BIT FIELDS

BIT FIELDS

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

struct { unsigned int widthValidated; unsigned int heightValidated; } status;

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 rewritten as follows:

struct { unsigned int widthValidated : 1; unsigned int heightValidated : 1; } status;

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:

#include <stdio.h> #include <string.h> /* define simple structure */ struct { unsigned int widthValidated; unsigned int heightValidated; } status1; /* define a structure with bit fields */ struct { unsigned int widthValidated : 1; unsigned int heightValidated : 1; } status2; int main( ) { printf( "Memory size occupied by status1 : %d\n", sizeof(status1)); printf( "Memory size occupied by status2 : %d\n", sizeof(status2)); return 0; }

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

Memory size occupied by status1 : 8 Memory size occupied by status2 : 4

Bit Field Declaration

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

struct { type [member_name] : width ; };

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

Elements        Description
type An integer type that determines how a bit-field's value is
interpreted. The type may be int, signed int, or unsigned int.
member_name The name of the bit-field.
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:

struct { unsigned int age : 3; } Age;

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:

#include <stdio.h> #include <string.h> struct { unsigned int age : 3; } Age; int main( ) { Age.age = 4; printf( "Sizeof( Age ) : %d\n", sizeof(Age) ); printf( "Age.age : %d\n", Age.age ); Age.age = 7; printf( "Age.age : %d\n", Age.age ); Age.age = 8; printf( "Age.age : %d\n", Age.age ); return 0; }

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

Sizeof( Age ) : 4 Age.age : 4 Age.age : 7 Age.age : 0