#define

This is used to define macros. Macros can be simple constants or function

Here is an example, here defined string PI and SQUARE(x) will be replace by compiler at compile time

#define PI 3.14159
#define SQUARE(x) ((x) * (x))

int main() {
    double radius = 5.0;
    double area = PI * SQUARE(radius);
    // ...
}

Here is another example

#define MAX(a, b) ((a) > (b) ? (a) : (b))

int main() {
    int x = 5, y = 7;
    printf("Max is: %d\n", MAX(x, y));  // Output: Max is: 7
    
    // Be careful with side effects:
    printf("Max is: %d\n", MAX(x++, y++));  // This might not behave as expected
}

Multiline Marcos

#define MULTI_LINE_MACRO do { \
    printf("This is a multi-line macro.\n"); \
    printf("It can contain multiple statements.\n"); \
} while(0)

int main() {
    MULTI_LINE_MACRO;
}