5.1.20 · D1String handling — char arrays, null terminator, strcpy, strc
Intuition Is poore topic ka EK core ideaC ki "string" koi special type nahi hoti — yeh sirf memory boxes ki ek row hoti hai (ek char array) jisme ek
andikhha STOP box ('\0') end mark karta hai. Har string function us row mein box by box chalta hai
jab tak use STOP nahi milta; agar STOP missing hai ya tum kisi aisi box mein likhte ho jo tumhari nahi, C kabhi
warn nahi karta.
Pehle tum strlen, strcpy, ya sprintf ko parent note mein padh sako, us par har ek tiny symbol ka pehle kuch matlab banana chahiye. Yeh page har ek cheez ko absolute zero se, ek dusre par lean karne ke order mein build karta hai.
char
char C mein sabse chhota storage box hai. Yeh exactly one byte hold karta hai — ek pura number
0 255 dikhawe karte hain ki woh number ek letter hai, ASCII naam ki ek lookup table use karke.
Letter 'A' actually ek shape ke roop mein store nahi hoti. Yeh number 65 'A' mein quotes C ka tarika hai yeh kehne ka "mujhe is letter ka ASCII number do" taaki tumhe yaad na rakhna pade ki 'A' 65
Intuition Ek box jo actually ek number hai — kyun?Computer sirf numbers store karta hai. Letters ek human idea hain. ASCII woh agreed dictionary hai jo number 72 H mein badalta hai aur wapas. Toh char c = 'H'; aur char c = 72; exactly same byte store karte hain — topic ko yeh chahiye kyunki STOP box actually number zero hai, '\0' likha jata hai, aur tumhe dikhna chahiye ki yeh ek number hai taaki samajh sako ki counting wahan kyun rukti hai.
Reveal check:
'0' (digit) versus '\0' (terminator) — same hain ya different?Bilkul different: '0' number 48 hai, '\0' number 0 hai.
Array ek hi tarah ke kai boxes hote hain jo memory mein ek dusre ke next rakhhe jaate hain , taaki hum
unhe front se steps ki number ke zariye reach kar sakein. char s[6]; ka matlab hai "ek row mein 6 char boxes."
s[i] mein number index hai — start se kitne boxes walk karne hain. Yeh 0 s[0] hai, s[1] nahi. Toh ek 6-box array mein valid indices s[0] se s[5] tak hain — last index hamesha size minus one hota hai.
Intuition 0 se count kyun karte hain?Index actually "kitna move karna hai" hota hai, aur pehle box ko zero movement chahiye. Yahi wajah hai ki topic baar baar warn karta hai "size = length + 1": ek size-6 array mein box s[5] last legal box hai, aur STOP us ke andar fit hona chahiye.
Common mistake Off-by-one: last index hai size − 1Kyun sahi lagta hai: "6 boxes hain, toh s[6] sixth box hai." Trap: s[6] seventh box hai — end ke ek baad. Use touch karna apne neighbour ke mailbox mein likhna hai. Legal indices hain s[0] … s[5].
Reveal check:
char s[6] mein sabse zyada legal index kya hai?s[5] (size − 1).
Definition Null terminatorNull terminator ek char box hai jisme number 0 '\0' likha jata hai. Yeh
andikhha marker hai jo kehta hai "word yahan khatam hota hai." Yeh print nahi hota; yeh ek signal hai.
Boxes ki ek row mein koi built-in sign nahi hota jo bataye word kahan rukta hai. Toh C last real letter ke baad ek STOP box plant karta hai. Har string function ka kaam hai: tab tak aage chalte raho jab tak STOP na mile.
Intuition Topic ke liye yeh kyun zaroori haiSTOP ke bina, strlen random memory mein forever count karta rahega; strcpy kabhi nahi janega kab copy karna band kare. Poora topic hai "is ek byte ko sahi se manage karo." Miss karo aur tumhe
Undefined Behaviour in C milta hai.
Reveal check:
Ek program '\0' ke bina kaise janega ki string kahan khatam hoti hai? char boxes ki ek bare row mein koi length store nahi hoti; '\0' hi akaula end marker hai.
Definition String literalDouble quotes "Hi" ek string literal hain: C boxes H, i, aur — free mein — end mein ek '\0' lay out karta hai. Single quotes 'H' sirf ek char hain, koi STOP add nahi hota.
Toh "Hi" silently 3 boxes occupy karta hai chahe tumne sirf 2 letters type kiye hon. Yeh hidden extra box woh +1 hai jiske baare mein tum baar baar sunta rehte ho.
Reveal check:
Literal "cat" ko kitne boxes chahiye? 4 — teen letters plus ek '\0'.
char *)
Pointer ek box hota hai jo ek address hold karta hai — pehle char box ki location, jaise ek street par house number. char *p ka matlab hai "p yaad rakhta hai jahan ek row of chars shuru hoti hai."
