Unions — overlapping memory — Hinglish notes

5.1.24 · Coding › C Programming

WHAT is a union?

union Data {
    int   i;     // 4 bytes
    float f;     // 4 bytes
    char  c[4];  // 4 bytes
};

Struct ke saath contrast karo:

Feature	`struct`	`union`
Memory	sum of all members (+padding)	size of largest member
Members live	all simultaneously	only one at a time
Use case	bundle related data	save space / reinterpret bytes

WHY is its size the largest member?

Worked example Haath se size nikalna

union Mix { char c; int i; double d; };

Sizes: char=1, int=4, double=8. Largest = 8. Yeh step kyun? Union ko sabse bade member, yaani double ko fit karna hai. double ki alignment 8 hai, aur 8 pehle se hi 8 ka multiple hai. $\Rightarrow$ sizeof(union Mix) == 8.

HOW bytes overlap hote hain (key picture)

Worked example Ek member likhne se doosre clobber ho jaate hain

union Data d;
d.i = 65;            // bytes ab integer 65 hold kar rahe hain
printf("%d\n", d.i); // 65   — valid, humne abhi i likha
d.f = 3.14f;         // SAME bytes float bits se overwrite ho gaye
printf("%d\n", d.i); // garbage! i ke bytes ab float bits hain
printf("%f\n", d.f); // 3.140000 — valid, last written

Yeh step kyun? d.i aur d.f same 4 bytes hain. d.f likhne par un bytes ko float pattern ki tarah reinterpret kiya jaata hai; baad mein d.i padhne par float bits ko aise padha jaata hai jaise woh int hon.

Worked example Type punning — bytes ko peek karna

Maano int i = 65 ek little-endian machine par. ASCII 'A' = 65.

union { int i; char c[4]; } u;
u.i = 65;
// bytes (little-endian): 41 00 00 00  (hex)
printf("%c\n", u.c[0]);  // 'A'

Yeh step kyun? 65 ka lowest-order byte 0x41 hai. Little-endian par yeh c[0] par aata hai, isliye isse char ke roop mein padhne par 'A' milta hai. Isi tarah unions raw representation inspect karne dete hain.

Ek practical use: tagged union (safe pattern)

struct Variant {
    enum { INT, FLOAT, STR } tag;   // kamre mein ab kaun rehta hai?
    union { int i; float f; char *s; } val;
};

KYUN: union ke andar koi runtime flag nahi hota jo bataye ki active member kaun sa hai. Tag tumhari bookkeeping hai taaki tum sirf wahi padho jo tumne last mein likha tha.

Common mistake Common errors ko steel-man karna

Galti 1: "Main d.i likh sakta hoon aur baad mein d.f padh kar int→float convert kar sakta hoon." Kyun sahi lagta hai: dono 4 bytes hain, isliye yeh "conversion jaisa lagta hai." Sachai: alag member padhna raw bit pattern ko reinterpret karta hai, yeh arithmetic conversion NAHI karta. 65 as int ka bit pattern 65.0 as float se bilkul alag hai. Fix: value conversion ke liye cast use karo float f = (float)d.i;; union ko tabhi use karo jab tumhe raw bits chahiye hon.

Galti 2: "sizeof(union) = sum of members." Kyun sahi lagta hai: structs ke liye yeh sach hai. Fix: unions overlap karte hain, isliye size = largest member (+alignment padding).

Galti 3: yeh bhool jaana ki union initialize karne par by default sirf PEHLA member set hota hai. union Data d = {65}; sirf d.i ko initialize karta hai, sab members ko nahi. Baaki ko padhna bekar hai.

Recall Feynman: 12-saal ke bacche ko samjhao

Ek whiteboard imagine karo. Tum uspar ek number likh sakte ho, YA ek word, YA ek drawing — lekin sirf ek cheez fit hoti hai kyunki yeh usi board hai. Agar tum apne number ke upar word likhte ho, number chala jaata hai. Union woh single whiteboard hai jo alag-alag tarah ki "writing" share karti hai. Struct, iske ulta, ek notebook hai jisme har cheez ke liye alag page hai. Board exactly utna bada hota hai jitna sabse badi cheez hogi jo tum likh sakte ho.

Flashcards

Union ke members ki defining property kya hai?

Woh sab same memory share karte hain (offset 0 se shuru hote hain), isliye ek waqt mein sirf ek valid hota hai.

Union ka size kaise determine hota hai?

Yeh uske largest member ke size ke barabar hota hai, alignment satisfy karne ke liye round up hoke.

Struct mein size kaise determine hota hai vs union mein?

struct = sum of members (+padding); union = size of largest member (+padding).

Jab tum ek union member likhte ho toh doosre members ka kya hota hai?

Unke bytes overwrite/reinterpret ho jaate hain — unhe ab padhne par meaningless data milta hai.

Kya alag union member padhna type conversion karta hai?

Nahi — yeh raw bit pattern ko reinterpret karta hai (type punning), arithmetic conversion nahi.

Tagged union kya hota hai aur kyun zaroori hai?

Ek struct jo union ke saath ek enum tag hold karta hai jo active member record karta hai, kyunki union khud yaad nahi rakhta ki kaun sa member live hai.

union { char c; int i; double d; } ke liye sizeof kya hai?

8 (sabse bade member, double, ka size, jo pehle se 8-aligned hai).

Little-endian par u.i = 65 ke saath, u.c[0] kya dega?

'A' (0x41), 65 ka lowest-order byte.

Connections

Structs — separate memory for each member
Memory alignment and padding
Endianness — byte order
Type punning and bit-level reinterpretation
Enums (tagged unions mein tag ke liye use hote hain)
sizeof operator