All about programming

FAQ Interview

2011-06-10T10:17:00.000-07:00

simple programs to ask
================
#STRCAT
#STRCPY
#SWAP
#SUM of Digits

Macros
======
#No of Seconds in a year
#Server IP address
#MIN/MAX of two
#Allocation and deallocation of mem
#FOREVER loop

Declarations
=========
Pointer to int
Array of pointers
Pointer to an array
Struct and Union
Function Pointers

BitWise Operators
=============
how bitwise operators are used in error status flags. ( check declaring bits for particular err, setting more than one error, reading what error occured, clearing the error)

Misc Quetions
==========
#Static, Extern vars and functions scopes
#const
#How do u copy two structs
#Linked lists

===================================================================================================
If he answers 3 out 5 for above questions(except link list) then ask below to judge the depth

#Program to eliminate the duplicate elements in array. (Chk array init syntax, loops, function fomal args)
#Program to reverse the array without using the temp storage array
#Sort array using bubble sort and search using binary.

#Fibonacci Or factorial using recursion. ( Good if he explains the stack flow b/w normal function and recursion)
===================================================================================================

If he answers 1 of above good to go deeper

#List list - Create, add the end, add at the begining, delete, sort

=================================================================================================

if he is 5+ then ask classical typical interview question like

Dictionary creation - Chk the approach, Data structure building, tree formation, binary search on trees. ( This is good enough to discuss for 2 hrs)

=================================================================================================

C++ Questions

# what is constructor and destructor - usage , declaration

# Can we overload cons and desc - why do we do so ?

# What are virtual function - usage ?

# Is it possible to ve virtual cons/dest - If no why ?

#scope - public, private and protected.

# Friend function and usage ..

# Different types of inheritance.

# Define a class and member functions to add and ask add to overload for any data type . (Chk class , object, functions, logic)

#what r pure virtual class .. - usage.

==============================================================================================

Operating system

# Memory Laypout of a program

# Job of OS. - how in between

# Difference b/w process and thread.

# IPC ( pipe, msg queques, shared memory, sockets )

# Socket programming : client side calls and server side calls ( Y for the difference)

# Virtual Memory concepts

=============================================================================================

Network questions

# TCP/IP 3 way hand shake
# MAC / IP address conversion

Check whether given number is Fibonacci OR not

2009-11-04T01:01:00.000-08:00

Algo : A given number is said to be fibonacci, if 5*N*N -4 or 5*N*N +4 is a square

#include
int isSquare(int num);
main()
{
int num, temp;
printf("Enter the number\n");
scanf("%d", &num);

temp = 5 *(num * num) - 4;

if(isSquare(temp) || isSquare(temp+8)){
printf("It is finonacci number\n");
}else {
printf("It is NOT finonacci number\n");
}
}
int isSquare(int num)
{
int i=1;
while(num>0)
{
num = num - i;
i = i+2;
}
if(num==0)
return 1;
else
return 0;
}

Fibonacci Numbers

2009-11-02T20:58:00.000-08:00

C program to generate Fibonacci numbers till the user entered value.

#include
main()
{
int num;
int *a, i;;
printf("Enter the number\n");
scanf("%d", &num);

a = (int *)malloc(sizeof(int)*num);

a[0] = 0;
a[1] = 1;
printf("FIBONACCI Numbers are\n");
printf("%d\n%d\n", a[0],a[1]);

for(i = 2; i
{
a[i] = a[i-1] + a[i-2];
printf("%d\n", a[i]);
}

}

Basic C programming answers

2008-05-06T05:10:00.000-07:00

1)Program to find number of 1s and zeros in given integer.
Find the answer here http://gucatheprogrammer.blogspot.com/2007/10/simple-yet-useful-c-programs.html

2)Write a C program to check whether machine is Big endian OR little endian
#include

int main()
{
unsigned int n = 1;
char *p;

p = (char*)&n;
if (*p == 1)
printf("Little Endian\n");
else if (*(p + sizeof(int) - 1) == 1)
printf("Big Endian\n");
else
printf("What the crap?\n");

return 0;
}
3)C program to Convert decimal to binary
Find the answer here http://gucatheprogrammer.blogspot.com/2007/10/simple-yet-useful-c-programs.html

