[syockit]

It's copyrighted, but nothing stops you from looking at it. It's akin to looking a t a mural painted at someone's house. You can also comment as long as you don't post snippets of the code (except for when fair use is applicable).

[rurban]

Legally yes, but in reality not. In most companies or projects legal tells me not to look at it all, esp. the FSF. I can look at the API, but not the implementation.

That's why it's called https://en.wikipedia.org/wiki/Clean-room_design.