C LANGUAGE INTERVIEW QUESTIONS
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.
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
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 the static specifier also have static scope.
Memory obtained from calls to malloc(), alloc() or realloc() belongs to allocated storage
class.
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.
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?
Answer 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.
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
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()
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?
%c", sCur[0]);
strcat(sNewBegin, cFLetter);
debugprint(sNewBegin, sCur+1);
}
}
}
#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);
}
String Processing --- Write out a function that prints out all the permutations of a string. For example, abc would give you abc, acb, bac, bca, cab, cba.
void PrintPermu (char *sBegin, char* sRest) {
int iLoop;
char cTmp;
char cFLetter[1];
char *sNewBegin;
char *sCur;
int iLen;
static int iCount;
iLen = strlen(sRest);
if (iLen == 2) {
iCount++;
printf("%d: %s%s\n",iCount,sBegin,sRest);
iCount++;
printf("%d: %s%c%c\n",iCount,sBegin,sRest[1],sRest[0]);
return;
} else if (iLen == 1) {
iCount++;
printf("%d: %s%s\n", iCount, sBegin, sRest);
return;
} else {
// swap the first character of sRest with each of
// the remaining chars recursively call debug print
sCur = (char*)malloc(iLen);
sNewBegin = (char*)malloc(iLen);
for (iLoop = 0; iLoop < iLen; iLoop ++) {
strcpy(sCur, sRest);
strcpy(sNewBegin, sBegin);
cTmp = sCur[iLoop];
sCur[iLoop] = sCur[0];
sCur[0] = cTmp;
sprintf(cFLetter, "void main() {
char s[255];
char sIn[255];
printf("\nEnter a string:");
scanf("%s%*c",sIn);
memset(s,0,255);
PrintPermu(s, sIn);
}
What will be the result of the following code?
#define TRUE 0 // some code while(TRUE) { // some code }
This will not go into the loop as TRUE is defined as 0.
What will be printed as the result of the operation below:
int x;
int modifyvalue()
return(x+=10);
}
int changevalue(int x)
{
return(x+=1);
}
void main()
{
int x=10;
x++;
changevalue(x);
x++;
modifyvalue();
printf("First output:%d\n",x);
x++;
changevalue(x);
printf("Second output:%d\n",x);
modifyvalue();
printf("Third output:%d\n",x);
}
Answer: 12 , 13 , 13
What will be printed as the result of the operation below:
main()
{
int x=10, y=15;
x = x++;
y = ++y;
printf(“%d %d\n”,x,y);
}
Answer: 11, 16
What will be printed as the result of the operation below:
main()
{
int a=0;
if(a==0)
printf(“Tech Preparation\n”);
printf(“Tech Preparation\n”);
}
Answer: Two lines with “Tech Preparation” will be printed.
What will the following piece of code do
int f(unsigned int x)
{
int i;
for (i=0; x!0; x>>=1){
if (x & 0X1)
i++;
}
return i;
}
Answer: returns the number of ones in the input parameter X
What will happen in these three cases?
if(a=0){
//somecode
}
if (a==0){
//do something
}
if (a===0){
//do something
}
What are x, y, y, u
#define Atype int*
typedef int *p;
p x, z;
Atype y, u;
Answer: x and z are pointers to int. y is a pointer to int but u is just an integer variable
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 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.
Write down the equivalent pointer expression for referring the same element a[i][j][k][l]?
a[i] == *(a+i)
a[i][j] == *(*(a+i)+j)
a[i][j][k] == *(*(*(a+i)+j)+k)
a[i][j][k][l] == *(*(*(*(a+i)+j)+k)+l)
Which bit wise operator is suitable for checking whether a particular bit is on or off?
The bitwise AND operator. Here is an example:enum {
KBit0 = 1,
KBit1,
…
KBit31,
};
if ( some_int & KBit24 )
printf ( “Bit number 24 is ON\n” );
else
printf ( “Bit number 24 is OFF\n” );
Which bit wise operator is suitable for turning off a particular bit in a number?
The bitwise AND operator, again. In the following code snippet, the bit number 24 is reset
to zero.
some_int = some_int & ~KBit24;
Which bit wise operator is suitable for putting on a particular bit in a number?
The bitwise OR operator. In the following code snippet, the bit number 24 is turned ON:
some_int = some_int | KBit24;