4)C program to do STRCPY without using strcpy
I will write just the function :
char * mystrcpy(char * dest,const char *src)
{
char *temp = dest;
while ((*dest++ = *src++) != '\0')
return temp;
}

5)C program to STRCAT without using strcat
char * mystrcat(char * dest, const char * src)
{
char *temp = dest;
while (*dest)
dest++;
while ((*dest++ = *src++) != '\0') ;
return temp;
}

6)Write a C macro which returns number of seconds in year
This is too general. This is just to check guy knows about the MACRO and its syntax.
#define NUMER_OF_SECONDS_YEAR (365*24*60*60)

7)C program to find sum of digits of any given integer number .
#include
int add_digits(int a);
main()
{
int a ;
printf("Enter the number\n");
scanf("%d", &a);
printf("Sum of digits = %d\n", add_digits(a));
}
int add_digits(int num)
{
int sum = 0;
while(num)
{
sum += num%10;
num /=10;
}
return sum;
}

8)C program to fine leap year or not
This is just to check how quick he can write ..
Better would be macro or function.
#include
main()
{
int year;
printf("Enter the year\n");
scanf("%d", &year);
if(year % 4 == 0){
printf("Given year is leap\n");
} else {
printf("Given year is NOT leap\n");
}
}

9)C program to swap two variables (using pointers), without using temp variable.
#include
void swap(int *p1, int *p2);
main()
{
int a, b;
printf("Enter the values for a and b\n");
scanf("%d%d",&a,&b);
swap(&a, &b);

printf("a = %d\n and b = %d\n", a, b);
}

void swap(int *p1, int *p2)
{
*p1 = *p1 + *p2;
*p2 = *p1 - *p2;
*p1 = *p1 - *p2;
return;

}

10)C program to find sum of given Array.
#include
int arr_sum(int *a, int size);
main()
{
int a[10] = {1,2,3,4,5,6,7,8,9,10};
int sum = arr_sum(a,10);
printf("SUM = %d\n", sum);
}
int arr_sum(int *a, int size)
{
int sum = 0, i;
for(i =0; i
#define MIN(a,b) ((a>b)?b:a)
main()
{
int a=100, b=20;
printf("MIN = %d\n", MIN(a,b));

}

The below ones are left as exercise to students :) .. Actually I got bored to write :)
12)C program to find largest of three numbers
13)C program to store data for 10 student info
14)Write all infinite loops u can think off

Campus to Corporate- 3

2008-03-23T22:28:00.000-07:00

Since many of freshers fail to answer simple HR question, I though of making a single page for most frequently asked questions and their answers. I have couple of the links, which I found to be useful. Spend some time in browsing some good stuff :) .. Just kidding :) ..

Please have a look at the following links

http://hitechskill.com/HR-Interview-Questions/display.jsp?type=HR&page=1

The below one is not that good.... But worth visiting once

http://www.yuvajobs.com/hr-questions.asp

I will come up with my answers and update this page asap. Keep visiting this page.

Campus to Corporate Gateway-2

2008-03-04T00:41:00.000-08:00

OPERATING SYSTEM BASICS
---------------------------------------
1)Explain the basic architecture of OS(Kernal, shell, user apps etc)
2)The same above question can be asked to trace the OS flow for simple commands like "ls -l"
3)What is process?
4)What is thread ?
5)what are the differences b/w process and thread?
6)What are the different states of process?
7)Explain the basics of scheduling? May be by taking simple round robin algorithm
8)What are interrupts?
9)what are different types of IPC mechanism available in the OS?
10)Difference between mutex and semaphore?
11)What is deadlock and how do we can avoid it?
12)Small shell script writting..
13)What is real time operating system?
14)What is the OS you have used for academic projects and why only that?
15)what is the difference between virtual memory and physical memory ??

