What is the method for defining constants in the C language?

In C language, constants can be defined using the #define preprocessor directive or the const keyword.

1. define

#define PI 3.14159
#define MAX_NUM 100

1. variable that cannot be changed

const double PI = 3.14159;
const int MAX_NUM = 100;

Constants defined using #define are valid throughout the program, while constants defined using the const keyword are only valid within the scope of their definition. It is generally recommended to use the const keyword to define constants because it has advantages such as type safety and scope.