Type qualifiers modify how an object can be accessed or used. The three standard C type qualifiers are const, volatile, and restrict.

#include <stdio.h>

int main()
{
    const int maximum = 100;
    volatile int status = 0;

    printf("%d\n", maximum);
    printf("%d\n", status);

    return 0;
}

Example Output #

100
0

Explanation #

const indicates that an object cannot be modified through the qualified lvalue:

const int maximum = 100;

An assignment such as:

maximum = 200;

is not allowed.

volatile indicates that accesses to an object may have observable effects outside the normal flow assumed by the compiler:

volatile int status = 0;

It is commonly used for objects that can change because of hardware or other execution contexts.

restrict is a qualifier used with pointers. It provides an optimization-related aliasing guarantee when the pointer is used according to the requirements of the qualifier.

int * restrict ptr;

Type Qualifiers #

The qualifiers can also be combined:

const volatile int status;

Here, status is both const and volatile.