Preprocessor Directives & Macros

#include <stdio.h>    // angle brackets: search system library folders
#include "myfile.h"    // quotes: search your own project folder first

This directive literally copies the entire contents of the named file directly into your source code at that exact point, before the compiler ever sees it — which is why header files typically only contain declarations, not full implementations.

#define MAX_SCORE 100
#define PI 3.14159

int main() {
    printf("%d\n", MAX_SCORE);   // preprocessor replaces this with: printf("%d\n", 100);
    return 0;
}

The preprocessor performs a simple find-and-replace: every occurrence of MAX_SCORE in your code is swapped for 100 before compilation, with zero runtime cost at all.

#define SQUARE(x) ((x) * (x))

int main() {
    printf("%d\n", SQUARE(5));   // expands to ((5) * (5)) = 25
    return 0;
}

Always wrap macro parameters in parentheses like this — without them, SQUARE(2 + 3) would expand incorrectly into 2 + 3 * 2 + 3 (11) instead of the intended (2 + 3) * (2 + 3) (25), a classic macro pitfall.

#define DEBUG 1

int main() {
#if DEBUG
    printf("Debug mode is on\n");
#endif
    printf("Program running normally\n");
    return 0;
}

#if/#endif let you include or exclude entire blocks of code depending on a condition, decided at compile time rather than while the program runs — useful for toggling debug logging or supporting multiple platforms from one codebase.

#include <stdio.h>

#define CUBE(x) ((x) * (x) * (x))

int main() {
    printf("%d\n", CUBE(3));   // 27
    printf("%d\n", CUBE(2 + 1)); // still 27, thanks to correct parentheses
    return 0;
}