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How to Debug Using GDB.md

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How to Debug Using GDB.md

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🐞 Debugging a Program with a Logical Error using GDB

This guide walks through debugging a C++ program that contains logical errors.
The program is supposed to compute:

[ (X^0)/0! + (X^1)/1! + (X^2)/2! + (X^3)/3! + (X^4)/4! + \dots + (X^n)/n! ]

for given inputs x and n.
However, it always outputs infinity (inf), regardless of the input.

We will trace the issue step by step using GDB.

📂 Step 1: Get the Sample Program

Download the buggy program:

➡️ broken.cpp

⚙️ Step 2: Compile and Run

Compile with debugging info: g++ -g broken.cpp -o broken ./broken

🐞 Step 3: Start GDB

gdb broken

🎯 Step 4: Set a Breakpoint

Set a breakpoint at line 43:

(gdb) b 43

This corresponds to: double seriesValue = ComputeSeriesValue(x, n);

▶️ Step 5: Run Program in Debugger

Run the program:

(gdb) run

When prompted, enter:

x = 2 n = 3

Expected output: 5

Debugger stops at the breakpoint: Breakpoint 1, main () at broken.cpp:43 43 double seriesValue = ComputeSeriesValue(x, n);

🧭 Step 6: Step Into ComputeSeriesValue

To step into the function:

(gdb) step

Execution moves to: 17 double seriesValue=0.0;

Continue stepping: (gdb) next 18 double xpow=1; (gdb) n 20 for (int k = 0; k <= n; k++) { (gdb) n 21 seriesValue += xpow / ComputeFactorial(k) ; (gdb) step

Now inside ComputeFactorial(): 7 int fact=0; // ⚠️ suspicious

🔍 Step 7: Inspect Execution

Use backtrace to see where we are:

(gdb) bt #0 ComputeFactorial (number=0) at broken.cpp:7 #1 0x08048907 in ComputeSeriesValue (x=2, n=3) at broken.cpp:21 #2 0x08048a31 in main () at broken.cpp:43

Step through the loop:

(gdb) next 9 for (int j = 0; j <= number; j++) { (gdb) n 10 fact = fact * j; (gdb) n 9 for (int j = 0; j <= number; j++) {

Check variable value:

(gdb) print fact $2 = 0 Continue: (gdb) n 13 return fact; (gdb) quit

🚨 Bug Found

fact was initialized as 0 Factorial is computed as: fact = fact * j;

💥 Debugging a Program that Produces a Core Dump

This guide explains how to debug a program that crashes due to a segmentation fault and produces a core dump.

📂 Step 1: Download the Program

Download the sample program:

➡️ testit.c

⚙️ Step 2: Compile the Program

Compile with debugging information: gcc -g testit.c -o testit

▶️ Step 3: Run the Program

Execute the binary: ./testit

Output: Segmentation fault (core dumped) A core dump file named core will be generated.

🐞 Step 4: Debug Using GDB

  1. Enable Core Dumps Temporarily Run this before executing your program:

ulimit -c unlimited

This tells the shell to allow core files of unlimited size.

Start GDB with both the program and the core file:

gdb testit core

Expected Output: GNU gdb 19991004 Core was generated by `testit'. Program terminated with signal 11, Segmentation fault. Reading symbols from /usr/lib/libstdc++-libc6.1-1.so.2...done. Reading symbols from /lib/libm.so.6...done. Reading symbols from /lib/libc.so.6...done. Reading symbols from /lib/ld-linux.so.2...done. #0 0x804851a in main () at testit.c:10 10 temp[3] = 'F';

🔎 Step 5: Analyze the Error

The debugger shows the crash occurred at line 10:

temp[3] = 'F'; Looking at the declaration on line 5:

char *temp = "Paras"; temp is a pointer to a string literal.

String literals in C/C++ are read-only, so attempting to modify them causes a segmentation fault.