Importantly, ek pointer sirf starting address hold karta hai. Usse yaad nahi rehta ki aage kitne boxes hain.
Yahi wajah hai C ko '\0' chahiye — pointer akele tumhe nahi bata sakta kahan rukna hai. Deep dive:
Pointers in C .
sizeof pointer par buffer ke baare mein jhoot bolta hai
Kyun sahi lagta hai: sizeof buf ne array size paane ke liye kaam kiya. Trap: jab buf ek char * ban jaata hai (jaise ek function parameter), tab sizeof buf pointer ka size deta hai (aksar 8 Arrays vs Pointers ki kyun ek array pointer mein "decay" hoti hai. Fix: size ko function boundaries ke across pass karo.
Reveal check:
Kya ek char * yaad rakhta hai ki us ke address ke baad kitne characters aate hain? Nahi — sirf start; '\0' end mark karta hai.
size_t
size_t woh type hai jo C sizes aur indices ke liye use karta hai — ek pura number jo kabhi negative nahi hota (tumhare paas negative box count nahi ho sakta). Parent ke code mein loop variables n, i, j is type ke hain.
Reveal check:
Length ke liye int ki jagah size_t kyun use karte hain? Ek length ya index kabhi negative nahi hota; size_t unsigned counting type hai.
Definition Woh do symbols jo log confuse karte hain= ka matlab hai is value ko store karo ("right side ko left box mein daalo"). != ka matlab hai equal nahi — ek haan/na ka sawaal. while (test) body ka matlab hai body ko tab tak repeat karo jab tak test true ho .
Parent ki core line hai while (s[n] != '\0') n++; — ise padho: "jab tak main jis box ko dekh raha hoon woh STOP box nahi, use count karo aur next par step karo." Yeh har string routine ki shape hai.
(dst[i] = src[i]) != '\0' ek clever line kyun hai
Andar ki dst[i] = src[i] ek box copy karti hai aur copied value return karti hai; bahari != '\0' phir us value ko test karti hai. Toh terminator pehle copy hoti hai, phir test fail hota hai — STOP dst mein safely land hoti hai rokne se pehle.
Reveal check:
= aur != mein kya antar hai?= ek value store karta hai; != poochta hai "kya yeh dono unequal hain?".
O ( n ) O ( n ) kaam size n ." Kyunki pointer koi length store nahi karta, strlen ko STOP tak har box touch karna padta hai — n n Time Complexity .
Yahi wajah hai ki loop ke andar strlen (ya strcat, jo secretly end dhundhne ke liye scan karta hai) call karna slow ho sakta hai: har call poori row ko dobara walk karta hai.
Reveal check:
strlen O ( n ) Koi stored length nahi, toh ise '\0' tak box by box scan karna padta hai — woh n
Definition Buffer overflowBuffer overflow tab hota hai jab tum ek array ke last legal box ke baad likhte ho. Kyunki C kabhi bounds check nahi karta, extra bytes neighbouring memory mein spill ho jaate hain. Consequence hai
Buffer Overflow & Memory Safety / Undefined Behaviour in C — crashes ya exploits.
Classic cause hai STOP box ka +1 bhool jaana: char buf[5]; strcpy(buf, "Hello"); ko 5 mein 6 boxes chahiye.
Reveal check:
C mein agar tum ek array ke end ke ek box baad likhte ho toh kya hota hai? Undefined behaviour — koi bounds check nahi; memory corrupt ho sakti hai, crash ho sakta hai, ya exploit ho sakta hai.
null terminator zero byte
string literal adds STOP for free
pointer holds only the start address
while loop walks until STOP
buffer overflow if no room
Kya tumhare paas har piece hai? Sirf apni awaaz mein answer dene ke baad reveal karo.
Ek char actually kya store karta hai? Ek byte — ek number 0 255
char s[6] ka pehla element kaise likha jata hai, aur aakhri?s[0] pehla, s[5] aakhri (index = size − 1).
'\0' kya hai aur uski numeric value kya hai?Null terminator, ek char jo number 0
"cat" kitne boxes occupy karta hai?4 — teen letters plus '\0'.
'0' aur '\0' mein antar?'0' 48 '\0' 0
Ek char * kya store karta hai, aur kya store NAHI karta? Yeh start address store karta hai; yeh length store NAHI karta.
sizeof buf galat kyun ho sakta hai?Agar buf ek pointer hai, toh sizeof pointer ka size deta hai, buffer ka nahi.
while (s[n] != '\0') n++; ko plain words mein padho.Jab tak yeh box STOP box nahi, use count karo aur aage step karo.
strlen O ( n ) Koi stored length nahi, toh yeh '\0' tak har box scan karta hai.
Ek line mein buffer overflow kya hai? Ek array ke last legal box ke baad likhna — C mein undefined behaviour.
