First face-to-face with Undefined Behavior in C++

Mathieu — Wed, 08 Jul 2026 23:25:43 +0000

Today I spent way too long debugging my toy compiler.

The tokenizer seemed to work perfectly. GDB showed the expected tokens:

exit
69
;

Yet the generated assembly file was empty.

I checked:

the parser
the code generator
the file writing
the assembler
the linker

Everything looked fine. The culprit?

size_t m_ind;

I had forgotten to initialize it. It should have been:

size_t m_ind = 0;

or initialized in the constructor.

The funny part is that tokenize() appeared to work. That's the trap with undefined behavior in C++.

An uninitialized variable doesn't always crash your program. Sometimes it "works" just long enough to convince you the bug is somewhere else.

Today's reminder:

If you're seeing impossible behavior, suspect undefined behavior.
DO Initialize your member variables.
Never trust code just because it worked once.

First personal project: Quartzine

Mathieu — Wed, 27 May 2026 19:48:31 +0000

Just recently, I started a new project alongside my studies. Here at school, we plug USB drives in and out all the time and... there's a little virus going around.

So I came up with a simple idea: sandbox the USB drive and only retrieve the files you actually want from it.

That's how I started coding Quartzine, a sandbox for USB drives. You plug in the USB drive, Quartzine detects it, creates a virtual machine, passes the USB through to the VM, you do your stuff and... that's all I originally had in mind.

Nevertheless, since I'm not really someone who focuses on the present, I decided to go further: Integrate malware analysis into it using eBPF (through bpftrace).

That led me to learning bpftrace, and I'm still doing that haha.

If you want to take a look, here's where the code lives:
https://github.com/Mathos6/Quartzine (The project isn't fully functional yet, but I think it'll be by the end of the summer.)

I'd love to hear what you think about it, things I could improve, things I should rethink, etc.

DEV Community: Mathieu

First face-to-face with Undefined Behavior in C++

First personal project: Quartzine