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Using GDB to Debug C and C++

· 7 min read
Brendan Smiley
Brendan Smiley
Software Engineering Student @ University of Calgary

A beginner's guides to debugging C and C++ programs with GDB.

TLDR​

The goal of this page is to be a reference to come back to when you want to debug with GNU Debugger (GDB). The first few sections are more educational and provide a basic understanding of GDB and Cygwin. The later sections are more practical and provide a reference for using GDB in Cygwin.

GDB Usage

Other Options​

Since originally writing this, I have found that using Visual Studio Code with the C/C++ extension is a much better option for debugging C and C++ programs. However, in my opinion, there is still a steeper learning curve to configuring C++ in Visual Studio Code. A big reason it is harder to use for a beginner is because you have to understand how VsCode is configuring the debugger and how it is using GDB or LLDB under the hood.

You might opt for cygwin to get a more basic and native experience with C and C++ debugging. You will also understand the Visual Studio Code debugger better if you start with GDB in Cygwin (VsCode uses GDB, LLDB, etc under the hood for C and C++ debugging).

Setting Up Cygwin​

What is Cygwin? Cygwin is a Unix-like environment for Windows. It allows developers to have access to a more Unix-like terminal and tools. I say "Unix-like" because it is not a full Unix system. Cygwin uses a DLL called cygwin1.dll to interact with Windows. To have a full Unix, Linux system on your Windows machine, see WSL or Virtual Machines.

To set up Cygwin, you can follow the instructions on the Cygwin website. The Cygwin installer is named setup-x86_64.exe. There are different mirrors you can select from, they all should provide the packages being downloaded. Continue to the download page. On the download page, there are different buttons like filters and version selections you can use to customize your Cygwin install. If you rerun the installer (setup-x86_64.exe), you can update your custom install. You do not need to install everything (MASSIVE DOWNLOAD!). The recommended installs for this environment setup are the following:

  • Base Category:
    • All packages in the Base category (May be automatically selected)
  • Devel Category:
    • gcc-core
    • gcc-g++
    • clang
    • gdb
  • Debug Category:
    • gdb-debuginfo
    • gdbm-debuginfo
  • nano

Continue the install until it is complete. You can also install other packages, such as make, cmake, and git if you want to level up your development environment.

The Cygwin Folder Structure​

Just a quick note on the Cygwin folder structure. The Cygwin install will create a folder called C:\cygwin64. Cygwin will use its own folder structure outside of the Windows folder structure. If you want your project to exist within the Windows folder structure, use cd C:// && ls to get to the root of the C drive. To get back to the Cygwin user folder, use cd ~.

Getting Started with GDB​

This section will cover a basic step-by-step example of using GDB to debug a binary executable.

  1. Validate installs: gdb --version, gcc --version, g++ --version,

  2. Create a simple C or C++ program to debug. For example, create a file with the following steps:

    1. Create a file either with Notepad++ or with nano example.cpp

    2. Write and save a simple program in the file:

      #include <iostream>

      int main() {
          int a = 5;
          int b = 0;
          std::cout << "Enter a number: ";
          std::cin >> b;
          std::cout << "The sum is: " << (a + b) << std::endl;
          return 0;
      }

    3. Validate the file is saved by running cat example.cpp to see the contents of the file.

    4. Use g++ -Wall -g -o example example.cpp to compile the program. The -Wall flag enables all warnings, and the -g flag includes debugging information in the binary. -g is required for GDB to work properly.

    5. ls to see the compiled binary example in the current directory.

    6. To run the program with GDB, use gdb ./example -tui. The -tui flag enables the text user interface for GDB, which is more user-friendly. You don't have to use -tui, but I find it significantly easier to use and closer to a Visual Studio Code experience.

    7. Press enter to start GDB. Once you see (gdb), you are in the GDB terminal. You can use gdb terminal commands to debug the program.

    8. r to run the program. If you have not set any breakpoints, it will run to completion and exit. If you want to set a breakpoint, see the next step.

    9. b 4 to set a breakpoint at line 4 of the source code.

    10. r to run the program again. This time, it will stop at the breakpoint you set.

    11. n to step to the next line of code.

    12. print a to print the value of a. Definitely experiment with printing other types of variables and expressions. You can do pointer dereferencing, arithmetic, and more. This is extremely useful for memory management debugging. The print statement can do A LOT like math print a + 6 or print &a.

    13. n until you enter a number. After you enter a number, it will look messy on the terminal. This happens when you use std::cin or std::cout. The terminal will show the output of the program, which can make it hard to read. You can clear the terminal with CTRL + L to make it easier to read.

    14. c to continue running the program until it exits or hits another breakpoint.

That's it for the example! Now you can use the commands below to debug the program. I will be adding to this section as I learn more about the GDB terminal.

Useful commands​

How I use a minimal C and C++ setup and useful commands. This section is for reference only and therefore the explainations are minimal. You can always search for more information on the commands you are interested in.

Compile and Starting in GDB GUI​

For C: gcc -Wall -g -o <OutputName> <file1.c> <file2.c> && gdb ./<OutputName> -tui

For C++: g++ -Wall -g -o <OutputName> <file1.c> <file2.c> && gdb ./<OutputName> -tui

Using Auto Complete​

tab

Running the application​

press enter on GDB start r for run (make sure you have breakpoints set first or it will just run) You can restart the program by calling r again

Use Previous Command​

Enter to run the previous command again

Refresh the GUI screen​

CTRL + L

Next (Step Over)​

n

Step Into​

s

Continue​

c

Exit/Quit GDB​

q

Set a Breakpoint​

b <classname>::<methodname> or b <linenumber> or b <functionName>

Changing Window Focus​

focus cmd for repeating commands (can use mouse scroll to move source code in this mode) focus src to move the displayed source code window (can use arrow keys to move source code in this mode)

Printing​

can use print(<variable>) for a quick check can use printf("<regular c string formatter here>"); for a more complex check

Example: print this->idM print &this print a + 6

Segmentation Fault - Backtrace​

bt or backtrace

References​

Below are some additional references that I found useful learning GDB. Definitely check them out to learn much more!