Static libraries — .a - .lib, creation and linking

5.3.3 · Coding › Build Systems & Toolchain

WHY — static libraries kyun exist karti hain?

Do competing goals jo ek static library balance karti hai:

Reuse — code ek baar likho, kahin bhi link karo.
Selectivity — apne executable ko un functions se bloat mat karo jo tum kabhi call hi nahi karte.

WHAT — actually ek `.a` / `.lib` kya hota hai?

HOW — ek library kaise banate hain (first principles)

Ek library teen conceptual stages mein banti hai. Aao command sequence derive karein.

Stage 1 — Object files mein compile karo (link mat karo abhi)

Humhe machine code chahiye lekin koi finished program nahi, isliye hum compiler ko linking se pehle -c ke saath rok dete hain.

gcc -c add.c    -o add.o     # Why -c? compile only, produce relocatable .o
gcc -c sub.c    -o sub.o     # no main() needed; libraries have no entry point

Stage 2 — `.o` files ko ek `.a` mein archive karo

ar rcs libmath.a add.o sub.o

Har flag decode karna (yahi reason hai ki command aise dikhti hai):

flag	matlab	kyun chahiye
`r`	members ko replace/insert karo	`.o`s add karo, purani versions overwrite karke
`c`	quietly create karo	archive exist nahi karta toh banao, koi warning nahi
`s`	ek symbol index likho	taaki linker symbols fast dhoond sake (warna `ranlib` chalao)

Stage 3 — Ek program ko iske against link karo

gcc main.o -L. -lmath -o app

-L. — current directory ko library search path mein add karo.
-lmath — libmath.a dhoondo (lib prefix aur .a suffix tumhare liye add ho jaate hain).

Figure — Static libraries — .a - .lib, creation and linking

HOW — linking actually code select kaise karta hai (deep part)

Yahi famous link order rule ka source hai.

Worked example Classic broken order

gcc -lmath main.o -o app      # ✗ undefined reference to `add`
gcc main.o -lmath -o app      # ✓ works

Yeh step kyun? Jab linker pehle -lmath se milta hai, tab tak koi symbol undefined nahi hai, isliye woh archive se kuch nahi leta. Phir main.o add ko reference karta hai → unresolved → error. main.o pehle rakho taaki add already "wanted" list pe ho.

Worked Examples

Worked example Example 1 — Scratch se pura build

Files: add.c (add define karta hai), main.c (add call karta hai).

gcc -c add.c  -o add.o     # Why? relocatable code lo, main needed nahi
ar rcs libmath.a add.o     # Why? bundle + index ek archive mein
gcc -c main.c -o main.o    # Why? caller ko alag compile karo
gcc main.o -L. -lmath -o app   # Why? main.o pehle, phir provider library
./app

Result: app mein add ke machine code ki ek copy hai; libmath.a delete karne se ./app nahi tootega. Yahi static guarantee hai.

Worked example Example 2 — Archive inspect karna

ar t libmath.a      # list members  → add.o  sub.o
nm libmath.a        # show symbols; 'T' = defined text, 'U' = undefined

Yeh step kyun? nm ka T vs U batata hai ki koi symbol library dwara provide kiya gaya hai ya usse chahiye — exactly wahi information jo linker use karta hai.

Worked example Example 3 — Windows / MSVC equivalent

cl /c add.c          :: produces add.obj
lib /OUT:math.lib add.obj   :: 'lib' tool = ar's cousin
cl main.c math.lib /Fe:app.exe

Yeh step kyun? Wahi teen stages — compile, archive (lib), link — bas alag tool names. MSVC ka lib.exe wahi role play karta hai jo ar karta hai.

Common Mistakes (Steel-manned)

Active Recall

Recall Static lib build aur use karne ka minimal 3-stage pipeline kya hai?

gcc -c *.c → object files. 2. ar rcs libX.a *.o → archive. 3. gcc main.o -L. -lX -o app → link, library apne user ke baad.

Recall

.a delete karne se executable kyun nahi toot-ta? Kyunki static linking ne build time pe zaroorat ka machine code executable mein copy kar diya tha; archive sirf ek build-time source tha.

Recall

ar rcs mein s tumhe kya deta hai? Ek symbol index taaki linker har member scan kiye bina symbols resolve kar sake.

Recall (Feynman, ek 12-saal ke bacche ko samjhao)

Socho ek LEGO bricks ka dabba (.a). Tumhare toy ke instructions (main.o) kehte hain "mujhe ek red 2×4 brick chahiye." Builder (linker) dabba kholega, sirf wahi red 2×4 nikalega jo tumne manga, toy pe lagaega, aur dabba band kar dega. Ek baar toy ban gaya, tum dabba phek sakte ho — brick already glued on hai. Lekin builder sirf wahi bricks uthata hai jo tum pehle se maang chuke ho, isliye dabbe tak pahunche se pehle maangna padega, baad mein nahi.

Flashcards

Unix vs Windows pe static library ka file extension kya hota hai?

lib<name>.a (Unix) aur <name>.lib (Windows/MSVC).

Unix pe .a archive kaunsa tool banata hai?

ar (jaise ar rcs libX.a *.o).

ar rcs mein flags decode karo.

r = members insert/replace karo, c = quietly create karo, s = symbol index likho.

Library sources compile karte waqt -c kyun use karna padta hai?

Relocatable object files pe rokne ke liye (no linking, koi main entry point required nahi).

Unix link order rule batao.

Ek library unke objects/libraries ke BAAD aani chahiye jo iske symbols reference karti hain (single-pass left-to-right resolution).

gcc -lmath main.o fail kyun hota hai lekin gcc main.o -lmath kaam karta hai?

Linker sirf wahi archive members pull karta hai jo currently-undefined symbols satisfy karti hain; lib pehle hone pe kuch bhi undefined nahi hota isliye kuch liya hi nahi jaata.

Kya runtime pe static .a present honi chahiye?

Nahi — iska code link time pe executable mein copy ho jaata hai; yeh sirf ek build-time dependency hai.

nm mein T vs U ka kya matlab hai?

T = symbol defined hai (text/code mein), U = symbol undefined hai (kahin aur se chahiye).

"Archive has no index" ko kya fix karta hai?

ar rcs se re-archive karo ya ranlib libX.a chalao.

Search karte waqt -lfoo kismein expand hota hai?

-L search paths mein libfoo.a (ya libfoo.so) naam ki file mein.

GNU ld mein do static libs ke beech cyclic dependencies kaise handle karte ho?

Unhe wrap karo: -Wl,--start-group -lA -lB -Wl,--end-group.

Connections

Object files and relocation
Shared libraries — .so / .dll (dynamic-link counterpart)
The Linker — symbol resolution & relocation
Compilation pipeline — preprocess, compile, assemble, link
Make and build dependency graphs
ELF and PE file formats