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See right interview answers on 30 common job interview questions


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1. Can a variable be both const and volatile?

Yes. The const modifier means that this code cannot change the value
 of the variable, but that does not mean that the value cannot be
 changed by means outside this code. For instance, in the example in
 FAQ 8, the timer structure was accessed through a volatile const
 pointer. The function itself did not change the value of the timer, so it
 was declared const. However, the value was changed by hardware on
 the computer, so it was declared volatile. If a variable is both const and
 volatile, the two modifiers can appear in either order.

2. Can include files be nested?

Yes. Include files can be nested any number of times. As long as you
 use precautionary measures , you can avoid including the same file
 twice. In the past, nesting header files was seen as bad programming
 practice, because it complicates the dependency tracking function of
 the MAKE program and thus slows down compilation. Many of today’s
 popular compilers make up for this difficulty by implementing a
 concept called precompiled headers, in which all headers and
 associated dependencies are stored in a precompiled state.
 Many programmers like to create a custom header file that has
 #include statements for every header needed for each module. This is
 perfectly acceptable and can help avoid potential problems relating to
 #include files, such as accidentally omitting an #include file in a
 module.

3. Can static variables be declared in a header file?

You can’t declare a static variable without defining it as well (this is
 because the storage class modifiers static and extern are mutually
 exclusive). A static variable can be defined in a header file, but this
 would cause each source file that included the header file to have its

own private copy of the variable, which is probably not what was
 intended.

4. Can you tell me how to check whether a linked list is circular?

Create two pointers, and set both to the start of the list. Update each
 as follows:
 while (pointer1) {
 pointer1 = pointer1->next;
 pointer2 = pointer2->next;
 if (pointer2) pointer2=pointer2->next;
 if (pointer1 == pointer2) {
 print ("circular");
 }}

If a list is circular, at some point pointer2 will wrap around and be
 either at the item just before pointer1, or the item before that. Either
 way, its either 1 or 2 jumps until they meet.

5. Differentiate between an internal static and external static variable?

An internal static variable is declared inside a block with static storage
 class whereas an external static variable is declared outside all the
 blocks in a file.An internal static variable has persistent storage,block
 scope and no linkage.An external static variable has permanent
 storage,file scope and internal linkage.

6. Given an array of 10 doubles named data, write a loop that loads the array with user input?

double data[10];
 for (int i=0;i<10;i++)
 scanf("%lf",&data[i]);

7. How can you determine the size of an allocated portion of memory?

You can’t, really. free() can , but there’s no way for your program to
 know the trick free() uses. Even if you disassemble the library and
 discover the trick, there’s no guarantee the trick won’t change with the
 next release of the compiler.

8. How many levels deep can include files be nested?

Even though there is no limit to the number of levels of nested include
 files you can have, your compiler might run out of stack space while
 trying to include an inordinately high number of files. This number
 varies according to your hardware configuration and possibly your
 compiler.

9. Is using exit() the same as using return?

No. The exit() function is used to exit your program and return control
 to the operating system. The return statement is used to return from a
 function and return control to the calling function. If you issue a return
 from the main() function, you are essentially returning control to the
 calling function, which is the operating system. In this case, the return
 statement and exit() function are similar.

10. What are the differences between malloc() and calloc()?

There are 2 differences.
 First, is in the number of arguments. malloc() takes a single
 argument(memory required in bytes), while calloc() needs 2
 arguments(number of variables to allocate memory, size in bytes of a
 single variable).
 Secondly, malloc() does not initialize the memory allocated, while
 calloc() initializes the allocated memory to ZERO.

11. What are the different storage classes in C?

C has three types of storage: automatic, static and allocated.
 Variable having block scope and without static specifier have
 automatic storage duration.
 Variables with block scope, and with static specifier have static scope.
 Global variables (i.e, file scope) with or without the static specifier also
 have static scope.
 Memory obtained from calls to malloc(), alloc() or realloc() belongs to
 allocated storage class.

12. What does static variable mean?

there are 3 main uses for the static.
 1. If you declare within a function:
 It retains the value between function calls
 2.If it is declared for a function name:
 By default function is extern..so it will be visible from other files if the
 function declaration is as static..it is invisible for the outer files
 3. Static for global variables:
 By default we can use the global variables from outside files If it is
 static global..that variable is limited to with in the file

13. What is a function and built-in function?

A large program is subdivided into a number of smaller programs or
 subprograms. Each subprogram specifies one or more actions to be
 performed for a large program. such subprograms are functions.
 The function supports only static and extern storage classes. By
 default, function assumes extern storage class. functions have global
 scope. Only register or auto storage class is allowed in the function

parameters. Built-in functions that predefined and supplied along with
 the compiler are known as built-in functions. They are also known as
 library functions.

