What do the terms "in-tree" and "out-of-tree" exactly mean? Also, does "source tree" specifically refer to the official kernel released from / maintained at kernel.org or is it a more general term which can refer to any (modified) Linux kernel source?
1 Answers
"source tree" is not a term specific to kernel source development, so it has to be a more general term and its meaning with regards to kernel source is context dependent.
I have not come across "in-tree" and "out-of-tree" outside of the Linux kernel source development and then only for working with modules. All modules start out as "out-of-tree" developments, that can be compiled using the context of a source-tree. Once a module gets accepted to be included, it becomes an in-tree module. A I have not come across an official definition for both terms though, maybe that was never necessary as it was clear to those working with modules what was meant.
E.g. while Reiserfs module was still an out-of-tree module I did the RPM package generation for SuSE, once it became in-tree there was no longer need for that.

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has at some point (they didn't when I first used it), started to useout-of-tree
the way you describe, but they usein-place
instead ofin-tree
in their doc. – Anthon Jul 03 '18 at 16:43in-tree
andin-place
are synonyms for the same term in my experience. I don't really have references for it, it's been something I've picked up working in the industry across a number of companies and sub-fields for the last ~15 years rather than something I read in a book. I was just pointing out that in terms of technical jargon it's something you're likely to come across in other contexts with slightly different meanings. I wasn't critiquing either, just adding some additional context for future readers. – mtalexan Aug 02 '18 at 01:03"in-tree" and "out-of-tree" are actually generic software development terms
based on your, limited, experience alone. There is more than books, and if using a tool like Google cannot easily give some support to your claims, things might not be as generic as you state, and then others are better of not being fed potentially incorrect information in the way you did. YMMV – Anthon Aug 02 '18 at 06:43GNU radio
project also useout-of-tree
terminology. – matt Aug 22 '23 at 10:07