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 re-written 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