14. What is a null pointer?

There are times when it’s necessary to have a pointer that doesn’t
 point to anything. The macro NULL, defined in , has a value that’s
 guaranteed to be different from any valid pointer. NULL is a literal zero,
 possibly cast to void* or char*. Some people, notably C++
 programmers, prefer to use 0 rather than NULL.
 The null pointer is used in three ways:
 1) To stop indirection in a recursive data structure
 2) As an error value
 3) As a sentinel value

15. What is a void pointer?

A void pointer is a C convention for a raw address. The compiler has no

idea what type of object a void Pointer really points to. If you write
 int *ip;
 ip points to an int. If you write
 void *p;
 p doesn’t point to a void!
 In C and C++, any time you need a void pointer, you can use another
 pointer type. For example, if you have a char*, you can pass it to a
 function that expects a void*. You don’t even need to cast it. In C (but
 not in C++), you can use a void* any time you need any kind of
 pointer, without casting. (In C++, you need to cast it).
 A void pointer is used for working with raw memory or for passing a
 pointer to an unspecified type.
 Some C code operates on raw memory. When C was first invented,
 character pointers (char *) were used for that. Then people started
 getting confused about when a character pointer was a string, when it
 was a character array, and when it was raw memory.

16. What is an lvalue?

An lvalue is an expression to which a value can be assigned. The lvalue
 expression is located on the left side of an assignment statement,
 whereas an rvalue is located on the right side of an assignment
 statement. Each assignment statement must have an lvalue and an
 rvalue. The lvalue expression must reference a storable variable in
 memory. It cannot be a constant.

17. What is C language?

The C programming language is a standardized programming language
 developed in the early 1970s by Ken Thompson and Dennis Ritchie for
 use on the UNIX operating system. It has since spread to many other
 operating systems, and is one of the most widely used programming
 languages. C is prized for its efficiency, and is the most popular
 programming language for writing system software, though it is also
 used for writing applications.

18. What is hashing?

To hash means to grind up, and that’s essentially what hashing is all
 about. The heart of a hashing algorithm is a hash function that takes
 your nice, neat data and grinds it into some random-looking integer.
 The idea behind hashing is that some data either has no inherent
 ordering (such as images) or is expensive to compare (such as
 images). If the data has no inherent ordering, you can’t perform
 comparison searches.
 If the data is expensive to compare, the number of comparisons used

even by a binary search might be too many. So instead of looking at
 the data themselves, you’ll condense (hash) the data to an integer (its
 hash value) and keep all the data with the same hash value in the
 same place. This task is carried out by using the hash value as an
 index into an array.
 To search for an item, you simply hash it and look at all the data whose
 hash values match that of the data you’re looking for. This technique
 greatly lessens the number of items you have to look at. If the
 parameters are set up with care and enough storage is available for
 the hash table, the number of comparisons needed to find an item can
 be made arbitrarily close to one.
 One aspect that affects the efficiency of a hashing implementation is
 the hash function itself. It should ideally distribute data randomly
 throughout the entire hash table, to reduce the likelihood of collisions.
 Collisions occur when two different keys have the same hash value.
 There are two ways to resolve this problem. In open addressing, the
 collision is resolved by the choosing of another position in the hash
 table for the element inserted later. When the hash table is searched, if
 the entry is not found at its hashed position in the table, the search
 continues checking until either the element is found or an empty
 position in the table is found.
 The second method of resolving a hash collision is called chaining. In
 this method, a bucket or linked list holds all the elements whose keys
 hash to the same value. When the hash table is searched, the list must
 be searched linearly.

19. What is indirection?

If you declare a variable, its name is a direct reference to its value. If
 you have a pointer to a variable, or any other object in memory, you
 have an indirect reference to its value.

20. What is modular programming?

If a program is large, it is subdivided into a number of smaller
 programs that are called modules or subprograms. If a complex
 problem is solved using more modules, this approach is known as
 modular programming.

21. What is Operator overloading ?

When an operator is overloaded, it takes on an additional meaning
 relative to a certain class. But it can still retain all of its old meanings.
 Examples:
 1) The operators >> and << may be used for I/O operations because
 in the header, they are overloaded.
 2) In a stack class it is possible to overload the + operator so that it
 appends the contents of one stack to the contents of another. But the
 + operator still retains its original meaning relative to other types of
 data.