Campus to Corporate Gateway

2008-02-26T04:45:00.000-08:00

This is my effort to consolidate FAQ for the freshers. The most common things on which freshers can be tested is through logical analysis. In spite the candidate clears the test, he has to face the technical interview. In most of the technical interviews freshers will be judged on the basis of the basics he knows.

To test the basics interviewer can choose any of the following.

C concepts and programming
Operating System Basics
C++ programming
Computer Networks
Microprocessors or Micro controllers.

Even though candidate is always given to the choice to choose his favorite subject, it is not only the criteria to select the candidate, Meaning even though candidate is good at his favorite subject interviewer always makes sure what he knows apart from that and exactly that's where other subject peek in.

I have tried my best to list down most probable questions in each area. If you them useful give a comment else update the missing info through comments :)

Let us see Fresh C :)
------------------------
1)Program to find number of 1s and zeros in given integer.
2)Write a C program to check whether machine is Big endian OR little endian
3)C program to Convert decimal to binary
4)C program to do STRCPY without using strcpy
5)C program to STRCAT without using strcat
6)Write a C macro which returns number of seconds in year
7)C program to find sum of digits of any given integer number .
8)C program to fine leap year or not
9)C program to swap two variables (using pointers), without using temp variable.
10)C program to find sum of given Array.
11)Write Macro to return minimum of two numbers
12)C program to find largest of three numbers
13)C program to store data for 10 student info
14)Write all infinite loops u can think off

Here is the most critical part, most of the freshers fail to undersatnd or answer this. If they answer, assume that he is good in C and try to interview him on some other basics.
15)a) An integer ->int a;
b) A pointer to an integer -> int *a;
c) A pointer to a pointer to an integer -> int **a;
d) An array of 10 integers -> int a[10];
e) An array of 10 pointers to integers -> int *a[10];
f) A pointer to an array of 10 integers-> int (*a)[10];
g) A pointer to a function that takes an integer as an argument and returns an integer -> int (*fun)(int)
h) An array of ten pointers to functions that take an integer argument and return an integer int (*fun[10])(int)

16)Difference between struct n union
17)What are uses of keyword "static" ?
18)Test knowledge on const
const int a;
int const a;
const int *a;
int * const a;
int const * a const;

19)C Program to set/clear 3rd bit in given int
20) What is self referntail structure
21)List down all file operation calls u ve used so far.
22) Give them small programs n ask the output
1) Check if catches seg faults
2)Relations operators (Especially logical operators)
3)const value increament/decrement rules.
4)extern initialzation
5)recursive functions
6)pointer operations

23) Ask him to explain basic data types and derived data types.
24)C program to remove the duplicate elements of the array.
25)C program to find the simple interst.(Or any formula)

Networking Basics
-----------------------
These questions will be asked to check the potential of the candidate, if he chooses his favorite subject as computer networks OR if the company is core networking company.
1)Different types of ethernet and their speed.
2)IP addres basics (classes etc)
3)Wireless and wired connections
4)Difference between router/bridge/switch
6)TCP/IP model explaination
7) OSI model (layers explaination)
8)CSMA/CD algorithm
9)TCP/UDP difference or try to get from him what are all he/she knows about it.
10)What is DHCP/DNS?
11)Ethernet header. (If he can explain MAC address alone,that is sufficient)
12) Tunneling theory.
13)Routing table - discovery (Any of the congestion algorithm)
14)Favourite layer and its operation
15)server/client model
16)Network topolopy and their selection
17)How do u choose the port number for any of the protocol
18)Any of the application layer protocol
19)Basics about the LAN
20)Some random problem about the network and try to see how he approaches towards it.

This will be updated on different subjects, when I find the time :)

SCJP FAQ

2007-11-18T00:17:00.000-08:00

This is my attempt to gather all the links required for SCJP. Even though it's not the area where I am very strong ... I tried collecting the links from my roomie, who tried hard to attend n clear the exam.. Upon his input these links helps novice to prepare, to take mock tests before attending the real exam ..

