What are Constants - C Language Basics

Introduction

You can define constant variable by adding the const keyword either before or after the data type.

const modifier makes the variable read-only, and you can only assign a value when it is declared.

const int var = 5;   /* recommended order */
int const var2 = 10; /* alternative order */

For pointers, const can be used in two ways.

First, the constant pointer means that it cannot point to another location.

int myPointee;
int* const p = &myPointee; /* constant pointer */

Second, the constant pointee means that the variable pointed to cannot be modified through this pointer.

const int* q = &var; /* constant pointee */

To declare both the pointer and the pointee as constant.

const int* const r = &var; /* constant pointer & pointee */

Function parameters can be marked as constant to prevent them from being updated by the function.

void foo(const int* x) {
  if (x != NULL) {
    int i = *x; /* allowed */
    *x = 1;     /* compile-time error */
  }
}