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, whereas #define can be used to define alias for values as well, e.g., you can define 1 as ONE, etc.  typedef interpretation is performed by the compiler whereas #define statements are processed by the preprocessor. 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