Let me elucidate again .. It's not the area I am pretty good at :( ... But I trust my roomie . So posted the links ..
All the best... Enjoy JAVA

Here we go ..

http://www.akgupta.com/Java/mock_exam.htm

http://www.geocities.com/wahjava/dctester.html

http://www.mihaiu.name/

http://www.wbrogden.com/java/javacert/TestApplet6.htm

http://www.wbrogden.com/exams/myexam

http://www.wbrogden.com/tests/index.html

http://www.google.com/Top/Computers/Programming/Languages/Java/Resources/Certification/

http://www.jiris.com/online/userpwd2.do

http://www.aniyog.com/java/certification.htm

http://www.geocities.com/sunjava4u/moc_ex.html

http://www.javacamp.org/testengine/comprehensive.html

http://www.certgear.com/products/info/scjp.htm

http://www.geocities.com/sunjava4u/moc_ex.html

http://valiveru.tripod.com/java/index.html/#certlinks

http://www.go4java.20m.com/mock2.htm

http://www.valoxo.ch/jr/mocks/mock01a.html

http://www.geocities.com/SiliconValley/Screen/5046/index.html

http://bmil.freeservers.com/tester/test.html

http://www21.brinkster.com/infoway02/SCJP2.asp

http://www21.brinkster.com/infoway02/html/SCJP_20.html

http://jdiscuss.com/Enthuse/jsp/ShowAvailableTests.jsp

http://www.geocities.com/sunjava4u/mock1.html

Linux Concluded

2007-11-16T23:36:00.000-08:00

1)Very much basics of Operating system. I feel its best to glance these once, to refresh the learning done.
http://www.techinterviews.com/?p=170

2) Threads demystified
Linux threads FAQ
http://pauillac.inria.fr/~xleroy/linuxthreads/faq.html

Posix threads tutorial with example
http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html

Thread with java .. Find FAQ here
http://www.jguru.com/faq/Linux

Programming with POSIX threads
http://www.llnl.gov/computing/tutorials/pthreads/

Debugging posix threads, use simple two files debug.c and debug.h listed here http://www.humanfactor.com/pthreads/debugging.html

3)Find the linux documentation at
http://www.linux.org/docs/

Simple yet useful C programs

2007-10-27T02:28:00.000-07:00

All functions written below are compiled on gcc.
1) Function to count number of ones and zeros in a given integer
Input Params :
a)num: The integer for which count needs to be dome
b)one_count : pass the address of variable in which 1's count is expected
c)one_count : pass the address of variable in which 0's count is expected

void count_OneAndZero(int num, int *one_count, int *zero_count)
{
while(num != 0){
if(num&1)
*one_count +=1;
num >>=1;
}
*zero_count = sizeof(int)*8 - *one_count;
return;
}

2) Function to convert decimal to binary
Input Params :
a)decimal: The integer to be converted into binary
b)binary : The fixed size array in which binary string is expected.
void dec2bin(long decimal, char *binary)
{
int i = 0, j = 0;
int neg_flag = 0;
int remain;
char temp[80];

/* what if i/p is -ve */
if (decimal < 0)
{
decimal = -decimal;
neg_flag = 1;
}

if (neg_flag)
temp[i++] = '-'; // add - sign
else
temp[i++] = ' '; // space
while (i >= 0)
binary[j++] = temp[--i];
binary[j-1] = '\0'; // end with NULL
return;
}

See C Links

2007-10-27T00:15:00.000-07:00

This is to make sure we have too many things on net to learn, But scattered here n there. One more attempt to put the things together for reference.

The site to learn C FAQ by chapter wise
http://c-faq.com/

You can find the answers to most of the above questions here.
http://www.faqs.org/faqs/C-faq/faq/

A systematic approach to question and answers
http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

Practical view of looking at C
http://www.geocities.com/vijoeyz/faq/

Solutions to K&R 2nd edition
http://users.powernet.co.uk/eton/kandr2/

Good and simple tutorial on C with graphical illustrations
http://www.cs.cf.ac.uk/Dave/C/

Find good C programming books here
http://freecomputerbooks.com/

Embedded C FAQ

2007-10-26T23:43:00.000-07:00

This is my attempt to to consolidate all the things available in the net at one spot for my reference as well as for others, if they wish too.. . The below one is from Nigel Jones to identify the candidate's potential in embedded C programming. U can find the same at http://www.embedded.com/2000/0005/0005feat2.htm. Anyway here is the content.

