C – Typedef

• Typedef is a keyword that is used to give a new symbolic name for the existing name in a C program. This is same like defining alias for the commands.
• Consider the below structure.

struct student
{
         int mark [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 <tag_name> 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 FOR C TYPEDEF:

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

// 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, "Raju");   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: Raju Percentage is: 86.500000

• 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 FOR C TYPEDEF:

C

1 2 3 4 5 6 7 8 9 10 11 12

#include <stdio.h> #include <limits.h>   int main() {    typedef long long int LLI;      printf("Storage size for long long int data " \           "type  : %ld \n", sizeof(LLI));      return 0; }

 OUTPUT:

Storage size for long long int data type : 8