Getting Started with GDB

GDB is a very powerful debugging tool for C/C++ and many other languages. Here I'm gonna show you how to get started with GDB.

testgdb.cpp

#include <iostream>
#include <string>
using namespace std;

string globalmsg = "Hello World";

void half(int& a) {
    a /= 2;
}

int whatever() {
    int i = 5;
    i *= 3;
    return i;
}

int main() {
    int a = 10;
    half(a);
    cout << a << endl;
    cout << whatever() << endl;
    return 0;
}

1. Compile the code

g++ -c -g -Wall -O0 testgdb.cpp

The -O0 flag is useful for debugging information because the compiler may do its own optimization that can eliminate some debugging information, such as -O2 flag for example.
2. Link it

g++ -o testgdb testgdb.o

3. Run it

gdb ./testgdb

After running this statement, there will be gdb prompt.
4. Set the breakpoint in line 18 and 19

break testgdb.cpp:18

break testgdb.cpp:19

5. See the list of breakpoints

info break

6. Remove the second breakpoint

delete 2

7. Run the program

run

8. See the current line

list

9. Step over to the next line

next

10. See the current line again

list

11. Step in to the half() function

step

12. Print the arguments

info args

13. Go to the next line

next

14. Print the value a now

print a

The new value should be 5.
15. Step out from half() function

finish

16. Print all the local variables

info locals

17. Set another breakpoint in line 13

break testgdb.cpp:13

18. Jump to that newly-set breakpoint

continue

19. Step over

next

20. Print the new value of i

print i

The value of i should be 15
21. Step out from the whatever() function

finish

22. Get the length of globalmessage string

print globalmessage.size()

23. Quit gdb

quit

I hope this brief introduction is useful. For more information, type

help <command>