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.
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.
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.
What is the difference between declaring a variable and defining a variable?
Declaring a variable means describing its type to the compiler but not allocating any
space for it. Defining a variable means declaring it and also allocating space to hold the variable. You can also initialize
a variable at the time it is defined.
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.
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.
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.
What is an
argument? Differentiate
between formal arguments and actual arguments?
An argument is an entity used to pass the data from calling
function to
the
called function. Formal arguments are the arguments
available in the function definition. They
are preceded by their own data types. Actual arguments are available
in the function call.
What are advantages and
disadvantages of external storage
class?
Advantages of external storage class
1)Persistent storage
of a variable
retains the latest value
2)The value is globally available
Disadvantages of external storage class
1)The storage for an external variable exists even when the variable is not needed
2)The side effect may produce surprising output
3)Modification of the program is difficult
4)Generality of a program is affected
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.
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.
When is a
switch statement better than multiple if
statements?
A switch statement is
generally best to use when you have more than two conditional expressions based on a single variable
of numeric type.
What is a static function?
A static function is a function whose
scope is limited to the current
source file. Scope refers to the visibility of a function or variable. If the function or
variable is visible outside
of the current source file, it is said to have global,
or external, scope. If
the
function or variable is not visible outside of the current source file, it is said to have local, or static, scope.
What is a pointer variable?
A pointer variable is
a variable that may contain the address of
another variable
or any valid address in the memory.
What is a pointer value and address?
A pointer value is
a data object that refers to a memory location. Each memory location is numbered in the memory. The number attached to a memory location is called the address of the location.
What is a modulus operator?
What are the restrictions
of a modulus operator?
A Modulus operator gives the remainder
value. The result of x%y is obtained by (x-(x/y)*y). This operator is
applied only to integral operands and cannot be applied
to float or double.
Differentiate between
a linker and linkage?
A linker converts
an object code into an executable
code by linking together the necessary build in functions. The form and place of declaration where the variable is declared in a program determine the
linkage of variable.
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.
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.
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.
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.
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.