Preprocessor
1. Using the #define statement, how would you declare a manifest constant that returns the number of seconds in a year? Disregard leap years in your answer.

#define SECONDS_PER_YEAR
(60 * 60 * 24 * 365)UL

I'm looking for several things here:

Basic knowledge of the #define syntax (for example, no semi-colon at the end, the need to parenthesize, and so on)

An understanding that the pre-processor will evaluate constant expressions for you. Thus, it is clearer, and penalty-free, to spell out how you are calculating the number of seconds in a year, rather than actually doing the calculation yourself

A realization that the expression will overflow an integer argument on a 16-bit machine-hence the need for the L, telling the compiler to treat the variable as a Long

As a bonus, if you modified the expression with a UL (indicating unsigned long), then you are off to a great start. And remember, first impressions count!

2. Write the "standard" MIN macro-that is, a macro that takes two arguments and returns the smaller of the two arguments.

#define MIN(A,B)
((A)
< = (B) ? (A) : (B))

The purpose of this question is to test the following:

Basic knowledge of the #define directive as used in macros. This is important because until the inline operator becomes part of standard C, macros are the only portable way of generating inline code. Inline code is often necessary in embedded systems in order to achieve the required performance level

Knowledge of the ternary conditional operator. This operator exists in C because it allows the compiler to produce more optimal code than an if-then-else sequence. Given that performance is normally an issue in embedded systems, knowledge and use of this construct is important

Understanding of the need to very carefully parenthesize arguments to macros

I also use this question to start a discussion on the side effects of macros, for example, what happens when you write code such as:

least = MIN(*p++, b);

3. What is the purpose of the preprocessor directive #error?

Either you know the answer to this, or you don't. If you don't, see Reference 1. This question is useful for differentiating between normal folks and the nerds. Only the nerds actually read the appendices of C textbooks to find out about such things. Of course, if you aren't looking for a nerd, the candidate better hope she doesn't know the answer.

Infinite loops
4. Infinite loops often arise in embedded systems. How does you code an infinite loop in C?

There are several solutions to this question. My preferred solution is:


while(1)
{
ý
}

Many programmers seem to prefer:


for(;;)
{
ý
}

This construct puzzles me because the syntax doesn't exactly spell out what's going on. Thus, if a candidate gives this as a solution, I'll use it as an opportunity to explore their rationale for doing so. If their answer is basically, "I was taught to do it this way and I haven't thought about it since," it tells me something (bad) about them.

A third solution is to use a goto :


Loop:
...
goto Loop;

Candidates who propose this are either assembly language programmers (which is probably good), or else they are closet BASIC/FORTRAN programmers looking to get into a new field.

Data declarations
5. Using the variable a, give definitions for the following:
a) An integer
b) A pointer to an integer
c) A pointer to a pointer to an integer
d) An array of 10 integers
e) An array of 10 pointers to integers
f) A pointer to an array of 10 integers
g) A pointer to a function that takes an integer as an argument and returns an integer
h) An array of ten pointers to functions that take an integer argument and return an integer

The answers are:
a) int a; // An integer
b) int *a; // A pointer to an integer
c) int **a; // A pointer to a pointer to an integer
d) int a[10]; // An array of 10 integers
e) int *a[10]; // An array of 10 pointers to integers
f) int (*a)[10]; // A pointer to an array of 10 integers
g) int (*a)(int); // A pointer to a function a that takes an integer argument and returns an integer
h) int (*a[10])(int); // An array of 10 pointers to functions that take an integer argument and return an integer

People often claim that a couple of these are the sorts of thing that one looks up in textbooks-and I agree. While writing this article, I consulted textbooks to ensure the syntax was correct. However, I expect to be asked this question (or something close to it) when I'm being interviewed. Consequently, I make sure I know the answers, at least for the few hours of the interview. Candidates who don't know all the answers (or at least most of them) are simply unprepared for the interview. If they can't be prepared for the interview, what will they be prepared for?

