GDB debugging — breakpoints, watchpoints, step, next, backtrace

5.3.15 · Coding › Build Systems & Toolchain

KAUNSA tool kya hai

KYUN breakpoints kaam karte hain (first principles, koi magic nahi)

Toh ek breakpoint byte substitution + ek trap hai. Koi formula derive karne wala nahi hai, lekin neeche ek overhead model zaroor hai (neeche).

Cost model derive karna (Forecast-then-Verify)

Figure — GDB debugging — breakpoints, watchpoints, step, next, backtrace

Ek poora worked debugging session

Buggy program (bug.c):

int factorial(int n) {
    int result = 1;
    for (int i = 1; i <= n; i++)
        result *= i;          // overflow for big n
    return result;
}
int main() {
    int x = factorial(5);
    return 0;
}

Worked example GDB mein step-by-step

$ gcc -g -O0 bug.c -o bug      # -g = debug symbols, -O0 = don't optimize away vars
$ gdb ./bug
(gdb) break factorial          # stop when we enter factorial
(gdb) run                      # start the program
(gdb) bt                       # show call stack: factorial() <- main()
(gdb) watch result             # hardware watchpoint on result
(gdb) continue                 # runs until result CHANGES
(gdb) print i                  # inspect loop index
(gdb) next                     # do one line, stay in factorial
(gdb) step                     # (if a call existed) dive in
(gdb) finish                   # run till current function returns, print return value

-g kyun? Debug symbols ke bina GDB raw addresses dekhta hai, result ya line 3 nahi. -O0 kyun? Optimizers variables delete/merge kar sakte hain; watch result tab "no symbol" bol sakta hai. Pehle bt kyun? Confirm karta hai ki tum kaise pahunche — crashes ke liye sabse useful command.

Worked example Segfault debug karna

(gdb) run
Program received signal SIGSEGV, Segmentation fault.
0x... in process (p=0x0) at list.c:88
(gdb) bt                # frame 0: process, frame 1: main — caller passed NULL
(gdb) frame 1           # switch to main's frame
(gdb) print head        # see it was never initialized

Yeh step kyun? Crash line tumhe batati hai ki kahan mara; bt + frame batate hain ki kisne bad value di — actual bug usually ek frame upar hoti hai.

Common mistakes (Steel-man + fix)

Flashcards

GDB x86 pe software breakpoint implement karne ke liye kaun sa instruction byte insert karta hai?

0xCC (the INT3 trap), original byte save karne ke baad.

step aur next mein kya fark hai?

step function calls ke andar utarta hai; next unhe completion tak run karta hai aur current level par rehta hai.

Watchpoint kya hota hai (breakpoint se alag)?

Ruk jaata hai jab kisi expression ki value change ho (data), na ki jab koi line reach ho (location).

Software watchpoints slow kyun hain par hardware wale fast?

Software har line ko single-step karta hai aur value dobara read karta hai (

T_{0} + N c

); hardware CPU debug registers use karta hai jo write par automatically trap karte hain (~koi overhead nahi).

bt (backtrace) kya dikhata hai, aur crash pe pehle use kyun karo?

Active function calls ki chain (the stack); reveal karta hai ki crashing function ko kisne call kiya, wahi real bug aksar hoti hai.

Debugging se pehle -g -O0 se compile kyun karo?

-g debug symbols add karta hai (names/line numbers); -O0 optimizer ko variables delete/merge karne se rokta hai jinhe tum inspect karna chahte ho.

finish kya karta hai?

Current function ke return hone tak run karta hai aur uski return value print karta hai.

Software watchpoint ka slowdown formula kya hai?

S = 1 + N c / T_{0}

jahan

N

=lines,

c

=per-line trap cost,

T_{0}

=normal time.

x86 typically kitne hardware watchpoints allow karta hai?

~4 (debug registers DR0–DR3), har ek ≤8 bytes watch karta hai.

Tum kaun sa stack frame ke variables inspect kar rahe ho woh switch karne ka command?

frame <n> (ya up/down).

Recall Feynman: 12-saal ke bachche ko samjhao

Ek super-fast cartoon flipbook imagine karo jo ek second mein ek million pages flip karta hai — tum koi ek drawing nahi dekh sakte. Ek breakpoint ek chosen page par bookmark lagana hai taaki flipping wahan ruk jaaye aur tum dekh sako. Ek watchpoint kehna hai "jis pal bhi is character ki hat ka color badle wahan ruk jao" chahe koi bhi page ho. Step matlab hai "bilkul ek page palto, aur agar woh page tumhe kisi sub-story mein bheje, toh andar jaao." Next matlab hai "ek page palto, lekin agar woh tumhe kisi sub-story mein bheje, toh poori sub-story khatam hone do aur wapas aao." Backtrace unhe bookmarks ka trail hai jo dikhata hai ki tum yahan pahunchne ke liye kaun se doors khole the. Bugs fast motion mein chhupte hain; debugger time ko rokta hai taaki tum unhe pakad sako.

Connections

Compilation & -g debug symbols
Optimization levels -O0 -O2 -O3
Stack frames & calling conventions
Signals — SIGSEGV SIGTRAP
CPU debug registers DR0–DR7
Memory debugging — Valgrind & AddressSanitizer
The scientific method in debugging