I recommend at least basic knowledge of C++, Linux and debugging to understand most of the things explained in the following part.

Assume we have this small C++ code block:

int MyDivide(int, int);

int main()
{
    int x, y, result;
    x = 10, y = 2;
    result = MyDivide(x, y);
    x = 4; y = 0;
    result = MyDivide(x, y); // Division by zero
    return result;       
}                        

int MyDivide(int _x, int _y)
{   
    return _x / _y;
}

Now put this in a file, called “divide.cpp”, and compile it to a binary, called “divide”:

$ g++ -g divide.cpp -o divide

After doing this, let us start with the GDB magic and load the “divide” binary:

$ gdb divide 
...
(gdb)

Everything is prepared, so here are some useful things you can do with GDB:

1. Start application:

(gdb) run
Starting program: /home/dennis/Entwicklung/Misc/divide 

Program received signal SIGFPE, Arithmetic exception.
0x0000000000400513 in MyDivide (_x=4, _y=0) at divide.cpp:15
15          return _x / _y;

In our case the startup of the application results in a crash, due to division by zero.

2. Backtraces:

Because of the crash, we now can get more detailed information with the backtrace.

(gdb) bt
#0  0x0000000000400513 in MyDivide (_x=4, _y=0) at divide.cpp:15
#1  0x00000000004004f9 in main () at divide.cpp:9
(gdb) bt full
#0  0x0000000000400513 in MyDivide (_x=4, _y=0) at divide.cpp:15
No locals.
#1  0x00000000004004f9 in main () at divide.cpp:9
        x = 4
        y = 0
        result = 5

3. Breakpoints:

(gdb) break divide.cpp:7
Breakpoint 1 at 0x4004ca: file divide.cpp, line 7.
(gdb) break divide.cpp:9
Breakpoint 2 at 0x4004ea: file divide.cpp, line 9.

4. Step debugging:

(gdb) run
Starting program: /home/dennis/Entwicklung/Misc/divide 

Breakpoint 1, main () at divide.cpp:7
7           result = MyDivide(x, y);
(gdb) next
8           x = 4; y = 0;
(gdb) continue
Continuing.

Breakpoint 2, main () at divide.cpp:9
9           result = MyDivide(x, y); // Division by zero

With the “next” command, GDB will go to the next line, which is executed by the application. As opposed to this the “continue” command will simply continue the application.

5. Variable manipulation:

(gdb) print y
$1 = 0
(gdb) set y = 1
(gdb) print y
$2 = 1

6. Exiting application:

(gdb) quit

I hope this tutorial helps you to work with GDB and to get some personal achievements with debugging your applications.

If you have feedback, regards, corrections or questions please let me know and do not hesitate to comment!