6.1 Macros and the Preprocessor

The preprocessor handles your source code before the real compilation starts. #define creates a macro, which is a text substitution done before compiling.

Object-like macros are often used to name constants:

#define PI 3.1415926
#define MAX_USERS 100

double area = PI * r * r;

Function-like macros look like functions but are still text substitution. Wrap parameters in parentheses to avoid precedence bugs.

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

int n = SQUARE(3 + 1);   // expands to ((3 + 1) * (3 + 1)) = 16

If you write #define SQUARE(x) x * x, then SQUARE(3 + 1) expands to 3 + 1 * 3 + 1, which is 7 — the classic missing-parentheses trap.

Conditional compilation includes different code depending on whether a macro is defined. It is common for debug switches or cross-platform code:

#define DEBUG 1

#if DEBUG
    printf("x = %d\n", x);
#endif

Macros have no type checking, so prefer const constants and real functions when you can; macros fit best for naming constants and conditional compilation.