C – typedef

The C programming language provides a keyword called typedef, which you can use to give a type, a new name. Following is an example to define a term BYTE for one-byte numbers −

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example..

BYTE  b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows −

typedef unsigned char byte;

You can use typedef to give a name to your user defined data types as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows −

#include<stdio.h>

#include<string.h>

typedefstructBooks{

char title[50];

char author[50];

char subject[100];

int book_id;

}Book;

int main(){

Book book;

strcpy( book.title,"C Programming");

strcpy( book.author,"Vishwa Prabhu");

strcpy( book.subject,"C Programming Softecks");

   book.book_id =123456;

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);

return0;

}

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

Book  title : C Programming

Book  author : Vishwa Prabhu

Book  subject : C Programming Softecks

Book  book_id : 123456

typedef vs #define

#define is a C-directive which is also used to define the aliases for various data types similar to typedef but with the following differences −

· typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, q., you can define 1 as ONE etc.

· typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.

The following example shows how to use #define in a program −

#include<stdio.h>

#defineTRUE  1

#define FALSE 0

int main(){

printf("Value of TRUE : %d\n", TRUE);

printf("Value of FALSE : %d\n", FALSE);

return0;

}

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

Value of TRUE : 1

Value of FALSE : 0