The Preprocessor and Macros

Beyond #include and #define constants, the preprocessor supports macro functions and conditional compilation:

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

printf("%d\n", SQUARE(5));    // expands to ((5) * (5)) -> 25

#ifdef DEBUG
    printf("Debug mode is on\n");
#endif

Common mistakes

• Writing #define SQUARE(x) x * x without the parentheses — SQUARE(1 + 2) then expands to 1 + 2 * 1 + 2, evaluating to 5 instead of the intended 9. Always wrap both the macro body and each parameter in parentheses.