Most Asked Bit Field Interview Questions

Q1. Predict the output of the following program. Assume that unsigned int takes 4 bytes and long int takes 8 bytes.

Ans:

C

#include <stdio.h>   struct test {     // Unsigned integer member x     unsigned int x;     // Bit-field member y with 33 bits     unsigned int y : 33;     // Unsigned integer member z     unsigned int z; };   int main() {     // Print the size of struct test     printf("%lu", sizeof(struct test));       return 0; }

Error:

./3ec6d9b7-7ae2-411a-a60e-66992a7fc29b.c:4:5: error: width of 'y' exceeds its type
     unsigned int y : 33;
     ^

 Q2. Predict the output of the following program.

Ans:

C

#include <stdio.h>   struct test {     // Unsigned integer member x     unsigned int x;     // Bit-field member y with 33 bits     long int y : 33;     // Unsigned integer member z     unsigned int z; };   int main() {     // Declare a variable t of type struct test     struct test t;     // Pointer to unsigned int, pointing to member x     unsigned int* ptr1 = &t.x;     // Pointer to unsigned int, pointing to member z     unsigned int* ptr2 = &t.z;       // Print the difference between the two pointers     printf("%d", ptr2 - ptr1);       return 0; }

Output

4

Q3. Predict the output of the following program.

Ans:

C

#include <stdio.h>   union test {     // Bit-field member x with 3 bits     unsigned int x : 3;     // Bit-field member y with 3 bits     unsigned int y : 3;     // Regular integer member z     int z; };   int main() {     // Declare a variable t of type union test     union test t;     // Assign the value 5 to x (3 bits)     t.x = 5;     // Assign the value 4 to y (3 bits)     t.y = 4;     // Assign the value 1 to z (32 bits)     t.z = 1;     // Print the values of x, y, and z     printf("t.x = %d, t.y = %d, t.z = %d", t.x, t.y, t.z);       return 0; }

Output

t.x = 1, t.y = 1, t.z = 1

In C, we can specify the size (in bits) of the structure and union members. The idea of bit-field is to use memory efficiently when we know that the value of a field or group of fields will never exceed a limit or is within a small range. C Bit fields are used when the storage of our program is limited.

Need of Bit Fields in C

• Reduces memory consumption.
• To make our program more efficient and flexible.
• Easy to Implement.