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Bit Fields In C 

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In C language, we can specify size (in bits) of structure and union members. The idea 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 withing a small range.

Suppose your C program contains a number of TRUE/FALSE variables grouped in a structure called status, as follows −

This structure requires 8 bytes of memory space but in actual, we are going to store either 0 or 1 in each of the variables. The C programming language offers a better way to utilize the memory space in such situations.

If you are using such variables inside a structure then you can define the width of a variable which tells the C compiler that you are going to use only those number of bytes. For example, the above structure can be re-written as follows −

The above structure requires 4 bytes of memory space for status variable, but only 2 bits will be used to store the values.

If you will use up to 32 variables each one with a width of 1 bit, then also the status structure will use 4 bytes. However as soon as you have 33 variables, it will allocate the next slot of the memory and it will start using 8 bytes. Let us check the following example to understand the concept

When the above code is executed, it produces following result:

Bit Field Declaration

The declaration of a bit-field has the following form inside a structure 

The following table describes the variable elements of a bit field −

Sr.No. Element & Description
1

type

An integer type that determines how a bit-field’s value is interpreted. The type may be int, signed int, or unsigned int.

2

member_name

The name of the bit-field.

3

width

The number of bits in the bit-field. The width must be less than or equal to the bit width of the specified type.

The variables defined with a predefined width are called bit fields. A bit field can hold more than a single bit; for example, if you need a variable to store a value from 0 to 7, then you can define a bit field with a width of 3 bits as follows

The above structure definition instructs the C compiler that the age variable is going to use only 3 bits to store the value. If you try to use more than 3 bits, then it will not allow you to do so. Let us try the following example

When the above code is compiled it will compile with a warning and when executed, it produces the following result

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