C Qualifiers - Constant and Volatile type Qualifier

C has some basic data types like 'int', 'float', 'char' etc. Each of which can hold a data of a specific type and has a set of specific properties defined for them. But what if you need to modify the properties of them? For example, if you want to make an 'int' variable constant, that means it's value shouldn't be changed from any where else in the program except the variable definition part. C type qualifiers can be used for such purposes.

'Const' Qualifier in C

Two type qualifiers available in C are 'const' and 'volatile'.

'Const' qualifier will impose a restriction on the variable, such a way that its value can't be changed or modified. The main use of 'const' is to define constants in your program; so that it's values can't be changed. Please take a look at the example given below to understand the use case of 'const' qualifier

#include <stdio.h>
// PI is defined as 'const' so that you may not change it's value accidentally.
const float PI = 3.14;
 
double find_area(float radius) {
 return PI * radius * radius;
}
 
int main() {
 printf("Area: %f", find_area(5.5));
 return 0;
}
 
Output:
Area: 94.985001

'Volatile' Type Qualifier in C

If a variable is declared as 'volatile', its value can be changed from outside the program. Declaring a variable with 'volatile' is actually a hint to the compiler to not perform any optimizations on the access restrictions of the variable. Let's take a look at it with the help of an example using 'volatile' keyword.

Variables

Void Type

Void type is defined using keyword 'void'. It's used to represent absence of data. For example, if a functions is declared with 'void' return type, it means that function doesn't return any values. But you cannot define a variable with 'void' type, program will not compile. You can declare void pointers though. 

Variables

In C programs, sometimes you may need to store some values so that you can later do some calculation or operation on the stored values. To hold these values we need variables. The variable can be considered as a type of identifiers that are used to represent some type of value or information in a designated portion of a program. A variable can be assigned different values in the course of its life span, i.e. the value stored in a variable may change. However, the type of data associated with a variable can never be changed.

Variable declaration

Declaration means associating a variable with some data type. Declaring variable will enforce that the variable can only represent values of already specified data types. A declaration has a specific format. Let’s consider following declaration as an example.

int number;

By this declaration, we are specifying that the number is a variable of data type integer. Until now, the variable number does not contain the value we want. The values can be assigned to the variable as follows

Enumeration Types

Enumeration types are used to hold a value that belongs to a particular set. Keyword 'enum' is used to define enumeration types. Each element in the enumeration type is assigned with a consecutive integer constant starting with 0 by default.

Syntax for declaring enum is given below

enum type_name{value_1, value_2, value_3,...}

Each of the values in the value set (ie; value_1, value_2 etc) are integer constants. By default, they start with 0 and increments by 1. You can override this values your self. Let's take a look at it with the help of an example.

#include <stdio.h>
enum day_of_week{ sunday, monday, tuesday, wednesday, thursday, friday, saturday};
int main(){
    enum day_of_week today;
 //Assigns enum value sunday to the variable today.
    today = sunday;
    printf("%d", today);
    return 0;
}
 
Output:
0

In the above program enum type "day_of_week" consists of 7 values; Sunday to Saturday. Each of them is internally assigned with an integer constant starting with 0 by default. 'Sunday' gets 0, 'Monday' gets 1 and so on. Here the 'printf' statement will output the value 0, because we've assigned enum value 'Sunday' to the variable 'today' of type 'day_of_week'.