Category Archives: _Twitter Archive 🐤

SerenityOS Day 1: Debugging the piano app

New screencast! I filmed myself debugging the SerenityOS Piano app having zero experience with the codebase or OS. 🐞

In this, I show my approach to working with and debugging unknown systems. A super useful skill to have!

cc @awesomekling 🫡https://t.co/5GTXmVYxG6
— Mark Mossberg (@offlinemark) September 16, 2023

I love spelunking into unknown codebases with nothing but find and grep. It’s one of the most valuable skills one can develop as a programmer imo and in this video you can see how I approach it.

This video focuses on debugging GUI event handling. At first the bug seemed related to the app’s waveform selection, but I then realized it was a more general topic with the SerenityOS GUI UX — selecting a dropdown entry retains focus, and requires an explicit escape key.

Ultimately I made progress accidentally by hitting the keyboard while the selection was still active, revealing to me that fact (which I hadn’t noticed before).

You can see my general debugging flow:

Get things building
How to run app from command line (to see stdout)?
How to print to stdout?
Using debug prints to understand the GUI event handling

Overall I’m quite impressed with SerenityOS. I only realized after looking into the code exactly how much code they had written and how fully featured the system is. Well done to the team.

Do you need to learn how to implement a red-black tree?

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confession: I've never implemented a red-black tree. I don't even really know what one is.

I didn't really study comp sci, so I never took an algo/DS course that covered them. It was always something I figured I could learn when needed. So far I've never needed to 🙂
— Mark Mossberg (@offlinemark) September 12, 2023

I hear you. The point isn't pride about ignorance. The points are:

– Learning is personal.
– Given limited resources, prioritize learning about what matters most to you, first.
– Certain topics may be less important for an individual than the discourse would lead you to believe.
— Mark Mossberg (@offlinemark) September 13, 2023

Pitfalls with fork() in real-time contexts

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This thread went viral. The main takeaways:

After calling fork(), a parent process gets its entire address space write protected to facilitate COW. This causes page faults.
This makes fork() unsafe to call from anywhere in a process with realtime deadlines — including non realtime threads! Usually non RT can do what they want, but that is an interesting exception.
On modern glibc, system() doesn’t use fork(), it uses posix_spawn(). But is posix_spawn() safe from a non RT thread?
posix_spawn() doesn’t COW — the parent/child literally share memory — so the page fault issue doesn’t apply. However the parent is suspended to prevent races between the child and parent. This seems RT unsafe…
However, only the caller thread of the parent is suspended, meaning the RT threads are not suspended and continue running with no page faults.
So it is safe to use system() or posix_spawn() from a non RT thread.

linux virtual memory fun fact of the day:

after a fork+exec to run a subshell, a parent process will page fault on every first write to every single writable page in its address space
— Mark Mossberg (@offlinemark) April 4, 2023

Save all the code you write

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Programming advice: Save all the code you write. All of it. Especially those dinky little one-off projects that went "nowhere".

I can't tell you the number of times I've gone back to reference some code snippet I wrote years ago. Happens more often than you'd think.
— Mark Mossberg (@offlinemark) February 1, 2023

Pick an impressive project, then checkout to the first commit

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A fun exercise: Pick an impressive software project that you respect, then checkout to the first commit and take a look around.

It's a great way to remind yourself that everyone starts somewhere 🐣

Some github links below:
(llvm/chromium/serenityos/cpython/react/curl/blender)
— Mark Mossberg (@offlinemark) February 4, 2023

A takeaway I often get from exploit writeups

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A takeaway I often get from exploit writeups: it's probably easier than you think to become one of the world's leading experts on impl details of a niche corner of some widely used software.

It's just a matter of time spent. I'd guess you can get v far with 20 focused hours. https://t.co/uWrkmlVqWo
— Mark Mossberg (@offlinemark) January 24, 2023

Can we have non-atomic std::shared_ptr in C++?

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Turns out, we do have them. libstdc++ contains a heuristic ~~kludge~~ where it checks if libpthread is linked into the process and conditionally executes an atomic add or non-atomic add depending on the result. Post 2020, it doesn’t directly use the libpthread hack, but uses glibc’s native support for this.

These kinds of things always make me sad about C++ (especially compared to how nice and clean things seem to be in Rust world), but at least it’s nice knowing that the committee did consider it and there are good reasons why to not have this in the C++ stdlib. The most compelling reason is that in the absence of a borrow checker, it is all too easy to silently use a non-atomic shared_ptr in multithreaded code.

Full thread:

me: can we have a non-atomic refcount shared_ptr in c++ like they do in rust?

mom: we have those at home

at home: pic.twitter.com/NDYnwvezyB
— Mark Mossberg (@offlinemark) January 23, 2023

Short thread about LLDB pretty printing internals

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Interesting fact about lldb: It provides built-in pretty printers for common libc++ types, which actually requires heroic effort.

This requires reimplementing the data structure directly at the private impl level — using knowledge of the internal layout and private member names. pic.twitter.com/mvEsPOrL7N
— Mark Mossberg (@offlinemark) October 13, 2022

My audio development journey

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Wrote a thread about it:

So excited for @audiodevcon soon! It means a lot; ADC 2019 started the incredible journey the last few years have been:

ADC 2019: Outsider, total noob
ADC 2020: Speaker
ADC 2021: Attendee, via my new employer @ableton ❤️

here's my story (how i broke into my dream industry)
— Mark Mossberg (@offlinemark) November 11, 2021

“Menu reference” shaped solutions

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I'm in my hometown and visited the burger joint I worked at during my gap year in 2020. Before I left, I wanted to find some way improve their system and I was delighted to see my contribution still there 1 year later, and very heavily used 🙂

backstory: pic.twitter.com/7zuRP6Oq0W
— Mark Mossberg (@offlinemark) October 28, 2021