Macros & Conditional Compilation

#define is a text swap

The preprocessor runs before the compiler and blindly replaces macro names with their text — no type checking happens at this stage.

Object-like and function-like macros

Wrap macro parameters in parentheses to avoid operator-precedence bugs.

#define MAX_SIZE 100
#define SQUARE(x) ((x) * (x))

int arr[MAX_SIZE];
printf("%d\n", SQUARE(3 + 2)); // 25, not 11

Output:

25

#ifdef for conditional code

Code inside #ifdef blocks is only included by the preprocessor if the macro is defined — common for debug logging or platform-specific code.

#define DEBUG

#ifdef DEBUG
    printf("debug: x = %d\n", x);
#endif