Lesson 14 — Advanced

Preprocessor & Macros

Compiler chalne se ek second pehle ek chhota program chalta hai jo sirf text badalta hai — preprocessor. #include yahin expand hota hai, #define ke macros yahin replace hote hain. Is lesson mein hum directives, function-like macros (unke traps ke saath) aur conditional compilation dekhenge.

Preprocessor kya hai

#include <stdio.h>     // poora header FILE khol kar paste kar do
#define PI 3.14159     // aage PI likha = 3.14159 likha

int main() {
    printf("%f\n", PI);   // → printf("%f\n", 3.14159);
    return 0;
}

Output: 3.141590

• Ye TEXT replacement hai: koi function call nahi, koi runtime cost nahi — compiler ko milta hai pehle hi badla hua code.
• # se shuru: har directive. Semicolon nahi chahiye (#define PI 3.14; likhoge toh ";" bhi replace hoga — galti!).
• Stages: preprocessor → compiler → assembler → linker. Ye step pehla hai (lesson 1 mein dekha tha).

Macro types — object-like aur function-like

#include <stdio.h>

#define SQUARE(x) ((x) * (x))   // function-like (sahi)
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define BAD(x) x * x             // GALAT (brackets nahi)

int main() {
    printf("%d\n", SQUARE(5));        // 25
    printf("%d\n", SQUARE(2 + 3));    // 25 ((2+3)*(2+3)) — sahi!
    printf("%d\n", MAX(10, 20));      // 20
    printf("%d\n", 10 / SQUARE(2));   // 2 (10 / (2*2)) — sahi
    printf("%d\n", 10 / BAD(2));      // 5 — 10/2*2 = 5 GALAT!
    return 0;
}

Output:
25
25
20
2
5   ← BAD macro ka jaadu

• Object-like: #define PI 3.14 — bas value replace.
• Function-like: #define SQ(x) ((x)*(x)) — function jaisa dikhta hai par andha text hai (type check nahi).
• Double evaluation: SQUARE(i++) mein i DO baar badhega — macro mein side-effect wale argument se bacho.
• Type-safe alternative: C99 inline function — check bhi, call overhead bhi nahi.

Conditional compilation — #ifdef ka khel

#define DEBUG 1

#ifdef DEBUG
    printf("Debug mode ON: line %d\n", __LINE__);
#endif

#ifndef HEADER_H        // include guard
#define HEADER_H
// ... header ka content ...
#endif

// Real use — platform specific:
#ifdef _WIN32
    system("cls");
#else
    system("clear");
#endif

Ye tab chalta hai jab preprocessor shart poori ho: #ifdef (defined hai?), #ifndef (defined NAHI hai?), #else, #endif (band). File ke beech mein rukna mat bhoolo — #endif ke baad sab normal.

• Include guards: #ifndef FILE_H / #define FILE_H — ek hi header do baar include ho toh definition dup nahi hoga. Har header file mein standard hai.
• #pragma once: include guard ka modern (compiler wala) version — ek line ka.
• Debug code: #ifdef DEBUG se debugging prints ek jagah — release build mein hat jaate hain (program tez).
• Makhluk: #undef PI — macro khatam (defined nahi rahega).

Predefined macros aur directives ki poori list

#include <stdio.h>

int main() {
    printf("File = %s\n", __FILE__);    // main.c
    printf("Line = %d\n", __LINE__);    // 6 (isi line ka number)
    printf("Date = %s\n", __DATE__);    // Oct 18 2026
    printf("Time = %s\n", __TIME__);    // 10:30:00

    // #error compiler ko rok deta hai:
    // #error "Yahan compilation fail karo"
    return 0;
}

Directive        Kya karta hai
#include         header paste
#define / #undef  macro banao / mitao
#ifdef/#ifndef   condition (defined?)
#else / #endif   conditional ka else / end
#line            line number badal do
#error           compilation error message
#pragma          compiler ko hint (once, pack)

Predefined: __FILE__ __LINE__ __DATE__ __TIME__ __STDC__

Macro vs function: macro fast hai (no call overhead, par code badh sakta hai), function safe hai (type check, debugging). Aaj kal small helpers ke liye inline function hi pasand kiya jaata hai — macro sirf constants aur conditionals ke liye rakho.

Exercise

Common mistakes

• Macro arguments ko bina bracket ke likhna: #define SQ(x) x*x → 10/SQ(2) = 5 (galat). Hamesha ((x)*(x)) — har argument bracket mein.
• Macro ko function samajhna: Macro mein type check nahi, side-effect ho sakta hai (i++ do baar). Function jaisa dikhta hai, hota nahi.
• #define ke baad semicolon laga dena: #define PI 3.14; — semicolon bhi replace ho jaata hai aur aage expressions toot-te hain. Directive pe semicolon nahi.

#define CHAI_RATE 15

bill = CHAI_RATE * 3;   // → 15 * 3 = 45

#ifdef SUNDAY
    rate = CHAI_RATE / 2;   // sirf Sunday code
#endif

Ek jagah badlo → poore program mein badal gaya