Understanding typedef in C: Simplifying Complex Declarations

The C programming language provides a keyword called typedef, which you can use to give a type a new name. OR WE CAN SAY, Typedef is a keyword that is used to give a new symbolic name for the existing name in a C program. This is same as defining alias for the commands. Following is an example to define a term MAN for one-byte numbers ?

typedef unsigned int MAN;

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

MAN  a1, a2;

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 int man;

Example Program:

 #include<stdio.h>
void main(){ 
	typedef unsigned int MAN;
	MAN a1, a2 ;
	a1 = 12;
	a2 = 13;
	
	printf("%u \n",a1);
	printf("%u \n",a2);
}
 

Output:

12
13
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typedef : typedef is used to give data type a new name, for example

Another Example Program:

// C program to demonstrate typedef
#include <stdio.h>
 
// After this line BYTE can be used
// in place of unsigned char
typedef unsigned char BYTE;
 
int main()
{
    BYTE b1, b2;
    b1 = 'c';
    printf("%c ", b1);
    return 0;
}
 

Output:

c Press any key to continue . . .

Typedef can be used to simplify the real commands as per our need. For example, consider below statement.

typedef long long int LLI;

In above statement, LLI is the type definition for the real C command long long int. We can use type definition LLI instead of using full command long long int in a C program once it is defined.

Another Example Program:

#include <stdio.h>
#include <limits.h>

int main()
{
   typedef long long int LLI ;
   LLI  a = 768783793;
   printf("Storage size for long long int data type : %ld \n", sizeof(LLI));
   printf(" %ld  \n",a);

   return 0;
}
 

Output:

Storage size for long long int data type : 8
 768783793
Press any key to continue . . .

---

typedef in structure

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

Consider the below structure.

struct student
{
    int marks[2];
    char name[10];
    float average;
}

Variable for the above structure can be declared in two ways.

1st way :

struct student record;       /* for normal variable */
struct student *record;     /* for pointer variable */

2nd way :

typedef struct student status;

• When we use typedef keyword before struct like above, after that we can simply use type definition “status” in the C program to declare structure variable.
• Now, structure variable declaration will be, status record.
• This is equal to “struct student record”. Type definition for struct student is status. i.e. status = struct student

An alternative way for structure declaration using typedef in C

typedef struct student
{
         int mark [2];
         char name [10];
         float average;
} status;

To declare structure variable, we can use the below statements.

status record1;                 /* record 1 is structure variable */
status record2;                 /* record 2 is structure variable */

Example Program:

// Structure using typedef:

#include <stdio.h>
#include <string.h>

typedef struct student 
{
  int id;
  char name[20];
  float percentage;
} status;

int main() 
{
  status record;
  record.id=1;
  strcpy(record.name, "atnyla");
  record.percentage = 86.5;
  printf(" Id is: %d \n", record.id);
  printf(" Name is: %s \n", record.name);
  printf(" Percentage is: %f \n", record.percentage);
  return 0;
}
 

Output:

Id is: 1
 Name is: atnyla
 Percentage is: 86.500000
Press any key to continue . . .

Another Example Program:

#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, "ANSCII C");
   strcpy( book.author, "Dennis sir"); 
   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;
}
 

Output:

Book title : ANSCII C
Book author : Dennis sir
Book subject : C Programming Tutorial
Book book_id : 6495407
Press any key to continue . . .