🐞 Introduction to GDB (GNU Debugger)

1. What is GDB?

GDB (GNU Debugger) is a powerful tool that allows you to:

• Run your program step by step

• Pause at specific lines or functions (breakpoints)

• Inspect variables and memory in real-time

• Trace the call stack when something goes wrong

In short, GDB helps you understand how your program is running under the hood and catch bugs more effectively.

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2. Hands-on: Debugging example.cpp with GDB

0) Source

// example.cpp
#include <iostream>
using namespace std;

int add(int a, int b) {
    int c = a + b;
    return c;
}

int main() {
    int x = 5, y = 10;
    int result = add(x, y);
    cout << "Result: " << result << endl;
    return 0;
}

1) Build with debug symbols

g++ -g example.cpp -o example

• -g adds line/variable info for GDB.

2) Start GDB and set a breakpoint

gdb ./example
(gdb) break add            # Stop at the start of add()
Breakpoint 1 at 0x...: file example.cpp, line 5.

3) Run until the breakpoint hits

(gdb) run
Starting program: /path/to/example
Breakpoint 1, add (a=5, b=10) at example.cpp:5
5       int c = a + b;

4) Inspect variables

(gdb) print a
$1 = 5
(gdb) print b
$2 = 10

5) Step over and check the new value

(gdb) next                 # executes 'int c = a + b;'
6       return c;
(gdb) print c
$3 = 15

6) See where we came from (call stack)

(gdb) backtrace
#0  add (a=5, b=10) at example.cpp:6
#1  0x... in main() at example.cpp:12

7) Finish the function and continue

(gdb) finish               # run until add() returns
Run till exit from #0  add (a=5, b=10) at example.cpp:6
0x... in main() at example.cpp:12
(gdb) continue
Result: 15
[Inferior 1 (process ...) exited normally]

8) Quit GDB

(gdb) quit

3. Basic GDB Commands

Starting GDB

gdb ./example

Running the Program

(gdb) run
(gdb) run arg1 arg2    # Run with arguments

Breakpoints

(gdb) break main       # Stop at main()
(gdb) break 15         # Stop at line 15
(gdb) delete           # Remove all breakpoints
(gdb) info breakpoints # Show current breakpoints

Controlling Execution

(gdb) next     # Execute next line (skip into functions)
(gdb) step     # Step into a function
(gdb) continue # Run until the next breakpoint
(gdb) finish   # Run until the current function returns

Inspecting Variables

(gdb) print x         # Print value of variable x
(gdb) print arr[3]    # Print array element
(gdb) display x       # Automatically show x after each step
(gdb) watch x         # Pause when x changes

Program State

(gdb) backtrace   # Show call stack
(gdb) info locals # Show local variables
(gdb) info args   # Show function arguments

Exiting GDB

(gdb) quit

4. Why Use GDB?

• Find logic errors that print statements can’t show

• Debug segmentation faults by checking where the crash happened

• Inspect call stacks in recursive or complex programs

• Essential foundation for advanced debugging tools (like cuda-gdb for GPU code)

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