Static
6. What are the uses of the keyword static?

This simple question is rarely answered completely. Static has three distinct uses in C:

A variable declared static within the body of a function maintains its value between function invocations

A variable declared static within a module, (but outside the body of a function) is accessible by all functions within that module. It is not accessible by functions within any other module. That is, it is a localized global

Functions declared static within a module may only be called by other functions within that module. That is, the scope of the function is localized to the module within which it is declared

Most candidates get the first part correct. A reasonable number get the second part correct, while a pitiful number understand the third answer. This is a serious weakness in a candidate, since he obviously doesn't understand the importance and benefits of localizing the scope of both data and code.

Const
7. What does the keyword const mean?

As soon as the interviewee says "const means constant," I know I'm dealing with an amateur. Dan Saks has exhaustively covered const in the last year, such that every reader of ESP should be extremely familiar with what const can and cannot do for you. If you haven't been reading that column, suffice it to say that const means "read-only." Although this answer doesn't really do the subject justice, I'd accept it as a correct answer. (If you want the detailed answer, read Saks' columns-carefully!)

If the candidate gets the answer correct, I'll ask him these supplemental questions:

What do the following declarations mean?


const int a;
int const a;
const int *a;
int * const a;
int const * a const;

The first two mean the same thing, namely a is a const (read-only) integer. The third means a is a pointer to a const integer (that is, the integer isn't modifiable, but the pointer is). The fourth declares a to be a const pointer to an integer (that is, the integer pointed to by a is modifiable, but the pointer is not). The final declaration declares a to be a const pointer to a const integer (that is, neither the integer pointed to by a, nor the pointer itself may be modified). If the candidate correctly answers these questions, I'll be impressed. Incidentally, you might wonder why I put so much emphasis on const, since it is easy to write a correctly functioning program without ever using it. I have several reasons:

The use of const conveys some very useful information to someone reading your code. In effect, declaring a parameter const tells the user about its intended usage. If you spend a lot of time cleaning up the mess left by other people, you'll quickly learn to appreciate this extra piece of information. (Of course, programmers who use const , rarely leave a mess for others to clean up.)

const has the potential for generating tighter code by giving the optimizer some additional information

Code that uses const liberally is inherently protected by the compiler against inadvertent coding constructs that result in parameters being changed that should not be. In short, they tend to have fewer bugs

Volatile
8. What does the keyword volatile mean? Give three different examples of its use.

A volatile variable is one that can change unexpectedly. Consequently, the compiler can make no assumptions about the value of the variable. In particular, the optimizer must be careful to reload the variable every time it is used instead of holding a copy in a register. Examples of volatile variables are:

Hardware registers in peripherals (for example, status registers)

Non-automatic variables referenced within an interrupt service routine

Variables shared by multiple tasks in a multi-threaded application

Candidates who don't know the answer to this question aren't hired. I consider this the most fundamental question that distinguishes between a C programmer and an embedded systems programmer. Embedded folks deal with hardware, interrupts, RTOSes, and the like. All of these require volatile variables. Failure to understand the concept of volatile will lead to disaster.

On the (dubious) assumption that the interviewee gets this question correct, I like to probe a little deeper to see if they really understand the full significance of volatile . In particular, I'll ask them the following additional questions:

Can a parameter be both const and volatile ? Explain.

Can a pointer be volatile ? Explain.

