C Bit Fields

In this tutorial you will learn about the C Bit Fields and its application with practical example.

In C Programming Language we have the option to store integer members into memory spaces smaller than the compiler would ordinarily allow. These can be achieved using the space-saving structure members are called bit fields, and their width in bits can be explicitly declared.

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This is very useful especially when memory or data storage is at a premium. A bit field is set up with a structure declaration that labels each field and determines its width.

Bit Field Declaration

A bit-field declaration contains a type specifier followed by an optional member_name, a colon, a constant integer expression that indicates the field width in bits, and a semicolon.

Bit fields with a width of 0 must be unnamed. Unnamed bit-fields cannot be referenced or initialized. If a series of bit fields do not makeup up the size of an int, padding can take place.

The following example demonstrates padding. Suppose that an int occupies 4 bytes. The example declares the identifier room to be of type struct app_state:

The structure room contains eight members with a width of 16 bytes. The following table describes the storage that each member occupies:

Member Name Storage Occupied
light 1 bit
fan 1 bit
(padding — 30 bits) To the next int boundary
count The size of an int (4 bytes)
ac 4 bits
(unnamed field) 4 bits
clock 1 bit
(padding — 23 bits) To the next int boundary (unnamed field)
flag 1 bit
(padding — 31 bits) To the next int boundary

You can not access the field by direct its name.


We can access the member “light” as “room. light”, The following expression sets the light field to 1

When you assign to a bit field a value that is out of its range, the bit pattern is preserved and the appropriate bits are assigned. The following expression sets the “fan” field of the room structure to a 0 (zero) because only the least significant bit is assigned to the fan field:

Note:- The maximum bit field length is 64 bits. For portability, do not use bit fields greater than 32 bits in size.


In this tutorial we have learn about the C Bit Fields and its application with practical example. I hope you will like this tutorial.