22. What is Polymorphism ?

'Polymorphism' is an object oriented term. Polymorphism may be
 defined as the ability of related objects to respond to the same
 message with different, but appropriate actions. In other words,
 polymorphism means taking more than one form. Polymorphism leads
 to two important aspects in Object Oriented terminology - Function
 Overloading and Function Overriding. Overloading is the practice of
 supplying more than one definition for a given function name in the
 same scope. The compiler is left to pick the appropriate version of the
 function or operator based on the arguments with which it is called.
 Overriding refers to the modifications made in the sub class to the
 inherited methods from the base class to change their behavior.

23. What is static memory allocation and dynamic memory allocation?

Static memory allocation: The compiler allocates the required memory
 space for a declared variable.By using the address of operator,the
 reserved address is obtained and this address may be assigned to a
 pointer variable.Since most of the declared variable have static
 memory,this way of assigning pointer value to a pointer variable is
 known as static memory allocation. memory is assigned during
 compilation time.
 Dynamic memory allocation: It uses functions such as malloc( ) or
 calloc( ) to get memory dynamically.If these functions are used to get
 memory dynamically and the values returned by these functions are
 assingned to pointer variables, such assignments are known as
 dynamic memory allocation.memory is assined during run time.

24. What is the difference between "calloc(...)" and "malloc(...)"?

1. calloc(...) allocates a block of memory for an array of elements of a
 certain size. By default the block is initialized to 0. The total number of
 memory allocated will be (number_of_elements * size).
 malloc(...) takes in only a single argument which is the memory
 required in bytes. malloc(...) allocated bytes of memory and not blocks
 of memory like calloc(...).
 2. malloc(...) allocates memory blocks and returns a void pointer to the
 allocated space, or NULL if there is insufficient memory available.
 calloc(...) allocates an array in memory with elements initialized to 0
 and returns a pointer to the allocated space. calloc(...) calls malloc(...)
 in order to use the C++ _set_new_mode function to set the new
 handler mode.

25. What is the difference between "printf(...)" and "sprintf(...)"?

sprintf(...) writes data to the character array whereas printf(...) writes data to the
 standard output device.

26. What is the difference between a string and an array?

An array is an array of anything. A string is a specific kind of an array
 with a well-known convention to determine its length.
 There are two kinds of programming languages: those in which a string
 is just an array of characters, and those in which it’s a special type. In
 C, a string is just an array of characters (type char), with one wrinkle: a
 C string always ends with a NUL character.
 The “value” of an array is the same as the address of (or a pointer to)
 the first element; so, frequently, a C string and a pointer to char are
 used to mean the same thing.
 An array can be any length. If it’s passed to a function, there’s no way
 the function can tell how long the array is supposed to be, unless some
 convention is used. The convention for strings is NUL termination; the
 last character is an ASCII NUL (‘’) character.

27. What is the difference between goto and longjmp() and setjmp()?

A goto statement implements a local jump of program execution, and
 the longjmp() and setjmp() functions implement a nonlocal, or far,

jump of program execution.
 Generally, a jump in execution of any kind should be avoided because
 it is not considered good programming practice to use such statements
 as goto and longjmp in your program.
 A goto statement simply bypasses code in your program and jumps to
 a predefined position. To use the goto statement, you give it a labeled
 position to jump to. This predefined position must be within the same
 function. You cannot implement gotos between functions.
 When your program calls setjmp(), the current state of your program is
 saved in a structure of type jmp_buf. Later, your program can call the
 longjmp() function to restore the program’s state as it was when you
 called setjmp().Unlike the goto statement, the longjmp() and setjmp()
 functions do not need to be implemented in the same function.
 However, there is a major drawback to using these functions: your
 program, when restored to its previously saved state, will lose its
 references to any dynamically allocated memory between the
 longjmp() and the setjmp(). This means you will waste memory for
 every malloc() or calloc() you have implemented between your
 longjmp() and setjmp(), and your program will be horribly inefficient.
 It is highly recommended that you avoid using functions such as
 longjmp() and setjmp() because they, like the goto statement, are
 quite often an indication of poor programming practice.

28. What is the difference between strings and character arrays?

A major difference is: string will have static storage duration, whereas
 as a character array will not, unless it is explicity specified by using the
 static keyword.
 Actually, a string is a character array with following properties:

* the multibyte character sequence, to which we generally call string,
 is used to initialize an array of static storage duration. The size of this
 array is just sufficient to contain these characters plus the terminating
 NUL character.
 * it not specified what happens if this array, i.e., string, is modified.
 * Two strings of same value[1] may share same memory area. For
 example, in the following declarations:
 char *s1 = “Calvin and Hobbes”;
 char *s2 = “Calvin and Hobbes”;
 the strings pointed by s1 and s2 may reside in the same memory
 location. But, it is not true for the following:
 char ca1[] = “Calvin and Hobbes”;
 char ca2[] = “Calvin and Hobbes”;
 [1] The value of a string is the sequence of the values of the contained
 characters, in order.

