C compilation — preprocessor, compiler, assembler, linker

5.1.1 · Coding › C Programming

Compilation mein stages KYU hote hain?

CPU sirf raw bytes (opcodes) samajhta hai. C source mein human ke liye bahut saari cheezein hain: #include, macros, comments, main, printf jaisi names. Yeh gap bahut bada hai, isliye hum divide and conquer karte hain — har tool ek hi sub-problem solve karta hai:

Preprocessor → text substitution handle karta hai (#-directives).
Compiler → real C ko tumhare CPU ke liye assembly mein translate karta hai.
Assembler → assembly ko machine code mein convert karta hai (ek object file).
Linker → object files + libraries ko ek executable mein jod deta hai.

Figure — C compilation — preprocessor, compiler, assembler, linker

Stage 1 — Preprocessor (`cpp`)

YEH KYA KARTA HAI:

#include <stdio.h> → us header file ka literally poora content wahan paste kar deta hai.
#define PI 3.14 → macro: har PI token ko 3.14 se replace karta hai.
#ifdef / #ifndef / #endif → conditional compilation (code tab hi include karo jab ek macro defined ho).
Saare comments strip kar deta hai.

DEKHNE KA TARIKA:

gcc -E hello.c          # output preprocessing ke baad ruk jaata hai

Worked example Macro expansion

#define SQUARE(x) ((x)*(x))
int y = SQUARE(3+1);

Preprocessing ke baad: int y = ((3+1)*(3+1)); → 16. Yeh step kyun? Preprocessor dumb text replacement karta hai, isliye hum x ko parentheses mein wrap karte hain. Unke bina, SQUARE(3+1) ban jaata hai 3+1*3+1 = 7 — galat!

Stage 2 — Compiler (`cc1`)

YEH KYA PRODUCE KARTA HAI: ek .s text file jisme human-readable assembly mnemonics hain (mov, add, call).

DEKHNE KA TARIKA:

gcc -S hello.c          # hello.s produce karta hai

Stage 3 — Assembler (`as`)

OBJECT FILE MEIN KYA HOTA HAI:

Tumhare functions ka machine code.
Ek symbol table jo yahan defined names (jaise main) aur woh names jo chahiye hain lekin abhi nahi mile (jaise printf) — undefined mark karke — list karta hai.

DEKHNE KA TARIKA:

gcc -c hello.c          # hello.o produce karta hai (abhi runnable nahi!)

Stage 4 — Linker (`ld`)

YEH KYA SOLVE KARTA HAI:

Har undefined symbol (printf) ko libc mein uski defined location se match karta hai.
Saare code/data sections merge karta hai aur final memory addresses assign karta hai.
Startup code (_start) add karta hai jo main ko call karta hai.

Worked example Classic linker error

int main(){ foo(); }   // foo declared but never defined

Compile theek se hota hai (compiler declaration trust karta hai), lekin link fail hota hai: undefined reference to 'foo'. Yeh step kyun? Har stage alag errors pakdta hai. Compiler errors = galat syntax/types. Linker errors = missing/duplicate definitions. Yeh jaanna ki kaunsa tool complain kar raha hai tumhe batata hai ki kis tarah ka bug hai.

Preprocessor = tumhara assistant jo koi bhi borrowed recipe pages paste karta hai jinka tumne reference diya tha ("cake recipe dekho") aur shorthand jo tumne define ki thi copy karta hai.
Compiler = tumhari English recipe ko robot ki step-by-step instruction language mein translate karta hai.
Assembler = un instructions ko exact button-press codes mein convert karta hai jo robot padhta hai.
Linker = agar tumhari recipe kehti hai "Grandma's frosting use karo," toh yeh jaata hai Grandma ke frosting steps lata hai aur staple karta hai, taaki robot ke paas ek complete booklet mein sab kuch ho jo woh start to finish run kar sake.

Active-Recall Flashcards

What are the 4 stages of C compilation in order?

Preprocessor → Compiler → Assembler → Linker

Which stage handles #include and #define?

The preprocessor (text substitution, before real compilation)

What does the compiler output?

Assembly language (.s), specific to the target CPU

What does the assembler output and what's special about it?

An object file (.o) with machine code + a symbol table containing undefined references

Why can't you run a .o file directly?

It has unresolved symbols (e.g. printf) and no entry-point setup; the linker must resolve them first

Which stage produces an "undefined reference" error?

The linker — a needed symbol's definition was never found

Do header files contain function code?

No — only declarations/prototypes; the actual code is in libraries, pulled in by the linker

gcc flag to stop after preprocessing?

gcc -E

gcc flag to stop after compiling (get assembly)?

gcc -S

gcc flag to stop after assembling (get .o, no link)?

gcc -c

Why wrap macro parameters in parentheses?

Macros are dumb text substitution; without parens, operator precedence breaks the intended math

What is relocation (linker)?

Assigning final memory addresses and patching references across merged object files

What's the file trail of the pipeline?

.c → .i → .s → .o → executable

Connections

C Preprocessor Directives
Header Files and #include guards
Static vs Dynamic Linking
Object Files and Symbol Tables
Makefiles and Incremental Builds
Assembly Language Basics
Compiler Optimization Levels (-O0..-O3)

C compilation — preprocessor, compiler, assembler, linker

Compilation mein stages KYU hote hain?

Stage 1 — Preprocessor (`cpp`)

Stage 2 — Compiler (`cc1`)

Stage 3 — Assembler (`as`)

Stage 4 — Linker (`ld`)

Forecast-then-Verify

Common Mistakes (Steel-manned)

Mnemonic

Feynman: ek 12-saal ke bacche ko explain karo

Active-Recall Flashcards

Connections

Concept Map