merge Sort
• Quick Sort

Big O notation

Big O notation is a mathematical way of representing an approximate time required for an expression to complete by checking for its major dependencies. For a example, f(n) = n^5 + n.

As we go on increasing the value of n the dependency on the second half equation on the right-hand side will reduce.

so, we can also say that f(n) = n^5 or complexity is O(n^5) for n equal to infinity.

Note: For comparison of big O complexity for different algorithms, refer following site: http://bigocheatsheet.com/

Source Code

• Gitlab Source Code Link

Searching Algorithms

Information retrieval is critical in any computer application. Large sets of records are stored in computer system, from which a particular record is fetched in which required information would be stored. Each record stored in the computer environment would have a key associated to it. Based on the search criteria system scans through the records and extract the matching records only. To Increase the speed or reduce the information retrieval duration, various searching techniques are used.

Below are the basic commonly used searching algorithms.

• Linear Search
• Binary Search

Embedded C Interview Questions and Answers

What is the use of volatile keyword?

The C's volatile keyword is a qualifier that tells the compiler not to optimize when applied to a variable. By declaring a variable volatile, we can tell the compiler that the value of the variable may change any moment from outside of the scope of the program. A variable should be declared volatile whenever its value could change unexpectedly and beyond the comprehension of the compiler.

Can a variable be both const and volatile?

The const keyword make sure that the value of the variable declared as const can't be changed. This statement holds true in the scope of the program. The value can still be changed by outside intervention. So, the use of const with volatile keyword makes perfect sense.

Can a pointer be volatile?

If we see the declaration volatile int *p, it means that the pointer itself is not volatile and points to an integer that is volatile. This is to inform the compiler that pointer p is pointing to an integer and the value of that integer may change unexpectedly even if there is no code indicating so in the program.

What is size of character, integer, integer pointer, character pointer?

• The sizeof character is 1 byte.

• Size of integer is 4 bytes.

• Size of integer pointer and character is 8 bytes on 64 bit machine and 4 bytes on 32 bit machine.

What is void pointer and what is its use?

The void pointer means that it points to a variable that can be of any type. Other pointers points to a specific type of variable while void pointer is a somewhat generic pointer and can be pointed to any data type, be it standard data type(int, char etc) or user define data type (structure, union etc.). We can pass any kind of pointer and reference it as a void pointer. But to dereference it, we have to type the void pointer to correct data type.

What is ISR?

An ISR(Interrupt Service Routine) is an interrupt handler, a callback subroutine which is called when a interrupt is encountered.

What is return type of ISR?

ISR does not return anything. An ISR returns nothing because there is no caller in the code to read the returned values.

What is interrupt latency?

Interrupt latency is the time required for an ISR responds to an interrupt.

How to reduce interrupt latency?

Interrupt latency can be minimized by writing short ISR routine and by not delaying interrupts for more time.

Write a short code using C

1)Code to print out all odd number from 1 to 100 using a for loop

    for( unsigned int i = 1; i < = 100; i++ )

    if( i & 0x00000001 )

    printf(“i=%u”,&i);

2)Write out a function that prints out all the permutations of a string. For example, abc would give you abc, acb, bac, bca, cab, cba.

// C program to print all permutations with duplicates allowed
#include <stdio.h>
#include <string.h>
 
/* Function to swap values at two pointers */

void swap(char *x, char *y)

{
    char temp;
    temp = *x;
    *x = *y;
    *y = temp;
}
 
/* Function to print permutations of string
   This function takes three parameters:
   1. String
   2. Starting index of the string
   3. Ending index of the string. */

void permute(char *a, int l, int r)
{
   int i;
   if (l == r)
     printf("%s\n", a);
   else
   {
       for (i = l; i <= r; i++)
       {
          swap((a+l), (a+i));
          permute(a, l+1, r);
          swap((a+l), (a+i)); //backtrack
       }
   }
}
 
/* Driver program to test above functions */
int main()
{
    char str[] = "ABC";
    int n = strlen(str);
    permute(str, 0, n-1);
    return 0;
}