Module 15: The C Preprocessor & Macro Metaprogramming

// DANGEROUS MACRO (Missing parentheses):
#define BAD_SQUARE(x) x * x
// BAD_SQUARE(1 + 2) expands to: 1 + 2 * 1 + 2 = 1 + 2 + 2 = 5! (NOT 9)

// CORRECT MACRO (Fully parenthesized):
#define SAFE_SQUARE(x) ((x) * (x))
// SAFE_SQUARE(1 + 2) expands to: ((1 + 2) * (1 + 2)) = 9!

// DOUBLE-EVALUATION TRAP:
int a = 3;
int res = SAFE_SQUARE(a++); // Expands to: ((a++) * (a++)) -> a is incremented TWICE!

// 1. Stringification Operator (#): Converts parameter name to string literal
#define PRINT_VAR(x) printf(#x " = %d\\n", x)
int total = 42;
PRINT_VAR(total); // Prints: "total = 42"

// 2. Token-Pasting Operator (##): Glues two tokens together
#define BUILD_STRUCT(type, name) type name##_instance
BUILD_STRUCT(int, score); // Expands to: int score_instance;

// Traditional C89 Header Guard:
#ifndef MY_HEADER_H
#define MY_HEADER_H

void exported_function(void);

#endif // MY_HEADER_H

// Modern Alternative:
#pragma once // Supported on all modern compilers (GCC, Clang, MSVC)

#define LOG_ERROR(msg) \\
    fprintf(stderr, "[ERROR] in %s (%s:%d): %s\\n", __func__, __FILE__, __LINE__, msg)

void connect_server(void) {
    LOG_ERROR("Connection timeout!");
    // Output: [ERROR] in connect_server (main.c:14): Connection timeout!
}