What's wrong with the following function?:


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

The answers are as follows:

Yes. An example is a read-only status register. It is volatile because it can change unexpectedly. It is const because the program should not attempt to modify it

Yes, although this is not very common. An example is when an interrupt service routine modifies a pointer to a buffer

This one is wicked. The intent of the code is to return the square of the value pointed to by *ptr . However, since *ptr points to a volatile parameter, the compiler will generate code that looks something like this:


int square(volatile int *ptr)
{
int a,b;
a = *ptr;
b = *ptr;
return a * b;
}

Because it's possible for the value of *ptr to change unexpectedly, it is possible for a and b to be different. Consequently, this code could return a number that is not a square! The correct way to code this is:


long square(volatile int *ptr)
{
int a;
a = *ptr;
return a * a;
}

Bit manipulation
9. Embedded systems always require the user to manipulate bits in registers or variables. Given an integer variable a, write two code fragments. The first should set bit 3 of a. The second should clear bit 3 of a. In both cases, the remaining bits should be unmodified.

These are the three basic responses to this question:

No idea. The interviewee cannot have done any embedded systems work

Use bit fields. Bit fields are right up there with trigraphs as the most brain-dead portion of C. Bit fields are inherently non-portable across compilers, and as such guarantee that your code is not reusable. I recently had the misfortune to look at a driver written by Infineon for one of their more complex communications chips. It used bit fields and was completely useless because my compiler implemented the bit fields the other way around. The moral: never let a non-embedded person anywhere near a real piece of hardware!

Use #defines and bit masks. This is a highly portable method and is the one that should be used. My optimal solution to this problem would be:


#define BIT3 (0x1
< <>



void set_bit3(void) {
a |= BIT3;
}
void clear_bit3(void) {
a &= ~BIT3;
}

Some people prefer to define a mask together with manifest constants for the set and clear values. This is also acceptable. The element that I'm looking for is the use of manifest constants, together with the |= and &= ~ constructs

Accessing fixed memory locations
10. Embedded systems are often characterized by requiring the programmer to access a specific memory location. On a certain project it is required to set an integer variable at the absolute address 0x67a9 to the value 0xaa55. The compiler is a pure ANSI compiler. Write code to accomplish this task.

This problem tests whether you know that it is legal to typecast an integer to a pointer in order to access an absolute location. The exact syntax varies depending upon one's style. However, I would typically be looking for something like this:


int *ptr;
ptr = (int *)0x67a9;
*ptr = 0xaa55;

A more obscure approach is:


*(int * const)(0x67a9) = 0xaa55;

Even if your taste runs more to the second solution, I suggest the first solution when you are in an interview situation.

Interrupts
11. Interrupts are an important part of embedded systems. Consequently, many compiler vendors offer an extension to standard C to support interrupts. Typically, this new keyword is __interrupt. The following code uses __interrupt to define an interrupt service routine (ISR). Comment on the code.


__interrupt double compute_area
(double
radius)
{
double area = PI * radius *
 radius;
printf("\nArea = %f", area);
return area;
}

This function has so much wrong with it, it's hard to know where to start:

ISRs cannot return a value. If you don't understand this, you aren't hired

ISRs cannot be passed parameters. See the first item for your employment prospects if you missed this

On many processors/compilers, floating-point operations are not necessarily re-entrant. In some cases one needs to stack additional registers. In other cases, one simply cannot do floating point in an ISR. Furthermore, given that a general rule of thumb is that ISRs should be short and sweet, one wonders about the wisdom of doing floating-point math here

In a vein similar to the third point, printf() often has problems with reentrancy and performance. If you missed points three and four, I wouldn't be too hard on you. Needless to say, if you got these last two points, your employment prospects are looking better and better

Code examples
12. What does the following code output and why?


void foo(void)
{
unsigned int a = 6;
int b = -20;
 (a+b > 6) ? puts("> 6") :
   puts("
< = 6"); }

This question tests whether you understand the integer promotion rules in C-an area that I find is very poorly understood by many developers. Anyway, the answer is that this outputs "> 6." The reason for this is that expressions involving signed and unsigned types have all operands promoted to unsigned types. Thus ý20 becomes a very large positive integer and the expression evaluates to greater than 6. This is a very important point in embedded systems where unsigned data types should be used frequently (see Reference 2). If you get this one wrong, you are perilously close to not getting the job.