 Difference between const char* p and char const* p 
 In const char* p, the character pointed by ‘p’ is constant, so u cant
 change the value of character pointed by p but u can make ‘p’ refer to
 some other location.
 in char const* p, the ptr ‘p’ is constant not the character referenced by
 it, so u cant make ‘p’ to reference to any other location but u can
 change the value of the char pointed by ‘p’.

29. What is the difference between text and binary modes?

Streams can be classified into two types: text streams and binary
 streams. Text streams are interpreted, with a maximum length of 255
 characters. With text streams, carriage return/line feed combinations
 are translated to the newline n character and vice versa. Binary
 streams are uninterrupted and are treated one byte at a time with no
 translation of characters. Typically, a text stream would be used for
 reading and writing standard text files, printing output to the screen or
 printer, or receiving input from the keyboard.
 A binary text stream would typically be used for reading and writing
 binary files such as graphics or word processing documents, reading
 mouse input, or reading and writing to the modem.

30. What is the output of printf("%d")?

1. When we write printf("%d",x); this means compiler will print the
 value of x. But as here, there is nothing after %d so compiler will show
 in output window garbage value.
 2. When we use %d the compiler internally uses it to access the
 argument in the stack (argument stack). Ideally compiler determines
 the offset of the data variable depending on the format specification
 string. Now when we write printf("%d",a) then compiler first accesses
 the top most element in the argument stack of the printf which is %d
 and depending on the format string it calculated to offset to the actual
 data variable in the memory which is to be printed. Now when only %d
 will be present in the printf then compiler will calculate the correct
 offset (which will be the offset to access the integer variable) but as
 the actual data object is to be printed is not present at that memory
 location so it will print what ever will be the contents of that memory
 location.
 3. Some compilers check the format string and will generate an error
 without the proper number and type of arguments for things like
 printf(...) and scanf(...).

31. When does the compiler not implicitly generate the address of the first element of an array?

Whenever an array name appears in an expression such as
 - array as an operand of the sizeof operator
 - array as an operand of & operator
 - array as a string literal initializer for a character array
 Then the compiler does not implicitly generate the address of the
 address of the first element of an array.

32. When should a far pointer be used?

Sometimes you can get away with using a small memory model in
 most of a given program. There might be just a few things that don’t fit
 in your small data and code segments. When that happens, you can

use explicit far pointers and function declarations to get at the rest of
 memory. A far function can be outside the 64KB segment most
 functions are shoehorned into for a small-code model. (Often, libraries
 are declared explicitly far, so they’ll work no matter what code model
 the program uses.) A far pointer can refer to information outside the
 64KB data segment. Typically, such pointers are used with farmalloc()
 and such, to manage a heap separate from where all the rest of the
 data lives. If you use a small-data, large-code model, you should
 explicitly make your function pointers far.

33. When should a type cast not be used?

A type cast should not be used to override a const or volatile
 declaration. Overriding these type modifiers can cause the program to
 fail to run correctly.
 A type cast should not be used to turn a pointer to one type of
 structure or data type into another. In the rare events in which this
 action is beneficial, using a union to hold the values makes the
 programmer’s intentions clearer.

34. Why should I prototype a function?

A function prototype tells the compiler what kind of arguments a
 function is looking to receive and what kind of return value a function
 is going to give back. This approach helps the compiler ensure that
 calls to a function are made correctly and that no erroneous type
 conversions are taking place.

35. Write a C function that computes that maximum of a specific row R in a 2D array of size 6 by 5?

int MaxOfRow( int a[][5], int ROWS, int searched_row)
 { int max;
 max=a[searched_row][0];
 for (int c=0; c<5;c++)
 if (a[searched_row][c]>max)
 max=a[searched_row][c];
 return max;
 }

36. Write a C function that has three inputs which are integers. The function returns true if the first number raised to the power of the second number equals the third number?

bool f1( int a, int b, int c)
 {
 if (pow( (float) a, (float) b)==c)
 return true;
 else return false;
 }

37. Write a C function that searches for value key in a a 2D array of size 6 by 5. The function should return true if found false otherwise?

bool SearchArray (int a[][3], int ROWS, int key)
 {
 bool found=false;
 for ( int r=0; r<ROWS; r++)
 for ( int c=0; c<3;c++)
 if (a[r][c]==key)
 {
 printf(" found at location [%d,%d] \n",r,c);
 return true;
 }
 return found;
 }

38. Write a for loop to initialize the following array (int data[10]) with the values 10, 9, 8? 1?

int data[10];
 for (int i=0;i<10;i++)
 data[i]=10-i;