Bit Fields in C

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.

Declaration of C Bit Fields

Bit-fields are variables that are defined using a predefined width or size. Format and the declaration of the bit-fields in C are shown below:

Syntax of C Bit Fields

struct
{
    data_type member_name : width_of_bit-field;
};

where,

• data_type: It is an integer type that determines the bit-field value which is to be interpreted. The type may be int, signed int, or unsigned int.
• member_name: The member name is the name of the bit field.
• width_of_bit-field: The number of bits in the bit-field. The width must be less than or equal to the bit width of the specified type.

Applications of C Bit Fields

• If storage is limited, we can go for bit-field.
• When devices transmit status or information encoded into multiple bits for this type of situation bit-field is most efficient.
• Encryption routines need to access the bits within a byte in that situation bit-field is quite useful.

Example of C Bit Fields

In this example, we compare the size difference between the structure that does not specify bit fields and the structure that has specified bit fields.

Structure Without Bit Fields

Consider the following declaration of date without the use of bit fields.

Size of date is 12 bytes
Date is 31/12/2014

The above representation of ‘date’ takes 12 bytes on a compiler whereas an unsigned int takes 4 bytes. Since we know that the value of d is always from 1 to 31, and the value of m is from 1 to 12, we can optimize the space using bit fields.

Structure with Bit Field

The below code defines a structure named date with a single member month. The month member is declared as a bit field with 4 bits.

struct date
{
// month has value between 0 and 15, 
// so 4 bits are sufficient for month variable.
    int month : 4;
};

However, if the same code is written using signed int and the value of the fields goes beyond the bits allocated to the variable, something interesting can happen.

Below is the same code but with signed integers:

Size of date is 8 bytes
Date is -1/-4/2014

Explanation

The output comes out to be negative. What happened behind is that the value 31 was stored in 5 bit signed integer which is equal to 11111. The MSB is a 1, so it’s a negative number and you need to calculate the 2’s complement of the binary number to get its actual value which is what is done internally.

By calculating 2’s complement you will arrive at the value 00001 which is equivalent to the decimal number 1 and since it was a negative number you get a -1. A similar thing happens to 12 in which case you get a 4-bit representation as 1100 and on calculating 2’s complement you get the value of -4.

Interesting Facts About C Bit Fields

1. A special unnamed bit field of size 0 is used to force alignment on the next boundary.

Example: The below code demonstrates how to force alignment to the next memory boundary using bit fields.

Size of test1 is 4 bytes
Size of test2 is 8 bytes

2. We cannot have pointers to bit field members as they may not start at a byte boundary.

Example: The below code demonstrates that taking the address of a bit field member directly is not allowed.

Output

prog.c: In function 'main':
prog.c:14:1: error: cannot take address of bit-field 'x'
 printf("Address of t.x is %p", &t.x); 
 ^

3. It is implementation-defined to assign an out-of-range value to a bit field member.

Example: The below code demonstrates the usage of bit fields within a structure and assigns an out-of-range value to one of the bit field members.

Output

Implementation-Dependent

4. Array of bit fields is not allowed.

Example: The below C program defines an array of bit fields and fails in the compilation.

Output

prog.c:3:1: error: bit-field 'x' has invalid type
 unsigned int x[10]: 5;
 ^

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

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