Debugging C Programs

GDB is an incredibly useful tool for debugging C code. It allows you to see where errors happen and step through your code one line at a time, with the ability to see values of variables along the way. Learning how to use GDB effectively will be very important to you in this course.

Entering GDB Commandline Mode

First, make sure to compile your source files with the -g flag. This flag will add debugging symbols to the executable that will allow GDB to debug much more effectively. For example, running:

rv gcc -g -Wall -Wextra -Wpedantic -Wshadow -Wformat=2 -std=c23 hi.c

In order to use gdb in the 3410 container, you need to open two terminals: one for running qemu with the debug mode in the background; and the other for invoking gdb and iteract with it.

1. First, open a new terminal, and type the following commands:

   • docker run -i --rm -v `pwd`:/root --name cs3410 ghcr.io/sampsyo/cs3410-infra:latest. Feel free the change the “name” from cs3410 to any name you prefer.
   • gcc -g -Wall ... (more flags) EXECUTABLE SOURCE.c. Once you have entered the container, compile your source file with the -g flag and any other recommended commands.
   • qemu -g 1234 EXECUTABLE ARG1 ... (more arguments). Now you can start executing qemu with the debug mode and invoke the executable file EXECUTABLE with any arguments you need to pass in.

2. Then, open another terminal, and type the following commands:

   • docker exec -i cs3410 /bin/bash, where cs3410 is the placeholder for the name of the container you are running in the background via the first terminal.
   • gdb --args EXECUTABLE ARG1 ... (more arguments) to start executing the GDB.
   • target remote localhost:1234: execute this inside the GDB. It instructs GDB to perform remote debugging by connecting it to listen to the specified port.
   • Start debugging!

3. Once you quit a GDB session, you need to go back to the first terminal to spin up the qemu again (Step 1.3) and then invoke GDB again (Step 2.2 and onwards).