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

STRUCTURES

STRUCTURES

Arrays allow to define type of variables that can hold several data items of the
same kind. Similarly, structure is another user-defined data type available in C
that allows to combine data items of different kinds.
Structures are used to represent a record. Suppose you want to keep track of
your books in a library. You might want to track the following attributes about
each book:

· Title
· Author
· Subject
· Book ID

Defining a Structure

To define a structure, you must use the struct statement. The struct statement
defines a new data type, with more than one member. The format of the struct
statement is as follows:

struct [structure tag] { member definition; member definition; ... member definition; } [one or more structure variables];

The structure tag is optional and each member definition is a normal variable
definition, such as int i; or float f; or any other valid variable definition. At the
end of the structure's definition, before the final semicolon, you can specify one
or more structure variables but it is optional. Here is the way you would declare
the Book structure:

struct Books { char title[50]; char author[50]; char subject[100]; int book_id; } book;

Accessing Structure Members

To access any member of a structure, we use the member access operator
(.). The member access operator is coded as a period between the structure
variable name and the structure member that we wish to access. You would
use the keyword struct to define variables of structure type. The following
example shows how to use a structure in a program:

#include <stdio.h> #include <string.h> struct Books { char title[50]; char author[50]; char subject[100]; int book_id; }; int main( ) { struct Books Book1; /* Declare Book1 of type Book */ struct Books Book2; /* Declare Book2 of type Book */ /* book 1 specification */ strcpy( Book1.title, "C Programming"); strcpy( Book1.author, "Nuha Ali"); strcpy( Book1.subject, "C Programming Tutorial"); Book1.book_id = 6495407; /* book 2 specification */ strcpy( Book2.title, "Telecom Billing"); strcpy( Book2.author, "Zara Ali"); strcpy( Book2.subject, "Telecom Billing Tutorial"); Book2.book_id = 6495700; /* print Book1 info */ printf( "Book 1 title : %s\n", Book1.title); printf( "Book 1 author : %s\n", Book1.author); printf( "Book 1 subject : %s\n", Book1.subject); printf( "Book 1 book_id : %d\n", Book1.book_id); /* print Book2 info */ printf( "Book 2 title : %s\n", Book2.title); printf( "Book 2 author : %s\n", Book2.author); printf( "Book 2 subject : %s\n", Book2.subject); printf( "Book 2 book_id : %d\n", Book2.book_id); return 0; }

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

Book 1 title : C Programming Book 1 author : Nuha Ali Book 1 subject : C Programming Tutorial Book 1 book_id : 6495407 Book 2 title : Telecom Billing Book 2 author : Zara Ali Book 2 subject : Telecom Billing Tutorial Book 2 book_id : 6495700

Structures as Function Arguments

You can pass a structure as a function argument in the same way as you pass
any other variable or pointer.

#include <stdio.h> #include <string.h> struct Books { char title[50]; char author[50]; char subject[100]; int book_id; }; /* function declaration */ void printBook( struct Books book ); int main( ) { struct Books Book1; /* Declare Book1 of type Book */ struct Books Book2; /* Declare Book2 of type Book */ /* book 1 specification */ strcpy( Book1.title, "C Programming"); strcpy( Book1.author, "Nuha Ali"); strcpy( Book1.subject, "C Programming Tutorial"); Book1.book_id = 6495407; /* book 2 specification */ strcpy( Book2.title, "Telecom Billing"); strcpy( Book2.author, "Zara Ali"); strcpy( Book2.subject, "Telecom Billing Tutorial"); Book2.book_id = 6495700; /* print Book1 info */ printBook( Book1 ); /* Print Book2 info */ printBook( Book2 ); return 0; } void printBook( struct Books book ) { printf( "Book title : %s\n", book.title); printf( "Book author : %s\n", book.author); printf( "Book subject : %s\n", book.subject); printf( "Book book_id : %d\n", book.book_id); }

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

Book title : C Programming Book author : Nuha Ali Book subject : C Programming Tutorial Book book_id : 6495407 Book title : Telecom Billing Book author : Zara Ali Book subject : Telecom Billing Tutorial Book book_id : 6495700

Pointers to Structures

You can define pointers to structures in the same way as you define pointer to
any other variable:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above-defined
pointer variable. To find the address of a structure variable, place the ‘&’
operator before the structure's name as follows:

struct_pointer = &Book1;

To access the members of a structure using a pointer to that structure, you must
use the -> operator as follows:

struct_pointer->title;

Let us rewrite the above example using structure pointer.

#include <stdio.h> #include <string.h> struct Books { char title[50]; char author[50]; char subject[100]; int book_id; }; /* function declaration */ void printBook( struct Books *book ); int main( ) { struct Books Book1; /* Declare Book1 of type Book */ struct Books Book2; /* Declare Book2 of type Book */ /* book 1 specification */ strcpy( Book1.title, "C Programming"); strcpy( Book1.author, "Nuha Ali"); strcpy( Book1.subject, "C Programming Tutorial"); Book1.book_id = 6495407; /* book 2 specification */ strcpy( Book2.title, "Telecom Billing"); strcpy( Book2.author, "Zara Ali"); strcpy( Book2.subject, "Telecom Billing Tutorial"); Book2.book_id = 6495700; /* print Book1 info by passing address of Book1 */ printBook( &Book1 ); /* print Book2 info by passing address of Book2 */ printBook( &Book2 ); return 0; } void printBook( struct Books *book ) { printf( "Book title : %s\n", book->title); printf( "Book author : %s\n", book->author); printf( "Book subject : %s\n", book->subject); printf( "Book book_id : %d\n", book->book_id); }

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

Book title : C Programming Book author : Nuha Ali Book subject : C Programming Tutorial Book book_id : 6495407 Book title : Telecom Billing Book author : Zara Ali Book subject : Telecom Billing Tutorial Book book_id : 6495700

Bit Fields

Bit Fields allow the packing of data in a structure. This is especially useful when
memory or data storage is at a premium. Typical examples include:

· Packing several objects into a machine word, e.g. 1 bit flags can be
compacted.

· Reading external file formats -- non-standard file formats could be read
in, e.g., 9-bit integers.

C allows us to do this in a structure definition by putting :bit length after the
variable. For example:

struct packed_struct { unsigned int f1:1; unsigned int f2:1; unsigned int f3:1; unsigned int f4:1; unsigned int type:4; unsigned int my_int:9; } pack;

Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4-bit
type, and a 9-bit my_int.
C automatically packs the above bit fields as compactly as possible, provided
that the maximum length of the field is less than or equal to the integer word
length of the computer. If this is not the case, then some compilers may allow
memory overlap for the fields, while others would store the next field in the next
word.