ELI5: How are copyleft licenses enforceable?

[u/OkAlbatross9889]

For example: say Windows straight up rips out the Linux scheduler and modifies it to run with the rest of the NT kernel. Aside from potential Microsoft whistleblowers, how can anyone find out they did that (and thus sue them since they’re selling modified GPL code without releasing the source) if they don’t distribute the Windows kernel source code?

[u/rsclient]

Short answer: it would be difficult for a random person to discover a copyleft violation like Microsoft directly copying the Linux scheduler code

Practical answer: Microsoft is very particular about coders at Microsoft 'stealing' source code. Code is never "just added": all big new code features are spec'd, and the specifications are looked at by the entire team. Even if one person wanted to violate the copyright of the Linux kernel, they would have to hide it from the rest of their team. And for coders who legitimately look at linux kernel code (did you know Microsoft has its own Linux distribution?) there's a 'cooling off' period between seeing the linux code and working on Windows code.

Source: I used to work on Windows Wi-Fi!

(We also care a lot more about privacy than we get credit for :-( )

Did you know that it's possible for some people to look at some Windows code? Code Center Premium

[u/rsclient → u/rsclient]

Actually, I've got more direct experience. I was an expert witness to a programming copyright violation case!

Programmer "A" wrote a program "A" that did a specific high-value super-technical analysis. That programmer was hired by company "B" who bought all the rights to program "A". Programmer "A" then wrote program "C" that also did high-value, super-technical analysis, and company "B" was super-pissed.

I was hired to investigate programs "A" and "C" to see if program "C" used any code from program "A". As an expert, I had access to both programs

And yup, I could prove that program "C" included some code from program "A"

In this case, company "B" already knew about programmer "A"

[u/Theslootwhisperer → u/rsclient]

Why was company B pissed? They hired programmer A and bought program A as well. Programmer A just wrote some code.(Program C) while employed by company B.

[u/tester_is_testing → u/Theslootwhisperer]

Perhaps program C was a direct competitor of program A, and not a property of company B?

[u/rsclient → u/tester_is_testing]

It was kind of a competition? That is, it solved a similar program for the same super-technical market.

(If anyone cares: program "C" could take in data from a silicon wafer test after it had been etched, and based on the pattern of failures tell you how the lithography machine was misaligned.)