C Programming Interview Questions
Bit manipulation
Write the logic for setting nth bit.
Write the logic for clearing nth bit.
Write the logic for toggling nth bit.
Write the logic for setting nth to mth bits, where n > m.
Write the logic for clearing nth to mth bits, where n > m.
Write the logic for toggling nth to mth bits, where n > m.
Program for finding number of 1s and 0s in a 32-bit number.
Program for finding whether a number is power of 2 or not.
if((n) & (n-1))
{n is a power of 2}
Program for finding whether a number is even or odd.
if((n) & (1))
{odd}
else
{even}
Write a function to swap even bits with consecutive odd bits in a number.
e.g. b0 swapped with b1, b2 swapped with b3 and so on.
unsigned int swap_bits(unsigned int num)
{
return (num>>1 & 0x55555555) | (num<<1 & 0xAAAAAAAA);
}
Write a function to swap odd bits in a number.
e.g. b1 swapped with b3, b5 swapped with b7 and so on.
unsigned int swap_odd_bits(unsigned int num)
{
return (num>>2 & 0x22222222) |
(num<<2 & 0x88888888) |
( num
& 0x55555555) ;
}
Write a function to swap even bits in a number.
e.g. b0 swapped with b2, b4 swapped with b6 and so on.
unsigned int swap_even_bits(unsigned int num)
{
return (num>>2 & 0x11111111) |
(num<<2 & 0x44444444) |( num
& 0xAAAAAAAA) ;
}
Write a function to find out the number of 1s in a number.
unsigned int num_of_ones(unsigned int num)
{
unsigned int count = 0;
while (num != 0) {
num = (num) & (num-1);
count++;
}
return count;
}
Write a function to check whether the number of 1s present in a number are even or odd.
enum {
EVEN,
ODD
} even_odd;
unsigned int check_even_odd_no_of_ones(unsigned int num)
{
if(num_of_ones(num) & 1)
return ODD;
else
return EVEN;
}
Write a function for finding the first lowest bit set in a number.
unsigned int first_lowest_bit_set(unsigned int num)
{
unsigned int count = 0;
while(num) {
count++;
if(num&1 == 1)
break;
num = num >> 1;
}
return count;
}
Write a function for finding the higest bit set in a number.
unsigned int first_highest_bit_set(unsigned int num)
{
unsigned int count = 0;
while(num) {
count++;
if(num&(1<<31) == 1)
break;
num = num << 1;}
return count;
}
Write a function for reversing the bits in a number.
unsigned int bit_reverse(unsigned int n)
{
unsigned int m = 0, i;
for (i = 0; i < 32; i++) {
m |= (n & 1) << (31-i);
n >>= 1;
}
return m;
}
Write the code for extracting nth to mth bits, where n < m.
31
0
------------------------
| |
m n
(num >> n) & ~(~0 << (m-n+1))
Write the code for toggling nth to mth bits, where n < m.
e.g.
4 2
| |
10101010 - num
11111111 ~0
11111100 (~0<<n)
00011111 (~0>>(31-m))
00011100 (~0<<n) & (~0>>(31-m))
--> mask
Solution: num ^ ((~0<<n) & (~0>>(31-m)))
Write the code for setting nth to mth bits, where n < m.num | ((~0<<n) & (~0>>(31-m)))
Write the code for clearing nth to mth bits, where n < m.
num & ~((~0<<n) & (~0>>(31-m)))
Write a piece of code for sizeof() implementation.
#define sizeof(a) ((char *)(&a+1)-(char *)&a)
Explain about ffs and ffz implementations in ARM linux.
/*
* On ARMv5 and above those functions can be implemented around
* the clz instruction for much better code efficiency.
*/
static inline int fls(int x)
{
int ret;
if (__builtin_constant_p(x))
return constant_fls(x);
asm("clz\t%0, %1" : "=r" (ret) : "r" (x));
ret = 32 - ret;
return ret;
}
#define
#define
#define
#define
__fls(x) (fls(x) -
ffs(x) ({ unsigned
__ffs(x) (ffs(x) -
ffz(x) __ffs( ~(x)
1)
long __t = (x); fls(__t & -__t); })
1)
)
Explain about container_of() and offsetof() implementations.
/**
* container_of - cast a member of a structure out to the
containing structure
* @ptr:
the pointer to the member.
* @type:
the type of the container struct this is embedded
in.
* @member:
the name of the member within the struct.
*
*/
#define container_of(ptr, type, member) ({
\
const typeof( ((type *)0)->member ) *__mptr = (ptr);
\
(type *)( (char *)__mptr - offsetof(type,member) );})
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)How to implement bit-wise operations without using bit-wise operators?
/* a ^ b */
c = 0;
for (x = 0; x <= 15; ++x) {
c += c;
if (a < 0) {
if (b >= 0) {
c += 1;
}
} else if (b < 0) {
c += 1;
}
a += a;
b += b;
}
/* a & b */
c = 0;
for (x = 0; x <= 15; ++x) {
c += c;
if (a < 0) {
if (b < 0) {
c += 1;
}
}
a += a;
b += b;
}
/* a | b */
c = 0;
for (x = 0; x <= 15; ++x) {
c += c;
if (a < 0) {
c += 1;
} else if (b < 0) {
c += 1;
}
a += a;
b += b;
}
Data Structures
Write a program for reversing a singly linked list?
struct node {int data;
struct node *next;
}
void reverse(struct node **head)
{
struct node *t0, *t1, *t2 = NULL;
t0 = *head;
while(t0 != NULL) {
t1 = t2;
t2 = t0;
t0 = t0->next;
t2->next = t1;
}
*head = t2;
}
