# 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>

typedef struct Books {
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, "Nuha Ali");
strcpy( book.subject, "C Programming Tutorial");
book.book_id = 6495407;

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);

return 0;
}


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


## 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>

#define TRUE  1
#define FALSE 0

int main( ) {
printf( "Value of TRUE : %d\n", TRUE);
printf( "Value of FALSE : %d\n", FALSE);

return 0;
}


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

Value of TRUE : 1
Value of FALSE : 0