How to do it...

In order to use some of the GDB commands, we need to modify the previous program and add some variables in it:

1. Open a shell and modify the hello.cpp file by typing in the following code:

 #include <iostream>
 int main()
 {
    int x = 10;
    x += 2;
    std::cout << "Hello World! x = " << x << std::endl;
    return 0;
 }

This is a very simple program: take a variable, add 2 to it, and print the result.

2. Let's make sure that the program is compiled by typing the following command:

root@bffd758254f8:~/Chapter1# make
 g++ -c hello.cpp
 g++ -o hello hello.o

3. Now that we have the executable, we will debug it. From the command line, type gdb hello:

root@bffd758254f8:~/Chapter1# gdb hello
 GNU gdb (Ubuntu 8.2.91.20190405-0ubuntu3) 8.2.91.20190405-git
 Copyright (C) 2019 Free Software Foundation, Inc.
 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
 This is free software: you are free to change and redistribute it.
 There is NO WARRANTY, to the extent permitted by law.
 Type "show copying" and "show warranty" for details.
 This GDB was configured as "x86_64-linux-gnu".
 Type "show configuration" for configuration details.
 For bug reporting instructions, please see:
 <http://www.gnu.org/software/gdb/bugs/>.
 Find the GDB manual and other documentation resources online at:
 <http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
 Type "apropos word" to search for commands related to "word"...
 Reading symbols from hello...
 (No debugging symbols found in hello)
 (gdb)

4. As you can see, the last line says (No debugging symbols found in hello). GDB doesn't have to debug symbols to debug the program, so we have to communicate to the compiler that the debug symbols are to be included during the compilation. We have to quit the current session; to do this, type q (Enter]. Then, edit the makefile, and add the -g option to the g++ compiler section (the hello.o target):

CC = g++
all: hello
hello: hello.o
    ${CC} -o hello hello.o
hello.o: hello.cpp
    $(CC) -c -g hello.cpp
clean:
    rm hello.o hello

5. Let's run it again, but, first, we have to rebuild the application with the make command:

root@bcec6ff72b3c:/BOOK/chapter1# gdb hello
GNU gdb (Ubuntu 8.2.91.20190405-0ubuntu3) 8.2.91.20190405-git
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
 <http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from hello...
(No debugging symbols found in hello)
(gdb)

We're ready to debug it. A debug session typically includes setting breakpoints, watching the content of variables, setting watchpoints, and many others. The next section will show the most common debug commands.