13. Comment on the following code fragment.


unsigned int zero = 0;
unsigned int compzero = 0xFFFF;
/*1's complement of zero */

On machines where an int is not 16 bits, this will be incorrect. It should be coded:


unsigned int compzero = ~0;

This question really gets to whether the candidate understands the importance of word length on a computer. In my experience, good embedded programmers are critically aware of the underlying hardware and its limitations, whereas computer programmers tend to dismiss the hardware as a necessary annoyance.

By this stage, candidates are either completely demoralized-or they're on a roll and having a good time. If it's obvious that the candidate isn't very good, then the test is terminated at this point. However, if the candidate is doing well, then I throw in these supplemental questions. These questions are hard, and I expect that only the very best candidates will do well on them. In posing these questions, I'm looking more at the way the candidate tackles the problems, rather than the answers. Anyway, have fun...

Dynamic memory allocation
14. Although not as common as in non-embedded computers, embedded systems do still dynamically allocate memory from the heap. What are the problems with dynamic memory allocation in embedded systems?

Here, I expect the user to mention memory fragmentation, problems with garbage collection, variable execution time, and so on. This topic has been covered extensively in ESP , mainly by P.J. Plauger. His explanations are far more insightful than anything I could offer here, so go and read those back issues! Having lulled the candidate into a sense of false security, I then offer up this tidbit:

What does the following code fragment output and why?


char *ptr;
if ((ptr = (char *)malloc(0)) ==
 NULL)
else
puts("Got a null pointer");
puts("Got a valid pointer");

This is a fun question. I stumbled across this only recently when a colleague of mine inadvertently passed a value of 0 to malloc and got back a valid pointer! That is, the above code will output "Got a valid pointer." I use this to start a discussion on whether the interviewee thinks this is the correct thing for the library routine to do. Getting the right answer here is not nearly as important as the way you approach the problem and the rationale for your decision.

Typedef
15. Typedef is frequently used in C to declare synonyms for pre-existing data types. It is also possible to use the preprocessor to do something similar. For instance, consider the following code fragment:


#define dPS  struct s *
typedef  struct s * tPS;

The intent in both cases is to define dPS and tPS to be pointers to structure s. Which method, if any, is preferred and why?

This is a very subtle question, and anyone who gets it right (for the right reason) is to be congratulated or condemned ("get a life" springs to mind). The answer is the typedef is preferred. Consider the declarations:


dPS p1,p2;
tPS p3,p4;

The first expands to:


struct s * p1, p2;

which defines p1 to be a pointer to the structure and p2 to be an actual structure, which is probably not what you wanted. The second example correctly defines p3 and p4 to be pointers.

Obscure syntax
16. C allows some appalling constructs. Is this construct legal, and if so what does this code do?


int a = 5, b = 7, c;
c = a+++b;

This question is intended to be a lighthearted end to the quiz, as, believe it or not, this is perfectly legal syntax. The question is how does the compiler treat it? Those poor compiler writers actually debated this issue, and came up with the "maximum munch" rule, which stipulates that the compiler should bite off as big (and legal) a chunk as it can. Hence, this code is treated as:


c = a++ + b;

Thus, after this code is executed, a = 6, b = 7, and c = 12.

If you knew the answer, or guessed correctly, well done. If you didn't know the answer then I wouldn't consider this to be a problem. I find the greatest benefit of this question is that it is good for stimulating questions on coding styles, the value of code reviews, and the benefits of using lint.

Well folks, there you have it. That was my version of the C test. I hope you had as much fun taking it as I had writing it. If you think the test is a good test, then by all means use it in your recruitment. Who knows, I may get lucky in a year or two and end up being on the receiving end of my own work.

Nigel Jones is a consultant living in Maryland. When not underwater, he can be found slaving away on a diverse range of embedded projects. He enjoys hearing from readers and can be reached at NAJones@compuserve.com .

References

Jones, Nigel, "In Praise of the #error directive," Embedded Systems Programming, September 1999, p. 114.

Jones, Nigel, " Efficient C Code for Eight-bit MCUs ," Embedded Systems Programming, November 1998, p. 66.