"C" IMPORTANT QUESTION

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.

printf() Function

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(...).

malloc() Function- 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.

printf() Function- What is the difference between "printf(...)" and

"sprintf(...)"?

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

standard output device.

Compilation How to reduce a final size of executable?

Size of the final executable can be reduced using dynamic linking for

libraries.

Linked Lists -- 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.

"union" Data Type What is the output of the following program? Why?

#include

main() {

typedef union {

int a;

char b[10];

float c;

}

Union;

Union x,y = {100};

x.a = 50;

strcpy(x.b,"hello");

x.c = 21.50;

printf("Union x : %d %s %f n",x.a,x.b,x.c);

printf("Union y : %d %s %f n",y.a,y.b,y.c);

}

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

Advantages of a macro over a function?

Macro gets to see the Compilation environment, so it can expand __

__TIME__ __FILE__ #defines. It is expanded by the preprocessor.

For example, you can’t do this without macros

#define PRINT(EXPR) printf( #EXPR “=%d\n”, EXPR)

PRINT( 5+6*7 ) // expands into printf(”5+6*7=%d”, 5+6*7 );

You can define your mini language with macros:

#define strequal(A,B) (!strcmp(A,B))

Macros are a necessary evils of life. The purists don’t like them, but

without it no real work gets done.

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.

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.

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’.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

How are pointer variables initialized?

Pointer variable are initialized by one of the following two ways

- Static memory allocation

- Dynamic memory allocation

Difference between arrays and pointers?

- Pointers are used to manipulate data using the address. Pointers use

* operator to access the data pointed to by them

- Arrays use subscripted variables to access and manipulate data.

Array variables can be equivalently written using pointer expression.

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.

What is a method?

Method is a way of doing something, especially a systematic way;

implies an orderly logical arrangement (usually in steps).

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.