You know what, if you have hesitation about posting something, then you probably shouldn't. It's one thing to have a specific concern about his project, but it's quite another to call into question his skill as a programmer and indeed quite arrogant to assume that you know what it takes to build an OS.
There is a big difference between saying "You don't have what it takes" and saying, "I think the obstacles you face will be very large, good luck!"
He is right though - I was really into messing with kernel development a few years ago, and despite being an experienced C/C++ developer and working on it on and off for a couple of years, nothing useful came out of it.
You really need to be extremely familiar with computer internals, have a very good grasp of the instruction set architecture of the platform you're targeting, have expert C knowledge, and at least be very comfortable programming with assembly.
And even then you're going to fail unless you're working with a large team - there's just too many parts of a modern operating system for one person to ever tackle. Even if you only want to support a single set of hardware (one graphics card, one network card, etc), you'd spend years just writing drivers for everything and by then the hardware would be long obsolete.
I'm not trying to say that he shouldn't do it - it is great for learning how computers work on a low level - but he really does need lots of experience, and should go into it with the correct expectations.
You are overstating what an operating system is. The core of most modern operating systems were written by a single person, or two people - including Windows.
Operating systems become large because of hardware support. They are also large now because what is considered 'operating system' has changed - from including a desktop environment to including a web browser.
The reason why OS X and Windows have large teams and year-long development cycles is because they are a complete stack of applications, not just a kernel, fs etc.
To get an OS written from scratch to boot on specific hardware with some basic functionality should be an 8-12 week job for a competent C developer.
The best thing I did in my teens was to grab x86 docs and attempt to write an OS. I thoroughly recommend it to everybody as a good project to learn system and development (along with writing a compiler and writing a simple database).
quite arrogant to assume that you know what it takes to build an OS
With all of the (largely web/app-centric) development work that I have done, there are numerous concepts in OS development that baffle me. I don't claim to know what it takes to build an OS, but I know that it is beyond me.
Also, I wasn't questioning his skill as a programmer, I was questioning the undertaking that he is discussing. Any solo developer would struggle to create an OS (especially when they have a full time job to contend with), let alone someone that is going to have to learn a lot along the way. So my suggestion was to go ahead, but treat it as a learning experience rather than a deliverable product.
Everyone that ever created an OS from scratch had to learn first. The only question is how long it will take, but there is no reason to say "you will fail".
treat it as a learning experience rather than a deliverable product.
But what is the purpose of mentioning this?
If you are recommending that he go ahead with the endeavour, then what does it matter if he ends up with a deliverable product or not.
Also, these kind of projects can have a timeframe of years, regardless of programming skill. If he finishes it, it will be a result of him sticking to it in the long run, not a result of how much C knowledge he had on the first day of the project.
There is a big difference between saying "You don't have what it takes" and saying, "I think the obstacles you face will be very large, good luck!"